Altitude Dependence of Nightside Martian Suprathermal Electron Depletions as Revealed by MAVEN Observations

The MAVEN (Mars Atmosphere and Volatile Evolution) spacecraft is providing new detailed observations of the Martian ionosphere thanks to its unique orbital coverage and its sophisticated instrument suite. From November 16 2014 to February 28 2015 its periapsis sampled the nightside Northern latitudes of Mars from 30° to 75° down to 125 km altitude above regions with and without significant crustal magnetic sources. On almost each periapsis in the nightside ionosphere suprathermal electron depletions were detected. A simple but robust criterion based on data recorded by the Solar Wind Electron Analyzer (SWEA) was implemented in order to detect all these electron depletions. This resulted in a dataset of 1742 depletions identified on 457 orbits among the 494 orbits where data were available during the time period under study. A statistical analysis reveals that the main ion and electron populations within the depletions are surprisingly constant in time and altitude. Absorption by CO2 is the main loss process for suprathermal electrons and electrons strongly peaked around 6 eV are resulting from this interaction. The observation of depletions appears however highly dependent on altitude. Depletions are mainly located above strong crustal magnetic sources above 170 km whereas the depletions observed for the first time below 170 km are globally scattered onto the Martian surface with no particular dependence on crustal fields. These results will be supplemented with new MAVEN data obtained above the southern hemisphere and will be contrasted with similar observations obtained from previous missions.