Density Spectrum in the Solar Wind Plasma

TL;DR

This paper demonstrates through 3D simulations that a Kolmogorov-like density spectrum in the solar wind arises from Alfvénic turbulence, despite the plasma's compressibility, addressing a key paradox in MHD turbulence theory.

Contribution

The study provides the first numerical evidence that Alfvénic cascades dominate the density spectrum in compressible MHD turbulence, explaining the observed Kolmogorov-like scaling in the solar wind.

Findings

Density spectrum follows a -5/3 slope in simulations.

Alfvénic cascades dominate over compressive modes.

Compressible turbulence can produce Kolmogorov-like spectra.

Abstract

The density fluctuation spectrum in the solar wind reveals a Kolmogorov-like scaling with a spectral slope of -5/3 in wavenumber space. The energy transfer process in the magnetized solar wind, characterized typically by MHD turbulence, over extended length-scales remains an unresolved paradox of modern turbulence theories, raising the question of how a compressible magnetofluid exhibits a turbulent spectrum that is characteristic of an incompressible hydrodynamic fluid. To address these questions, we have undertaken three-dimensional time dependent numerical simulations of a compressible magnetohydrodynamic fluid describing super-Alfv\'enic, supersonic and strongly magnetized plasma fluid. It is shown that a Kolmogorov-like density spectrum can develop by plasma motions that are dominated by Alfv\'enic cascades whereas compressive modes are dissipated.