Intermittency and electron heating in kinetic-Alfv\'en-wave turbulence

TL;DR

This paper investigates kinetic-Alfvén-wave turbulence at sub-ion scales in low-beta plasmas, revealing how electron Landau damping and velocity-space cascades influence spectral steepening and electron heating.

Contribution

It provides an analytical and numerical study of turbulence spectra and electron heating mechanisms, incorporating intermittency and kinetic effects beyond isothermal models.

Findings

Spectral steepening due to electron Landau damping.

Intermittency modifies turbulence cascade properties.

Velocity-space cascades contribute to electron heating.

Abstract

We report analytical and numerical investigations of sub-ion-scale turbulence in low-beta plasmas, focusing on the spectral properties of the fluctuations and electron heating. In the isothermal limit, the numerical results strongly support a description of the turbulence as a critically-balanced Kolmogorov-like cascade of kinetic Alfv\'en wave fluctuations, as amended by Boldyrev & Perez (Astrophys. J. Lett. 758, L44 (2012)) to include intermittent effects. When the constraint of isothermality is removed (i.e., with the inclusion of electron kinetic physics), the energy spectrum is found to steepen due to electron Landau damping, which is enabled by the local weakening of advective nonlinearities around current sheets, and yields significant energy dissipation via a velocity-space cascade. The use of a Hermite-polynomial representation to express the velocity-space dependence of the electron distribution function allows us to obtain an analytical, lowest-order solution for the Hermite moments of the distribution, which is borne out by numerical simulations.