Observations of kinetic-size magnetic holes in the magnetosheath

TL;DR

This study reports the first observation of kinetic-size magnetic holes in the Earth's magnetosheath using high-resolution MMS data, revealing unique electron dynamics and suggesting EMHD solitons as a possible formation mechanism.

Contribution

It provides the first evidence of kinetic-size magnetic holes in the magnetosheath and analyzes their electron dynamics with high-resolution measurements, proposing a new generation mechanism.

Findings

Magnetic holes of 10-20 electron gyroradii observed in the magnetosheath.

Distinct electron flux variations at different energies within the holes.

Electron vortex flow consistent with diamagnetic drift and EMHD soliton hypothesis.

Abstract

Magnetic holes (MHs), with a scale much greater than \r{ho}i (proton gyroradius), have been widely reported in various regions of space plasmas. On the other hand, kinetic-size magnetic holes (KSMHs), previously called small size magnetic holes (SSMHs), with a scale of the order of magnitude of or less than \r{ho}i have only been reported in the Earth's magnetospheric plasma sheet. In this study, we report such KSMHs in the magnetosheath whereby we use measurements from the Magnetospheric Multiscale (MMS) mission, which provides three-dimensional (3D) particle distribution measurements with a resolution much higher than previous missions. The MHs have been observed in a scale of 10 ~ 20 \r{ho}e (electron gyroradii) and lasted 0.1 ~ 0.3 s. Distinctive electron dynamics features are observed, while no substantial deviations in ion data are seen. It is found that at the 90{\deg} pitch angle, the flux of electrons with energy 34 ~ 66 eV decreased while for electrons of energy 109 ~ 1024 eV increased inside the MHs. We also find the electron flow vortex perpendicular to the magnetic field, a feature self-consistent with the magnetic depression. Moreover, the calculated current density is mainly contributed by the electron diamagnetic drift, and the electron vortex flow is the diamagnetic drift flow. The electron magnetohydrodynamics (EMHD) soliton is considered as a possible generation mechanism for the KSMHs with the scale size of 10 ~ 20 \r{ho}e.