  The Martian upper atmosphere is known to vary diurnally and seasonally due to
changing amounts of solar radiation. However, in the upper thermosphere and
exosphere, the neutrals are also subject to ion precipitation. This can
increase the temperature in the region of precipitation, resulting in density
changes that might be seen in in situ data (Fang et al. 2013). Therefore, we
examine neutral density data from the MAVEN Neutral Gas and Ion Mass
Spectrometer (NGIMS) in Mars-Solar-Electric (MSE) coordinates, where location
is determined by the direction of the solar wind convective electric field,
resulting in a hemispherical asymmetry in the ion precipitation. By examining
densities in MSE coordinates (Hara et al. 2013) we are able to look for a
detectable effect in the region where ion precipitation is more likely. Using
the NGIMS neutral data and Key Parameters in situ solar wind data from February
2015 to August 2017 we look for asymmetries by constructing average density
maps in Mars-Solar-Orbital (MSO) and MSE coordinates near the exobase. The
NGIMS densities for O, Ar, and CO2 from 180-220 km altitude for each orbit are
averaged and then binned by location in MSO coordinates and transformed to MSE
coordinates. The resulting MSE map exhibits a small density increase in the
southern hemisphere, where one would expect to see enhanced precipitation.
Although suggestive, the change is not statistically significant, so that the
effect of ion precipitation, thought to be an important driver in the evolution
of Mars' atmosphere remains elusive.
