Direct kinetic Alfv\'en wave energy cascade in the presence of imbalance

TL;DR

This study uses a gyrofluid model to analyze the energy cascade of kinetic Alfvén waves, revealing a steep spectral transition near the ion scale influenced by wave imbalance and helicity interactions.

Contribution

It introduces a two-field gyrofluid model that captures ion finite Larmor radius effects and magnetic fluctuations to study turbulence from MHD to sub-ion scales.

Findings

Spectral transition zone appears near the ion scale under certain conditions.

Perpendicular energy flux remains nearly constant across scales.

Interactions between co-propagative waves influence the transition zone development.

Abstract

A two-field Hamiltonian gyrofluid model for kinetic Alfv\'en waves retaining ion finite Larmor radius corrections and parallel magnetic field fluctuations is used to study direct turbulent cascades from the MHD to the sub-ion scales. For moderate energy imbalance and weak enough magnetic fluctuations, the spectrum of the transverse magnetic field and that of the most energetic wave display a steep transition zone near the ion scale, while the parallel transfer (and thus the parallel dissipation) remains weak. In this regime, the perpendicular flux of generalized cross helicity displays a significant decay past the ion scale, while the perpendicular energy flux remains almost constant. A phenomenological model suggests that the interactions between co-propagative waves present at the sub-ion scales can play a central role in the development of a transition zone in the presence of a helicity barrier.