Dual-Spacecraft Observation of Density Features in the Near-Terminator Martian Ionosphere: Mars Express Ionospheric Sounding and MAVEN

Abstract:

<span" roman"="Roman"" new="New">The Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS), on the Mars Express spacecraft, incorporates the Active Ionospheric Sounding (AIS) mode, which is used to sound the topside of the Martian ionosphere. This instrument has been used over the last ten years to identify and study a wide variety of ionospheric features. The advent of the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft, inserted into Mars orbit in September 2014, allows us to look at the ionosphere with two spacecraft. In this study, we find density features, primarily depressions with one apparent ionopause, identifiable within a few hours and a few tens of degrees in solar zenith angle in the MARSIS AIS and MAVEN Neutral Gas and Ion Mass Spectrometer (NGIMS) data, and study them with a variety of instruments from both spacecraft. Most of the features identified occur in the vicinity of weak crustal magnetic fields and are slowly migrating toward higher solar zenith angles. During these events, MARSIS scalar magnetic fields are slightly elevated above the model crustal fields. Two of the three depressions are coincident with abrupt increases in the solar wind density, as measured by the MAVEN Solar Wind Ion Analyzer (SWIA), and with abrupt changes in the solar wind magnetic field clock angle, from MAVEN Magnetometer (MAG). We believe that both changes in the solar wind and proximity to crustal field anomalies are related to the existence of these dropouts.