Seasonal and Mach Number Variation of the Martian Bow Shock Structure

Jacob Fruchtman; Jasper Halekas; Jacob Gruesbeck; David Mitchell; Christian Mazelle

doi:10.1029/2023JA031759

Like its terrestrial counterpart, the Martian bow shock can exhibit foot, ramp, and overshoot substructures. The shock extent is known to vary with solar wind and season, but it remains an open question whether other features of the bow shock also vary seasonally. By comparing parameters of the shock during different seasons and solar wind conditions, we investigated whether the presence of seasonal planetary ions has any effect on the shock structure. We utilized a series of algorithms to automatically collect and analyze 7056 shock crossings from MAVEN spacecraft data between November 2014 and 2019. We compared the magnetic shock jump and overshoot amplitude as functions of upstream parameters and season. We found that the magnetic shock jump agrees to first order with Rankine‐Hugoniot predictions, and that the overshoot amplitude dependence on critical ratio, beta and Alfvén Mach number agree with the results of previous studies. These trends are identical for all seasons. We also found that the shock jump and fast Mach number correlate primarily with solar zenith angle, whereas overshoot amplitude correlates primarily with shock extent. We found that the most likely cause of the latter was an unexpected strong correlation between overshoot amplitude and the solar wind flow fast Mach number. The seasonal variability of the Sun‐Mars system does not appear to be strongly reflected in the resulting shock structure. The bow shock structure is controlled mainly by the properties of the solar wind, rather than by Mars' location in its orbit around the Sun. Plain Language Summary The size of the Martian bow shock is known to vary with solar wind and season, but it remains an open question whether other features of the bow shock also vary seasonally. By comparing the shock during different seasons and solar wind conditions, we investigated whether the presence of seasonal planetary ions has any effect on the shock structure. We wrote an algorithm to identify whenever the MAVEN spacecraft crossed the bow shock, and measure relevant quantities in the vicinity to characterize the shock's behavior. We found that the shocks' measured magnetic structures' dependencies on solar wind parameters roughly agree with theoretical predictions and results of previous studies. We found that the primary and secondary substructures have negligible seasonal variation, while the latter is most strongly correlated with an entirely different solar wind parameter than we predicted—the solar wind flow directed magnetosonic Mach number—which is independent of the direction of the shock surface. Thus, the bow shock structure is controlled mainly by the properties of the solar wind, rather than by Mars' location in its orbit around the Sun. Key Points We found that the bow shock magnetic field structure has at most a very weak seasonal variation in the magnetic jump and the overshoot The Martian bow shock overshoot varies much more significantly with the solar wind flow Mach number than with any local shock parameters The Martian shock responds to solar wind variations in the same way as the Earth's shock

Seasonal and Mach Number Variation of the Martian Bow Shock Structure

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