Examining MAVEN NGIMS Neutral Data Response to Solar Wind Drivers

Abstract

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.