The Width of the Martian Bow Shock and Implications on Thermalization

Abstract

<p>In this study we have identified wide quasi-perpendicular bow shock events at Mars. We have compared these to thin quasi perpendicular bow shock events in an effort to identify a cause. To ensure they are quasi-perpendicular we have used two methods to determine the normal of the shock, the local mixed-mode coplanarity method, and a global normal from a bow shock model. We found that the wide bow shocks were on average 370 km (5 r<sub>gi</sub>), and the thin ones on average 30 km (0.7 r<sub>gi</sub>). Both the wide and thin events were found at the nose and at the flank, with no difference in location. We looked into whether factors that affect standoff distance also affect width. Magnetosonic Mach number proved not to affect the width. At comets it is known that standoff distance of the shock is affected by mass loading, and we investigated whether that could be the cause also at Mars. We found however that the wide shocks on average have lower upstream ion density of H<sup>+</sup>, H<sub>2</sub><sup>+</sup>, O<sup>+</sup> and O<sub>2</sub><sup>+</sup>, leading to the conclusion that mass loading is unlikely to be the cause. We investigated the difference in downstream and upstream temperature for all events and found that the wide events had a higher difference in the flow direction, which could mean that more energy is transferred at the wider bow shock. </p> <p>Furthermore we have looked into bow shock asymmetry with respect to the electric convective field. The asymmetry has been observed at the infant bow shock at comets, and due to the similarity of Mars and comets, such as the lack of a dipole field and scale of system relative to ion gyroradius, there is grounds to believe it can affect also the Martian bow shock. </p>