Whistler Wave Cascade in Solar Wind Plasma

TL;DR

This study uses fluid simulations to explore how whistler waves influence turbulence in the solar wind, revealing different behaviors at scales above and below the electron inertial length, with implications for understanding plasma spectra.

Contribution

The paper demonstrates that whistler wave turbulence exhibits distinct dispersive behaviors at different scales, with a Kolmogorov-like spectrum at large scales and Navier-Stokes-like turbulence at small scales.

Findings

Large scale whistler modes are dispersive but do not alter the spectral cascade.

Small scale turbulence follows a Kolmogorov-like $k^{-7/3}$ spectrum.

Small scale fluctuations exhibit hydrodynamic $k^{-5/3}$ turbulence.

Abstract

Nonlinear three dimensional, time dependent, fluid simulations of whistler wave turbulence are performed to investigate role of whistler waves in solar wind plasma turbulence in which characteristic turbulent fluctuations are characterized typically by the frequency and length scales that are respectively bigger than ion gyro frequency and smaller than ion gyro radius. The electron inertial length is an intrinsic length scale in whistler wave turbulence that distinguishably divides the high frequency solar wind turbulent spectra into scales smaller and bigger than the electron inertial length. Our simulations find that the dispersive whistler modes evolve entirely differently in the two regimes. While the dispersive whistler wave effects are stronger in the large scale regime, they do not influence the spectral cascades which are describable by a Kolmogorov-like $k^{-7/3}$ spectrum. By contrast, the small scale turbulent fluctuations exhibit a Navier-Stokes like evolution where characteristic turbulent eddies exhibit a typical $k^{-5/3}$ hydrodynamic turbulent spectrum. By virtue of equipartition between the wave velocity and magnetic fields, we quantify the role of whistler waves in the solar wind plasma fluctuations.