The Martian Ion Foreshock Boundary

Planetary bow shocks are sites where a fraction of the solar wind is accelerated to suprathermal energies. In the terrestrial foreshock, sunward propagating ion beams of several keV collimated along the interplanetary field lines (Field-Aligned Beams) are usually observed within a region upstream from the quasi-perpendicular shock. Numerous observations indicate that these beams are not seen along IMF lines that make an angle larger than 70^o with the local shock normal (θ_Bn), thereby marking the spatial location of the ion foreshock boundary. The ion foreshock boundary reflects the maximum level of energization that solar wind ions can reach via coherent interaction with a planetary shock. In the present study, the Martian ion foreshock boundary is investigated for the first time using MAVEN particle and magnetic field data. More than fifty spacecraft orbit segments were scrutinized to identify the presence of FAB events. The shock geometry associated with each FAB event was determined using a bow shock model. The obtained results clearly indicate that no FAB is observed for a shock-θ_Bn larger than 51^o. Our results indicate that the Martian ion foreshock boundary is located downstream of the expected location based on the terrestrial case. This finding is in good agreement with a recent report showing that FABs observed in the Martian foreshock have noticeably lower speeds than those observed at Earth. The characteristics of both the terrestrial and Martian ion foreshock boundaries provide new and relevant insights into the mechanisms responsible for FAB formation at planetary bow shocks. Furthermore, the present results point to a spatial boundary where ultra-low frequency waves excited by the beams are found.