Multi-Instrument Observations of an Aurora-Type Event by Mars Express

Abstract : We present a new set of observations of Martian aurora obtained by SPICAM UV spectrometer on board Mars Express (MEX). Several auroral emissions are identified on the Martian night side near crustal magnetic fields. During several orbits consecutive events separated by several tens of seconds are observed, highlighting the role of closed and open field line structures in shaping spatially these events. For most of these events coordinated observations with MARSIS and ASPERA-3 on board Mars Express were possible. ASPERA-3 is composed of an ion mass analyzer (IMA), of two neutral particle imager (NPI and NPD) and of one electron spectrometer (ELS). For these particular events, data from the electron spectrometer were available so that a simultaneous measurement of the precipitating electron flux was possible. MARSIS is a multifrequency synthetic aperture orbital sounding radar which monitors in particular the Total Electron Content (TEC) and which was operating for some of these events. At the end, SPICAM UVS is a UV spectrograph covering the spectral range between 110 and 300 nm and which measures the atmospheric glow. It is this latter instrument which clearly provides the spectral evidence of the occurrence of an auroral-event. In order to avoid any ambiguity on the positions of the simultaneous measurements, we used orbits of MEX during which SPICAM UVS field of view was nadir oriented. This new set of observations shows quite strong coincidences between the occurrence of energetic precipitating electrons into the Martian atmosphere, the increase of the TEC, the presence of crustal magnetic field anomalies and auroral-type glow. Following the definition of Brain et al. (2007) of open / closed magnetic field lines, we observe that the aurora detected by SPICAM UVS occur on open field lines. This conclusion therefore suggests a significant relation between aurora events at Mars and the presence of cusp like magnetic field line structures.