The Width of the Martian Bow Shock and Implications on Thermalization

Abstract

In theory the width of the quasi-perpendicular bow shock ramp is on the scale of a few electron inertial lengths, but as this work will show the quasi-perpendicular bow shock at Mars is often wider. This is important because it implies that the conditions at Mars create a behaviour at the shock which cannot be described by current theory. Furthermore, the width could affect processes at the shock such as energy transfer of the ions and their subsequent thermalization. To investigate the cause of the width, two sets of quasi-perpendicular bow shock crossings measured by MAVEN are compared, one of unusual width (average 370 km or 5r$_{gi}$), and one of typical width (average 30 km or 0.7r$_{gi}$). These sets are labeled wide and thin shocks respectively. It is seen that the wide shocks have no distinct overshoot and have a higher level of magnetic field fluctuations than the thin shocks. Factors that are known to affect the standoff distance, such as the magnetosonic Mach number and mass loading of the solar wind by planetary species, were found not to affect the width of the bow shock. It is found that the temperature of the solar wind plasma increases more as it passes through a wide than a thin shock, indicating that ions are thermalized to a larger extent than at thin shocks. The larger-than-predicted by theory width of the Martian quasi-perpendicular bow shock indicate that there are conditions at Mars which we do not yet understand.