Electron scale magnetic holes generation driven by Whistler-to-Bernstein mode conversion in fully kinetic plasma turbulence

TL;DR

This study demonstrates a new mechanism for generating electron scale magnetic holes in turbulent plasma, involving mode conversion from whistler to Bernstein waves, supported by fully kinetic simulations aligned with MMS observations.

Contribution

The paper introduces a novel turbulence-driven process for electron scale magnetic hole formation through mode conversion in kinetic plasma simulations.

Findings

Electron scale MHs are generated by turbulence via mode conversion.

Generated MHs exhibit properties consistent with MMS observations.

The mechanism involves electron temperature anisotropy, wave instability, and current filament merging.

Abstract

Magnetic holes (MHs) are coherent structures characterized by a strong and localized magnetic field amplitude dip, commonly observed in the solar wind and planetary magnetosheaths. These structures come in different sizes, from magnetohydrodynamic to kinetic scales. Magnetospheric Multiscale (MMS) observations have revealed electron scale MHs to be ubiquitous in the turbulent Earth's magnetosheath, potentially playing an important role in the energy cascade and dissipation. Despite abundant observations, the origin of electron scale MHs is still unclear and debated. In this work, we use fully kinetic simulations to investigate the role of plasma turbulence in generating electron scale MHs. We perform a fully kinetic simulation of freely decaying plasma turbulence, initialized with typical Earth's magnetosheath parameters. We find that electron scale MHs can be generated by turbulence via the following mechanism: first, large-scale turbulent velocity shears produce regions with high electron temperature anisotropy; these localized regions become unstable, generating oblique electron scale whistler waves; as they propagate over the inhomogeneous turbulent background, whistler fluctuations develop an electrostatic component, turning into Bernstein-like modes; the strong electrostatic fluctuations produce current filaments that merge into an electron scale current vortex; the resulting electron vortex locally reduces the magnetic field amplitude, finally evolving into an electron scale MH. We show that MHs generated by this mechanism have properties consistent with MMS observations and nontrivial kinetic features. We provide numerical evidence of a new electron scale MH generation mechanism, driven by turbulence. Our results have potential implications for understanding the formation and occurrence of electron scale MHs in turbulent environments, such as the Earth's magnetosheath.