Subproton-scale cascades in solar wind turbulence: driven hybrid-kinetic simulations

TL;DR

This study uses high-resolution hybrid-kinetic simulations to investigate the nature of subproton-scale turbulence in the solar wind, revealing a dependence on plasma beta and supporting theoretical predictions.

Contribution

It provides the first detailed numerical analysis of driven hybrid-kinetic turbulence at subproton scales in two real-space and three velocity-space dimensions.

Findings

Kinetic Alfvén waves dominate at high beta

Whistler fluctuations dominate at low beta

Spectral properties align with theoretical models

Abstract

A long-lasting debate in space plasma physics concerns the nature of subproton-scale fluctuations in solar wind (SW) turbulence. Over the past decade, a series of theoretical and observational studies were presented in favor of either kinetic Alfv\'en wave (KAW) or whistler turbulence. Here, we investigate numerically the nature of the subproton-scale turbulent cascade for typical SW parameters by means of unprecedented high-resolution simulations of forced hybrid-kinetic turbulence in two real-space and three velocity-space dimensions. Our analysis suggests that small-scale turbulence in this model is dominated by KAWs at $\beta\gtrsim1$ and by magnetosonic/whistler fluctuations at lower $\beta$. The spectral properties of the turbulence appear to be in good agreement with theoretical predictions. A tentative interpretation of this result in terms of relative changes in the damping rates of the different waves is also presented. Overall, the results raise interesting new questions about the properties and variability of subproton-scale turbulence in the SW, including its possible dependence on the plasma $\beta$, and call for detailed and extensive parametric explorations of driven kinetic turbulence in three dimensions.