What drives Mars’ nightside Ionosphere?  Insights from in situ and remote sensing

Mars’ nightside ionosphere is highly variable and poorly understood, with several internal, external, and local factors affecting multiple sources of ionospheric plasma, all combining to drive highly complex behavior.  The three primary plasma sources are a) planetary rotation and cross-terminator ion winds transporting dayside plasma to the nightside, particularly at dusk, and impact ionization when magnetic topology connects nightside upper atmosphere to b) dayside suprathermal photoelectrons c) solar wind electrons (accelerated or not) precipitating from the magnetotail.  These sources are impacted by different subsets of: season, local time, subsolar longitude, IMF direction (which determines tail draping configuration), solar wind pressure, solar EUV flux, and the local strength and geometry of crustal magnetic fields that combine with global draping patterns to guide electrons toward or away from certain areas. We utilize 130.4 nm auroral observations from the Emirates Mars Ultraviolet Spectrograph, simulations of terminator ion winds, and in situ measurements from MAVEN of ion and electron density and temperature and suprathermal electron precipitation, as well as upstream measurements and proxies of IMF clock angle and solar wind pressure.  Analysis reveals some interesting features.  First, in regions of weak or no crustal magnetic fields, surprisingly, pre-dawn (~4 am) plasma densities vary by up to a factor of 30 in response to solar EUV varying by only a factor of ~3.  Second, 5R aurora occurrence rate in a given region can vary by up to a factor of 7 with IMF orientation, generally highest for southward IMF and lowest for westward IMF, with higher auroral occurrence post-dusk, decreasing toward and past midnight before increasing again towards dawn.  A broad diversity of IMF and local time dependence of plasma density and aurora occurrence is observed across Mars’ crustal magnetic fields.  The polarity of a given crustal field determines whether its field lines preferentially magnetically connect to dayside ionospheric or nightside magnetotail electron sources in the post-dusk or pre-dawn sectors.  These results highlight the complex and dynamic nature of Mars’ magnetic topology in controlling auroral electron access. Future studies integrating two-dimensional ionospheric models with in situ electron measurements with synoptic and limb auroral imaging will further constrain the relative contributions of the many factors controlling Mars’ enigmatic nightside ionosphere.