Significant Amplification of Turbulent Energy Dissipation through the Shock Transition at Mars

TL;DR

This work investigates how the Martian bow shock modifies the turbulent energy transfer in the solar wind using high-time-resolution MAVEN measurements. By applying exact compressible MHD turbulence relations (AS17) and, for comparison, incompressible PP98, linked via Taylor’s hypothesis, the authors quantify the energy cascade rate $\varepsilon_c$ across solar-wind and magnetosheath intervals and map its dependence on shock obliquity $\Theta_{\mathrm{Bn}}$. They find a dramatic amplification of $\varepsilon_c$ by roughly 2–3 orders of magnitude downstream of the bow shock, with magnetosheath cascade rates substantially larger for quasi-perpendicular shocks (peaking near $\Theta_{\mathrm{Bn}} \approx 45^\circ$) than for quasi-parallel shocks, and a regional enhancement toward the magnetosheath nose. The results provide the first quantitative view of shock-driven turbulence evolution in a compact, non-magnetized planetary environment and highlight the roles of compressibility, density perturbations, and turbulent Mach number in shaping energy transfer, with implications for similar systems where direct measurements are inaccessible.

Abstract

Turbulence is fundamental to energy transfer across scales in space and astrophysical plasmas. Bow shock interactions have long been hypothesized to significantly modify turbulence in planetary environments, yet the quantification of such effects and their parametric dependencies remain largely unaddressed. Using in situ long-term high-time resolution measurements from NASA's MAVEN mission, we report the first observational characterization of the evolution and parametric dependence of the turbulence energy cascade rate $\varepsilon_C$ at magnetohydrodynamic (MHD) scales. Key findings reveal an averaged three-order-of-magnitude enhancement in $\varepsilon_C$ when transitioning from the solar wind to the magnetosheath. Notably, downstream measurements of oblique and quasi-perpendicular shocks exhibit higher energy dissipation rates than those of quasi-parallel configurations. These results provide the first direct evidence linking shock obliquity to turbulence amplification, offering key insights into shock-mediated turbulence in similar but inaccessible systems.

Figures (4)

• Figure 1: MAVEN observations in the solar wind, quasi-parallel, and quasi-perpendicular magnetosheath regions at Mars. The panels display, from top to bottom, (a) the magnetic field components and magnitude, (b) the ion number density, (c) the ion bulk velocity components, (d) the ion temperature and $\beta^{*}$ parameter, (e) the ion differential energy flux spectrogram as a function of time, respectively. The red (blue) shaded areas indicate the magnetosheath regions behind the quasi-perpendicular (quasi-parallel) bow shock. The green shaded areas mark pristine solar wind intervals. Panels (f) show the energy cascade rates as a function of time lag, the horizontal blue and green dotted lines denote the average cascade rates. Panels (g) display the spacecraft's trajectory in the Mars Solar Electric $(X,Y)$ plane, $(X,\sqrt{Y^2+Z^2})$ plane, and $(Y,Z)$ plane, respectively. The colored lines in panels (g) depict the spacecraft trajectories during the quasi-perpendicular (red), quasi-parallel (blue) magnetosheath and pristine solar wind (green) intervals. The black dashed and solid lines represent the positions of the bow shock and the magnetic pile-up boundary.
• Figure 2: (a) Superposition of the compressible turbulent energy cascade rate as a function of time lag for all events. Black solid and dashed lines indicate the average cascade rates. (b) The distribution of the average ratios of compressible (blue) and incompressible (green) turbulent energy cascade rates, while the green and blue vertical lines denote its average.
• Figure 3: (a) Distribution map of compressible turbulence energy cascade rates (AS17) in the $\mathrm{(X,\mathrm{sign (Y)}\cdot\sqrt{Y^2+Z^2})}$ plane in MSE coordinates. The red dashed and black solid lines denote the nominal positions of the bow shock and the magnetic pile-up boundary. (b) Compressible turbulence energy cascade rates (AS17) plotted against distance from the center of Mars. The black vertical line indicates the nominal position of the bow shock. Compressible (blue) and incompressible (green) turbulence energy cascade rates as a function of the bow shock normal angle (c) in the solar wind and (d) in the magnetosheath. (e) Ratios of compressible (blue) and incompressible (green) turbulence energy cascade rates from the solar wind to the magnetosheath based on the bow shock normal angle.
• Figure 4: (a) The magnetosheath $M_\mathrm{turb}$ and root-mean-square ion density perturbation $\delta n_\mathrm{RMS}$ as functions of the shock normal angle. The error bars represent standard error of the mean value (vertical) and the extent of the bin (horizontal). (b) Solar wind and magnetosheath turbulence energy cascade rates as a function of the local turbulent Mach number $M_\text{turb}$. Dotted and solid lines represent linear scaling laws from fitting and previous studies at 1 au.