Nonuniform Bow Shock Boundary at Mars

The Martian bow shock is much smaller than its terrestrial counterpart. Average bow shock distances from Mars at the subsolar point and terminator are around 1.6 and 2.7 Mars radii, respectively; and the typical upstream solar wind magnetosonic Mach number is between 6-7. Lacking an intrinsic magnetic field, Mars' ionosphere and extended exosphere are the primary obstacles to the solar wind flow. A substantial population of heavy pickup ions exists upstream of Mars with gyroradii comparable to the entire bow shock width. Unlike the terrestrial bow shock where the solar wind proton convective gyroradius is much smaller than the radius of the curvature of the shock, at Mars this parameter is comparable to the size of the bow shock. These length scales suggest that kinetic effects can change the bow shock dynamics.

In this study, we use data from the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission to investigate the nonuniform shape of the bow shock surface at Mars. In particular, we observe short intervals of shocked solar wind after the spacecraft leaves the magnetosheath region. We also observe instances of unshocked solar wind inside the magnetosheath. These events lack some common shock features including foot and ramp regions, but all show a distinctive jump/drop in magnetic field magnitude and ion density anti-correlated with variations in the solar wind bulk flow velocity.

We typically observe these events when the MAVEN spacecraft skims along and very close to a modeled bow shock boundary. Minimum variance analysis shows that the shock normal direction for some of these crossings is inconsistent with an averaged well-defined global conic section shape. This could be caused by many factors including an overall bow shock motion or a bow shock surface deformed by ripples, ridges and troughs. We will discuss the size and time scales of these events and analyze their plasma properties and possible causes of formation.