Kinetic cascade beyond magnetohydrodynamics of solar wind turbulence in two-dimensional hybrid simulations

TL;DR

This study uses two-dimensional hybrid simulations to explore the kinetic cascade in solar wind turbulence beyond magnetohydrodynamics, revealing complex wave interactions and dissipation processes at small scales.

Contribution

It introduces a hybrid simulation model to analyze the kinetic cascade in solar wind turbulence, highlighting wave mode interactions and dissipation mechanisms beyond MHD.

Findings

Identification of Alfvén/ion-cyclotron wave cascade up to resonant absorption scales.

Observation of a broad spectrum of fast-mode and whistler waves.

Detection of pressure-balanced structures and ion-Bernstein waves at small scales.

Abstract

The nature of solar wind turbulence in the dissipation range at scales much smaller than the large MHD scales remains under debate. Here a two-dimensional model based on the hybrid code abbreviated as A.I.K.E.F. is presented, which treats massive ions as particles obeying the kinetic Vlasov equation and massless electrons as a neutralizing fluid. Up to a certain wavenumber in the MHD regime, the numerical system is initialized by assuming a superposition of isotropic Alfv\'en waves with amplitudes that follow the empirically confirmed spectral law of Kolmogorov. Then turbulence develops and energy cascades into the dispersive spectral range, where also dissipative effects occur. Under typical solar wind conditions, weak turbulence develops as a superposition of normal modes in the kinetic regime. Spectral analysis in the direction parallel to the background magnetic field reveals a cascade of left-handed Alfv\'en/ion-cyclotron waves up to wave vectors where their resonant absorption sets in, as well as a continuing cascade of right-handed fast-mode and whistler waves. Perpendicular to the background field, a broad turbulent spectrum is found to be built up of fluctuations having a strong compressive component. Ion-Bernstein waves seem to be possible normal modes in this propagation direction for lower driving amplitudes. Also signatures of short-scale pressure-balanced structures (very oblique slow-mode waves) are found.