The Turbulent Heating Rate in Strong MHD Turbulence with Nonzero Cross Helicity

TL;DR

This paper examines how different mechanisms generating counter-propagating Alfvén waves affect the cascade power in strong MHD turbulence with nonzero cross helicity, clarifying conditions for various results in the literature.

Contribution

It identifies the impact of the wave generation mechanism on cascade power, emphasizing the role of coherence time in strong MHD turbulence with cross helicity.

Findings

Cascade power is larger when z- is generated by reflection of z+ fluctuations.

The mechanism of z- generation influences the cascade efficiency.

Conditions for different cascade power results depend on wave interaction coherence.

Abstract

Different results for the cascade power in strong, incompressible MHD turbulence with nonzero cross helicity appear in the literature. In this paper, we discuss the conditions under which these different results are valid. We define z+ to be the rms amplitude of Alfven waves propagating parallel to the background magnetic field, and z- to be the rms amplitude of Alfven waves propagating anti-parallel to the background magnetic field. Nonzero cross helicity implies that z+ and z- differ, and we take z- to be less than z+. We find that the mechanism that generates the z- fluctuations strongly affects the cascade power, because it controls the coherence time for interactions between oppositely directed wave packets at the outer scale. In particular, for fixed values of z+ and z-, the cascade power is in many cases larger when the z- fluctuations are generated by the reflection of z+ fluctuations than when the z- fluctuations are generated by forcing that is only weakly correlated with the z+ fluctuations.