This study conducts a statistical analysis of the aurora observed by the Swedish satellite MATS. MATS' main instrument is a telescope that performs limb imaging at six different wavelength intervals, among them the 762 nm wavelength emission in the O2 atmospheric band. This emission, even though it can not be observed from the ground, is important at mesosphere/lower thermosphere altitudes for both atmospheric airglow and aurora. Here, some auroral properties of this emission, such as peak altitude, geomagnetic location, and auroral intensity, are examined and compared to the SME and Kp geomagnetic indices. A total of 378 events are analyzed. An average geomagnetic latitude of 67.7° is found in both hemispheres, and an average peak altitude of 103 km is obtained. The peak altitude shows dependence on the magnetic local time. Auroral intensities of the order of 102–103 kR are observed.

The Small Payloads for Investigation of Disturbances in Electrojet by Rockets 2 (SPIDER-2) sounding rocket was launched from Esrange, Sweden, on the 19th of February 2020 at 23:14 UT. It traversed a pulsating aurora event, deploying eight free falling units which provided in situ multi-point measurements of the electric field, magnetic field and plasma parameters. In this article, the measured plasma parameters have been analyzed and compared with each other and with optical measurements obtained by ground based instrumentation. Peaks in electron density, thermal ion flux and optical emission have been found in the E region. Electron density profiles have been derived from the data collected by the Langmuir probes in two free falling units, the electron probes in the main rocket and the wave propagation experiment. A generally good agreement has been found among the different measurements in the up-leg of the trajectory, while the effect of the rocket wake was evident in the down-leg. The observed electron density profile has been found to agree with an incoming flux of high energetic electrons with energies around 20 keV. Auroral pulsations with a periodicity of 1–2 s have been recorded by an onboard photometer, a ground-based high speed camera, and the in situ thermal ion flux. The percentages of variation between the ON and OFF phases of the pulsations have been quantified for these quantities. The brightness measured by the photometer varies up to 68%, while the thermal ion flux measurements show only a 2.5% variation.

MATS (Mesospheric Airglow/Aerosol Tomography and Spectroscopy) is a Swedish satellite mission designed to investigate atmospheric gravity waves. In order to observe wave patterns, MATS observes structures in the O2 atmospheric band airglow (light emitted by oxygen molecules in the mesosphere and lower thermosphere), as well as structures in noctilucent clouds (NLCs) which form around the mesopause. The main instrument is a telescope that continuously captures high-resolution images of the atmospheric limb. Using tomographic analysis of the acquired images, the MATS mission can reconstruct waves in three dimensions and provide a comprehensive global map of the properties of gravity waves. The data provided by the MATS satellite will thus be three-dimensional fields of airglow and NLC properties in 200 km-wide (across track) strips along the orbit at altitudes of 70 to 110 km. By adding spectroscopic analysis, by separating light into six distinct wavelength channels, it also becomes possible to derive temperature and microphysical NLC properties. Based on those data fields, further analysis will yield gravity wave parameters, such as the wavelengths, amplitudes, phase, and direction of the waves, on a global scale.The MATS satellite, funded by the Swedish National Space Agency, was launched in November 2022 into a 580 km sun-synchronous orbit with a 17.25 local time of the ascending node (LTAN). This paper accompanies the public release of the Level 1b (v. 1.0) dataset from the MATS limb imager. The purpose of the paper is to provide background information in order to assist users to correctly and efficiently handle the data. As such, it details the image processing and how instrumental artefacts are handled. It also describes the calibration efforts that have been carried out on the basis of laboratory and in-flight observations, and it discusses uncertainties that affect the dataset.

The paper describes technical characteristics and presents the first scientific results of a novel infrared imaging system (imager) for studies of nightglow emissions coming from the hydroxyl (OH) and molecular oxygen (O2) layers in the mesopause region (80–100 km) above northern Scandinavia. The OH imager was put into operation in November 2022 at the Swedish Institute of Space Physics in Kiruna (67.86° N, 20.42° E; 400 m altitude). The OH imager records selected emission lines in the OH(3-1) band near 1500 nm to obtain intensity and temperature maps at around 87 km altitude. In addition, the OH imager registers infrared emissions coming from the O2 IR A-band airglow at 1268.7 nm in order to obtain O2 intensity maps at a slightly higher altitude, around 94 km. This technique allows the tracing of wave disturbances in both horizontal and vertical domains in the mesopause region. Validation and comparison of the OH(3-1) rotational temperature with collocated lidar and Aura Microwave Limb Sounder (MLS) satellite temperatures are performed. The first scientific results obtained from the OH imager for the first winter season (2022–2023) are discussed.

The ground-based measurements obtained from a lidar network and the 6-year OSIRIS (optical spectrograph and infrared imager system) limb-scanning radiance measurements made by the Odin satellite are used to study the climatology of the middle- and low-latitude sodium (Na) layer. Up to January 2021, four Na resonance fluorescence lidars at Beijing (40.5∘ N, 116.0∘ E), Hefei (31.8∘ N, 117.3∘ E), Wuhan (30.5∘ N, 114.4∘ E), and Haikou (19.5∘ N, 109.1∘ E) collected vertical profiles of Na density for a total of 2136 nights (19 587 h). These large datasets provide multi-year routine measurements of the Na layer with exceptionally high temporal and vertical resolution. The lidar measurements are particularly useful for filling in OSIRIS data gaps since the OSIRIS measurements were not made during the dark winter months because they utilize the solar-pumped resonance fluorescence from Na atoms. The observations of Na layers from the ground-based lidars and the satellite are comprehensively compared with a global model of meteoric Na in the atmosphere (WACCM–Na). The lidars present a unique test of OSIRIS and WACCM (Whole Atmosphere Community Climate Model), because they cover the latitude range along 120∘ E longitude in an unusual geographic location with significant gravity wave generation. In general, good agreement is found between lidar observations, satellite measurements, and WACCM simulations. On the other hand, the Na number density from OSIRIS is larger than that from the Na lidars at the four stations within one standard deviation of the OSIRIS monthly average, particularly in autumn and early winter arising from significant uncertainties in Na density retrieved from much less satellite radiance measurements. WACCM underestimates the seasonal variability of the Na layer observed at the lower latitude lidar stations (Wuhan and Haikou). This discrepancy suggests the seasonal variability of vertical constituent transport modelled in WACCM is underestimated because much of the gravity wave spectrum is not captured in the model.