Strong Anisotropic MHD Turbulence with Cross Helicity

TL;DR

This paper introduces a new phenomenological and quantitative model for strong anisotropic incompressible MHD turbulence with nonzero cross helicity, revealing how cross helicity affects energy spectra and cascade dynamics.

Contribution

It develops a novel phenomenology and Fokker-Planck model for anisotropic MHD turbulence with cross helicity, providing insights into spectral steepening and cascade behavior.

Findings

Magnetic and total energy spectra are steeper than Kolmogorov and become steeper with increasing cross helicity.

Higher cross helicity increases the energy cascade time, reduces cascade power, and enhances small-scale anisotropy.

Results have implications for understanding turbulence in the solar wind and corona.

Abstract

This paper proposes a new phenomenology for strong incompressible MHD turbulence with nonzero cross helicity. This phenomenology is then developed into a quantitative Fokker-Planck model that describes the time evolution of the anisotropic power spectra of the fluctuations propagating parallel and anti-parallel to the background magnetic field. It is found that in steady state the power spectra of the magnetic field and total energy are steeper than a Kolmogorov spectrum and become increasingly steep as C/E increases, where C is the cross helicity and E is the fluctuation energy. Increasing C with fixed E increases the time required for energy to cascade to smaller scales, reduces the cascade power, and increases the anisotropy of the small-scale fluctuations. The implications of these results for the solar wind and solar corona are discussed in some detail.