Electron vortex magnetic holes: a nonlinear coherent plasma structure

TL;DR

This paper introduces electron vortex magnetic holes (EVMHs), a new type of plasma structure observed in simulations, characterized by magnetic depressions caused by trapped electrons, with implications for space plasma turbulence.

Contribution

The study identifies and models a novel sub-proton scale magnetic hole, demonstrating its formation, stability, and relevance to space plasma phenomena through detailed simulations.

Findings

EVMHs are stable, circular magnetic depressions with trapped electron populations.

They form from initial perturbations during plasma turbulence evolution.

Properties of EVMHs match observed magnetic holes in the terrestrial plasma sheet.

Abstract

We report the properties of a novel type of sub-proton scale magnetic hole found in two dimensional PIC simulations of decaying turbulence with a guide field. The simulations were performed with a realistic value for ion to electron mass ratio. These structures, electron vortex magnetic holes (EVMHs), have circular cross-section. The magnetic field depression is associated with a diamagnetic azimuthal current provided by a population of trapped electrons in petal-like orbits. The trapped electron population provides a mean azimuthal velocity and since trapping preferentially selects high pitch angles, a perpendicular temperature anisotropy. The structures arise out of initial perturbations in the course of the turbulent evolution of the plasma, and are stable over at least 100 electron gyroperiods. We have verified the model for the EVMH by carrying out test particle and PIC simulations of isolated structures in a uniform plasma. It is found that (quasi-)stable structures can be formed provided that there is some initial perpendicular temperature anisotropy at the structure location. The properties of these structures (scale size, trapped population etc.) are able to explain the observed properties of magnetic holes in the terrestrial plasma sheet. EVMHs may also contribute to turbulence properties, such as intermittency, at short scale lengths in other astrophysical plasmas.