Areas of enhanced ionization in the deep nightside ionosphere of Mars

Abstract

[1] We present observations of areas of enhanced ionization located in the deep nightside ionosphere of Mars as observed by Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) on board the Mars Express spacecraft. Oblique ionospheric echoes coming from the same region observed during several consecutive MARSIS measurements enable us to constrain the geographic location of the reflection area that is the source of the echoes. We have identified 90 such events in all available data. None of the reflection areas are located in regions typically having a closed magnetic field line configuration. The locations of the reflection areas of enhanced plasma density are consistent with the ionization due to precipitating electrons on field lines connected to the collisional atmosphere. Reflection areas observed in regions of stronger magnetic field are found to have larger plasma number density. We use a model of Martian crustal magnetic field lines to demonstrate that the magnetic field has a “focusing” effect on incident particles. Electron impact ionization is thus expected to take place only in well-defined areas, in agreement with our observations. Our observations show that the deep nightside ionosphere of Mars is very irregular, controlled primarily by the configuration of crustal magnetic fields.