Incompressive Energy Transfer in the Earth's Magnetosheath: Magnetospheric Multiscale Observations

TL;DR

This study uses MMS observational data to estimate energy transfer rates across different scales in Earth's magnetosheath, revealing that the cascade rate is significantly higher than in the solar wind and consistent with turbulence theories.

Contribution

First direct multi-scale measurement of incompressive energy cascade rates in Earth's magnetosheath using MMS data, confirming theoretical predictions and previous indirect estimates.

Findings

Inertial range cascade rate matches von Kármán law predictions.

Kinetic scale cascade rate is lower than inertial range, as expected.

Magnetosheath cascade rate is about 1000 times larger than in the solar wind.

Abstract

Using observational data from the \emph{Magnetospheric Multiscale} (MMS) Mission in the Earth's magnetosheath, we estimate the energy cascade rate using different techniques within the framework of incompressible magnetohydrodynamic (MHD) turbulence. At the energy containing scale, the energy budget is controlled by the von K\'arm\'an decay law. Inertial range cascade is estimated by fitting a linear scaling to the mixed third-order structure function. Finally, we use a multi-spacecraft technique to estimate the Kolmogorov-Yaglom-like cascade rate in the kinetic range, well below the ion inertial length scale. We find that the inertial range cascade rate is almost equal to the one predicted by the von K\'arm\'an law at the energy containing scale, while the cascade rate evaluated at the kinetic scale is somewhat lower, as anticipated in theory~\citep{Yang2017PoP}. Further, in agreement with a recent study~\citep{Hadid2018PRL}, we find that the incompressive cascade rate in the Earth's magnetosheath is about $1000$ times larger than the cascade rate in the pristine solar wind.