The Structure of the Martian Bow Shock as Seen by the MAVEN Spacecraft

With a mean stand-off distance of typically less than 2 planetary radii, the Martian bow shock is among the smallest of the solar system not only in absolute size, but also in terms of solar wind ion gyroradii (of the same order than the curvature radius). In addition, the Martian bow shock displays great complexity as the shock structure (with supercritical features) coexists with ultra-low frequency waves of exospheric origin.

Since 2014 the MAVEN spacecraft has sampled the Martian shock exhaustively providing unique observations of the properties of the plasma and fields across the boundary. In this work, we analyze the morphology of the Martian shock from magnetic field data from the MAVEN fluxgate magnetometer (MAG), and from solar wind electron and ion distribution functions and moments provided by the Solar Wind Electron Analyzer (SWEA) and the Solar Wind Ion Analyzer (SWIA).

We carefully analyzed the extent of the shock transition from the changes in plasma and field relevant parameters. Then, we calculated fundamental shock parameters such as the shock normal vector by applying different monosatellite methods, some of them used for the first time around Mars. In the case of quasi-perpendicular, supercritical crossings, we also identified and characterized the size and amplitude of the shock foot, ramp and overshoot. These are substructures arising from the reflection of solar wind ions as, due to the high Mach numbers, the Martian shock is unable to convert the total excess of solar wind kinetic energy into heat.

By an in-depth analysis of several case studies, we conclude that the Martian bow shock exhibits fast-magnetosonic shock features with well-defined supercritical substructures in quasi-perpendicular crossings. Our estimated foot thicknesses are in good agreement with the theory of the foot formation by ion reflection. We observed a ramp thickness of a few c/ω_pe, as has been already been found at the Earth bow shock from multi-spacecraft determination. We also observed an overshoot a couple of r_ci wide.