Ground-based observations of shock aurora, which is an aurora related to geomagnetic sudden commencement (SC), have mainly been limited to the dayside. On 26 February 2023, right after the SC onset at 19:24 UT, ground-based all-sky cameras and a wide-angle camera detected shock aurora at 21 MLT in the auroral zone. For this aurora, three different emissions are detected instead of the previously known two emissions: intensification of a pre-existing arc (19:25 UT), red diffuse aurora (19:28 UT), and a second, green discrete arc (19:31 UT). The relative location of these emissions differs from the dayside cases where the first and last types are not distinguished. Geomagnetic data under this aurora indicates a significant difference in the anti-sunward propagation velocity between the secondary discrete arc and the related field-aligned current.

A specialized ground-based system has been developed for simultaneous observations of pulsating aurora (PsA) and related magnetospheric phenomena with the Arase satellite. The instrument suite is composed of 1) six 100-Hz sampling high-speed all-sky imagers (ASIs), 2) two 10-Hz sampling monochromatic ASIs observing 427.8 and 844.6 nm auroral emissions, 3) Watec Monochromatic Imagers, 4) a 20-Hz sampling magnetometer and 5) a 5-wavelength photometer. The 100-Hz ASIs were deployed in four stations in Scandinavia and two stations in Alaska, which have been used for capturing the main pulsations and quasi 3 Hz internal modulations of PsA at the same time. The 10-Hz sampling monochromatic ASIs have been operative in Tromsø, Norway with the 20-Hz magnetometer and the 5-wavelength photometer. Combination of these multiple instruments with the European Incoherent SCATter (EISCAT) radar enables us to reveal the energetics/electrodynamics behind PsA and further to detect the low-altitude ionization due to energetic electron precipitation during PsA. In particular, we intend to derive the characteristic energy of precipitating electrons during PsA by comparing the 427.8 and 844.6 nm emissions from the two monochromatic ASIs. Since the launch of the Arase satellite, the data from these instruments have been examined in comparison with the wave and particle data from the satellite in the magnetosphere. In the future, the system will be utilized not only for studies of PsA but also for other categories of aurora in close collaboration with the planned EISCAT_3D project.

The Assimilative Canadian High Arctic Ionospheric Model (A-CHAIM) is an operational ionospheric data assimilation model that provides a 3D representation of the high latitude ionosphere in Near-Real-Time (NRT). A-CHAIM uses low-latency observations slant Total Electron Content (sTEC) from ground-based Global Navigation Satellite System (GNSS) receivers, ionosondes, and vertical TEC from the JASON-3 altimeter satellite to produce an updated electron density model above $45^o$ geomagnetic latitude. A-CHAIM is the first operational use of a particle filter data assimilation for space environment modeling, to account for the nonlinear nature of sTEC observations. The large number (>10^4) of simultaneous observations creates significant problems with particle weight degeneracy, which is addressed by combining measurements to form new composite observables. The performance of A-CHAIM is assessed by comparing the model outputs to unassimilated ionosonde observations, as well as to in-situ electron density observations from the SWARM and DMSP satellites. During moderately disturbed conditions from September 21st, 2021 through September 29th, 2021, A-CHAIM demonstrates a 40% to 50% reduction in error relative to the background model in the F2-layer critical frequency (foF2) at midlatitude and auroral reference stations, and little change at higher latitudes. The height of the F2-layer (hmF2) shows a small 5% to 15% improvement at all latitudes. In the topside, A-CHAIM demonstrates a 15% to 20% reduction in error for the Swarm satellites, and a 23% to 28% reduction in error for the DMSP satellites. The reduction in error is distributed evenly over the assimilation region, including in data-sparse regions.

Aurorae and nightglow are faint atmospheric emissions visible during night-time at several wavelengths. These emissions have been extensively studied but their polarization remains controversial. A great challenge is that light pollution from cities and scattering in the lower atmosphere interfere with polarization measurements. We introduce a new polarized radiative transfer model able to compute the polarization measured by a virtual instrument in a given nocturnal environment recreating real world conditions (atmospheric and aerosol profiles, light sources with complex geometries, terrain obstructions). The model, based on single scattering equations in the atmosphere, is tested on a few simple configurations to assess the effect of several key parameters in controlled environments. {Our model constitutes a proof of concept for polarization measurements in nocturnal conditions, that calls for further investigations. In particular, we discuss how multiple-scattering (neglected in the present study) {could} impact our observations and their interpretation, and the future need for inter-calibrating the source and the polarimeter in order to optimally extract the information contained in this kind of measurements. The model outputs are compared to field measurements in five wavelengths. A convincing fit between the model predictions and observations is found in the three most constrained wavelengths despite the single scattering approximation. Several applications of our model are discussed that concern the polarization of aurorae, the impact of light pollution, or aerosols and air pollution measurements.}