Ion Composition Boundary Layer Instabilities at Mars

The ion composition boundary layer that separates solar wind from ionospheric plasma at Mars typically contains a smooth compositional transition. However, similar to 10% of boundary crossings display large nonmonotonic variations in density, indicating an instability. We find that most of these instabilities occur on the flanks or downstream of the planet, and many fall into two distinct classes, which occur in different locations, under different preferred conditions. Plume events, which occur where the solar wind electric field points outward, likely represent the motion of the escaping ion plume across the spacecraft. Snowplow events, which occur where the electric field points inward, likely represent the escape of discrete parcels of plasma, which could account for similar to 10% of the total ion loss from Mars. Snowplow events occur under conditions that favor the development of ionospheric irregularities, which could form a seed for the snowplow mechanism that detaches and accelerates ionospheric plasma downstream.