The dependence of Martian ion escape on solar EUV irradiance as observed by MAVEN

Abstract

We analyze the planetary ion, solar wind, interplanetary magnetic field (IMF), and solar extreme ultraviolet (EUV) irradiance data from the NASA Mars Atmosphere and Volatile EvolutioN (MAVEN) mission to quantify the variation in ion escape with solar EUV irradiance. With relatively strict constraints on the upstream solar wind and IMF conditions, we divide the planetary ion data into 4 subsets with different solar EUV conditions to estimate ion escape rates. The results show that the total ion escape rate may increase nonlinearly with solar EUV irradiance and eventually approach an asymptotic limit as EUV increases. Further analysis on different ion species, escape channels, and energy ranges show that the dependence on solar EUV varies between these different ion populations. We also find that the spatial distributions of ion density and velocity vary between high and low solar EUV conditions, which is likely caused by the variations of electromagnetic field distributions. These results suggest that as solar EUV controls the ion production and affects the ion distributions near Mars, it will in turn influence the electromagnetic field distributions and thus affect the acceleration of escaping ions. This feedback mechanism may limit the total ion escape rate as solar EUV increases under constrained solar wind and IMF conditions.