Cascade and Damping of Alfv\'en-Cyclotron Fluctuations: Application to Solar Wind Turbulence

TL;DR

This study models the cascade and damping of Alfvén-cyclotron fluctuations in solar plasmas, explaining observed turbulence spectra and their breakpoints through numerical simulations considering anisotropic effects and kinetic processes.

Contribution

It introduces a numerical model that incorporates diffusion approximation and linear dissipation rates to analyze turbulence spectra and their anisotropic damping in solar wind plasmas.

Findings

Turbulence spectra follow a -3/2 power-law in the inertial range.

Kinetic effects soften the spectrum at higher wavenumbers.

The model reproduces observed broken power-law spectra in the solar wind.

Abstract

It is well-recognized that the presence of magnetic fields will lead to anisotropic energy cascade and dissipation of astrophysical turbulence. With the diffusion approximation and linear dissipation rates, we study the cascade and damping of Alfv\'en-cyclotron fluctuations in solar plasmas numerically. For an isotropic case the steady-state turbulence spectra are nearly isotropic in the inertial range and can be fitted by a single power-law function with a spectral index of -3/2, similar to the Iroshnikov-Kraichnan phenomenology. Beyond the MHD regime the kinetic effects make the spectrum softer at higher wavenumbers. In the dissipation range the turbulence spectrum cuts off at the wavenumber, where the damping rate becomes comparable to the cascade rate, and the cutoff wavenumber changes with the wave propagation direction. The angle averaged turbulence spectrum of the isotropic model resembles a broken power-law. Taking into account the Doppler effects, the model naturally reproduces the broken power-law turbulence spectra observed in the solar wind and predicts that a higher break frequency always comes along with a softer dissipation range spectrum that may be caused by the increase of the turbulence intensity, the reciprocal of the plasma \beta, and/or the angle between the solar wind velocity and the mean magnetic field. These predictions can be tested by detailed comparisons with more accurate observations.