Mean shear flows generated by nonlinear resonant Alfven waves

TL;DR

This paper analytically investigates the generation of mean shear flows by nonlinear resonant Alfvén waves, revealing flow characteristics, potential instabilities, and implications for solar atmospheric dynamics.

Contribution

It provides the first detailed analysis of mean shear flow generation at the Alfvén resonance, including flow properties and potential for Kelvin-Helmholtz instability.

Findings

Generated flow is parallel to magnetic surfaces with velocity ~10 km/s.

Flow velocity scales with the square root of perturbation amplitude.

Potential for Kelvin-Helmholtz instability in the solar atmosphere.

Abstract

In the context of resonant absorption, nonlinearity has two different manifestations. The first is the reduction in amplitude of perturbations around the resonant point (wave energy absorption). The second is the generation of mean shear flows outside the dissipative layer surrounding the resonant point. Ruderman et al. [Phys. Plasmas 4, 75 (1997)] studied both these effects at the slow resonance in isotropic plasmas. Clack et al. [Astron. Astrophys. 494}, 317 (2009)] investigated nonlinearity at the Alfven resonance, however, they did not include the generation of mean shear flow. In this present paper, we investigate the mean shear flow, analytically, and study its properties. We find that the flow generated is parallel to the magnetic surfaces and has a characteristic velocity proportional to $\epsilon^{1/2}$, where $\epsilon$ is the dimensionless amplitude of perturbations far away from the resonance. This is, qualitatively, similar to the flow generated at the slow resonance. The jumps in the derivatives of the parallel and perpendicular components of mean shear flow across the dissipative layer are derived. We estimate the generated mean shear flow to be of the order of $10{\rm kms}^{-1}$ in both the solar upper chromosphere and solar corona, however, this value strongly depends on the choice of boundary conditions. It is proposed that the generated mean shear flow can produce a Kelvin--Helmholtz instability at the dissipative layer which can create turbulent motions. This instability would be an additional effect, as a Kelvin--Helmholtz instability may already exist due to the velocity field of the resonant Alfven waves. This flow can also be superimposed onto existing large scale motions in the solar upper atmosphere.