Spectropolarimetry of Sunspots at 0.16 ARCSEC resolution

2008. Göran Scharmer (et al.). 12th European Solar Physics Meeting

Konferens

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CRISP spectropolarimetric imaging of penumbral fine structure

2008. Göran B. Scharmer (et al.). Astrophysical Journal Letters 689 (1), L69

Artikel

We discuss penumbral fine structure in a small part of a pore, observed with the CRISP imaging spectropolarimeter at the Swedish 1-m Solar Telescope (SST), close to its diffraction limit of 0.16 . Milne-Eddington inversions applied to these Stokes data reveal large variations of field strength and inclination angle over dark-cored penumbral intrusions and a dark-cored light bridge. The mid-outer part of this penumbra structure shows 0.3 wide spines, separated by 1.6 (1200 km) and associated with 30° inclination variations. Between these spines, there are no small-scale magnetic structures that easily can be identified with individual flux tubes. A structure with nearly 10° more vertical and weaker magnetic field is seen midway between two spines. This structure is cospatial with the brightest penumbral filament, possibly indicating the location of a convective upflow from below.

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Measuring the solar atmosphere

2010. Jaime de la Cruz Rodríguez.

Avhandling (Dok)

The new CRISP filter at the Swedish 1-m Solar Telescope provides opportunities for observing the solar atmosphere with unprecedented spatial resolution and cadence. In order to benefit from the high quality of observational data from this instrument, we have developed methods for calibrating and restoring polarized Stokes images, obtained at optical and near infrared wavelengths, taking into account field-of-view variations of the filter properties.

In order to facilitate velocity measurements, a time series from a 3D hydrodynamical granulation simulation is used to compute quiet Sun spectral line profiles at different heliocentric angles. The synthetic line profiles, with their convective blueshifts, can be used as absolute references for line-of-sight velocities.

Observations of the Ca II 8542 Å line are used to study magnetic fields in chromospheric fibrils. The line wings show the granulation pattern at mid-photospheric heights whereas the overlying chromosphere is seen in the core of the line. Using full Stokes data, we have attempted to observationally verify the alignment of chromospheric fibrils with the magnetic field. Our results suggest that in most cases fibrils are aligned along the magnetic field direction, but we also find examples where this is not the case.

Detailed interpretation of Stokes data from spectral lines formed in the chromospheric data can be made using non-LTE inversion codes. For the first time, we use a realistic 3D MHD chromospheric simulation of the quiet Sun to assess how well NLTE inversions recover physical quantities from spectropolarimetric observations of Ca II 8542 Å. We demonstrate that inversions provide realistic estimates of depth-averaged quantities in the chromosphere, although high spectral resolution and high sensitivity are needed to measure quiet Sun chromospheric magnetic fields.

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High-order aberration compensation with multi-frame blind deconvolution and phase diversity image restoration techniques

2010. Göran B. Scharmer (et al.). Astronomy and Astrophysics 521, A68

Artikel

Context. For accurately measuring intensities and determining magnetic field strengths of small-scale solar (magnetic) structure, knowledge of and compensation for the point spread function is crucial. For images recorded with the Swedish 1-meter Solar Telescope (SST), restoration with multi-frame blind deconvolution (MFBD) and joint phase diverse speckle (JPDS) methods lead to remarkable improvements in image quality but granulation contrasts that are too low, indicating additional stray light. Aims. We propose a method to compensate for stray light from high-order atmospheric aberrations not included in MFBD and JPDS processing. Methods. To compensate for uncorrected aberrations, a reformulation of the image restoration process is proposed that allows the average effect of hundreds of high-order modes to be compensated for by relying on Kolmogorov statistics for these modes. The applicability of the method requires simultaneous measurements of Fried's parameter r(0). The method is tested with simulations as well as real data and extended to include compensation for conventional stray light. Results. We find that only part of the reduction of granulation contrast in SST images is due to uncompensated high-order aberrations. The remainder is still unaccounted for and attributed to stray light from the atmosphere, the telescope with its re-imaging system and to various high-altitude seeing effects. Conclusions. We conclude that statistical compensation of high-order modes is a viable method to reduce the loss of contrast occurring when a limited number of aberrations is explicitly compensated for with MFBD and JPDS processing. We show that good such compensation is possible with only 10 recorded frames. The main limitation of the method is that already MFBD and JPDS processing introduces high-order compensation that, if not taken into account, can lead to over-compensation.

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THE EFFECT OF ISOTOPIC SPLITTING ON THE BISECTOR AND INVERSIONS OF THE SOLAR Ca II 854.2 nm LINE

2014. Jorrit Leenaarts (et al.). Astrophysical Journal Letters 784 (1), L17

Artikel

The Ca II 854.2 nm spectral line is a common diagnostic of the solar chromosphere. The average line profile shows an asymmetric core, and its bisector shows a characteristic inverse-C shape. The line actually consists of six components with slightly different wavelengths depending on the isotope of calcium. This isotopic splitting of the line has been taken into account in studies of non-solar stars, but never for the Sun. We performed non-LTE radiative transfer computations from three models of the solar atmosphere and show that the line-core asymmetry and inverse C-shape of the bisector of the 854.2 nm line can be explained by isotopic splitting. We confirm this finding by analyzing observations and showing that the line asymmetry is present irrespective of conditions in the solar atmosphere. Finally, we show that inversions based on the Ca II 854.2 nm line should take the isotopic splitting into account, otherwise the inferred atmospheres will contain erroneous velocity gradients and temperatures.

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EMERGENCE OF GRANULAR-SIZED MAGNETIC BUBBLES THROUGH THE SOLAR ATMOSPHERE. I. SPECTROPOLARIMETRIC OBSERVATIONS AND SIMULATIONS

2014. Ada Ortiz (et al.). Astrophysical Journal 781 (2)

Artikel

We study a granular-sized magnetic flux emergence event that occurred in NOAA 11024 in 2009 July. The observations were made with the CRISP spectropolarimeter at the Swedish 1 m Solar Telescope achieving a spatial resolution of 0.14. Simultaneous full Stokes observations of the two photospheric Fe i lines at 630.2 nm and the chromospheric Ca ii 854.2 nm line allow us to describe in detail the emergence process across the solar atmosphere. We report here on three-dimensional (3D) semi-spherical bubble events, where instead of simple magnetic footpoints, we observe complex semi-circular feet straddling a few granules. Several phenomena occur simultaneously, namely, abnormal granulation, separation of opposite-polarity legs, and brightenings at chromospheric heights. However, the most characteristic signature in these events is the observation of a dark bubble in filtergrams taken in the wings of the Ca ii 854.2 nm line. There is a clear coincidence between the emergence of horizontal magnetic field patches and the formation of the dark bubble. We can infer how the bubble rises through the solar atmosphere as we see it progressing from the wings to the core of Ca ii 854.2 nm. In the photosphere, the magnetic bubble shows mean upward Doppler velocities of 2 km s-1 and expands at a horizontal speed of 4 kms-1. In about 3.5minutes it travels some 1100 km to reach themid chromosphere, implying an average ascent speed of 5.2 km s-1. The maximum separation attained by the magnetic legs is 6.6. From an inversion of the observed Stokes spectra with the SIR code, we find maximum photospheric field strengths of 480 G and inclinations of nearly 90. in the magnetic bubble interior, along with temperature deficits of up to 250 K at log t = -2 and above. To aid the interpretation of the observations, we carry out 3D numerical simulations of the evolution of a horizontal, untwisted magnetic flux sheet injected in the convection zone, using the Bifrost code. The computational domain spans from the upper convection zone to the lower corona. In the modeled chromosphere, the rising flux sheet produces a large, cool, magnetized bubble. We compare this bubble with the observed ones and find excellent agreement, including similar field strengths and velocity signals in the photosphere and chromosphere, temperature deficits, ascent speeds, expansion velocities, and lifetimes.

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DYNAMIC PROPERTIES ALONG THE NEUTRAL LINE OF A DELTA SPOT INFERRED FROM HIGH-RESOLUTION OBSERVATIONS

2014. A. Cristaldi (et al.). Astrophysical Journal 789 (2), 162

Artikel

Delta (delta) spots are complex magnetic configurations of sunspots characterized by umbrae of opposite polarity sharing a common penumbra. In order to investigate the fine structure of the region separating the two magnetic polarities of a delta spot, we studied the morphology, the magnetic configuration, and the velocity field in such a region using observations of active region (AR) NOAA 11267 obtained with the CRisp Imaging SpectroPolarimeter (CRISP) at the Swedish Solar Telescope on 2011 August 6. The analysis of CRISP data shows upflows and downflows of similar to +/- 3 km s(-1) in proximity of the delta spot polarity inversion line (PIL), and horizontal motions along the PIL of the order of similar to 1 km s(-1). The results obtained from the SIR inversion of CRISP data also indicate that the transverse magnetic field in the brighter region separating the two opposite magnetic polarities of the delta spot is tilted about similar to 45 degrees with respect to the PIL. Solar Dynamics Observatory/Helioseismic and Magnetic Imager observations confirm the presence of motions of similar to +/- 3 km s(-1) in proximity of the PIL, which were observed to last 15 hr. From the data analyzed, we conclude that the steady, persistent, and subsonic motions observed along the delta spot PIL can be interpreted as being due to Evershed flows occurring in the penumbral filaments that show a curved, wrapped configuration. The fluting of the penumbral filaments and their bending, continuously increased by the approaching motion of the negative umbra toward the positive one, give rise to the complex line-of-sight velocity maps that we observed.

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Sparse inversion of Stokes profiles I. Two-dimensional Milne-Eddington inversions

2015. A. Asensio Ramos, Jaime de la Cruz Rodriguez. Astronomy and Astrophysics 577

Artikel

Context. Inversion codes are numerical tools used to infer physical properties from observations. Despite their success, the quality of current spectropolarimetric observations and those expected in the near future presents a challenge to current inversion codes. Aims. The pixel-by-pixel strategy of inverting spectropolarimetric data that we currently use needs to be surpassed and improved. The inverted physical parameters have to take into account the spatial correlation that is present in the data and that contains valuable physical information. Methods. We used the concept of sparsity or compressibility to develop a new generation of inversion codes for the Stokes parameters. The inversion code uses numerical optimization techniques based on the idea of proximal algorithms to impose sparsity. In so doing, we allow for the first time exploiting the spatial correlation on the maps of physical parameters. Sparsity also regularizes the solution by reducing the number of unknowns. Results. We compare the results of the new inversion code with pixel-by-pixel inversions to demonstrate the increased robustness of the solution. We also show how the method can easily compensate for the effect of the telescope point spread function, producing solutions with an enhanced contrast.

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Small-scale magnetic flux emergence in a sunspot light bridge

2015. Rohan E. Louis (et al.). Astronomy and Astrophysics 584

Artikel

Context: Light bridges are convective intrusions in sunspots that often show enhanced chromospheric activity.

Aims: We seek to determine the nature of flux emergence in a light bridge and the processes related to its evolution in the solar atmosphere.

Methods: We analyse a sequence of high-resolution spectropolarimetric observations of a sunspot taken at the Swedish 1-m Solar Telescope. The data consist of spectral scans of the photospheric Fe I line pair at 630 nm and the chromospheric Ca II 854.2 nm line. Bisectors were used to construct Dopplergrams from the Fe I 630.15 nm measurements. We employed LTE and non-LTE inversions to derive maps of physical parameters in the photosphere and chromosphere, respectively.

Results: We observe the onset of blueshifts of about 2 km s(-1) near the entrance of a granular light bridge on the limbward side of the spot. The blueshifts lie immediately next to a strongly redshifted patch that appeared six minutes earlier. Both patches can be seen for 25 min until the end of the sequence. The blueshifts coincide with an elongated emerging granule, while the redshifts appear at the end of the granule. In the photosphere, the development of the blueshifts is accompanied by a simultaneous increase in field strength of about 400 G. The field inclination increases by some 25 degrees, becoming nearly horizontal. At the position of the redshifts, the magnetic field is equally horizontal but of opposite polarity. An intense brightening is seen in the Ca II filtergrams over the blueshifts and redshifts, about 17 min after their detection in the photosphere. The brightening is due to emission in the blue wing of the Ca II 854.2 nm line, close to its knee. Non-LTE inversions reveal that this kind of asymmetric emission is caused by a temperature enhancement of similar to 700 K between -5.0 <= log tau <= -3.0 and a blueshift of 3 km s(-1) at log tau = - 2.3 that decreases to zero at log tau = - 6.0

Conclusions: The photospheric blueshifts and redshifts observed in a granular light bridge seem to be caused by the emergence of a small-scale, flat Omega-loop with highly inclined footpoints of opposite polarity that brings new magnetic field to the surface. The gas motions detected in the two footpoints are reminiscent of a siphon flow. The rising loop is probably confined to the lower atmosphere by the overlying sunspot magnetic field and the interaction between the two flux systems may be responsible for temperature enhancements in the upper photosphere/lower chromosphere. This is the first time that magnetic flux is observed to emerge in the strongly magnetised environment of sunspots, pushed upwards by the convective flows of a granular light bridge.

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Are solar chromospheric fibrils tracing the magnetic field?

Jaime de la Cruz Rodriguez, Hector Socas-Navarro.

Artikel

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Line formation of He I D₃ and He I 10830 Å in a small-scale reconnection event

Tine Libbrecht (et al.).

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Non-LTE inversions from a 3D MHD chromospheric model

Jaime de la Cruz Rodriguez (et al.).

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Solar velocity references from 3D HD photospheric models

Jaime de la Cruz Rodríguez, Dan Kiselman, Mats Carlsson.

Artikel

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Stokes imaging polarimetry using image restoration at the Swedish 1-m Solar Telescope II: A calibration strategy for Fabry-Perot based instruments

Roald Schnerr, Jaime de la Cruz Rodriguez, Michiel van Noort.

Artikel

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Exploring spectropolarimetric inversions using neural fields: Solar chromospheric magnetic field under the weak-field approximation

2025. C. J. Díaz Baso (et al.). Astronomy and Astrophysics 693

Artikel

Context. Full-Stokes polarimetric datasets, originating from slit-spectrograph or narrow-band filtergrams, are routinely acquired nowadays. The data rate is increasing with the advent of bi-dimensional spectropolarimeters and observing techniques that allow long-time sequences of high-quality observations. There is a clear need to go beyond the traditional pixel-by-pixel strategy in spectropolarimetric inversions by exploiting the spatiotemporal coherence of the inferred physical quantities that contain valuable information about the conditions of the solar atmosphere.

Aims. We explore the potential of neural networks as a continuous representation of the physical quantities over time and space (also known as neural fields), for spectropolarimetric inversions.

Methods. We have implemented and tested a neural field to perform one of the simplest forms of spectropolarimetric inversions, the inference of the magnetic field vector under the weak-field approximation (WFA). By using a neural field to describe the magnetic field vector, we regularized the solution in the spatial and temporal domain by assuming that the physical quantities are continuous functions of the coordinates. This technique can be trivially generalized to account for more complex inversion methods.

Results. We have tested the performance of the neural field to describe the magnetic field of a realistic 3D magnetohydrodynamic (MHD) simulation. We have also tested the neural field as a magnetic field inference tool (approach also known as physics-informed neural networks) using the WFA as our radiative transfer model. We investigated the results in synthetic and real observations of the Ca II 8542 Å line. We also explored the impact of other explicit regularizations, such as using the information of an extrapolated magnetic field, or the orientation of the chromospheric fibrils.

Conclusions. Compared to traditional pixel-by-pixel inversions, the neural field approach improves the fidelity of the reconstruction of the magnetic field vector, especially the transverse component. This implicit regularization is a way of increasing the effective signal to noise of the observations. Although it is slower than the pixel-wise WFA estimation, this approach shows a promising potential for depth-stratified inversions, by reducing the number of free parameters and inducing spatiotemporal constraints in the solution.

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High flow speeds and transition-region-like temperatures in the solar chromosphere during flux emergence: Evidence from imaging spectropolarimetry in Hea I 1083 nm and numerical simulations

2025. Jorrit Leenaarts (et al.). Astronomy and Astrophysics 696

Artikel

Context. Flux emergence in the solar atmosphere is a complex process that causes a release of magnetic energy as heat and acceleration of solar plasma on a variety of spatial scales.

Aims. We aim to investigate temperatures and velocities in small-scale reconnection episodes during flux emergence.

Methods. We analyzed imaging spectropolarimetric data taken in the He I 1083 nm line with a spatial resolution of 0.26″, a time cadence of 2.8 s, and a spectral range corresponding to ±220 km s⁻¹ around the line. This line is sensitive to temperatures higher than 15 kK, unlike diagnostics such as Mg II h&k, Ca II H&K, and Hα, which lose sensitivity already at 15 kK. The He I data is complemented by imaging spectropolarimetry in the Fe I 617.3 nm and Ca II 854.2 nm lines and imaging spectroscopy in Ca II K and Hα at a cadence between 12 s and 36 s. We employed inversions to determine the magnetic field and vertical velocity in the solar atmosphere. We computed He I 1083 nm profiles from a radiation-magneto-hydrodynamics simulation of the solar atmosphere to help in the interpretation of the observations.

Results. We find fast-evolving blob-like emission features in the He I 1083 nm triplet at locations where the magnetic field is rapidly changing direction, and these are likely sites of magnetic reconnection. We fit the line with a model consisting of an emitting layer located below a cold layer representing the fibril canopy. The modeling provides evidence that this model, while simple, catches the essential characteristics of the line formation. The morphology of the emission in the He I 1083 nm is localized and blob-like, unlike the emission in the Ca II K line, which is more filamentary.

Conclusions. The modeling shows that the He I 1083 nm emission features and their Doppler shifts can be caused by opposite-polarity reconnection and/or horizontal current sheets below the canopy layer in the chromosphere. Based on the high observed Doppler width and the blob-like appearance of the emission features, we conjecture that at least a fraction of them are produced by plasmoids. We conclude that transition-region-like temperatures in the deeper layers of the active region chromosphere are more common than previously thought.

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Transverse oscillations in 3D along Ca II K bright fibrils in the solar chromosphere

2025. Sepideh Kianfar (et al.). Astronomy and Astrophysics 698

Artikel

Context. Fibrils in the solar chromosphere carry transverse oscillations as determined from non-spectroscopic imaging data. They are estimated to carry an energy flux of several kW m⁻², which is a significant fraction of the average chromospheric radiative energy losses.

Aims. We aim to determine the oscillation properties of fibrils not only in the plane-of-the-sky (horizontal) direction, but also along the line-of-sight (vertical) direction.

Methods. We obtained imaging-spectroscopy data in Fe I 6173 Å, Ca II 8542 Å, and Ca II K with the Swedish 1-m Solar Telescope. We created a sample of 605 bright Ca II K fibrils and measured their horizontal motions. Their vertical motion was determined through non-local thermodynamic equilibrium (non-LTE) inversion of the observed spectra. We determined the periods and velocity amplitudes of the fibril oscillations, as well as phase differences between vertical and horizontal oscillations in the fibrils.

Results. The bright Ca II K fibrils carry transverse waves with a mean period of 2.1 × 10² s, and a horizontal velocity amplitude of 1 km s⁻¹, consistent with earlier results. The mean vertical velocity amplitude is 1.1 km s⁻¹. We find that 77% of the fibrils carry waves in both the vertical and horizontal directions, and 80% of this subsample exhibit oscillations with similar periods in both horizontal and vertical directions. For the latter, we find that all phase differences between 0 and 2π occur with a mild but significant preference for linearly polarised waves (a phase difference of 0 or π).

Conclusions. The results are consistent with the scenario where transverse waves are excited by granular buffeting at the photospheric footpoints of the fibrils. Estimates of transverse wave flux based only on imaging data are too low because they ignore the contribution of the vertical velocity.

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Constraints on Acoustic Wave Energy Fluxes and Radiative Losses in the Solar Chromosphere from Non-LTE Inversions

2024. J. M. da Silva Santos (et al.). Astrophysical Journal 976 (1)

Artikel

Accurately assessing the balance between acoustic wave energy fluxes and radiative losses is critical for understanding how the solar chromosphere is thermally regulated. We investigate the energy balance in the chromosphere by comparing deposited acoustic flux and radiative losses under quiet and active solar conditions using non–local thermodynamic equilibrium inversions with the Stockholm Inversion Code. To achieve this, we utilize spectroscopic observations from the Interferometric BIdimensional Spectrometer in the Na ɪ 5896 Å and Ca ɪɪ 8542 Å lines and from the Interface Region Imaging Spectrograph in the Mg ɪɪ h and k lines to self-consistently derive spatially resolved velocity power spectra and cooling rates across different heights in the atmosphere. Additionally, we use snapshots of a three-dimensional radiative magnetohydrodynamics simulation to investigate the systematic effects of the inversion approach, particularly the effect of attenuation on the velocity power spectra and the determination of the cooling rates. The results indicate that inversions potentially underestimate acoustic fluxes at all chromospheric heights while slightly overestimating the radiative losses when fitting these spectral lines. However, even after accounting for these biases, the ratio of acoustic flux to radiative losses remains below unity in most observed regions, particularly in the higher layers of the chromosphere. We also observe a correlation between the magnetic field inclination in the photosphere and radiative losses in the low chromosphere in plage, which is evidence that the field topology plays a role in the chromospheric losses.

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Improved reconstruction of solar magnetic fields from imaging spectropolarimetry through spatio-temporal regularisation

2024. Jaime de la Cruz Rodríguez, Jorrit Leenaarts. Astronomy and Astrophysics 685

Artikel

Context. Determination of solar magnetic fields with a spatial resolution set by the diffraction limit of a telescope is difficult because the time required to measure the Stokes vector with sufficient signal-to-noise ratio is long compared to the solar evolution timescale. This difficulty becomes greater with increasing telescope size as the photon flux per diffraction-limited resolution element remains constant but the evolution timescale decreases linearly with the diffraction-limited resolution. Aims. We aim to improve magnetic field reconstruction at the diffraction limit without averaging the observations in time or space, and without applying noise filtering. Methods. The magnetic field vector tends to evolve more slowly than the temperature, velocity, or microturbulence. We exploit this by adding temporal regularisation terms for the magnetic field to the linear least-squares fitting used in the weak-field approximation, as well as to the Levenberg-Marquardt algorithm used in inversions. The other model parameters are allowed to change in time without constraints. We infer the chromospheric magnetic field from Ca II 854.2 nm observations using the weak field approximation and the photospheric magnetic field from Fe I 617.3 nm observations, both with and without temporal regularisation. Results. Temporal regularisation reduces the noise in the reconstructed maps of the magnetic field and provides a better coherency in time in both the weak-field approximation and Milne-Eddington inversions. Conclusions. Temporal regularisation markedly improves magnetic field determination from spatially and temporally resolved observations.

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Jacobian-free Newton-Krylov method for multilevel nonlocal thermal equilibrium radiative transfer problems

2024. Dimitri Arramy, Jaime de la Cruz Rodríguez, Jorrit Leenaarts. Astronomy and Astrophysics 690

Artikel

Context. The calculation of the emerging radiation from a model atmosphere requires knowledge of the emissivity and absorption coefficients, which are proportional to the atomic level population densities of the levels involved in each transition. Due to the intricate interdependence of the radiation field and the physical state of the atoms, iterative methods are required in order to calculate the atomic level population densities. A variety of different methods have been proposed to solve this problem, which is known as the nonlocal thermodynamical equilibrium (NLTE) problem. Aims. Our goal is to develop an efficient and rapidly converging method to solve the NLTE problem under the assumption of statistical equilibrium. In particular, we explore whether the Jacobian-Free Newton-Krylov (JFNK) method can be used. This method does not require an explicit construction of the Jacobian matrix because it estimates the new correction with the Krylov-subspace method. Methods. We implemented an NLTE radiative transfer code with overlapping bound-bound and bound-free transitions. This solved the statistical equilibrium equations using a JFNK method, assuming a depth-stratified plane-parallel atmosphere. As a reference, we also implemented the Rybicki & Hummer (1992) method based on linearization and operator splitting. Results. Our tests with the Fontenla, Avrett and Loeser C model atmosphere (FAL-C) and two different six-level Ca II and H I atoms show that the JFNK method can converge faster than our reference case by up to a factor 2. This number is evaluated in terms of the total number of evaluations of the formal solution of the radiative transfer equation for all frequencies and directions. This method can also reach a lower residual error compared to the reference case. Conclusions. The JFNK method we developed poses a new alternative to solving the NLTE problem. Because it is not based on operator splitting with a local approximate operator, it can improve the convergence of the NLTE problem in highly scattering cases. One major advantage of this method is that it is expected to allow for a direct implementation of more complex problems, such as overlapping transitions from different active atoms, charge conservation, or a more efficient treatment of partial redistribution, without having to explicitly linearize the equations.

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One-dimensional, geometrically stratified semi-empirical models of the quiet-Sun photosphere and lower chromosphere

2024. J. M. Borrero (et al.). Astronomy and Astrophysics 688

Artikel

Context. One-dimensional, semi-empirical models of the solar atmosphere are widely employed in numerous contexts within solar physics, ranging from the determination of element abundances and atomic parameters to studies of the solar irradiance and from Stokes inversions to coronal extrapolations. These models provide the physical parameters (i.e. temperature, gas pressure, etc.) in the solar atmosphere as a function of the continuum optical depth τc. The transformation to the geometrical z scale (i.e. vertical coordinate) is provided via vertical hydrostatic equilibrium. Aims. Our aim is to provide updated, one-dimensional, semi-empirical models of the solar atmosphere as a function of z, but employing the more general case of three-dimensional magneto-hydrostatic equilibrium (MHS) instead of vertical hydrostatic equilibrium (HE). Methods. We employed a recently developed Stokes inversion code that, along with non-local thermodynamic equilibrium effects, considers MHS instead of HE. This code is applied to spatially and temporally resolved spectropolarimetric observations of the quiet Sun obtained with the CRISP instrument attached to the Swedish Solar Telescope. Results. We provide average models for granules, intergranules, dark magnetic elements, and overall quiet-Sun as a function of both τc and z from the photosphere to the lower chromosphere. Conclusions. We demonstrate that, in these quiet-Sun models, the effect of considering MHS instead of HE is negligible. However, employing MHS increases the consistency of the inversion results before averaging. We surmise that in regions with stronger magnetic fields (i.e. pores, sunspots, network) the benefits of employing the magneto-hydrostatic approximation will be much more palpable.

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Spatial resolution effects on the solar open flux estimates

2024. I. Milić (et al.). Astronomy and Astrophysics 683

Artikel

Context. Spectropolarimetric observations used to infer the solar magnetic fields are obtained with a limited spatial resolution. The effects of this limited resolution on the inference of the open flux over the observed region have not been extensively studied.

Aims. We aim to characterize the biases that arise in the inference of the mean flux density by performing an end-to-end study that involves the generation of synthetic data, its interpretation (inversion), and a comparison of the results with the original model.

Methods. We synthesized polarized spectra of the two magnetically sensitive lines of neutral iron around 630 nm from a state-of-the-art numerical simulation of the solar photosphere. We then performed data degradation to simulate the effect of the telescope with a limited angular resolution and interpreted (inverted) the data using a Milne-Eddington spectropolarimetric inversion code. We then studied the dependence of the inferred parameters on the telescope resolution.

Results. The results show a significant decrease in the mean magnetic flux density – related to the open flux observed at the disk center – with decreasing telescope resolution. The original net magnetic field flux is fully resolved by a 1m telescope, but a 20 cm aperture telescope yields a 30% smaller value. Even in the fully resolved case, the result is still biased due to the corrugation of the photospheric surface.

Conclusions. Even the spatially averaged quantities, such as the open magnetic flux in the observed region, are underestimated when the magnetic structures are unresolved. The reason for this is the presence of nonlinearities in the magnetic field inference process. This effect might have implications for the modeling of large-scale solar magnetic fields; for example, those corresponding to the coronal holes, or the polar magnetic fields, which are relevant to our understanding of the solar cycle.

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A reconnection-driven magnetic flux cancellation and a quiet Sun Ellerman bomb

2023. Anjali J. Kaithakkal (et al.). Monthly notices of the Royal Astronomical Society 521 (3), 3882-3897

Artikel

The focus of this investigation is to quantify the conversion of magnetic to thermal energy initiated by a quiet Sun cancellation event and to explore the resulting dynamics from the interaction of the opposite-polarity magnetic features. We used imaging spectroscopy in the Hα line, along with spectropolarimetry in the Fe I 6173 Å and Ca II 8542 Å lines from the Swedish Solar Telescope (SST) to study a reconnection-related cancellation and the appearance of a quiet Sun Ellerman bomb (QSEB). We observed, for the first time, QSEB signature in both the wings and core of the Fe I 6173 Å line. We also found that, at times, the Fe I line-core intensity reaches higher values than the quiet Sun continuum intensity. From FIRTEZ-dz inversions of the Stokes profiles in Fe I and Ca II lines, we found enhanced temperature, with respect to the quiet Sun values, at the photospheric (log τ_c = −1.5; ∼1000 K) and lower chromospheric heights (log τ_c = −4.5; ∼360 K). From the calculation of total magnetic energy and thermal energy within these two layers, it was confirmed that the magnetic energy released during the flux cancellation can support heating in the aforesaid height range. Further, the temperature stratification maps enabled us to identify cumulative effects of successive reconnection on temperature pattern, including recurring temperature enhancements. Similarly, Doppler velocity stratification maps revealed impacts on plasma flow pattern, such as a sudden change in the flow direction.

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Chromospheric Heating from Local Magnetic Growth and Ambipolar Diffusion under Nonequilibrium Conditions

2023. Juan Martínez-Sykora (et al.). Astrophysical Journal Letters 943 (2)

Artikel

The heating of the chromosphere in internetwork regions remains one of the foremost open questions in solar physics. In the present study, we tackle this old problem by using a very-high-spatial-resolution simulation of quiet-Sun conditions performed with radiative MHD numerical models and interface region imaging spectrograph (IRIS) observations. We have expanded a previously existing 3D radiative MHD numerical model of the solar atmosphere, which included self-consistently locally driven magnetic amplification in the chromosphere, by adding ambipolar diffusion and time-dependent nonequilibrium hydrogen ionization to the model. The energy of the magnetic field is dissipated in the upper chromosphere, providing a large temperature increase due to ambipolar diffusion and nonequilibrium ionization (NEQI). At the same time, we find that adding the ambipolar diffusion and NEQI in the simulation has a minor impact on the local growth of the magnetic field in the lower chromosphere and its dynamics. Our comparison between synthesized Mg ii profiles from these high-spatial-resolution models, with and without ambipolar diffusion and NEQI, and quiet-Sun and coronal hole observations from IRIS now reveal a slightly better correspondence. The intensity of profiles is increased, and the line cores are slightly broader when ambipolar diffusion and NEQI effects are included. Therefore, the Mg ii profiles are closer to those observed than in previous models, though some differences still remain.

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Decay of a photospheric transient filament at the boundary of a pore and the chromospheric response

2023. P. Lindner (et al.). Astronomy and Astrophysics 673

Artikel

Context. The intermediate stages between pores and the formation of sunspots are a rare phenomenon and can be manifested as transient photospheric penumbral-like filaments. Although the magnetic field changes rapidly during the evolution of such filaments, they have not yet been shown to be connected to magnetic reconnection events.

Aims. We characterize the evolution of transient photospheric filaments around a pore and search for possible signs of chromospheric responses.

Methods. We analyzed observations of a pore in NOAA AR 12739 from the Swedish Solar Telescope, including the spectropolarimetric data of the Fe I 6173 Å and the Ca II 8542 Å line and spectroscopic data of the Ca II K 3934 Å line. The VFISV Milne-Eddington inversion code and the multi-line non-LTE inversion code STiC were utilized to obtain atmospheric parameters in the photosphere and the chromosphere.

Results. Multiple filamentary structures of inclined magnetic fields are found in photospheric inclination maps at the boundary of the pore, although the pore had never developed a penumbra. One of the filaments shows a clear counterpart in continuum intensity maps, in addition to photospheric blueshifts. During its decay, a brightening in the blue wing of the Ca II 8542 Å line is observed. The Ca II K 3934 Å and the Ca II 8542 Å lines show complex spectral profiles in this region. Depth-dependent STiC inversion results using data from all available lines yield a temperature increase (roughly 1000 K) and bidirectional flows (magnitudes up to 8 km s⁻¹) at log τ = −3.5.

Conclusions. The temporal and spatial correlations of the decaying filament (observed in the photosphere) to the temperature increase and the bidirectional flows in the high photosphere and low chromosphere suggest that they are connected. We propose scenarios in which magnetic reconnection happens at the edge of a rising magnetic flux tube in the photosphere. This would lead to both the decay of the filament in the photosphere as well as the observed temperature increase and the bidirectional flows in the high photosphere and low chromosphere.

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Designing wavelength sampling for Fabry–Pérot observations: Information-based spectral sampling

2023. Carlos José Díaz Baso (et al.). Astronomy and Astrophysics 673

Artikel

Context. Fabry–Pérot interferometers (FPIs) have become very popular in solar observations because they offer a balance between cadence, spatial resolution, and spectral resolution through a careful design of the spectral sampling scheme according to the observational requirements of a given target. However, an efficient balance requires knowledge of the expected target conditions, the properties of the chosen spectral line, and the instrumental characteristics.

Aims. Our aim is to find a method that allows the optimal spectral sampling of FPI observations in a given spectral region to be found. The selected line positions must maximize the information content in the observation with a minimal number of points.

Methods. In this study, we propose a technique based on a sequential selection approach in which a neural network is used to predict the spectrum (or physical quantities, if the model is known) from the information at a few points. Only those points that contain relevant information and improve the model prediction are included in the sampling scheme.

Results. We have quantified the performance of the new sampling schemes by showing the lower errors in the model parameter reconstructions. The method adapts the separation of the points according to the spectral resolution of the instrument, the typical broadening of the spectral shape, and the typical Doppler velocities. The experiments that use the Ca II 8542 Å line show that the resulting wavelength scheme naturally places more points in the core than in the wings (by almost a factor of 4), consistent with the sensitivity of the spectral line at each wavelength interval. As a result, observations focused on magnetic field analysis should prioritize a denser grid near the core, while those focused on thermodynamic properties would benefit from a larger coverage. The method can also be used as an accurate interpolator to improve the inference of the magnetic field when using the weak-field approximation.

Conclusions. Overall, this method offers an objective approach for designing new instrumentation or observing proposals with customized configurations for specific targets. This is particularly relevant when studying highly dynamic events in the solar atmosphere with a cadence that preserves spectral coherence without sacrificing much information.

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Do Hα Stokes V Profiles Probe the Chromospheric Magnetic Field? An Observational Perspective

2023. Harsh Mathur (et al.). Astrophysical Journal 946 (1)

Artikel

We investigated the diagnostic potential of the Stokes V profile of the Hα line to probe the chromospheric line-of-sight (LOS) magnetic field (B_LOS) by comparing the B_LOS inferred from the weak field approximation (WFA) with that inferred from the multiline inversions of the CaII 8542 Å, Si I 8536 Å, and Fe I 8538 Å lines using the STiC inversion code. Simultaneous spectropolarimetric observations of a pore in the Ca II 8542 Å and Hα spectral lines obtained from the SPINOR at the Dunn Solar Telescope on 2008 December 4 are used in this study. The WFA was applied on the Stokes I and V profiles of Hα line over three wavelength ranges, viz., around line core (Δλ = ±0.35 Å), line wings (Δλ = [−1.5, −0.6], and [+0.6, +1.5] Å), and full spectral range of the line (Δλ = ± 1.5 Å) to derive the BLOS. We found the maximum B_LOS strengths of ∼+800 and ∼+600 G at log _t500 = −1 and −4.5, respectively, in the pore. The morphological map of the B_LOS inferred from the Hα line core is similar to the B_LOS map at log _t500 = −4.5 inferred from multiline inversions. The B_LOS map inferred from the Hα line wings and full spectral range have a similar morphological structure to the B_LOS map inferred at log _t500 = −1. The B_LOS estimated from Hα using WFA is weaker by a factor of ≈0.53 than that of inferred from the multiline inversions.

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Estimating the longitudinal magnetic field in the chromosphere of quiet-Sun magnetic concentrations

2023. S. Esteban Pozuelo (et al.). Astronomy and Astrophysics 672

Artikel

Context. Details of the magnetic field in the quiet-Sun chromosphere are key to our understanding of essential aspects of the solar atmosphere. However, the strength and orientation of this magnetic field have not been thoroughly studied at high spatial resolution.

Aims. We aim to determine the longitudinal magnetic field component (B_∥) of quiet-Sun regions depending on their size.

Methods. We estimated B_∥ by applying the weak-field approximation to high-spatial-resolution Ca II 854.2 nm data taken with the Swedish 1 m Solar Telescope. Specifically, we analyzed the estimates inferred for different spectral ranges using the data at the original cadence and temporally integrated signals.

Results. The longitudinal magnetic field in each considered plasma structure correlates with its size. Using a spectral range restricted to the line core leads to chromospheric longitudinal fields varying from ∼50 G at the edges to 150–500 G at the center of the structure. These values increase as the spectral range widens due to the photospheric contribution. However, the difference between this contribution and the chromospheric one is not uniform for all structures. Small and medium-sized concentrations show a steeper height gradient in B_∥ compared to their chromospheric values, so estimates for wider ranges are less trustworthy. Signal addition does not alleviate this situation as the height gradients in B_∥ are consistent with time. Finally, despite the amplified noise levels that deconvolving processes may cause, data restored with the destretching technique show similar results, though are affected by smearing.

Conclusions. We obtained B_∥ estimates similar to those previously found, except for large concentrations and wide spectral ranges. In addition, we report a correlation between the height variation of B_∥ compared to the chromospheric estimates and the concentration size. This correlation affects the difference between the photospheric and chromospheric magnetic flux values and the reliability of the estimates for wider spectral ranges.

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Ultra-high-resolution observations of plasmoid-mediated magnetic reconnection in the deep solar atmosphere

2023. Luc H. M. Rouppe van der Voort, Michiel van Noort, Jaime de la Cruz Rodríguez. Astronomy and Astrophysics 673

Artikel

Context. Magnetic reconnection in the deep solar atmosphere can give rise to enhanced emission in the Balmer hydrogen lines, a phenomenon referred to as Ellerman bombs.

Aims. To effectively trace magnetic reconnection below the canopy of chromospheric fibrils, we analyzed unique spectroscopic observations of Ellerman bombs in the Hα line.

Methods. We analyzed a 10 min data set of a young emerging active region observed with the prototype of the Microlensed Hyperspectral Imager (MiHI) at the Swedish 1-m Solar Telescope (SST). The MiHI instrument is an integral field spectrograph that is capable of achieving simultaneous ultra-high resolution in the spatial, temporal, and spectral domains. With the combination of the SST adaptive optics system and image restoration techniques, MiHI can deliver diffraction-limited observations if the atmospheric seeing conditions allow. The data set samples the Hα line over 4.5 Å with 10 mÅ pix⁻¹, with 0.″065 pix⁻¹ over a field of view of 8.″6 × 7.″7, and at a temporal cadence of 1.33 s. This constitutes a hyperspectral data cube that measures 132 × 118 spatial pixels, 456 spectral pixels, and 455 time steps.

Results. There were multiple sites with Ellerman bomb activity associated with strong magnetic flux emergence. The Ellerman bomb activity is very dynamic, showing rapid variability and a small-scale substructure. We found a number of plasmoid-like blobs with full-width-half-maximum sizes between 0.″1 and 0.″4 and moving with apparent velocities between 14 and 77 km s⁻¹. Some of these blobs have Ellerman bomb spectral profiles with a single peak at a Doppler offset between 47 and 57 km s⁻¹.

Conclusions. Our observations support the idea that fast magnetic reconnection in Ellerman bombs is mediated by the formation of plasmoids. These MiHI observations demonstrate that a microlens-based integral field spectrograph is capable of probing fundamental physical processes in the solar atmosphere.

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Active region chromospheric magnetic fields: Observational inference versus magnetohydrostatic modelling

2022. Gregal J. M. Vissers (et al.). Astronomy and Astrophysics 662

Artikel

Context. A proper estimate of the chromospheric magnetic fields is thought to improve modelling of both active region and coronal mass ejection evolution. However, because the chromospheric field is not regularly obtained for sufficiently large fields of view, estimates thereof are commonly obtained through data-driven models or field extrapolations, based on photospheric boundary conditions alone and involving pre-processing that may reduce details and dynamic range in the magnetograms.

Aims. We investigate the similarity between the chromospheric magnetic field that is directly inferred from observations and the field obtained from a magnetohydrostatic (MHS) extrapolation based on a high-resolution photospheric magnetogram.

Methods. Based on Swedish 1-m Solar Telescope Fe I 6173 Å and Ca II 8542 Å observations of NOAA active region 12723, we employed the spatially regularised weak-field approximation (WFA) to derive the vector magnetic field in the chromosphere from Ca II, as well as non-local thermodynamic equilibrium (non-LTE) inversions of Fe I and Ca II to infer a model atmosphere for selected regions. Milne-Eddington inversions of Fe I serve as photospheric boundary conditions for the MHS model that delivers the three-dimensional field, gas pressure, and density self-consistently.

Results. For the line-of-sight component, the MHS chromospheric field generally agrees with the non-LTE inversions and WFA, but tends to be weaker by 16% on average than these when larger in magnitude than 300 G. The observationally inferred transverse component is systematically stronger, up to an order of magnitude in magnetically weaker regions, but the qualitative distribution with height is similar to the MHS results. For either field component, the MHS chromospheric field lacks the fine structure derived from the inversions. Furthermore, the MHS model does not recover the magnetic imprint from a set of high fibrils connecting the main polarities.

Conclusions. The MHS extrapolation and WFA provide a qualitatively similar chromospheric field, where the azimuth of the former is better aligned with Ca II 8542 Å fibrils than that of the WFA, especially outside strong-field concentrations. The amount of structure as well as the transverse field strengths are, however, underestimated by the MHS extrapolation. This underscores the importance of considering a chromospheric magnetic field constraint in data-driven modelling of active regions, particularly in the context of space weather predictions.

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Bayesian Stokes inversion with normalizing flows

2022. Carlos José Díaz Baso, A. Asensio Ramos, Jaime de la Cruz Rodríguez. Astronomy and Astrophysics 659

Artikel

Stokes inversion techniques are very powerful methods for obtaining information on the thermodynamic and magnetic properties of solar and stellar atmospheres. In recent years, highly sophisticated inversion codes have been developed that are now routinely applied to spectro-polarimetric observations. Most of these inversion codes are designed to find an optimum solution to the nonlinear inverse problem. However, to obtain the location of potentially multimodal cases (ambiguities), the degeneracies and the uncertainties of each parameter inferred from the inversions algorithms – such as Markov chain Monte Carlo (MCMC) – require evaluation of the likelihood of the model thousand of times and are computationally costly. Variational methods are a quick alternative to Monte Carlo methods, and approximate the posterior distribution by a parametrized distribution. In this study, we introduce a highly flexible variational inference method to perform fast Bayesian inference, known as normalizing flows. Normalizing flows are a set of invertible, differentiable, and parametric transformations that convert a simple distribution into an approximation of any other complex distribution. If the transformations are conditioned on observations, the normalizing flows can be trained to return Bayesian posterior probability estimates for any observation. We illustrate the ability of the method using a simple Milne-Eddington model and a complex non-local thermodynamic equilibrium (NLTE) inversion. The method is extremely general and other more complex forward models can be applied. The training procedure need only be performed once for a given prior parameter space and the resulting network can then generate samples describing the posterior distribution several orders of magnitude faster than existing techniques.

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Heating of the solar chromosphere through current dissipation

2022. João Manuel da Silva Santos (et al.). Astronomy and Astrophysics 661

Artikel

Context. The solar chromosphere is heated to temperatures higher than predicted by radiative equilibrium. This excess heating is greater in active regions where the magnetic field is stronger.

Aims. We aim to investigate the magnetic topology associated with an area of enhanced millimeter (mm) brightness temperatures in a solar active region mapped by the Atacama Large Millimeter/submillimeter Array (ALMA) using spectropolarimetric co-observations with the 1-m Swedish Solar Telescope (SST).

Methods. We used Milne–Eddington inversions, nonlocal thermodynamic equilibrium (non-LTE) inversions, and a magnetohydrostatic extrapolation to obtain constraints on the three-dimensional (3D) stratification of temperature, magnetic field, and radiative energy losses. We compared the observations to a snapshot of a magnetohydrodynamics simulation and investigate the formation of the thermal continuum at 3 mm using contribution functions.

Results. We find enhanced heating rates in the upper chromosphere of up to ∼5 kW m⁻², where small-scale emerging loops interact with the overlying magnetic canopy leading to current sheets as shown by the magnetic field extrapolation. Our estimates are about a factor of two higher than canonical values, but they are limited by the ALMA spatial resolution (∼1.2″). Band 3 brightness temperatures reach about ∼10⁴ K in the region, and the transverse magnetic field strength inferred from the non-LTE inversions is on the order of ∼500 G in the chromosphere.

Conclusions. We are able to quantitatively reproduce many of the observed features including the integrated radiative losses in our numerical simulation. We conclude that the heating is caused by dissipation in current sheets. However, the simulation shows a complex stratification in the flux emergence region where distinct layers may contribute significantly to the emission in the mm continuum.

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Physical properties of a fan-shaped jet backlit by an X9.3 flare

2022. Alexander G. M. Pietrow (et al.). Astronomy and Astrophysics 659

Artikel

Context. Fan-shaped jets sometimes form above light bridges and are believed to be driven by the reconnection of the vertical umbral field with the more horizontal field above the light bridges. Because these jets are not fully opaque in the wings of most chromospheric lines, it is not possible to study their spectra without highly complex considerations of radiative transfer in spectral lines from the atmosphere behind the fan.

Aims. We take advantage of a unique set of observations of the Hα line along with the Ca II 8542 Å and Ca II K lines obtained with the CRISP and CHROMIS instrument of the Swedish 1-m Solar Telescope to study the physical properties of a fan-shaped jet that was backlit by an X9.3 flare. For what we believe to be the first time, we report an observationally derived estimate of the mass and density of material in a fan-shaped jet.

Methods. The Hα flare ribbon emission profiles from behind the fan are highly broadened and flattened, allowing us to investigate the fan with a single slab via Beckers’ cloud model, as if it were backlit by a flat spectral profile of continuum emission. Using this model we derived the opacity and velocity of the material in the jet. Using inversions of Ca II 8542 Å emission via the STockholm inversion Code, we were also able to estimate the temperature and to cross-check the velocity of the material in the jet. Finally, we used the masses and the plane-of-sky and line-of-sight velocities as functions of time to investigate the downward supply of energy and momentum to the photosphere in the collapse of this jet, and evaluated it as a potential driver for a sunquake beneath.

Results. We find that the physical properties of the fan material are reasonably chromospheric in nature, with a temperature of 7050 ± 250 K and a mean density of 2 ± 0.3 × 10⁻¹¹ g cm⁻³.

Conclusions. The total mass observed in Hα was found to be 3.9 ± 0.7 × 10₁₃ g and the kinetic energy delivered to the base of the fan in its collapse was nearly two orders of magnitude below typical sunquake energies. We therefore rule out this jet as the sunquake driver, but cannot completely rule out larger fan jets as potential drivers.

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Prospects and challenges of numerical modeling of the Sun at millimeter wavelengths

2022. Sven Wedemeyer (et al.). Frontiers in Astronomy and Space Sciences 9

Artikel

The Atacama Large Millimeter/submillimeter Array (ALMA) offers new diagnostic possibilities that complement other commonly used diagnostics for the study of the Sun. In particular, ALMA’s ability to serve as an essentially linear thermometer of the chromospheric gas at unprecedented spatial resolution at millimeter wavelengths and future polarization measurements has great diagnostic potential. Solar ALMA observations are therefore expected to contribute significantly to answering long-standing questions about the structure, dynamics, and energy balance of the outer layers of the solar atmosphere. In this regard, current and future ALMA data are also important for constraining and further developing numerical models of the solar atmosphere, which in turn are often vital for the interpretation of observations. The latter is particularly important given the Sun’s highly intermittent and dynamic nature that involves a plethora of processes occurring over extended ranges in spatial and temporal scales. Realistic forward modeling of the Sun therefore requires time-dependent three-dimensional radiation magnetohydrodynamics that account for non-equilibrium effects and, typically as a separate step, detailed radiative transfer calculations, resulting in synthetic observables that can be compared to observations. Such artificial observations sometimes also account for instrumental and seeing effects, which, in addition to aiding the interpretation of observations, provide instructive tools for designing and optimizing ALMA’s solar observing modes. In the other direction, ALMA data in combination with other simultaneous observations enable the reconstruction of the solar atmospheric structure via data inversion techniques. This article highlights central aspects of the impact of ALMA for numerical modeling of the Sun and their potential and challenges, together with selected examples.

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Radiative losses in the chromosphere during a C-class flare

2022. Rahul Yadav (et al.). Astronomy and Astrophysics 665

Artikel

Context. Solar flares release an enormous amount of energy (∼10³² erg) into the corona. A substantial fraction of this energy is transported to the lower atmosphere, which results in chromospheric heating. The mechanisms that transport energy to the lower solar atmosphere during a flare are still not fully understood.

Aims. We aim to estimate the temporal evolution of the radiative losses in the chromosphere at the footpoints of a C-class flare, in order to set observational constraints on the electron beam parameters of a RADYN flare simulation.

Methods. We estimated the radiative losses from hydrogen, and singly ionized Ca and Mg using semiempirical model atmospheres, which were inferred from a multiline inversion of observed Stokes profiles obtained with the CRISP and CHROMIS instruments on the Swedish 1-m Solar Telescope. The radiative losses were computed taking into account the effect of partial redistribution and non-local thermodynamic equilibrium. To estimate the integrated radiative losses in the chromosphere, the net cooling rates were integrated between the temperature minimum and the height where the temperature reaches 10 kK. We also compared our time series of radiative losses with those from the RADYN flare simulations.

Results. We obtained a high spatial-resolution map of integrated radiative losses around the flare peak time. The stratification of the net cooling rate suggests that the Ca IR triplet lines are responsible for most of the radiative losses in the flaring atmosphere. During the flare peak time, the contribution from Ca II H and K and Mg II h and k lines are strong and comparable to the Ca IR triplet (∼32 kW m⁻²). Since our flare is a relatively weak event, the chromosphere is not heated above 11 kK, which in turn yields a subdued Lyα contribution (∼7 kW m⁻²) in the selected limits of the chromosphere. The temporal evolution of total integrated radiative losses exhibits sharply rising losses (0.4 kW m⁻² s⁻¹) and a relatively slow decay (0.23 kW m⁻² s⁻¹). The maximum value of total radiative losses is reached around the flare peak time, and can go up to 175 kW m⁻² for a single pixel located at footpoint. After a small parameter study, we find the best model-data consistency in terms of the amplitude of radiative losses and the overall atmospheric structure with a RADYN flare simulation in the injected energy flux of 5 × 10¹⁰ erg s⁻¹ cm⁻².

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Spatio-temporal analysis of chromospheric heating in a plage region

2022. Roberta Morosin (et al.). Astronomy and Astrophysics 664

Artikel

Context. Our knowledge of the heating mechanisms that are at work in the chromosphere of plage regions remains highly unconstrained from observational studies. While many heating candidates have been proposed in theoretical studies, the exact contribution from each of them is still unknown. The problem is rather difficult because there is no direct way of estimating the heating terms from chromospheric observations.

Aims: The purpose of our study is to estimate the chromospheric heating terms from a multi-line high-spatial-resolution plage dataset, characterize their spatio-temporal distribution and set constraints on the heating processes that are at work in the chromosphere.

Methods: We used nonlocal thermodynamical equilibrium inversions in order to infer a model of the photosphere and chromosphere of a plage dataset acquired with the Swedish 1-m Solar Telescope (SST). We used this model atmosphere to calculate the chromospheric radiative losses from the main chromospheric cooler from H I, Ca II, and Mg II atoms. In this study, we approximate the chromospheric heating terms by the net radiative losses predicted by the inverted model. In order to make the analysis of time-series over a large field of view computationally tractable, we made use of a neural network which is trained from the inverted models of two non-consecutive time-steps. We have divided the chromosphere in three regions (lower, middle, and upper) and analyzed how the distribution of the radiative losses is correlated with the physical parameters of the model.

Results: In the lower chromosphere, the contribution from the Ca II lines is dominant and predominantly located in the surroundings of the photospheric footpoints. In the upper chromosphere, the H I contribution is dominant. Radiative losses in the upper chromosphere form a relatively homogeneous patch that covers the entire plage region. The Mg II also peaks in the upper chromosphere. Our time analysis shows that in all pixels, the net radiative losses can be split in a periodic component with an average amplitude of amp̅Q = 7.6 kW m−2 and a static (or very slowly evolving) component with a mean value of −26.1 kW m−2. The period of the modulation present in the net radiative losses matches that of the line-of-sight velocity of the model.

Conclusions: Our interpretation is that in the lower chromosphere, the radiative losses are tracing the sharp lower edge of the hot magnetic canopy that is formed above the photosphere, where the electric current is expected to be large. Therefore, Ohmic current dissipation could explain the observed distribution. In the upper chromosphere, both the magnetic field and the distribution of net radiative losses are room-filling and relatively smooth, whereas the amplitude of the periodic component is largest. Our results suggest that acoustic wave heating may be responsible for one-third of the energy deposition in the upper chromosphere, whereas other heating mechanisms must be responsible for the rest: turbulent Alfvén wave dissipation or ambipolar diffusion could be among them. Given the smooth nature of the magnetic field in the upper chromosphere, we are inclined to rule out Ohmic dissipation of current sheets in the upper chromosphere.

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The European Solar Telescope

2022. C. Quintero Noda (et al.). Astronomy and Astrophysics 666

Artikel

The European Solar Telescope (EST) is a project aimed at studying the magnetic connectivity of the solar atmosphere, from the deep photosphere to the upper chromosphere. Its design combines the knowledge and expertise gathered by the European solar physics community during the construction and operation of state-of-the-art solar telescopes operating in visible and near-infrared wavelengths: the Swedish 1m Solar Telescope, the German Vacuum Tower Telescope and GREGOR, the French Télescope Héliographique pour l’Étude du Magnétisme et des Instabilités Solaires, and the Dutch Open Telescope. With its 4.2 m primary mirror and an open configuration, EST will become the most powerful European ground-based facility to study the Sun in the coming decades in the visible and near-infrared bands. EST uses the most innovative technological advances: the first adaptive secondary mirror ever used in a solar telescope, a complex multi-conjugate adaptive optics with deformable mirrors that form part of the optical design in a natural way, a polarimetrically compensated telescope design that eliminates the complex temporal variation and wavelength dependence of the telescope Mueller matrix, and an instrument suite containing several (etalon-based) tunable imaging spectropolarimeters and several integral field unit spectropolarimeters. This publication summarises some fundamental science questions that can be addressed with the telescope, together with a complete description of its major subsystems.

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ALMA and IRIS Observations of the Solar Chromosphere. I. An On-disk Type II Spicule

2021. Georgios Chintzoglou (et al.). Astrophysical Journal 906 (2)

Artikel

We present observations of the solar chromosphere obtained simultaneously with the Atacama Large Millimeter/submillimeter Array (ALMA) and the Interface Region Imaging Spectrograph. The observatories targeted a chromospheric plage region of which the spatial distribution (split between strongly and weakly magnetized regions) allowed the study of linear-like structures in isolation, free of contamination from background emission. Using these observations in conjunction with a radiative magnetohydrodynamic 2.5D model covering the upper convection zone all the way to the corona that considers nonequilibrium ionization effects, we report the detection of an on-disk chromospheric spicule with ALMA and confirm its multithermal nature.

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ALMA and IRIS Observations of the Solar Chromosphere. II. Structure and Dynamics of Chromospheric Plages

2021. Georgios Chintzoglou (et al.). Astrophysical Journal 906 (2)

Artikel

We propose and employ a novel empirical method for determining chromospheric plage regions, which seems to better isolate a plage from its surrounding regions than other methods commonly used. We caution that isolating a plage from its immediate surroundings must be done with care in order to successfully mitigate statistical biases that, for instance, can impact quantitative comparisons between different chromospheric observables. Using this methodology, our analysis suggests that lambda = 1.25 mm free-free emission in plage regions observed with the Atacama Large Millimeter/submillimeter Array (ALMA)/Band6 may not form in the low chromosphere as previously thought, but rather in the upper chromospheric parts of dynamic plage features (such as spicules and other bright structures), i.e., near geometric heights of transition-region temperatures. We investigate the high degree of similarity between chromospheric plage features observed in ALMA/Band6 (at 1.25 mm wavelengths) and the Interface Region Imaging Spectrograph (IRIS)/Si iv at 1393 angstrom. We also show that IRIS/Mg ii h and k are not as well correlated with ALMA/Band6 as was previously thought, and we discuss discrepancies with previous works. Lastly, we report indications of chromospheric heating due to propagating shocks supported by the ALMA/Band6 observations.

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An observationally constrained model of strong magnetic reconnection in the solar chromosphere: Atmospheric stratification and estimates of heating rates

2021. Carlos J. Díaz Baso, Jaime de la Cruz Rodríguez, Jorrit Leenaarts. Astronomy and Astrophysics 647

Artikel

Context. The evolution of the photospheric magnetic field plays a key role in the energy transport into the chromosphere and the corona. In active regions, newly emerging magnetic flux interacts with the pre-existent magnetic field, which can lead to reconnection events that convert magnetic energy into thermal energy.

Aims. We aim to study the heating caused by a strong reconnection event that was triggered by magnetic flux cancelation.

Methods. We use imaging and spectropolarimetric data in the Fei6301& 6302 Å, Caii8542 Å, and CaiiK spectral lines obtained with the CRISP and CHROMIS instruments at the Swedish 1-m Solar Telescope. These data were inverted with the STiC code by performing multi-atom, multi-line, non-local thermodynamic equilibrium inversions. These inversions yielded a three-dimensional model of the reconnection event and surrounding atmosphere, including temperature, velocity, microturbulence, magnetic field, and radiative loss rate.

Results. The model atmosphere shows the emergence of magnetic loops with a size of several arcseconds into a pre-existing pre-dominantly unipolar field. Where the reconnection region is expected to be, we see an increase in the chromospheric temperature of roughly 2000 K as well as bidirectional flows of the order of 10 km s−1 emanating from there. We see bright blobs of roughly 0.2 arcsec in diameter in the CaiiK, moving at a plane-of-the-sky velocity of the order of 100 km s−1 and a blueshift of 100 km s−1, which we interpret as ejected plasmoids from the same region. This scenario is consistent with theoretical reconnection models, and therefore provides evidence of a reconnection event taking place. The chromospheric radiative losses at the reconnection site are as high as160 kW m−2, providing a quantitative constraint on theoretical models that aim to simulate reconnection caused by flux emergence in the chromosphere.

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Design and Performance Analysis of a Highly Efficient Polychromatic Full Stokes Polarization Modulator for the CRISP Imaging Spectrometer

2021. A. G. de Wijn (et al.). Astronomical Journal 161 (2)

Artikel

We present the design and performance of a polychromatic polarization modulator for the CRisp Imaging SpectroPolarimeter (CRISP) Fabry-Perot tunable narrow-band imaging spectropolarimer at the Swedish 1 m Solar Telescope (SST). We discuss the design process in depth, compare two possible modulator designs through a tolerance analysis, and investigate thermal sensitivity of the selected design. The trade-offs and procedures described in this paper are generally applicable in the development of broadband polarization modulators. The modulator was built and has been operational since 2015. Its measured performance is close to optimal between 500 and 900 nm, and differences between the design and as-built modulator are largely understood. We show some example data, and briefly review scientific work that used data from SST/CRISP and this modulator.

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Downflowing umbral flashes as evidence of standing waves in sunspot umbrae

2021. T. Felipe (et al.). Astronomy and Astrophysics 645

Artikel

Context. Umbral flashes are sudden brightenings commonly visible in the core of some chromospheric lines. Theoretical and numerical modeling suggests that they are produced by the propagation of shock waves. According to these models and early observations, umbral flashes are associated with upflows. However, recent studies have reported umbral flashes in downflowing atmospheres.

Aims. We aim to understand the origin of downflowing umbral flashes. We explore how the existence of standing waves in the umbral chromosphere impacts the generation of flashed profiles.

Methods. We performed numerical simulations of wave propagation in a sunspot umbra with the code MANCHA. The Stokes profiles of the CaII 8542 Å line were synthesized with the NICOLE code.

Results. For freely propagating waves, the chromospheric temperature enhancements of the oscillations are in phase with velocity upflows. In this case, the intensity core of the CaII 8542 Å atmosphere is heated during the upflowing stage of the oscillation. However, a different scenario with a resonant cavity produced by the sharp temperature gradient of the transition region leads to chromospheric standing oscillations. In this situation, temperature fluctuations are shifted backward and temperature enhancements partially coincide with the downflowing stage of the oscillation. In umbral flash events produced by standing oscillations, the reversal of the emission feature is produced when the oscillation is downflowing. The chromospheric temperature keeps increasing while the atmosphere is changing from a downflow to an upflow. During the appearance of flashed CaII 8542 Å cores, the atmosphere is upflowing most of the time, and only 38% of the flashed profiles are associated with downflows.

Conclusions. We find a scenario that remarkably explains the recent empirical findings of downflowing umbral flashes as a natural consequence of the presence of standing oscillations above sunspot umbrae.

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Line formation of He I D₃ and He I 10 830 Å in a small-scale reconnection event

2021. Tine Libbrecht (et al.). Astronomy and Astrophysics 652

Artikel

Context. Ellerman bombs (EBs) and UV bursts are small-scale reconnection events that occur in the region of the upper photosphere to the chromosphere. It has recently been discovered that these events can have emission signatures in the He I D₃ and He I 10 830 Å lines, suggesting that their temperatures are higher than previously expected.

Aims. We aim to explain the line formation of He I D₃ and He I 10 830 Å in small-scale reconnection events.

Methods. We used a simulated EB in a Bifrost-generated radiative magnetohydrodynamics snapshot. The resulting He I D₃ and He I 10 830 Å line intensities were synthesized in 3D using the non-local thermal equilibrium (non-LTE) Multi3D code. The presence of coronal extreme UV (EUV) radiation was included self-consistently. We compared the synthetic helium spectra with observed raster scans of EBs in He I 10 830 Å and He I D₃ obtained at the Swedish Solar Telescope with the TRI-Port Polarimetric Echelle-Littrow Spectrograph.

Results. Emission in He I D₃ and He I 10 830 Å is formed in a thin shell around the EB at a height of ∼0.8 Mm, while the He I D₃ absorption is formed above the EB at ∼4 Mm. The height at which the emission is formed corresponds to the lower boundary of the EB, where the temperature increases rapidly from 6 × 10³ K to 10⁶ K. The synthetic line profiles at a heliocentric angle of μ = 0.27 are qualitatively similar to the observed profiles at the same μ-angle in dynamics, broadening, and line shape: emission in the wing and absorption in the line core. The opacity in He I D₃ and He I 10 830 Å is generated through photoionization-recombination driven by EUV radiation that is locally generated in the EB at temperatures in the range of 2 × 10⁴ − 2 × 10⁶ K and electron densities between 10¹¹ and 10¹³ cm⁻³. The synthetic emission signals are a result of coupling to local conditions in a thin shell around the EB, with temperatures between 7 × 10³ and 10⁴ K and electron densities ranging from ∼10¹² to 10¹³ cm⁻³. This shows that both strong non-LTE and thermal processes play a role in the formation of He I D₃ and He I 10 830 Å in the synthetic EB/UV burst that we studied.

Conclusions. In conclusion, the synthetic He I D₃ and He I 10 830 Å emission signatures are an indicator of temperatures of at least 2 × 10⁴ K; in this case, as high as ∼10⁶ K.

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Non-LTE inversions of a confined X2.2 flare I. The vector magnetic field in the photosphere and chromosphere

2021. Gregal J. M. Vissers (et al.). Astronomy and Astrophysics 645

Artikel

Context. Obtaining an accurate measurement of magnetic field vector in the solar atmosphere is essential for studying changes in field topology during flares and reliably modelling space weather.

Aims. We tackle this problem by applying various inversion methods to a confined X2.2 flare that occurred in NOAA AR 12673 on 6 September 2017 and comparing the photospheric and chromospheric magnetic field vector with the results of two numerical models of this event.

Methods. We obtained the photospheric magnetic field from Milne-Eddington and (non-)local thermal equilibrium (non-LTE) inversions of Hinode SOT/SP FeI 6301.5 angstrom and 6302.5 angstrom. The chromospheric field was obtained from a spatially regularised weak-field approximation (WFA) and non-LTE inversions of CaII 8542 angstrom observed with CRISP at the Swedish 1 m Solar Telescope. We investigated the field strengths and photosphere-to-chromosphere shear in the field vector.

Results. The LTE- and non-LTE-inferred photospheric magnetic field components are strongly correlated across several optical depths in the atmosphere, with a tendency towards a stronger field and higher temperatures in the non-LTE inversions. For the chromospheric field, the non-LTE inversions correlate well with the spatially regularised WFA, especially in terms of the line-of-sight field strength and field vector orientation. The photosphere exhibits coherent strong-field patches of over 4.5 kG, co-located with similar concentrations exceeding 3 kG in the chromosphere. The obtained field strengths are up to two to three times higher than in the numerical models, while the photosphere-to-chromosphere shear close to the polarity inversion line is more concentrated and structured.

Conclusions. In the photosphere, the assumption of LTE for FeI line formation does not yield significantly different magnetic field results in comparison to the non-LTE case, while Milne-Eddington inversions fail to reproduce the magnetic field vector orientation where FeI is in emission. In the chromosphere, the non-LTE-inferred field is excellently approximated by the spatially regularised WFA. Our inversions confirm the locations of flux rope footpoints that have been predicted by numerical models. However, pre-processing and lower spatial resolution lead to weaker and smoother field in the models than what our data indicate. This highlights the need for higher spatial resolution in the models to better constrain pre-eruptive flux ropes.

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On the (Mis)Interpretation of the Scattering Polarization Signatures in the Ca ii 8542 Å Line through Spectral Line Inversions

2021. Rebecca Centeno, Jaime de la Cruz Rodríguez, Tanausú del Pino Alemán. Astrophysical Journal 918 (1)

Artikel

Scattering polarization tends to dominate the linear polarization signals of the Ca ii 8542 Å line in weakly magnetized areas (B ≲ 100 G), especially when the observing geometry is close to the limb. In this paper, we evaluate the degree of applicability of existing non-LTE spectral line inversion codes (which assume that the spectral line polarization is due to the Zeeman effect only) at inferring the magnetic field vector and, particularly, its transverse component. To this end, we use the inversion code STiC to extract the strength and orientation of the magnetic field from synthetic spectropolarimetric data generated with the Hanle-RT code. The latter accounts for the generation of polarization through scattering processes as well as the joint actions of the Hanle and the Zeeman effects. We find that, when the transverse component of the field is stronger than ∼80 G, the inversion code is able to retrieve accurate estimates of the transverse field strength as well as its azimuth in the plane of the sky. Below this threshold, the scattering polarization signatures become the major contributors to the linear polarization signals and often mislead the inversion code into severely over- or underestimating the field strength. Since the line-of-sight component of the field is derived from the circular polarization signal, which is not affected by atomic alignment, the corresponding inferences are always good.

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SSTRED: Data- and metadata-processing pipeline for CHROMIS and CRISP

2021. Mats G. Löfdahl (et al.). Astronomy and Astrophysics 653

Artikel

Context. Data from ground-based, high-resolution solar telescopes can only be used for science with calibrations and processing, which requires detailed knowledge about the instrumentation. Space-based solar telescopes provide science-ready data, which are easier to work with for researchers whose expertise is in the interpretation of data. Recently, data-processing pipelines for ground-based instruments have been constructed.

Aims. We aim to provide observers with a user-friendly data pipeline for data from the Swedish 1-meter Solar Telescope (SST) that delivers science-ready data together with the metadata needed for proper interpretation and archiving.

Methods. We briefly describe the CHROMospheric Imaging Spectrometer (CHROMIS) instrument, including its (pre)filters, as well as recent upgrades to the CRisp Imaging SpectroPolarimeter (CRISP) prefilters and polarization optics. We summarize the processing steps from raw data to science-ready data cubes in FITS files. We report calibrations and compensations for data imperfections in detail. Misalignment of Ca II data due to wavelength-dependent dispersion is identified, characterized, and compensated for. We describe intensity calibrations that remove or reduce the effects of filter transmission profiles as well as solar elevation changes. We present REDUX, a new version of the MOMFBD image restoration code, with multiple enhancements and new features. It uses projective transforms for the registration of multiple detectors. We describe how image restoration is used with CRISP and CHROMIS data. The science-ready output is delivered in FITS files, with metadata compliant with the SOLARNET recommendations. Data cube coordinates are specified within the World Coordinate System (WCS). Cavity errors are specified as distortions of the WCS wavelength coordinate with an extension of existing WCS notation. We establish notation for specifying the reference system for Stokes vectors with reference to WCS coordinate directions. The CRIsp SPectral EXplorer (CRISPEX) data-cube browser has been extended to accept SSTRED output and to take advantage of the SOLARNET metadata.

Results. SSTRED is a mature data-processing pipeline for imaging instruments, developed and used for the SST/CHROMIS imaging spectrometer and the SST/CRISP spectropolarimeter. SSTRED delivers well-characterized, science-ready, archival-quality FITS files with well-defined metadata. The SSTRED code, as well as REDUX and CRISPEX, is freely available through git repositories.

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Stratification of physical parameters in a C-class solar flare using multiline observations

2021. Rahul Yadav (et al.). Astronomy and Astrophysics 649

Artikel

We present high-resolution and multiline observations of a C2-class solar flare (SOL2019-05-06T08:47), which occurred in NOAA AR 12740 on May 6, 2019. The rise, peak, and decay phases of the flare were recorded continuously and quasi-simultaneously in the Ca II K line with the CHROMIS instrument and in the Ca II 8542 Å and Fe I 6173 Å lines with the CRISP instrument at the Swedish 1 m Solar Telescope. The observations in the chromospheric Ca II lines exhibit intense brightening near the flare footpoints. At these locations, a nonlocal thermodynamic equilibrium inversion code was employed to infer the temperature, magnetic field, line-of-sight (LOS) velocity, and microturbulent velocity stratification in the flaring atmosphere. The temporal analysis of the inferred temperature at the flare footpoints shows that the flaring atmosphere from log τ₅₀₀ ∼ −2.5 to −3.5 is heated up to 7 kK, whereas from log τ₅₀₀ ∼ −3.5 to −5 the inferred temperature ranges between ∼7.5 kK and ∼11 kK. During the flare peak time, the LOS velocity shows both upflows and downflows around the flare footpoints in the upper chromosphere and lower chromosphere, respectively. Moreover, the temporal analysis of the LOS magnetic field at the flare points exhibits a maximum change of ∼600 G. After the flare, the LOS magnetic field decreases to the non-flaring value, exhibiting no permanent or step-wise change. The analysis of response functions to the temperature, LOS magnetic field, and velocity shows that the Ca II lines exhibit enhanced sensitivity to the deeper layers (i.e., log τ₅₀₀ ∼ −3) of the flaring atmosphere, whereas for the non-flaring atmosphere they are mainly sensitive around log τ₅₀₀ ∼ −4. We suggest that a fraction of the apparent increase in the LOS magnetic field at the flare footpoints may be due to the increase in the sensitivity of the Ca II 8542 Å line in the deeper layers, where the field strength is relatively strong. The rest may be due to magnetic field reconfiguration during the flare. In the photosphere, we do not notice significant changes in the physical parameters during the flare or non-flare times. Our observations illustrate that even a less intense C-class flare can heat the deeper layers of the solar chromosphere, mainly at the flare footpoints, without affecting the photosphere.

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ALMA observations of transient heating in a solar active region

2020. João Manuel da Silva Santos (et al.). Astronomy and Astrophysics 643

Artikel

Aims. We aim to investigate the temperature enhancements and formation heights of solar active-region brightenings such as Ellerman bombs (EBs), ultraviolet bursts (UVBs), and flaring active-region fibrils (FAFs) using interferometric observations in the millimeter (mm) continuum provided by the Atacama Large Millimeter/submillimeter Array (ALMA).

Methods. We examined 3 mm signatures of heating events identified in Solar Dynamics Observatory observations of an active region and compared the results with synthetic spectra from a 3D radiative magnetohydrodynamic simulation. We estimated the contribution from the corona to the mm brightness using differential emission measure analysis.

Results. We report the null detection of EBs in the 3 mm continuum at ∼1.2″ spatial resolution, which is evidence that they are sub-canopy events that do not significantly contribute to heating the upper chromosphere. In contrast, we find the active region to be populated with multiple compact, bright, flickering mm-bursts – reminiscent of UVBs. The high brightness temperatures of up to ∼14 200 K in some events have a contribution (up to ∼7%) from the corona. We also detect FAF-like events in the 3 mm continuum. These events show rapid motions of > 10 kK plasma launched with high plane-of-sky velocities (37 − 340 km s⁻¹) from bright kernels. The mm FAFs are the brightest class of warm canopy fibrils that connect magnetic regions of opposite polarities. The simulation confirms that ALMA should be able to detect the mm counterparts of UVBs and small flares and thus provide a complementary diagnostic for localized heating in the solar chromosphere.

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High-resolution observations of the solar photosphere, chromosphere, and transition region: A database of coordinated IRIS and SST observations

2020. L. H. M. Rouppe van der Voort (et al.). Astronomy and Astrophysics 641

Artikel

NASA’s Interface Region Imaging Spectrograph (IRIS) provides high-resolution observations of the solar atmosphere through ultraviolet spectroscopy and imaging. Since the launch of IRIS in June 2013, we have conducted systematic observation campaigns in coordination with the Swedish 1 m Solar Telescope (SST) on La Palma. The SST provides complementary high-resolution observations of the photosphere and chromosphere. The SST observations include spectropolarimetric imaging in photospheric Fe I lines and spectrally resolved imaging in the chromospheric Ca II 8542 Å, Hα, and Ca II K lines. We present a database of co-aligned IRIS and SST datasets that is open for analysis to the scientific community. The database covers a variety of targets including active regions, sunspots, plages, the quiet Sun, and coronal holes.

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Inference of the chromospheric magnetic field configuration of solar plage using the CaII 8542 Å line

2020. Alexander G. M. Pietrow (et al.). Astronomy and Astrophysics 644

Artikel

Context. It has so far proven impossible to reproduce all aspects of the solar plage chromosphere in quasi-realistic numerical models. The magnetic field configuration in the lower atmosphere is one of the few free parameters in such simulations. The literature only offers proxy-based estimates of the field strength, as it is difficult to obtain observational constraints in this region. Sufficiently sensitive spectro-polarimetric measurements require a high signal-to-noise ratio, spectral resolution, and cadence, which are at the limit of current capabilities.

Aims. We use critically sampled spectro-polarimetric observations of the CaII 8542 angstrom line obtained with the CRISP instrument of the Swedish 1-m Solar Telescope to study the strength and inclination of the chromospheric magnetic field of a plage region. This will provide direct physics-based estimates of these values, which could aid modelers to put constraints on plage models.

Methods. We increased the signal-to-noise ratio of the data by applying several methods including deep learning and PCA. We estimated the noise level to be 1x10(-3)I(c). We then used STiC, a non-local thermodynamic equilibrium inversion code to infer the atmospheric structure and magnetic field pixel by pixel.

Results. We are able to infer the magnetic field strength and inclination for a plage region and for fibrils in the surrounding canopy. In the plage we report an absolute field strength of |B| = 440 +/- 90 G, with an inclination of 10 degrees +/- 16 degrees with respect to the local vertical. This value for |B| is roughly double of what was reported previously, while the inclination matches previous studies done in the photosphere. In the fibrillar region we found |B| = 300 +/- 50 G, with an inclination of 50 degrees +/- 13 degrees.

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Physical properties of bright Ca II K fibrils in the solar chromosphere

2020. Sepideh Kianfar (et al.). Astronomy and Astrophysics 637

Artikel

Context. Broad-band images of the solar chromosphere in the Ca II H&K line cores around active regions are covered with fine bright elongated structures called bright fibrils. The mechanisms that form these structures and cause them to appear bright are still unknown.

Aims. We aim to investigate the physical properties, such as temperature, line-of-sight velocity, and microturbulence, in the atmosphere that produces bright fibrils and to compare those to the properties of their surrounding atmosphere.

Methods. We used simultaneous observations of a plage region in Fe I 6301-2 Å, Ca II 8542 Å, Ca II K, and Hα acquired by the CRISP and CHROMIS instruments on the Swedish 1 m Solar Telescope. We manually selected a sample of 282 Ca II K bright fibrils. We compared the appearance of the fibrils in our sample to the Ca II 8542 Å and Hα data. We performed non-local thermodynamic equilibrium inversions using the inversion code STiC on the Fe I 6301-2 Å, Ca II 8542 Å, and Ca II K lines to infer the physical properties of the atmosphere.

Results. The line profiles in bright fibrils have a higher intensity in their K-2 peaks compared to profiles formed in the surrounding atmosphere. The inversion results show that the atmosphere in fibrils is on average -100 K hotter at an optical depth log (τ500 nm) = -4.3 compared to their surroundings. The line-of-sight velocity at chromospheric heights in the fibrils does not show any preference towards upflows or downflows. The microturbulence in the fibrils is on average 0.5 km s(-1) higher compared to their surroundings. Our results suggest that the fibrils have a limited extent in height, and they should be viewed as hot threads pervading the chromosphere.

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Signatures of ubiquitous magnetic reconnection in the lower solar atmosphere

2020. Jayant Joshi, Luc H. M. Rouppe van der Voort, Jaime de la Cruz Rodríguez. Astronomy and Astrophysics 641

Artikel

Ellerman Bomb-like brightenings of the hydrogen Balmer line wings in the quiet Sun, also known as quiet Sun Ellerman bombs (QSEBs), are a signature of the fundamental process of magnetic reconnection at the smallest observable scale in the lower solar atmosphere. We analyze high spatial resolution observations (0_.^″1) obtained with the Swedish 1-m Solar Telescope to explore signatures of QSEBs in the Hβ line. We find that QSEBs are ubiquitous and uniformly distributed throughout the quiet Sun, predominantly occurring in intergranular lanes. We find up to 120 QSEBs in the field of view for a single moment in time; this is more than an order of magnitude higher than the number of QSEBs found in earlier Hα observations. This suggests that about half a million QSEBs could be present in the lower solar atmosphere at any given time. The QSEB brightenings found in the Hβ line wings also persist in the line core with a temporal delay and spatial offset toward the nearest solar limb. Our results suggest that QSEBs emanate through magnetic reconnection along vertically extended current sheets in the lower solar atmosphere. The apparent omnipresence of small-scale magnetic reconnection may play an important role in the energy balance of the solar chromosphere.

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Stratification of canopy magnetic fields in a plage region: Constraints from a spatially-regularized weak-field approximation method

2020. Roberta Morosin (et al.). Astronomy and Astrophysics 642

Artikel

Context. The role of magnetic fields in the chromospheric heating problem remains greatly unconstrained. Most theoretical predictions from numerical models rely on a magnetic configuration, field strength, and connectivity; the details of which have not been well established with observational studies for many chromospheric scenarios. High-resolution studies of chromospheric magnetic fields in plage are very scarce or non existent in general.

Aims. Our aim is to study the stratification of the magnetic field vector in plage regions. Previous studies predict the presence of a magnetic canopy in the chromosphere that has not yet been studied with full-Stokes observations. We use high-spatial resolution full-Stokes observations acquired with the CRisp Imaging Spectro-Polarimeter (CRISP) at the Swedish 1-m Solar Telescope in the MgI 5173 angstrom, NaI 5896 angstrom and CaII 8542 angstrom lines.

Methods. We have developed a spatially-regularized weak-field approximation (WFA) method, based on the idea of spatial regularization. This method allows for a fast computation of magnetic field maps for an extended field of view. The fidelity of this new technique has been assessed using a snapshot from a realistic 3D magnetohydrodynamics simulation.

Results. We have derived the depth-stratification of the line-of-sight component of the magnetic field from the photosphere to the chromosphere in a plage region. The magnetic fields are concentrated in the intergranular lanes in the photosphere and expand horizontally toward the chromosphere, filling all the space and forming a canopy. Our results suggest that the lower boundary of this canopy must be located around 400-600 km from the photosphere. The mean canopy total magnetic field strength in the lower chromosphere (z approximate to 760 km) is 658 G. At z=1160 km, we estimate B approximate to 417 G.

Conclusions. In this study we propose a modification to the WFA that improves its applicability to data with a worse signal-to-noise ratio. We have used this technique to study the magnetic properties of the hot chromospheric canopy that is observed in plage regions. The methods described in this paper provide a quick and reliable way of studying multi layer magnetic field observations without the many difficulties inherent to other inversion methods.

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The Formation Height of Millimeter-wavelength Emission in the Solar Chromosphere

2020. Juan Martínez-Sykora (et al.). Astrophysical Journal Letters 891 (1)

Artikel

In the past few years, the ALMA radio telescope has become available for solar observations. ALMA diagnostics of the solar atmosphere are of high interest because of the theoretically expected linear relationship between the brightness temperature at millimeter wavelengths and the local gas temperature in the solar atmosphere. Key for the interpretation of solar ALMA observations is understanding where in the solar atmosphere the ALMA emission originates. Recent theoretical studies have suggested that ALMA bands at 1.2 (band 6) and 3 mm (band 3) form in the middle and upper chromosphere at significantly different heights. We study the formation of ALMA diagnostics using a 2.5D radiative MHD model that includes the effects of ion-neutral interactions (ambipolar diffusion) and nonequilibrium ionization of hydrogen and helium. Our results suggest that in active regions and network regions, observations at both wavelengths most often originate from similar heights in the upper chromosphere, contrary to previous results. Nonequilibrium ionization increases the opacity in the chromosphere so that ALMA mostly observes spicules and fibrils along the canopy fields. We combine these modeling results with observations from IRIS, SDO, and ALMA to suggest a new interpretation for the recently reported dark chromospheric holes, regions of very low temperatures in the chromosphere.

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The Sun at millimeter wavelengths: I. Introduction to ALMA Band 3 observations

2020. Sven Wedemeyer (et al.). Astronomy and Astrophysics 635

Artikel

Context. The Atacama Large Millimeter/submillimeter Array (ALMA) started regular observations of the Sun in 2016, first offering receiver Band 3 at wavelengths near 3 mm (100 GHz) and Band 6 at wavelengths around 1.25 mm (239 GHz).Aims. Here we present an initial study of one of the first ALMA Band 3 observations of the Sun. Our aim is to characterise the diagnostic potential of brightness temperatures measured with ALMA on the Sun.Methods. The observation covers a duration of 48 min at a cadence of 2 s targeting a quiet Sun region at disc-centre. Corresponding time series of brightness temperature maps are constructed with the first version of the Solar ALMA Pipeline and compared to simultaneous observations with the Solar Dynamics Observatory (SDO).Results. The angular resolution of the observations is set by the synthesised beam, an elliptical Gaussian that is approximately 1.4 '' x2.1 '' in size. The ALMA maps exhibit network patches, internetwork regions, and elongated thin features that are connected to large-scale magnetic loops, as confirmed by a comparison with SDO maps. The ALMA Band 3 maps correlate best with the SDO/AIA 171 angstrom, 131 angstrom, and 304 angstrom channels in that they exhibit network features and, although very weak in the ALMA maps, imprints of large-scale loops. A group of compact magnetic loops is very clearly visible in ALMA Band 3. The brightness temperatures in the loop tops reach values of about 8000-9000 K and in extreme moments up to 10 000 K.Conclusions. ALMA Band 3 interferometric observations from early observing cycles already reveal temperature differences in the solar chromosphere. The weak imprint of magnetic loops and the correlation with the 171, 131, and 304 SDO channels suggests, however, that the radiation mapped in ALMA Band 3 might have contributions from a wider range of atmospheric heights than previously assumed, but the exact formation height of Band 3 needs to be investigated in more detail. The absolute brightness temperature scale as set by total power measurements remains less certain and must be improved in the future. Despite these complications and the limited angular resolution, ALMA Band 3 observations have a large potential for quantitative studies of the small-scale structure and dynamics of the solar chromosphere.

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The multi-thermal chromosphere Inversions of ALMA and IRIS data

2020. Joäo Manuel da Silva Santos (et al.). Astronomy and Astrophysics 634

Artikel

Context. Numerical simulations of the solar chromosphere predict a diverse thermal structure with both hot and cool regions. Observations of plage regions in particular typically feature broader and brighter chromospheric lines, which suggests that they are formed in hotter and denser conditions than in the quiet Sun, but also implies a nonthermal component whose source is unclear. Aims. We revisit the problem of the stratification of temperature and microturbulence in plage and the quiet Sun, now adding millimeter (mm) continuum observations provided by the Atacama Large Millimiter Array (ALMA) to inversions of near-ultraviolet Interface Region Imaging Spectrograph (IRIS) spectra as a powerful new diagnostic to disentangle the two parameters. We fit cool chromospheric holes and track the fast evolution of compact mm brightenings in the plage region. Methods. We use the STiC nonlocal thermodynamic equilibrium (NLTE) inversion code to simultaneously fit real ultraviolet and mm spectra in order to infer the thermodynamic parameters of the plasma. Results. We confirm the anticipated constraining potential of ALMA in NLTE inversions of the solar chromosphere. We find significant differences between the inversion results of IRIS data alone compared to the results of a combination with the mm data: the IRIS+ALMA inversions have increased contrast and temperature range, and tend to favor lower values of microturbulence (similar to 3-6 km s(-1) in plage compared to similar to 4-7 km s(-1) from IRIS alone) in the chromosphere. The average brightness temperature of the plage region at 1.25 mm is 8500 K, but the ALMA maps also show much cooler (similar to 3000 K) and hotter (similar to 11000 K) evolving features partially seen in other diagnostics. To explain the former, the inversions require the existence of localized low-temperature regions in the chromosphere where molecules such as CO could form. The hot features could sustain such high temperatures due to non-equilibrium hydrogen ionization effects in a shocked chromosphere - a scenario that is supported by low-frequency shock wave patterns found in the MgII lines probed by IRIS.

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A method for global inversion of multi-resolution solar data

2019. Jaime de la Cruz Rodríguez. Astronomy and Astrophysics 631

Artikel

Understanding the complex dynamics and structure of the upper solar atmosphere strongly benefits from the use of a combination of several diagnostics. Frequently, such diverse diagnostics can only be obtained from telescopes and/or instrumentation operating at widely different spatial resolution. To optimize the utilization of such data, we propose a new method for the global inversion of data acquired at different spatial resolution. The method has its roots in the Levenberg-Marquardt algorithm but involves the use of linear operators to transform and degrade the synthetic spectra of a highly resolved guess model to account for the effects of spatial resolution, data sampling, alignment, and image rotation of each of the datasets. We have carried out a list of numerical experiments to show that our method allows for the extraction of spatial information from two simulated datasets that have gone through two different telescope apertures and that are sampled in different spatial grids. Our results show that each dataset contributes in the inversion by constraining information at the spatial scales that are present in each of the datasets, and no negative effects are derived from the combination of multiple resolution data. This method is especially relevant for chromospheric studies that attempt to combine datasets acquired with different telescopes and/or datasets acquired at different wavelengths. The techniques described in the present study will also help to address the ever increasing resolution gap between space-borne missions and forthcoming ground-based facilities.

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Chromospheric condensations and magnetic field in a C3.6-class flare studied via He I D-3 spectro-polarimetry

2019. Tine Libbrecht (et al.). Astronomy and Astrophysics 621

Artikel

Context. Magnetic reconnection during flares takes place in the corona, but a substantial part of flare energy is deposited in the chromosphere. However, high-resolution spectro-polarimetric chromospheric observations of flares are very rare. The most used observables are Ca II 8542 angstrom and He I 10830 angstrom.

Aims. We aim to study the chromosphere during a C3.6 class flare via spectro-polarimetric observations of the He I D-3 line.

Methods. We present the first SST/CRISP spectro-polarimetric observations of He I D3. We analyzed the data using the inversion code HAZEL, and estimate the line-of-sight velocity and the magnetic field vector.

Results. Strong He I D-3 emission at the flare footpoints, as well as strong He I D(3 )absorption profiles tracing the flaring loops are observed during the flare. The He I D-3 traveling emission kernels at the flare footpoints exhibit strong chromospheric condensations of up to similar to 60 km s(-1) at their leading edge. Our observations suggest that such condensations result in shocking the deep chromosphere, causing broad and modestly blueshifted He I D-3 profiles indicating subsequent upflows. A strong and rather vertical magnetic field of up to similar to 2500 G is measured in the flare footpoints, confirming that the He I D-3 line is likely formed in the deep chromosphere at those locations. We provide chromospheric line-of-sight velocity and magnetic field maps obtained via He I D-3 inversions. We propose a fan-spine configuration as the flare magnetic field topology.

Conclusions. The He I D-3 line is an excellent diagnostic to study the chromosphere during flares. The impact of strong condensations on the deep chromosphere has been observed. Detailed maps of the flare dynamics and the magnetic field are obtained.

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Chromospheric polarimetry through multiline observations of the 850nm spectral region III: Chromospheric jets driven by twisted magnetic fields

2019. C. Quintero Noda (et al.). Monthly notices of the Royal Astronomical Society 486 (3), 4203-4215

Artikel

We investigate the diagnostic potential of the spectral lines at 850 nm for understanding the magnetism of the lower atmosphere. For that purpose, we use a newly developed 3D simulation of a chromospheric jet to check the sensitivity of the spectral lines to this phenomenon as well as our ability to infer the atmospheric information through spectropolarimetric inversions of noisy synthetic data. We start comparing the benefits of inverting the entire spectrum at 850 nm versus only the Ca II 8542 angstrom spectral line. We found a better match of the input atmosphere for the former case, mainly at lower heights. However, the results at higher layers were not accurate. After several tests, we determined that we need to weight more the chromospheric lines than the photospheric ones in the computation of the goodness of the fit. The new inversion configuration allows us to obtain better fits and consequently more accurate physical parameters. Therefore, to extract the most from multiline inversions, a proper set of weights needs to be estimated. Besides that, we conclude again that the lines at 850 nm, or a similar arrangement with Ca II 8542 angstrom plus Zeeman-sensitive photospheric lines, pose the best-observing configuration for examining the thermal and magnetic properties of the lower solar atmosphere.

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Diagnostic potential of the Ca II 8542 Ångstrom line for solar filaments

2019. Carlos José Díaz Baso (et al.). Astronomy and Astrophysics 623

Artikel

Aims. In this study we explore the diagnostic potential of the chromospheric Ca II line at 8542 angstrom for studying the magnetic and dynamic properties of solar filaments. We have acquired high spatial resolution spectropolarimetric observations in the Ca II 8542 angstrom line using the CRISP instrument at the Swedish 1 m Solar Telescope. Methods. We used the NICOLE inversion code to infer physical properties from observations of a solar filament. We discuss the validity of the results due to the assumption of hydrostatic equilibrium. We have used observations from other telescopes such as CHROTEL and SDO, in order to study large scale dynamics and the long term evolution of the filament. Results. We show that the Ca II 8542 angstrom line encodes information of the temperature, line-of-sight velocity and magnetic field vector from the region where the filament is located. The current noise levels only allow us to estimate an upper limit of 260 G for the total magnetic field of the filament. Our study also reveals that if we consider information from the aforementioned spectral line alone, the geometric height, the temperature and the density could be degenerated parameters outside the hydrostatic equilibrium approach.

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Is the sky the limit? Performance of the revamped Swedish 1-m Solar Telescope and its blue- and red-beam reimaging systems

2019. Göran B. Scharmer (et al.). Astronomy and Astrophysics 626

Artikel

We discuss the use of measurements of the solar granulation contrast as a measure of optical quality. We demonstrate that for data recorded with a telescope that uses adaptive optics and/or post-processing to compensate for many low- and high-order aberrations, the RMS granulation contrast is directly proportional to the Strehl ratio calculated from the residual (small-scale) wavefront error (static and/or from seeing). We demonstrate that the wings of the high-order compensated point spread function for the Swedish 1-m Solar Telescope (SST) are likely to extend to a radius of not more than about 2 '', which is consistent with earlier conclusions drawn from stray-light compensation of sunspot images. We report on simultaneous measurements of seeing and solar granulation contrast averaged over 2 s time intervals at several wavelengths from 525 nm to 853.6 nm on the red-beam (CRISP beam) and wavelengths from 395 nm to 484 nm on the blue-beam (CHROMIS beam). These data were recorded with the SST, which has been revamped with an 85-electrode adaptive mirror and a new tip-tilt mirror, both of which were polished to exceptionally high optical quality. Compared to similar data obtained with the previous 37-electrode adaptive mirror in 2009 and 2011, there is a significant improvement in image contrast. The highest 2 s average image contrasts measured in April 2015 through 0.3-0.9 nm interference filters at 525 nm, 557 nm, 630 nm, and 853.5 nm with compensation only for the diffraction limited point spread function of SST are 11.8%, 11.8%, 10.2%, and 7.2%, respectively. Similarly, the highest 2 s contrasts measured at 395 nm, 400 nm, and 484 nm in May 2016 through 0.37-1.3 nm filters are 16%, 16%, and 12.5%, respectively. The granulation contrast observed with SST compares favorably to measured values with SOT on Hinode and with Sunrise as well as major ground-based solar telescopes. Simultaneously with the above wideband red-beam data, we also recorded narrowband continuum images with the CRISP imaging spectropolarimeter. We find that contrasts measured with CRISP are entirely consistent with the corresponding wideband contrasts, demonstrating that any additional image degradation by the CRISP etalons and telecentric optical system is marginal or even insignificant. Finally, we discuss the origin of the 48 nm RMS wavefront error needed to bring consistency between the measured granulation contrast and that obtained from 3D simulations of convection.

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Observationally Based Models of Penumbral Microjets

2019. Sara Esteban Pozuelo (et al.). Astrophysical Journal 870 (2)

Artikel

We study the polarization signals and physical parameters of penumbral microjets (PMJs) by using high spatial resolution data taken in the Fe I 630 nm pair, Ca II 854.2 nm, and Ca II K lines with the CRISP and CHROMIS instruments at the Swedish 1 m Solar Telescope. We infer their physical parameters, such as physical observables in the photosphere and chromospheric velocity diagnostics, by different methods, including inversions of the observed Stokes profiles with the STiC code. PMJs harbor overall brighter Ca II K line profiles and conspicuous polarization signals in Ca II 854.2 nm, specifically in circular polarization that often shows multiple lobes mainly due to the shape of Stokes I They usually overlap photospheric regions with a sheared magnetic field configuration, suggesting that magnetic reconnections could play an important role in the origin of PMJs. The discrepancy between their low LOS velocities and the high apparent speeds reported on earlier, as well as the existence of different vertical velocity gradients in the chromosphere, indicate that PMJs might not be entirely related to mass motions. Instead, PMJs could be due to perturbation fronts induced by magnetic reconnections occurring in the deep photosphere that propagate through the chromosphere. This reconnection may be associated with current heating that produces temperature enhancements from the temperature minimum region. Furthermore, enhanced collisions with electrons could also increase the coupling to the local conditions at higher layers during the PMJ phase, giving a possible explanation for the enhanced emission in the overall Ca II K profiles emerging from these transients.

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Recovering Thermodynamics from Spectral Profiles observed by IRIS: A Machine and Deep Learning Approach

2019. Alberto Sainz Dalda (et al.). Astrophysical Journal Letters 875 (2)

Artikel

Inversion codes allow the reconstruction of a model atmosphere from observations. With the inclusion of optically thick lines that form in the solar chromosphere, such modeling is computationally very expensive because a non-LTE evaluation of the radiation field is required. In this study, we combine the results provided by these traditional methods with machine and deep learning techniques to obtain similar-quality results in an easy-to-use, much faster way. We have applied these new methods to Mg II h and k lines observed by the Interface Region Imaging Spectrograph (IRIS). As a result, we are able to reconstruct the thermodynamic state (temperature, line-of-sight velocity, nonthermal velocities, electron density, etc.) in the chromosphere and upper photosphere of an area equivalent to an active region in a few CPU minutes, speeding up the process by a factor of 10(5) - 10(6). The opensource code accompanying this Letter will allow the community to use IRIS observations to open a new window to a host of solar phenomena.

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STiC: A multiatom non-LTE PRD inversion code for full-Stokes solar observations

2019. Jaime de la Cruz Rodriguez (et al.). Astronomy and Astrophysics 623

Artikel

The inference of the underlying state of the plasma in the solar chromosphere remains extremely challenging because of the nonlocal character of the observed radiation and plasma conditions in this layer. Inversion methods allow us to derive a model atmosphere that can reproduce the observed spectra by undertaking several physical assumptions. The most advanced approaches involve a depth-stratified model atmosphere described by temperature, line-of-sight velocity, turbulent velocity, the three components of the magntic field vector, and gas and electron pressure. The parameters of the radiative transfer equation are computed from a solid ground of physical principles. In order to apply these techniques to spectral lines that sample the chromosphere, nonlocal thermodynamical equilibrium effects must be included in the calculations. We developed a new inversion code STiC (STockholm inversion Code) to study spectral lines that sample the upper chromosphere. The code is based on the RH forward synthesis code, which we modified to make the inversions faster and more stable. For the first time, STiC facilitates the processing of lines from multiple atoms in non-LTE, also including partial redistribution effects (PRD) in angle and frequency of scattered photons. Furthermore, we include a regularization strategy that allows for model atmospheres with a complex depth stratification, without introducing artifacts in the reconstructed physical parameters, which are usually manifested in the form of oscillatory behavior. This approach takes steps toward a node-less inversion, in which the value of the physical parameters at each grid point can be considered a free parameter. In this paper we discuss the implementation of the aforementioned techniques, the description of the model atmosphere, and the optimizations that we applied to the code. We carry out some numerical experiments to show the performance of the code and the regularization techniques that we implemented. We made STiC publicly available to the community.

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Solar image denoising with convolutional neural networks

2019. Carlos José Diaz Baso, Jaime de la Cruz Rodriguez, Sanja Danilovic. Astronomy and Astrophysics 629

Artikel

The topology and dynamics of the solar chromosphere are greatly affected by the presence of magnetic fields. The magnetic field can be inferred by analyzing polarimetric observations of spectral lines. Polarimetric signals induced by chromospheric magnetic fields are, however, particularly weak, and in most cases very close to the detection limit of current instrumentation. Because of this, there are only few observational studies that have successfully reconstructed the three components of the magnetic field vector in the chromosphere. Traditionally, the signal-to-noise ratio of observations has been improved by performing time-averages or spatial averages, but in both cases, some information is lost. More advanced techniques, like principal-component analysis, have also been employed to take advantage of the sparsity of the observations in the spectral direction. In the present study, we use the spatial coherence of the observations to reduce the noise using deep-learning techniques. We designed a neural network that is capable of recovering weak signals under a complex noise corruption (including instrumental artifacts and non-linear post-processing). The training of the network is carried out without a priori knowledge of the clean signals, or an explicit statistical characterization of the noise or other corruption. We only use the same observations as our generative model. The performance of this method is demonstrated on both synthetic experiments and real data. We show examples of the improvement in typical signals obtained in current telescopes such as the Swedish 1 m Solar Telescope. The presented method can recover weak signals equally well no matter what spectral line or spectral sampling is used. It is especially suitable for cases when the wavelength sampling is scarce.

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The Dark Side of Penumbral Microjets: Observations in H alpha

2019. David Bühler (et al.). Astrophysical Journal 876 (1)

Artikel

We present data of 10 penumbral microjets (PMJs) observed in a H alpha, Ca II 8542 angstrom, and Fe I 6302 angstrom line pair with the Swedish 1 m Solar Telescope (SST) with CRISP and Ca II K with SST/CHROMIS in active region NOAA 12599 on 2016 October 12 at mu = 0.68. All four Stokes parameters of the Ca II 8542 angstrom and Fe I 6302 angstrom lines were observed and a series of test pixels were inverted using the Stockholm inversion code. Our analysis revealed for the first time that PMJs are visible in H alpha, where they appear as dark features with average line-of-sight (LOS) upflows of 1.1 +/- 0.6 km s(-1), matching the LOS velocities from the inversions. Based on the H alpha observations we extend the previous average length and lifetime of PMJs to 2815 +/- 530 km and 163 +/- 25 s, respectively. The plane-of-sky (POS) velocities of our PMJs of up to 17 km s(-1) tend to give increased velocities with distance traveled. Furthermore, two of our PMJs with significant Stokes V signal indicate that the PMJs possess an increased LOS magnetic field of up to 100 G compared to the local pre-/post- PMJ magnetic field, which propagates as quickly as the PMJs' POS velocities. Finally, we present evidence that PMJs display an on average 1 minute gradual precursory brightening that only manifests itself in the cores of the Ca II lines. We conclude that PMJs are not ordinary jets but likely are manifestations of heat fronts that propagate at the local Alfven velocity.

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Three-dimensional magnetic field structure of a flux-emerging region in the solar atmosphere

2019. Rahul Yadav (et al.). Astronomy and Astrophysics 632

Artikel

We analyze high-resolution spectropolarimetric observations of a flux-emerging region (FER) in order to understand its magnetic and kinematic structure. Our spectropolarimetric observations in the HeI 10830 angstrom spectral region of a FER were recorded with GRIS at the 1.5 m aperture GREGOR telescope. A Milne-Eddington-based inversion code was employed to extract the photospheric information of the SiI spectral line, whereas the HeI triplet line was analyzed with the Hazel inversion code, which takes into account the joint action of the Hanle and the Zeeman effects. The spectropolarimetric analysis of the SiI line reveals a complex magnetic structure near the vicinity of the FER, where a weak (350-600 G) and horizontal magnetic field was observed. In contrast to the photosphere, the analysis of the HeI triplet presents a smooth variation of the magnetic field vector (ranging from 100 to 400 G) and velocities across the FER. Moreover, we find supersonic downflows of similar to 40 km s(-1) appearing near the foot points of loops connecting two pores of opposite polarity, whereas strong upflows of 22 km s(-1) appear near the apex of the loops. At the location of supersonic downflows in the chromosphere, we observed downflows of 3 km s(-1) in the photosphere. Furthermore, nonforce-free field extrapolations were performed separately at two layers in order to understand the magnetic field topology of the FER. We determine, using extrapolations from the photosphere and the observed chromospheric magnetic field, that the average formation height of the HeI triplet line is similar to 2 Mm from the solar surface. The reconstructed loops using photospheric extrapolations along an arch filament system have a maximum height of similar to 10.5 Mm from the solar surface with a foot-point separation of similar to 19 Mm, whereas the loops reconstructed using chromospheric extrapolations reach around similar to 8.4 Mm above the solar surface with a foot-point separation of similar to 16 Mm at the chromospheric height. The magnetic topology in the FER suggests the presence of small-scale loops beneath the large loops. Under suitable conditions, due to magnetic reconnection, these loops can trigger various heating events in the vicinity of the FER.

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Three-dimensional modeling of chromospheric spectral lines in a simulated active region

2019. Johan P. Bjørgen (et al.). Astronomy and Astrophysics 631

Artikel

Context. Because of the complex physics that governs the formation of chromospheric lines, interpretation of solar chromospheric observations is difficult. The origin and characteristics of many chromospheric features are, because of this, unresolved.

Aims. We focus on studying two prominent features: long fibrils and flare ribbons. To model these features, we use a 3D magnetohydrodynamic simulation of an active region, which self-consistently reproduces both of these features.

Methods. We modeled the Ha, Mg it k, Call K, and Call 8542 A lines using the 3D non-LTE radiative transfer code Multi3D. To obtain non-LTE electron densities, we solved the statistical equilibrium equations for hydrogen simultaneously with the charge conservation equation. We treated the Call K and Mg It k lines with partially coherent scattering.

Results. This simulation reproduces long fibrils that span between the opposite -polarity sunspots and go up to 4 Mm in height. They can be traced in all lines owing to density corrugation. In contrast to previous studies, Ha, Mg II h&k, and Call H&K are formed at similar height in this model. Although some of the high fibrils are also visible in the Call 8542 A line, this line tends to sample loops and shocks lower in the chromosphere. Magnetic field lines are aligned with the Ha fibrils, but the latter holds to a lesser extent for the Call 8542 A line. The simulation shows structures in the Ha line core that look like flare ribbons. The emission in the ribbons is caused by a dense chromosphere and a transition region at high column mass. The ribbons are visible in all chromospheric lines, but least prominent in Call 8542 A line. In some pixels, broad asymmetric profiles with a single emission peak are produced similar to the profiles observed in flare ribbons. They are caused by a deep onset of the chromospheric temperature rise and large velocity gradients.

Conclusions. The simulation produces long fibrils similar to what is seen in observations. It also produces structures similar to flare ribbons despite the lack of nonthermal electrons in the simulation. The latter suggests that thermal conduction might be a significant agent in transporting flare energy to the chromosphere in addition to nonthermal electrons.

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Chromospheric Heating due to Cancellation of Quiet Sun Internetwork Fields

2018. M. Gošić (et al.). Astrophysical Journal 857 (1)

Artikel

The heating of the solar chromosphere remains one of the most important questions in solar physics. Our current understanding is that small-scale internetwork (IN) magnetic fields play an important role as a heating agent. Indeed, cancellations of IN magnetic elements in the photosphere can produce transient brightenings in the chromosphere and transition region. These bright structures might be the signature of energy release and plasma heating, probably driven by the magnetic reconnection of IN field lines. Although single events are not expected to release large amounts of energy, their global contribution to the chromosphere may be significant due to their ubiquitous presence in quiet Sun regions. In this paper, we study cancellations of IN elements and analyze their impact on the energetics and dynamics of the quiet Sun atmosphere. We use high-resolution, multiwavelength, coordinated observations obtained with the Interface Region Imaging Spectrograph and the Swedish 1 m Solar Telescope (SST) to identify cancellations of IN magnetic flux patches and follow their evolution. We find that, on average, these events live for similar to 3. minutes in the photosphere and similar to 12. minutes in the chromosphere and/or transition region. Employing multi-line inversions of the Mg II h and k lines, we show that cancellations produce clear signatures of heating in the upper atmospheric layers. However, at the resolution and sensitivity accessible to the SST, their number density still seems to be one order of magnitude too low to explain the global chromospheric heating.

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Chromospheric heating during flux emergence in the solar atmosphere

2018. Jorrit Leenaarts (et al.). Astronomy and Astrophysics 612

Artikel

Context. The radiative losses in the solar chromosphere vary from 4 kW m(-2) in the quiet Sun, to 20 kW m(-2) in active regions. The mechanisms that transport non-thermal energy to and deposit it in the chromosphere are still not understood. Aims. We aim to investigate the atmospheric structure and heating of the solar chromosphere in an emerging flux region. Methods. We have used observations taken with the CHROMIS and CRISP instruments on the Swedish 1-m Solar Telescope in the Ca II K, Ca II 854.2 nm, H alpha, and Fe I 630.1 nm and 630.2 nm lines. We analysed the various line profiles and in addition perform multi-line, multi-species, non-local thermodynamic equilibrium (non-LTE) inversions to estimate the spatial and temporal variation of the chromospheric structure. Results. We investigate which spectral features of Ca II K contribute to the frequency-integrated Ca II K brightness, which we use as a tracer of chromospheric radiative losses. The majority of the radiative losses are not associated with localised high-Ca II K-brightness events, but instead with a more gentle, spatially extended, and persistent heating. The frequency-integrated Ca II K brightness correlates strongly with the total linear polarization in the Ca II 854.2 nm, while the Ca II K profile shapes indicate that the bulk of the radiative losses occur in the lower chromosphere. Non-LTE inversions indicate a transition from heating concentrated around photospheric magnetic elements below log tau(500) = -3 to a more space-filling and time-persistent heating above log tau(500) = -4. The inferred gas temperature at log tau(500) = -3.8 correlates strongly with the total linear polarization in the Ca II 854.2 nm line, suggesting that that the heating rate correlates with the strength of the horizontal magnetic field in the low chromosphere.

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Chromospheric observations and magnetic configuration of a supergranular structure

2018. Carolina Robustini (et al.). Astronomy and Astrophysics 621

Artikel

Context. Unipolar magnetic regions are often associated with supergranular cells. The chromosphere above these regions is regulated by the magnetic field, but the field structure is poorly known. In unipolar regions, the fibrillar arrangement does not always coincide with magnetic field lines, and polarimetric observations are needed to establish the chromospheric magnetic topology. Aims. In an active region close to the limb, we observed a unipolar annular network of supergranular size. This supergranular structure harbours a radial distribution of the fibrils converging towards its centre. We aim to improve the description of this structure by determining the magnetic field configuration and the line-of-sight velocity distribution in both the photosphere and the chromosphere. Methods. We observed the supergranular structure at different heights by taking data in the Fe I 6301-6302 angstrom, H alpha, Ca II 8542 angstrom, and the Ca II H&K spectral lines with the CRisp Imaging SpectroPolarimeter (CRISP) and CHROMospheric Imaging Spectrometer (CHROMIS) at the Swedish 1-m Solar Telescope. We performed Milne-Eddington inversions of the spectropolarimetric data of Fe I 6301-6302 angstrom and applied the weak field approximation to Ca II 8542 angstrom data to retrieve the magnetic field in the photosphere and chromosphere. We used photospheric magnetograms of CRISP, Hinode Solar Optical Telescope spectropolarimeter, and Helioseismic and Magnetic Imager to calculate the magnetic flux. We investigated the velocity distribution using the line-of-sight velocities computed from the Milne-Eddington inversion and from the Doppler shift of the K-3 feature in the Ca II K spectral line. To describe the typical spectral profiles characterising the chromosphere above the inner region of the supergranular structure, we performed a K-mean clustering of the spectra in Ca II K. Results. The photospheric magnetic flux shows that the supergranular boundary has an excess of positive polarity and the whole structure is not balanced. The magnetic field vector at chromospheric heights, retrieved by the weak field approximation, indicates that the field lines within the supergranular cell tend to point inwards, and might form a canopy above the unipolar region. In the centre of the supergranular cell hosting the unipolar region, we observe a persistent chromospheric brightening coinciding with a strong gradient in the line-of-sight velocity.

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Magnetic field variations associated with umbral flashes and penumbral waves

2018. Jayant Joshi, Jaime de la Cruz Rodríguez. Astronomy and Astrophysics 619

Artikel

Context. Umbral flashes (UF) and running penumbral waves (RPWs) in sunspot chromospheres leave a dramatic imprint in the intensity profile of the Can 8542 angstrom line. Recent studies have focussed on also explaining the observed polarization profiles, which show even more dramatic variations during the passage of these shock fronts. While most of these variations can be explained with an almost constant magnetic field as a function of time, several studies have reported changes in the inferred magnetic field strength during UF phases. These changes could be explained by opacity effects or by intrinsic changes in the magnetic field strength.

Aims. In this study we investigate the origin of these periodic variations of the magnetic field strength by analyzing a time-series of high-temporal-cadence observations acquired in the Can 8542 angstrom line with the CRISP instrument at the Swedish 1-m Solar Telescope. In particular, we analyze how the inferred geometrical height scale changes between quiescent and UF phases, and whether those changes are enough to explain the observed changes in the magnetic field, B.

Methods. We have performed non local thermodynamical equilibrium (non-LTE) data inversions with the NICOLE code of a timeseries of very high spatio-temporal-resolution observations in the Can 8542 angstrom, Fei 6301.5, and Fei 6302.5 angstrom lines. We analyze in detail the variations of the different physical parameters of the model as a function of time.

Results. Our results indicate that the Can 8542 angstrom line in sunspots is greatly sensitive to magnetic fields at log tau(500 )= -5 (hereafter log tau = -5) during UFs and quiescence. However this optical depth value does not correspond to the same geometrical height during the two phases. Our results indicate that during UFs and RPWs the log tau = -5 is located at a higher geometrical height than during quiescence. Additionally, the inferred magnetic field values are higher in UFs (up to similar to 270 G) and in RPWs (similar to 100 G).

Conclusions. Our results suggest that opacity changes caused by UFs and RPWs cannot explain the observed temporal variations in the magnetic field, as the line seems to form at higher geometrical heights where the field is expected to be lower.

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Modeling of the Hydrogen Lyman Lines in Solar Flares

2018. Stephen A. Brown (et al.). Astrophysical Journal 862 (1)

Artikel

The hydrogen Lyman lines (91.2 nm < lambda < 121.6 nm) are significant contributors to the radiative losses of the solar chromosphere, and they are enhanced during flares. We have shown previously that the Lyman lines observed by the Extreme Ultraviolet Variability instrument onboard the Solar Dynamics Observatory exhibit Doppler motions equivalent to speeds on the order of 30 km s(-1). However, contrary to expectations, both redshifts and blueshifts were present and no dominant flow direction was observed. To understand the formation of the Lyman lines, particularly their Doppler motions, we have used the radiative hydrodynamic code, RADYN, along with the radiative transfer code, RH, to simulate the evolution of the flaring chromosphere and the response of the Lyman lines during solar flares. We find that upflows in the simulated atmospheres lead to blueshifts in the line cores, which exhibit central reversals. We then model the effects of the instrument on the profiles, using the Extreme Ultraviolet Variability Experiment (EVE) instrument's properties. What may be interpreted as downflows (redshifted emission) in the lines, after they have been convolved with the instrumental line profile, may not necessarily correspond to actual downflows. Dynamic features in the atmosphere can introduce complex features in the line profiles that will not be detected by instruments with the spectral resolution of EVE, but which leave more of a signature at the resolution of the Spectral Investigation of the Coronal Environment instrument onboard the Solar Orbiter.

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Real-time, multiframe, blind deconvolution of solar images

2018. A. Asensio Ramos, Jaime de la Cruz Rodríguez, A. Pastor Yabar. Astronomy and Astrophysics 620

Artikel

The quality of images of the Sun obtained from the ground are severely limited by the perturbing effect of the Earth's turbulent atmosphere. The post-facto correction of the images to compensate for the presence of the atmosphere require the combination of high-order adaptive optics techniques, fast measurements to freeze the turbulent atmosphere, and very time-consuming blind deconvolution algorithms. Under mild seeing conditions, blind deconvolution algorithms can produce images of astonishing quality. They can be very competitive with those obtained from space, with the huge advantage of the flexibility of the instrumentation thanks to the direct access to the telescope. In this contribution we make use of deep learning techniques to significantly accelerate the blind deconvolution process and produce corrected images at a peak rate of similar to 100 images per second. We present two different architectures that produce excellent image corrections with noise suppression while maintaining the photometric properties of the images. As a consequence, polarimetric signals can be obtained with standard polarimetric modulation without any significant artifact. With the expected improvements in computer hardware and algorithms, we anticipate that on-site real-time correction of solar images will be possible in the near future.

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Spectropolarimetric Inversions of the Ca II 8542 angstrom Line in an M-class Solar Flare

2018. D. Kuridze (et al.). Astrophysical Journal 860 (1)

Artikel

We study the M1.9-class solar flare SOL2015-09-27T10:40 UT using high-resolution full Stokes imaging spectropolarimetry of the Ca II 8542 angstrom line obtained with the CRISP imaging spectropolarimeter at the Swedish 1-m Solar Telescope. Spectropolarimetric inversions using the non-LTE code NICOLE are used to construct semiempirical models of the flaring atmosphere to investigate the structure and evolution of the flare temperature and magnetic field. A comparison of the temperature stratification in flaring and nonflaring areas reveals strong heating of the flare ribbon during the flare peak. The polarization signals of the ribbon in the chromosphere during the flare maximum become stronger when compared to its surroundings and to pre- and post-flare profiles. Furthermore, a comparison of the response functions to perturbations in the line-of-sight magnetic field and temperature in flaring and nonflaring atmospheres shows that during the flare, the Ca II 8542 angstrom line is more sensitive to the lower atmosphere where the magnetic field is expected to be stronger. The chromospheric magnetic field was also determined with the weak-field approximation, which led to results similar to those obtained with the NICOLE inversions.

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Temperature constraints from inversions of synthetic solar optical, UV, and radio spectra

2018. Joäo Manuel da Silva Santos, Jaime de la Cruz Rodriguez, Jorrit Leenaarts. Astronomy and Astrophysics 620

Artikel

Context. High-resolution observations of the solar chromosphere at millimeter wavelengths are now possible with the Atacama Large Millimeter Array (ALMA), bringing with them the promise of tackling many open problems in solar physics. Observations from other ground and space-based telescopes will greatly benefit from coordinated endeavors with ALMA, yet the diagnostic potential of combined optical, ultraviolet and mm observations has remained mostly unassessed. Aims. In this paper we investigate whether mm-wavelengths could aid current inversion schemes to retrieve a more accurate representation of the temperature structure of the solar atmosphere. Methods. We performed several non-LTE inversion experiments of the emergent spectra from a snapshot of 3D radiation-MHD simulation. We included common line diagnostics such as Ca II K, 8542 angstrom and Mg II h and k, taking into account partial frequency redistribution effects, along with the continuum around 1.2 mm and 3 mm. Results. We find that including the mm-continuum in inversions allows a more accurate inference of temperature as function of optical depth. The addition of ALMA bands to other diagnostics should improve the accuracy of the inferred chromospheric temperatures between log tau similar to [-6, -4.5] where the Ca II and Mg II lines are weakly coupled to the local conditions. However, we find that simultaneous multiatom, non-LTE inversions of optical and UV lines present equally strong constraints in the lower chromosphere and thus are not greatly improved by the 1.2 mm band. Nonetheless, the 3 mm band is still needed to better constrain the mid-upper chromosphere.

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The chromosphere above a delta-sunspot in the presence of fan-shaped jets

2018. Carolina Robustini, Jorrit Leenaarts, Jaime de la Cruz Rodríguez. Astronomy and Astrophysics 609

Artikel

Context. Delta-sunspots are known to be favourable locations for fast and energetic events like flares and coronal mass ejections. The photosphere of this sunspot type has been thoroughly investigated in the past three decades. The atmospheric conditions in the chromosphere are not as well known, however. Aims. This study is focused on the chromosphere of a delta-sunspot that harbours a series of fan-shaped jets in its penumbra. The aim of this study is to establish the magnetic field topology and the temperature distribution in the presence of jets in the photosphere and the chromosphere. Methods. We use data from the Swedish 1m Solar Telescope (SST) and the Solar Dynamics Observatory. We invert the spectropolarimetric Fe I 6302 angstrom and Ca II 8542 angstrom data from the SST using the non-LTE inversion code NICOLE to estimate the magnetic field configuration, temperature, and velocity structure in the chromosphere. Results. A loop-like magnetic structure is observed to emerge in the penumbra of the sunspot. The jets are launched from this structure. Magnetic reconnection between this emerging field and the pre-existing vertical field is suggested by hot plasma patches on the interface between the two fields. The height at which the reconnection takes place is located between log tau(500) = 2 and log tau(500) = -3. The magnetic field vector and the atmospheric temperature maps show a stationary configuration during the whole observation.

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Three-dimensional modeling of the Ca II H and K lines in the solar atmosphere

2018. Johan P. Bjørgen (et al.). Astronomy and Astrophysics 611

Artikel

Context. CHROMIS, a new imaging spectrometer at the Swedish 1-m Solar Telescope (SST), can observe the chromosphere in the H and K lines of Ca II at high spatial and spectral resolution. Accurate modeling as well as an understanding of the formation of these lines are needed to interpret the SST/CHROMIS observations. Such modeling is computationally challenging because these lines are influenced by strong departures from local thermodynamic equilibrium, three-dimensional radiative transfer, and partially coherent resonance scattering of photons. Aims. We aim to model the Ca II H and K lines in 3D model atmospheres to understand their formation and to investigate their diagnostic potential for probing the chromosphere. Methods. We model the synthetic spectrum of Ca II using the radiative transfer code Multi3D in three different radiation-magnetohydrodynamic model atmospheres computed with the Bifrost code. We classify synthetic intensity profiles according to their shapes and study how their features are related to the physical properties in the model atmospheres. We investigate whether the synthetic data reproduce the observed spatially-averaged line shapes, center-to-limb variation and compare this data with SST/CHROMIS images. Results. The spatially-averaged synthetic line profiles show too low central emission peaks, and too small separation between the peaks. The trends of the observed center-to-limb variation of the profiles properties are reproduced by the models. The Ca II H and K line profiles provide a temperature diagnostic of the temperature minimum and the temperature at the formation height of the emission peaks. The Doppler shift of the central depression is an excellent probe of the velocity in the upper chromosphere.

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A Hot Downflowing Model Atmosphere for Umbral Flashes and the Physical Properties of Their Dark Fibrils

2017. V. M. J. Henriques (et al.). Astrophysical Journal 845 (2)

Artikel

We perform non-LTE inversions in a large set of umbral flashes, including the dark fibrils visible within them, and in the quiescent umbra by using the inversion code NICOLE on a set of full Stokes high-resolution Ca II lambda 8542 observations of a sunspot at disk center. We find that the dark structures have Stokes profiles that are distinct from those of the quiescent and flashed regions. They are best reproduced by atmospheres that are more similar to the flashed atmosphere in terms of velocities, even if with reduced amplitudes. We also find two sets of solutions that finely fit the flashed profiles: a set that is upflowing, featuring a transition region that is deeper than in the quiescent case and preceded by a slight dip in temperature, and a second solution with a hotter atmosphere in the chromosphere but featuring downflows close to the speed of sound at such heights. Such downflows may be related, or even dependent, on the presence of coronal loops, rooted in the umbra of sunspots, as is the case in the region analyzed. Similar loops have been recently observed to have supersonic downflows in the transition region and are consistent with the earlier sunspot plumes, which were invariably found to display strong downflows in sunspots. Finally, we find, on average, a magnetic field reduction in the flashed areas, suggesting that the shock pressure is moving field lines in the upper layers.

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Chromospheric polarimetry through multiline observations of the 850-nm spectral region

2017. C. Quintero Noda (et al.). Monthly notices of the Royal Astronomical Society 464 (4), 4534-4543

Artikel

Future solar missions and ground-based telescopes aim to understand the magnetism of the solar chromosphere. We performed a supporting study in Quintero Noda et al. focused on the infrared Ca (II) 8542 angstrom line and we concluded that it is one of the best candidates because it is sensitive to a large range of atmospheric heights, from the photosphere to the middle chromosphere. However, we believe that it is worth trying to improve the results produced by this line observing additional spectral lines. In that regard, we examined the neighbourhood solar spectrum looking for spectral lines which could increase the sensitivity to the atmospheric parameters. Interestingly, we discovered several photospheric lines which greatly improve the photospheric sensitivity to the magnetic field vector. Moreover, they are located close to a second chromospheric line which also belongs to the Ca (II) infrared triplet, i.e. the Ca (II) 8498 angstrom line, and enhances the sensitivity to the atmospheric parameters at chromospheric layers. We conclude that the lines in the vicinity of the Ca (II) 8542 A line not only increase its sensitivity to the atmospheric parameters at all layers, but also they constitute an excellent spectral window for chromospheric polarimetry.

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Chromospheric polarimetry through multiline observations of the 850-nm spectral region - II. A magnetic flux tube scenario

2017. C. Quintero Noda (et al.). Monthly notices of the Royal Astronomical Society 472 (1), 727-737

Artikel

In this publication, we continue the work started in Quintero Noda et al., examining this time a numerical simulation of a magnetic flux tube concentration. Our goal is to study if the physical phenomena that take place in it, in particular, the magnetic pumping, leaves a specific imprint on the examined spectral lines. We find that the profiles from the interior of the flux tube are periodically doppler shifted following an oscillation pattern that is also reflected in the amplitude of the circular polarization signals. In addition, we analyse the properties of the Stokes profiles at the edges of the flux tube discovering the presence of linear polarization signals for the Ca II lines, although they are weak with an amplitude around 0.5 per cent of the continuum intensity. Finally, we compute the response functions to perturbations in the longitudinal field, and we estimate the field strength using the weak-field approximation. Our results indicate that the height of formation of the spectral lines changes during the magnetic pumping process, which makes the interpretation of the inferred magnetic field strength and its evolution more difficult. These results complement those from previous works, demonstrating the capabilities and limitations of the 850-nm spectrum for chromospheric Zeeman polarimetry in a very dynamic and complex atmosphere.

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Inference of the chromospheric magnetic field orientation in the Ca II 8542 angstrom line fibrils

2017. A. Asensio Ramos (et al.). Astronomy and Astrophysics 599

Artikel

Context. Solar chromospheric fibrils, as observed in the core of strong chromospheric spectral lines, extend from photospheric field concentrations suggesting that they trace magnetic field lines. These images have been historically used as proxies of magnetic fields for many purposes.

Aims. Use statistical analysis to test whether the association between fibrils and magnetic field lines is justified.

Methods. We use a Bayesian hierarchical model to analyze several tens of thousands of pixels in spectro- polarimetric chromospheric images of penumbrae and chromospheric fibrils. We compare the alignment between the field azimuth inferred from the linear polarization signals through the transverse Zeeman effect and the direction of the fibrils in the image.

Results. We conclude that, in the analyzed fields of view, fibrils are often well aligned with the magnetic field azimuth. Despite this alignment, the analysis also shows that there is a non-negligible dispersion. In penumbral filaments, we find a dispersion with a standard deviation of similar to 16 degrees, while this dispersion goes up to similar to 34 degrees in less magnetized regions.

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Intermittent Reconnection and Plasmoids in UV Bursts in the Low Solar Atmosphere

2017. L. Rouppe van der Voort (et al.). Astrophysical Journal Letters 851 (1)

Artikel

Magnetic reconnection is thought to drive a wide variety of dynamic phenomena in the solar atmosphere. Yet, the detailed physical mechanisms driving reconnection are difficult to discern in the remote sensing observations that are used to study the solar atmosphere. In this Letter, we exploit the high-resolution instruments Interface Region Imaging Spectrograph and the new CHROMIS Fabry Perot instrument at the Swedish 1-m Solar Telescope (SST) to identify the intermittency of magnetic reconnection and its association with the formation of plasmoids in socalled UV bursts in the low solar atmosphere. The Si IV 1403 angstrom UV burst spectra from the transition region show evidence of highly broadened line profiles with often non-Gaussian and triangular shapes, in addition to signatures of bidirectional flows. Such profiles had previously been linked, in idealized numerical simulations, to magnetic reconnection driven by the plasmoid instability. Simultaneous CHROMIS images in the chromospheric Ca 11 K 3934 angstrom line now provide compelling evidence for the presence of plasmoids by revealing highly dynamic and rapidly moving brightenings that are smaller than 0.12 and that evolve on timescales of the order of seconds. Our interpretation of the observations is supported by detailed comparisons with synthetic observables from advanced numerical simulations of magnetic reconnection and associated plasmoids in the chromosphere. Our results highlight how subarcsecond imaging spectroscopy sensitive to a wide range of temperatures combined with advanced numerical simulations that are realistic enough to compare with observations can directly reveal the small-scale physical processes that drive the wide range of phenomena in the solar atmosphere.

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Observations of Ellerman bomb emission features in He I D-3 and He I 10 830 angstrom

2017. Tine Libbrecht (et al.). Astronomy and Astrophysics 598

Artikel

Context. Ellerman bombs (EBs) are short-lived emission features, characterised by extended wing emission in hydrogen Balmer lines. Until now, no distinct signature of EBs has been found in the He I 10 830 angstrom line, and conclusive observations of EBs in He I D-3 have never been reported.

Aims. We aim to study the signature of EBs in neutral helium triplet lines.

Methods. The observations consisted of ten consecutive SST/TRIPPEL raster scans close to the limb, featuring the H beta, He I D-3 and He I 10 830 angstrom spectral regions. We also obtained raster scans with IRIS and made use of the SDO/AIA 1700 angstrom channel. We used HAZEL to invert the neutral helium triplet lines.

Results. Three EBs in our data show distinct emission signatures in neutral helium triplet lines, most prominently visible in the He I D-3 line. The helium lines have two components: a broad and blueshifted emission component associated with the EB, and a narrower absorption component formed in the overlying chromosphere. One of the EBs in our data shows evidence of strong velocity gradients in its emission component. The emission component of the other two EBs could be fitted using a constant slab. Our analysis hints towards thermal Doppler motions having a large contribution to the broadening for helium and IRIS lines. We conclude that the EBs must have high temperatures to exhibit emission signals in neutral helium triplet lines. An order of magnitude estimate places our observed EBs in the range of T similar to 2 x 10(4) 10(5) K.

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Radiative Diagnostics in the Solar Photosphere and Chromosphere

2017. Jaime de la Cruz Rodríguez, M. van Noort. Space Science Reviews 210 (1-4), 109-143

Artikel

Magnetic fields on the surface of the Sun and stars in general imprint or modify the polarization state of the electromagnetic radiation that is leaving from the star. The inference of solar/ stellar magnetic fields is performed by detecting, studying and modeling polarized light from the target star. In this review we present an overview of techniques that are used to study the atmosphere of the Sun, and particularly those that allow to infer magnetic fields. We have combined a small selection of theory on polarized radiative transfer, inversion techniques and we discuss a number of results from chromospheric inversions.

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EMERGENCE OF GRANULAR-SIZED MAGNETIC BUBBLES THROUGH THE SOLAR ATMOSPHERE. III. THE PATH TO THE TRANSITION REGION

2016. Ada Ortiz (et al.). Astrophysical Journal 825 (2)

Artikel

We study, for the first time, the ascent of granular-sized magnetic bubbles from the solar photosphere through the chromosphere into the transition region and above. Such events occurred in a flux emerging region in NOAA 11850 on 2013 September 25. During that time, the first co-observing campaign between the Swedish 1-m Solar Telescope (SST) and the Interface Region Imaging Spectrograph (IRIS) spacecraft was carried out. Simultaneous observations of the chromospheric Ha 656.28 nm and Ca II 854.2 nm lines, plus the photospheric Fe I 630.25 nm line, were made with the CRISP spectropolarimeter at the Spitzer Space Telescope (SST) reaching a spatial resolution of 0 ''.14. At the same time, IRIS was performing a four-step dense raster of the emerging flux region, taking slit jaw images at 133 (C II, transition region), 140 (Si IV, transition region), 279.6 (Mg II k, core, upper chromosphere), and 283.2 nm (Mg II k, wing, photosphere). Spectroscopy of several lines was performed by the IRIS spectrograph in the far-and near-ultraviolet, of which we have used the Si IV 140.3 and the Mg II k 279.6 nm lines. Coronal images from the Atmospheric Imaging Assembly of the Solar Dynamics Observatory were used to investigate the possible coronal signatures of the flux emergence events. The photospheric and chromospheric properties of small-scale emerging magnetic bubbles have been described in detail in Ortiz et al. Here we are able to follow such structures up to the transition region. We describe the properties, including temporal delays, of the observed flux emergence in all layers. We believe this may be an important mechanism of transporting energy and magnetic flux from subsurface layers to the transition region and corona.

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Fan-shaped jets above the light bridge of a sunspot driven by reconnection

2016. Carolina Robustini (et al.). Astronomy and Astrophysics 590

Artikel

We report on a fan-shaped set of high-speed jets above a strongly magnetized light bridge (LB) of a sunspot observed in the H alpha line. We study the origin, dynamics, and thermal properties of the jets using high-resolution imaging spectroscopy in H alpha from the Swedish 1m Solar Telescope and data from the Solar Dynamics Observatory and Hinode. The H alpha jets have lengths of 7-38 Mm, are impulsively accelerated to a speed of similar to 100 km s(-1) close to photospheric footpoints in the LB, and exhibit a constant deceleration consistent with solar effective gravity. They are predominantly launched from one edge of the light bridge, and their footpoints appear bright in the H alpha wings. Atmospheric Imaging Assembly data indicates elongated brightenings that are nearly co-spatial with the H alpha jets. We interpret them as jets of transition region temperatures. The magnetic field in the light bridge has a strength of 0.8-2 kG and it is nearly horizontal. All jet properties are consistent with magnetic reconnection as the driver.

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Inversion of Stokes profiles with systematic effects

2016. A. Asensio Ramos (et al.). Astronomy and Astrophysics 590

Artikel

Quantitative thermodynamical, dynamical and magnetic properties of the solar and stellar plasmas are obtained by interpreting their emergent non-polarized and polarized spectrum. This inference requires the selection of a set of spectral lines that are particularly sensitive to the physical conditions in the plasma and a suitable parametric model of the solar/stellar atmosphere. Nonlinear inversion codes are then used to fit the model to the observations. However, the presence of systematic effects, like nearby or blended spectral lines, telluric absorption, or incorrect correction of the continuum, among others, can strongly affect the results. We present an extension to current inversion codes that can deal with these effects in a transparent way. The resulting algorithm is very simple and can be applied to any existing inversion code with the addition of a few lines of code as an extra step in each iteration.

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NON-LTE INVERSIONS OF THE Mg II h & k AND UV TRIPLET LINES

2016. Jaime de la Cruz Rodriguez, Jorrit Leenaarts, Andrés Asensio Ramos. Astrophysical Journal Letters 830 (2)

Artikel

The Mg II h & k lines are powerful diagnostics for studying the solar chromosphere. They have become particularly popular with the launch of the Interface Region Imaging Spectrograph (IRIS) satellite, and a number of studies that include these lines have lead to great progress in understanding chromospheric heating, in many cases thanks to the support from 3D MHD simulations. In this study, we utilize another approach to analyze observations: non-LTE inversions of the Mg II h & k and UV triplet lines including the effects of partial redistribution. Our inversion code attempts to construct a model atmosphere that is compatible with the observed spectra. We have assessed the capabilities and limitations of the inversions using the FALC atmosphere and a snapshot from a 3D radiation-MHD simulation. We find that Mg II h & k allow reconstructing a model atmosphere from the middle photosphere to the transition region. We have also explored the capabilities of a multi-line/multi-atom setup, including the Mg IIh & k, the Ca II 854.2. nm, and the Fe I. 630.25 lines to recover the full stratification of physical parameters, including the magnetic field vector, from the photosphere to the chromosphere. Finally, we present the first inversions of observed IRIS spectra from quiet-Sun, plage, and sunspot, with very promising results.

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ON THE MAGNETISM AND DYNAMICS OF PROMINENCE LEGS HOSTING TORNADOES

2016. M. J. Martinez Gonzalez (et al.). Astrophysical Journal 825 (2)

Artikel

Solar tornadoes are dark vertical filamentary structures observed in the extreme ultraviolet associated with prominence legs and filament barbs. Their true nature and relationship to prominences requires an understanding of their magnetic structure and dynamic properties. Recently, a controversy has arisen: is the magnetic field organized forming vertical, helical structures or is it dominantly horizontal? And concerning their dynamics, are tornadoes really rotating or is it just a visual illusion? Here we analyze four consecutive spectro-polarimetric scans of a prominence hosting tornadoes on its legs, which helps us shed some light on their magnetic and dynamical properties. We show that the magnetic field is very smooth in all the prominence, which is probably an intrinsic property of the coronal field. The prominence legs have vertical helical fields that show slow temporal variation that is probably related to the motion of the fibrils. Concerning the dynamics, we argue that (1) if rotation exists, it is intermittent, lasting no more than one hour, and (2) the observed velocity pattern is also consistent with an oscillatory velocity pattern (waves).

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PROPERTIES OF SUPERSONIC EVERSHED DOWNFLOWS

2016. Sara Esteban Pozuelo, L. R. Bellot Rubio, Jaime de la Cruz Rodríguez. Astrophysical Journal 832 (2)

Artikel

We study supersonic Evershed downflows in a sunspot penumbra by means of high spatial resolution spectropolarimetric data acquired in the Fe I. 617.3 nm line with the CRISP instrument at the Swedish 1 m Solar Telescope. Physical observables, such as Dopplergrams calculated from line bisectors and Stokes. V zero-crossing wavelengths, and Stokes. V maps in the far red-wing, are used to find regions where supersonic Evershed downflows may exist. We retrieve the line-of-sight velocity and the magnetic field vector in these regions using two-component inversions of the observed Stokes profiles with the help of the SIR code. We follow these regions during their lifetime to study their temporal behavior. Finally, we carry out a statistical analysis of the detected supersonic downflows to characterize their physical properties. Supersonic downflows are contained in compact patches moving outward, which are located in the mid-and outer penumbra. They are observed as bright, roundish structures at the outer end of penumbral filaments that resemble penumbral grains. The patches may undergo fragmentations and mergings during their lifetime; some of them are recurrent. Supersonic downflows are associated with strong and rather vertical magnetic fields with a reversed polarity compared to that of the sunspot. Our results suggest that downflows returning back to the solar surface with supersonic velocities are abruptly stopped in dense deep layers and produce a shock. Consequently, this shock enhances the temperature and is detected as a bright grain in the continuum filtergrams, which could explain the existence of outward-moving grains in the mid-and outer penumbra.

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Spectropolarimetric capabilities of Ca II 8542 angstrom line

2016. C. Quintero Noda (et al.). Monthly notices of the Royal Astronomical Society 459 (3), 3363-3376

Artikel

The next generation of space-and ground-based solar missions aim to study the magnetic properties of the solar chromosphere using the infrared Ca II lines and the He I 10830 angstrom line. The former seem to be the best candidates to study the stratification of magnetic fields in the solar chromosphere and their relation to the other thermodynamical properties underlying the chromospheric plasma. The purpose of this work is to provide a detailed analysis of the diagnostic capabilities of the Ca II 8542 angstrom line, anticipating forthcoming observational facilities. We study the sensitivity of the Ca II 8542 angstrom line to perturbations applied to the physical parameters of reference semi-empirical 1D model atmospheres using response functions and we make use of 3D magnetohydrodynamics simulations to examine the expected polarization signals for moderate magnetic field strengths. Our results indicate that the Ca II 8542 angstrom line is mostly sensitive to the layers enclosed in the range log tau = [0, -5.5], under the physical conditions that are present in our model atmospheres. In addition, the simulated magnetic flux tube generates strong longitudinal signals in its centre and moderate transversal signals, due to the vertical expansion of magnetic field lines, in its edge. Thus, observing the Ca II 8542 angstrom line we will be able to infer the 3D geometry of moderate magnetic field regions.

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An open-source, massively parallel code for non-LTE synthesis and inversion of spectral lines and Zeeman-induced Stokes profiles

2015. H. Socas-Navarro (et al.). Astronomy and Astrophysics 577

Artikel

With the advent of a new generation of solar telescopes and instrumentation, interpreting chromospheric observations (in particular, spectropolarimetry) requires new, suitable diagnostic tools. This paper describes a new code, NICOLE, that has been designed for Stokes non-LTE radiative transfer, for synthesis and inversion of spectral lines and Zeeman-induced polarization profiles, spanning a wide range of atmospheric heights from the photosphere to the chromosphere. The code features a number of unique features and capabilities and has been built from scratch with a powerful parallelization scheme that makes it suitable for application on massive datasets using large supercomputers. The source code is written entirely in Fortran 90/2003 and complies strictly with the ANSI standards to ensure maximum compatibility and portability. It is being publicly released, with the idea of facilitating future branching by other groups to augment its capabilities.

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CRISPRED: A data pipeline for the CRISP imaging spectropolarimeter

2015. Jaime de la Cruz Rodriguez (et al.). Astronomy and Astrophysics 573

Artikel

The production of science-ready data from major solar telescopes requires expertise beyond that of the typical observer. This is a consequence of the increasing complexity of instruments and observing sequences, which require calibrations and corrections for instrumental and seeing effects that are not only difficult to measure, but are also coupled in ways that require careful analysis in the design of the correction procedures. Modern space-based telescopes have data-processing pipelines capable of routinely producing well-characterized data products. High resolution imaging spectropolarimeters at ground-based telescopes need similar data pipelines. We present new methods for flat-fielding spectropolarimetric data acquired with telecentric Fabry-Perot instruments and a new approach for accurate camera co-alignment for image restoration. We document a procedure that forms the basis of current state-of- the-art processing of data from the CRISP imaging spectropolarimeter at the Swedish 1 m Solar Telescope (SST). By collecting, implementing, and testing a suite of computer programs, we have defined a data reduction pipeline for this instrument. This pipeline, CRISPRED, streamlines the process of making science-ready data. It is implemented and operated in IDL, with time-consuming steps delegated to C. CRISPRED will also be the basis for the data pipeline of the forthcoming CHROMIS instrument.

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EMERGENCE OF GRANULAR-SIZED MAGNETIC BUBBLES THROUGH THE SOLAR ATMOSPHERE. II. NON-LTE CHROMOSPHERIC DIAGNOSTICS AND INVERSIONS

2015. Jaime de la Cruz Rodriguez (et al.). Astrophysical Journal 810 (2)

Artikel

Magnetic flux emergence into the outer layers of the Sun is a fundamental mechanism for releasing energy into the chromosphere and the corona. In this paper, we study the emergence of granular-sized flux concentrations and the structuring of the corresponding physical parameters and atmospheric diagnostics in the upper photosphere and in the chromosphere. We make use of a realistic 3D MHD simulation of the outer layers of the Sun to study the formation of the Ca II 8542 line. We also derive semi-empirical 3D models from non-LTE inversions of our observations. These models contain information on the line-of-sight stratifications of temperature, velocity, and the magnetic field. Our analysis explains the peculiar Ca II 8542 angstrom profiles observed in the flux emerging region. Additionally, we derive detailed temperature and velocity maps describing the ascent of a magnetic bubble from the photosphere to the chromosphere. The inversions suggest that, in active regions, granular-sized bubbles emerge up to the lower chromosphere where the existing large-scale field hinders their ascent. We report hints of heating when the field reaches the chromosphere.

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LATERAL DOWNFLOWS IN SUNSPOT PENUMBRAL FILAMENTS AND THEIR TEMPORAL EVOLUTION

2015. S. Esteban Pozuelo, L. R. Bellot Rubio, Jaime de la Cruz Rodriguez. Astrophysical Journal 803 (2)

Artikel

We study the temporal evolution of downflows observed at the lateral edges of penumbral filaments in a sunspot located very close to the disk center. Our analysis is based on a sequence of nearly diffraction-limited scans of the Fe I 617.3 nm line taken with the CRisp Imaging Spectro-Polarimeter instrument at the Swedish 1 m Solar Telescope. We compute Dopplergrams from the observed intensity profiles using line bisectors and filter the resulting velocity maps for subsonic oscillations. Lateral downflows appear everywhere in the center-side penumbra as small, weak patches of redshifts next to or along the edges of blueshifted flow channels. These patches have an intermittent life and undergo mergings and fragmentations quite frequently. The lateral downflows move together with the hosting filaments and react to their shape variations, very much resembling the evolution of granular convection in the quiet Sun. There is a good relation between brightness and velocity in the center-side penumbra, with downflows being darker than upflows on average, which is again reminiscent of quiet Sun convection. These results point to the existence of overturning convection in sunspot penumbrae, with elongated cells forming filaments where the flow is upward but very inclined, and weak lateral downward flows. In general, the circular polarization profiles emerging from the lateral downflows do not show sign reversals, although sometimes we detect three-lobed profiles that are suggestive of opposite magnetic polarities in the pixel.

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MAGNETIC UPFLOW EVENTS IN THE QUIET-SUN PHOTOSPHERE. I. OBSERVATIONS

2015. S. Jafarzadeh, L. Rouppe Van Der Voort, Jaime De La Cruz Rodriguez. Astrophysical Journal 810 (1)

Artikel

Rapid magnetic upflows in the quiet-Sun photosphere were recently uncovered from both SUNRISE/IMaX and Hinode/SOT observations. Here, we study magnetic upflow events (MUEs) from high-quality, high- (spatial, temporal, and spectral) resolution, and full Stokes observations in four photospheric magnetically sensitive Fe I lines centered at 5250.21, 6173.34, 6301.51, and 6302.50 angstrom acquired with the Swedish Solar Telescope (SST)/CRISP. We detect MUEs by subtracting in-line Stokes V signals from those in the far blue wing whose signal-to-noise ratio (S/N) >= 7. We find a larger number of MUEs at any given time (2.0 x 10(-2) arcsec(-2)), larger by one to two orders of magnitude, than previously reported. The MUEs appear to fall into four classes presenting different shapes of Stokes V profiles with (I) asymmetric double lobes, (II) single lobes, (III) double-humped (two same-polarity lobes), and (IV) three lobes (an extra blueshifted bump in addition to double lobes), of which less than half are single-lobed. We also find that MUEs are almost equally distributed in network and internetwork areas and they appear in the interior or at the edge of granules in both regions. Distributions of physical properties, except for horizontal velocity, of the MUEs (namely, Stokes V signal, size, line-of-sight velocity, and lifetime) are almost identical for the different spectral lines in our data. A bisector analysis of our spectrally resolved observations shows that these events host modest upflows and do not show a direct indication of the presence of supersonic upflows reported earlier. Our findings reveal that the numbers, types (classes), and properties determined for MUEs can strongly depend on the detection techniques used and the properties of the employed data, namely, S/Ns, resolutions, and wavelengths.

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SPECTRO-POLARIMETRIC IMAGING REVEALS HELICAL MAGNETIC FIELDS IN SOLAR PROMINENCE FEET

2015. M. J. Martinez Gonzalez (et al.). Astrophysical Journal 802 (1)

Artikel

Solar prominences are clouds of cool plasma levitating above the solar surface and insulated from the million-degree corona by magnetic fields. They form in regions of complex magnetic topology, characterized by non-potential fields, which can evolve abruptly, disintegrating the prominence and ejecting magnetized material into the heliosphere. However, their physics is not yet fully understood because mapping such complex magnetic configurations and their evolution is extremely challenging, and must often be guessed by proxy from photometric observations. Using state-of-the-art spectro-polarimetric data, we reconstruct the structure of the magnetic field in a prominence. We find that prominence feet harbor helical magnetic fields connecting the prominence to the solar surface below.

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Jaime de la Cruz RodriguezForskare

Publikationer

Spectropolarimetry of Sunspots at 0.16 ARCSEC resolution

CRISP spectropolarimetric imaging of penumbral fine structure

Measuring the solar atmosphere

High-order aberration compensation with multi-frame blind deconvolution and phase diversity image restoration techniques

THE EFFECT OF ISOTOPIC SPLITTING ON THE BISECTOR AND INVERSIONS OF THE SOLAR Ca II 854.2 nm LINE

EMERGENCE OF GRANULAR-SIZED MAGNETIC BUBBLES THROUGH THE SOLAR ATMOSPHERE. I. SPECTROPOLARIMETRIC OBSERVATIONS AND SIMULATIONS

DYNAMIC PROPERTIES ALONG THE NEUTRAL LINE OF A DELTA SPOT INFERRED FROM HIGH-RESOLUTION OBSERVATIONS

Sparse inversion of Stokes profiles I. Two-dimensional Milne-Eddington inversions

Small-scale magnetic flux emergence in a sunspot light bridge

Are solar chromospheric fibrils tracing the magnetic field?

Line formation of He I D3 and He I 10830 Å in a small-scale reconnection event

Non-LTE inversions from a 3D MHD chromospheric model

Solar velocity references from 3D HD photospheric models

Stokes imaging polarimetry using image restoration at the Swedish 1-m Solar Telescope II: A calibration strategy for Fabry-Perot based instruments

Exploring spectropolarimetric inversions using neural fields: Solar chromospheric magnetic field under the weak-field approximation

High flow speeds and transition-region-like temperatures in the solar chromosphere during flux emergence: Evidence from imaging spectropolarimetry in Hea I 1083 nm and numerical simulations

Transverse oscillations in 3D along Ca II K bright fibrils in the solar chromosphere

Constraints on Acoustic Wave Energy Fluxes and Radiative Losses in the Solar Chromosphere from Non-LTE Inversions

Improved reconstruction of solar magnetic fields from imaging spectropolarimetry through spatio-temporal regularisation

Jacobian-free Newton-Krylov method for multilevel nonlocal thermal equilibrium radiative transfer problems

One-dimensional, geometrically stratified semi-empirical models of the quiet-Sun photosphere and lower chromosphere

Spatial resolution effects on the solar open flux estimates

A reconnection-driven magnetic flux cancellation and a quiet Sun Ellerman bomb

Chromospheric Heating from Local Magnetic Growth and Ambipolar Diffusion under Nonequilibrium Conditions

Decay of a photospheric transient filament at the boundary of a pore and the chromospheric response

Designing wavelength sampling for Fabry–Pérot observations: Information-based spectral sampling

Do Hα Stokes V Profiles Probe the Chromospheric Magnetic Field? An Observational Perspective

Estimating the longitudinal magnetic field in the chromosphere of quiet-Sun magnetic concentrations

Ultra-high-resolution observations of plasmoid-mediated magnetic reconnection in the deep solar atmosphere

Active region chromospheric magnetic fields: Observational inference versus magnetohydrostatic modelling

Bayesian Stokes inversion with normalizing flows

Heating of the solar chromosphere through current dissipation

Physical properties of a fan-shaped jet backlit by an X9.3 flare

Prospects and challenges of numerical modeling of the Sun at millimeter wavelengths

Radiative losses in the chromosphere during a C-class flare

Spatio-temporal analysis of chromospheric heating in a plage region

The European Solar Telescope

ALMA and IRIS Observations of the Solar Chromosphere. I. An On-disk Type II Spicule

ALMA and IRIS Observations of the Solar Chromosphere. II. Structure and Dynamics of Chromospheric Plages

An observationally constrained model of strong magnetic reconnection in the solar chromosphere: Atmospheric stratification and estimates of heating rates

Design and Performance Analysis of a Highly Efficient Polychromatic Full Stokes Polarization Modulator for the CRISP Imaging Spectrometer

Downflowing umbral flashes as evidence of standing waves in sunspot umbrae

Line formation of He I D3 and He I 10 830 Å in a small-scale reconnection event

Non-LTE inversions of a confined X2.2 flare I. The vector magnetic field in the photosphere and chromosphere

On the (Mis)Interpretation of the Scattering Polarization Signatures in the Ca ii 8542 Å Line through Spectral Line Inversions

SSTRED: Data- and metadata-processing pipeline for CHROMIS and CRISP

Stratification of physical parameters in a C-class solar flare using multiline observations

ALMA observations of transient heating in a solar active region

High-resolution observations of the solar photosphere, chromosphere, and transition region: A database of coordinated IRIS and SST observations

Inference of the chromospheric magnetic field configuration of solar plage using the CaII 8542 Å line

Physical properties of bright Ca II K fibrils in the solar chromosphere

Signatures of ubiquitous magnetic reconnection in the lower solar atmosphere

Stratification of canopy magnetic fields in a plage region: Constraints from a spatially-regularized weak-field approximation method

The Formation Height of Millimeter-wavelength Emission in the Solar Chromosphere

The Sun at millimeter wavelengths: I. Introduction to ALMA Band 3 observations

The multi-thermal chromosphere Inversions of ALMA and IRIS data

A method for global inversion of multi-resolution solar data

Chromospheric condensations and magnetic field in a C3.6-class flare studied via He I D-3 spectro-polarimetry

Chromospheric polarimetry through multiline observations of the 850nm spectral region III: Chromospheric jets driven by twisted magnetic fields

Diagnostic potential of the Ca II 8542 Ångstrom line for solar filaments

Is the sky the limit? Performance of the revamped Swedish 1-m Solar Telescope and its blue- and red-beam reimaging systems

Observationally Based Models of Penumbral Microjets

Recovering Thermodynamics from Spectral Profiles observed by IRIS: A Machine and Deep Learning Approach

STiC: A multiatom non-LTE PRD inversion code for full-Stokes solar observations

Solar image denoising with convolutional neural networks

The Dark Side of Penumbral Microjets: Observations in H alpha

Three-dimensional magnetic field structure of a flux-emerging region in the solar atmosphere

Three-dimensional modeling of chromospheric spectral lines in a simulated active region

Chromospheric Heating due to Cancellation of Quiet Sun Internetwork Fields

Chromospheric heating during flux emergence in the solar atmosphere

Chromospheric observations and magnetic configuration of a supergranular structure

Magnetic field variations associated with umbral flashes and penumbral waves

Modeling of the Hydrogen Lyman Lines in Solar Flares

Real-time, multiframe, blind deconvolution of solar images

Spectropolarimetric Inversions of the Ca II 8542 angstrom Line in an M-class Solar Flare

Temperature constraints from inversions of synthetic solar optical, UV, and radio spectra

The chromosphere above a delta-sunspot in the presence of fan-shaped jets

Three-dimensional modeling of the Ca II H and K lines in the solar atmosphere

Line formation of He I D₃ and He I 10830 Å in a small-scale reconnection event

Line formation of He I D₃ and He I 10 830 Å in a small-scale reconnection event