Influence of the nonlinearity parameter on the solar-wind sub-ion magnetic energy spectrum: FLR-Landau fluid simulations

TL;DR

This study uses FLR-Landau fluid simulations to explore how the nonlinearity parameter influences the magnetic energy spectrum at sub-ion scales in the solar wind, highlighting the role of Landau damping and turbulence strength.

Contribution

It introduces a fluid simulation approach that incorporates Landau damping and finite Larmor radius effects to analyze the impact of the nonlinearity parameter on solar wind turbulence spectra.

Findings

Magnetic energy spectra follow power laws consistent with observations.

Magnetic field line meandering and ion temperature homogenization depend on turbulence strength.

Landau damping-induced homogenization along field lines is central to the phenomenological model.

Abstract

The cascade of kinetic Alfv\'en waves (KAWs) at the sub-ion scales in the solar wind is numerically simulated using a fluid approach that retains ion and electron Landau damping, together with ion finite Larmor radius corrections. Assuming initially equal and isotropic ion and electron temperatures, and an ion beta equal to unity, different simulations are performed by varying the propagation direction and the amplitude of KAWs that are randomly driven at a transverse scale of about one fifth of the proton gyroradius in order to maintain a prescribed level of turbulent fluctuations. The resulting turbulent regimes are characterized by the nonlinearity parameter, defined as the ratio of the characteristic times of Alfv\'en wave propagation and of the transverse nonlinear dynamics. The corresponding transverse magnetic energy spectra display power laws with exponents spanning a range of values consistent with spacecraft observations. The meandering of the magnetic field lines together with the ion temperature homogenization along these lines are shown to be related to the strength of the turbulence, measured by the nonlinearity parameter. The results are interpreted in terms of a recently proposed phenomenological model where the homogenization process along field lines induced by Landau damping plays a central role.