The Drivers of the Martian Bow Shock Location: A Statistical Analysis of Mars Atmosphere and Volatile EvolutioN and Mars Express Observations

The Martian interaction with the solar wind leads to the formation of a bow shock upstream of the planet. The shock dynamics appear complex, due to the combined influence of external and internal drivers. The extreme ultraviolet fluxes and magnetosonic Mach number are known major drivers of the shock location, while the influence of other possible drivers is less constrained or unknown such as crustal magnetic fields, solar wind dynamic pressure, or the Interplanetary Magnetic Field (IMF) intensity, and orientation. In this study, we compare the influence of the main drivers of the Martian shock location, based on several methods and published datasets from Mars Express (MEX) and Mars Atmosphere Volatile EvolutioN (MAVEN) missions. We include here the influence of the crustal fields, extreme ultraviolet fluxes, solar wind dynamic pressure, as well as (for MAVEN, thanks to magnetic field measurements) magnetosonic Mach number and Interplanetary Magnetic Field parameters (intensity and orientation angles). The bias due to the cross-correlations among the possible drivers is investigated with a partial correlations analysis. Several model selection methods (Akaike Information Criterion and Least Absolute Shrinkage Selection Operator regression) are also used to rank the relative importance of the physical parameters. We conclude that the major drivers of the shock location are extreme ultraviolet fluxes and magnetosonic Mach number, while crustal fields and solar wind dynamic pressure are secondary drivers at a similar level. The IMF orientation also plays a significant role, with larger distances for perpendicular shocks rather than parallel shocks. Key Points The relative importance of the drivers of the Martian shock location is studied according to the AIC and LASSO model selection methods The Mach number and EUV fluxes are the primary drivers of the shock location, crustal fields, and solar wind pressure are secondary drivers The angle between the shock normal and the Interplanetary Magnetic Field appears as a significant driver of the shock location Plain Language Summary The Mars Express (MEX) and Mars Atmosphere and Volatile Evolution (MAVEN) observations underlined the complex interaction between the solar wind plasma flowing out from the Sun and the Martian environment. The supersonic flow creates a bow shock (and several other plasma boundaries) upstream of the planet before it is deflected around the conductive ionized atmosphere of the planet that acts as an obstacle to the flow. Understanding the dynamics of the plasma boundaries that structure the interaction is of crucial importance to better understand the evolution of the Martian environment. In this study, we investigate in detail the complex influence of external and internal drivers of the Martian bow shock, including solar extreme ultraviolet fluxes, solar wind dynamic pressure, magnetosonic Mach number, crustal magnetic fields locked in the martian crust, as well as the Interplanetary Magnetic Field intensity and orientation. We use innovative methods to disentangle the influence of the various drivers that are correlated to each other.</p

The Influence of Crustal Magnetic Fields on the Martian Bow Shock Location: A Statistical Analysis of MAVEN and Mars Express Observations

Previous missions underlined the complex influence of the crustal magnetic fields on the Martian environment, including the plasma boundaries. Their influence on the bow shock is however poorly constrained, with most studies showing North/South differences attributed to the crustal fields, with various conclusions from little to strong variabilities. We analyze for the first time in detail the influence of crustal fields on the Martian shock location based on a multi-mission analysis (MAVEN and MEX). We introduce the angular distance to the strongest crustal field region in the southern hemisphere that induces the largest influence (but not unique, with a minimum pressure threshold analyzed). Its impact is at large scale (>40–60° around), is modulated by the local time of the strongest source region (with no influence beyond terminator), and maximizes when the Interplanetary Magnetic field (IMF) is stable during the preceding hours. We introduce a technique, that is, partial correlations, to provide a coherent picture for both MAVEN/MEX due to existing cross correlations with Extreme UltraViolet (EUV). A composite parameter is proposed, that represents the combined influence of EUV, magnetosonic mach number (two major drivers) and crustal fields, the latter having an impact of hundreds of km. The influence of crustal fields on the shock appears seasonal and correlated with the Total Electronic Content, revealing a large scale coupling between the crustal fields, the ionosphere and the shock. The crustal field influence on the shock is thus significant and complex, with a coupling to both the ionosphere below and the IMF above.</p