Refined study of the structure of the quasiperpendicular supercritical Martian shock: new results and methodology

The study of the structure of the Martian shock is crucial to understand its microphysics and it is of special interest to understand the solar wind interaction with an unmagnetized, atmospheric body. 

The Martian bow shock is a rich example of a supercritical, mass-loaded, collisionless shock and it is one of the smallest of the solar system (both in absolute size and in terms of the solar wind ion gyroradii, of the same order of the curvature radius). This raises questions related to which particle acceleration and energy dissipation mechanism can take place, when its small size means dissipation timescales are too long for a stationary shock to convert the excess kinetic energy into heat. In addition, this shock coexists with ultra-low frequency (ULF) upstream waves, that are generated from the pick-up of exospheric ions.  

We use MAVEN plasma and magnetic field data to show that the fine structure of the Martian supercritical quasi-perpendicular shock (given by the typical supercritical substructures: the foot, ramp and overshoot) is in many ways comparable with that of the Terrestrial shock, which presents a substantially different solar wind – planet interaction. We observe a shock foot of the order of an upstream ion convected gyroradius, that agrees with the model of specular reflection of foot formation (Woods, 1971; Livesey et al., 1984; Gosling and Thomsen, 1985). Also, we find that the shock ramp is typically very narrow, of the order of a few electron inertial lengths. The presence of a well-defined foot and overshoot confirm the importance of dissipative effects, even in such a small bow shock boundary. 

In this work we also provide a meticulous analysis methodology that stresses the importance on the correct processing of MAVEN data, and the clarity and consistency of the criteria used in the data selection and analysis. We pay special attention to the determination of the external limit of the entry to the ion foot and the identification of the main and secondary overshoots, where the presence of the ULF waves could mean an erroneous identification of these shock features. We also attempt to assess the non-stationarity of the shock substructures, even with the limitations of a single spacecraft mission, by computing a range of local shock speeds to obtain the substructures spatial widths from the timeseries within an upper and lower value.