Nightside ionosphere of Mars: Radar soundings by the Mars Express spacecraft

Abstract

[1] We present results of a survey of the nightside ionosphere of Mars as observed by Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) on board the Mars Express spacecraft. The occurrence rate of the nightside ionosphere is studied as a function of solar zenith angle (SZA), magnetic field magnitude, and magnetic field inclination. It is shown that at locations with weak crustal magnetic fields the occurrence rate of the nightside ionosphere decreases with increasing SZA up to about 125°, suggesting that plasma transport from the dayside plays a crucial role in its formation. However, at locations with strong crustal magnetic fields, the dependence on SZA is no longer apparent and the inclination of magnetic field becomes a crucial parameter: the occurrence rate of the nightside ionosphere is more than 4 times larger at locations with nearly vertical magnetic fields as compared to the locations with nearly horizontal magnetic fields. This indicates that impact ionization by precipitating electrons is the main ionization source at these locations. Observed peak electron densities are less than 2 × 10 4 cm −3 in the vast majority of cases. Lower estimates of altitudes of peak electron densities are mostly between 100 and 150 km.

Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) after nine years of operation: A summary

“…This is because the densities are much lower on the nightside; much of the time they are not visible at all. Several studies of the nightside ionosphere have been published, including Němec et al (2010), Duru et al (2011) (previously discussed), andNěmec et al (2011a). The last reference shows a strong relationship between the visible ionosphere in the deep nightside of Mars and regions of open crustal magnetic field lines.…”

The Aeronomy of Mars: Characterization by MAVEN of the Upper Atmosphere Reservoir That Regulates Volatile Escape

“…Mars ion flow may contribute to the near terminator (and maybe deeper nightside) ionosphere structure as well (e.g. Němec et al 2010Němec et al , 2011. Information on the cross-terminator flows is essential to distinguishing the transport source of ionization from the suggested particle precipitation sources of the nightside (e.g., Fox et al 1993;Dobe et al 1995;Fox 2009; Ma and Nagy 2007;Fränz et al 2010;Haider et al 2011).…”

On wind-driven electrojets at magnetic cusps in the nightside ionosphere of Mars

“…The short time duration and the shape of the radar echoes reflecting off of the high density structures indicate that the regions of enhanced density typically have scale sizes of a few tens of km (Duru et al, 2006). In addition, MARSIS measurements have shown localized increases of the ionospheric density and total electron content (TEC) on the nightside that are generally correlated with vertical magnetic fields in cusp regions Mouginot et al, 2008;Němec et al, 2010).…”