Probing upper thermospheric neutral densities at Mars using electron reflectometry

We present a new technique for inferring neutral densities in the Martian upper atmosphere from atmospheric absorption of magnetically reflected solar wind electrons. Using electron loss cone measurements from the Magnetometer/Electron Reflectometer (MAG/ER) experiment on board Mars Global Surveyor (MGS), we derive upper thermospheric (~150 - 200km altitude) densities at 2am local time, continuously from 1999 to 2005 in regions where crustal fields exist, approximately 40 percent of the planet. We find a mean density of 0.028kg/km3 at 160km and observe a latitude-dependent, repeatable seasonal variation of up to a factor of 4, with inter-annual differences and consistently lower winter densities at high latitudes, compared with the tropics (0° - 25°). The mean densities and overall seasonal variation are in general agreement with the Mars Thermosphere Global Circulation Model (MTGCM), while the specific latitude dependence is not. These measurements are important for improved understanding of the dynamics of Mars' upper atmosphere and for planning spacecraft aerobraking maneuvers.