MAVEN SWIA Observations of Solar Wind Signatures in the Collisional Atmosphere of Mars

Abstract

AbstractAt Mars, the solar wind is usually decelerated and heated at the bow shock, then diverted around the planet by the induced magnetosphere. A recent study by Crismani et al. (2019, https://doi.org/10.1029/2018ja026251), however, presented evidence of near‐pristine solar wind below the exobase of Mars (<200 km) during one Mars Atmosphere and Volatile EvolutioN (MAVEN) periapsis pass, implying the solar wind penetrated into the upper atmosphere with little modification. In this work, we search through 7 years of MAVEN Solar Wind Ion Analyzer (SWIA) periapsis observations to determine how often, and the conditions under which, these low‐altitude solar wind events occur. We find 23 candidate events that contain signatures of the solar wind below 200 km. The events are much more common at low solar zenith angles and tend to occur over weak crustal field regions where the radial component of the field is oriented downward. The observations also point to significant interactions between the incoming solar wind and the neutral atmosphere, including evidence of solar wind alphas becoming singly ionized helium through collisions with atmospheric CO2. Finally, we find the events are four times more likely to be detected when the interplanetary magnetic field (IMF) is radial (cone angle <30°), suggesting the events are triggered when the IMF is nearly aligned with the solar wind flow.