Nature and Scalings of Density Fluctuations of Compressible MHD Turbulence with Applications to the Solar Wind

TL;DR

This study uses 3D MHD simulations to explore how density fluctuations in solar wind turbulence depend on plasma parameters, revealing a linear scaling with turbulent Mach number and dominance of nonlinear structures over waves.

Contribution

The paper provides new insights into the scaling laws of density fluctuations in compressible MHD turbulence relevant to the solar wind, highlighting the role of nonlinear structures.

Findings

Density fluctuations scale linearly with turbulent Mach number.

Weak dependence of scaling coefficients on plasma parameters.

Density fluctuations dominated by low-frequency nonlinear structures.

Abstract

The solar wind is a magnetized and turbulent plasma. Its turbulence is often dominated by Alfv\'enic fluctuations and often deemed as nearly incompressible far away from the Sun, as shown by in-situ measurements near 1AU. However, for solar wind closer to the Sun, the plasma $\beta$ decreases (often lower than unity) while the turbulent Mach number $M_t$ increases (can approach unity, e.g., transonic fluctuations). These conditions could produce significantly more compressible effects, characterized by enhanced density fluctuations, as seen by several space missions. In this paper, a series of 3D MHD simulations of turbulence are carried out to understand the properties of compressible turbulence, particularly the generation of density fluctuations. We find that, over a broad range of parameter space in plasma $\beta$, cross helicity and polytropic index, the turbulent density fluctuations scale linearly as a function of $M_t$, with the scaling coefficients showing weak dependence on parameters. Furthermore, through detailed spatio-temporal analysis, we show that the density fluctuations are dominated by low-frequency nonlinear structures, rather than compressible MHD eigen-waves. These results could be important for understanding how compressible turbulence contributes to solar wind heating near the Sun.