Magnetic Field and Plasma Asymmetries Between the Martian Quasi-Perpendicular and Quasi-Parallel Magnetosheaths

TL;DR

This study tackles how the Martian magnetosheath exhibits systematic asymmetries between the quasi-perpendicular and quasi-parallel flanks. It analyzes nine years of MAVEN observations (MAG, STATIC, SWIA) using Magnetosheath-MSE coordinates and a binning framework to compute a dimensionless asymmetry parameter $A = 100 \times \frac{Q_\\parallel - Q_\\perp}{Q_\\parallel + Q_\\perp}$ across angular bins. Key findings show that $|B|/|B_{IMF}|$ is larger on the $Q_\\perp$ flank by $3\\%$–$14\\%$, dayside proton density is higher in the $Q_\\perp$ magnetosheath by about 5–10%, while planetary-oxygen ions $O^+$ and $O_2^+$ accumulate more on the tailward $Q_\\parallel$ side, with $O_2^+$ up to ~40% higher. The results suggest that mass loading by planetary ions and current-sheet/kink-like structures modulate the magnetosheath beyond single-fluid Rankine–Hugoniot expectations, with implications for comparative planetary space physics.

Abstract

The Martian magnetosheath acts as a conduit for mass and energy transfer between the upstream solar wind and its induced magnetosphere. However, our understanding of its global properties remains limited. Using nine years of data from NASA's Mars Atmosphere and Volatile EvolutioN (MAVEN) mission, we performed a quantitative statistical analysis to explore the spatial distribution of the magnetic fields, solar wind and planetary ions in the magnetosheath. We discovered significant asymmetries in the magnetic field, solar wind protons, and planetary ions between the quasi-perpendicular and quasi-parallel magnetosheaths. The asymmetries in the Martian magnetosheath exhibit both similarities and differences compared to those in the Earth's and Venus' magnetosheaths. These results indicate that the Martian magnetosheath is distinctly shaped by both shock geometry and planetary ions.

Figures (4)

• Figure 1: a) Two example IMF configurations in MSE coordinates. The IMF has a +$B_{x}$ component in the top half panel and a -$B_{x}$ component in the bottom half panel. The BS (black curve) is drawn with various vectors indicating the shock normal at different locations. The $Q_\parallel$ and $Q_\perp$ shocks are labeled. b) The conversion of both configurations to MMSE coordinates. c) A diagram of our angular cut method. The measurement area is shown as the orange shaded region, with $15^\circ$ bin intervals (the bin overlap is not shown). d) The spatial coverage of our data. $N$ is the bin count. This map is for STATIC ($O^+$, $O_2^+$), which has the most limited data coverage.
• Figure 2: Statistical mapping results for (a1) magnetic field strength with overplotted direction vectors projected into the $XY$ plane, (b1) proton density, and (c1) proton velocity, also with overplotted direction vectors. All values are normalized by upstream conditions. Panels (a2), (b2), and (c2) show the value of the angular bins and the number of data points in each bin $N$. Figures (a3), (b3), and (c3) show the percent asymmetry calculated from these bins.
• Figure 3: Following the method and format used in Figure \ref{['fig:hydrogen']}, panel (a) shows the number density of $O^+$, and panel (b) presents the number density of $O_2^+$.
• Figure 4: Summary diagram of results. The asymmetries are shaded in the region where they are strongest. The magnetic field is strongest near the MPB between $-30^\circ$ and $-60^\circ$ Proton density is highest in the $Q_\perp$ flank near $-60^\circ$. The kink-like magnetic structure is located on the $Q_\parallel$ nightside, which also corresponds to a region of increased proton speed and slight $Q_\parallel$-favored asymmetry. $O^+$ and $O_2^+$ are densest in the $Q_\parallel$ magnetosheath.