Exospheric number densities retrieved from pickup ion ring distributions and their temporal variations responding to the dust storm and comet approach events at Mars

Abstract

Pickup ions around Mars are originally produced from an exospheric neutral atmosphere and their measurements thus contain information on the exospheric neutral abundance. Here we present a method to retrieve exospheric number densities by analyzing the ion velocity distribution functions of pickup ions measured by the SupraThermal And Thermal Ion Composition instrument on the Mars Atmosphere and Volatile EvolutioN spacecraft. We successfully retrieved exospheric oxygen density distributions at altitudes ranging from 1,000–10,000 km around Mars (Masunaga et al., submitted). In this study, we also apply this method to analyze temporal variations of O exospheric densities. We particularly examined the case of the dust storm event in September 2016 during which thermospheric O and ionospheric O+ densities varied with a period of ~7-8 days (Masunaga et al., 2022, Hara et al., 2024) and the comet approach event in September 2014 during which the cometary atmosphere was transferred to the upper atmosphere of Mars (Crismani et al., 2015), finding that the Martian O exosphere likely responds to these drivers.