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

Abstract

The solar wind is a magnetized and turbulent plasma. Its turbulence is often dominated by Alfvénic 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.

References in corpus (8)

• The Athena++ Adaptive Mesh Refinement Framework: Design and Magnetohydrodynamic Solvers
• The Evolution and Role of Solar Wind Turbulence in the Inner Heliosphere
• Density Fluctuations in MHD Turbulence: Spectra, Intermittency and Topology
• Magnetic field kinks and folds in the solar wind
• Three-dimensional simulation of the fast solar wind driven by compressible magnetohydrodynamic turbulence
• Turbulence Transport Modeling and First Orbit Parker Solar Probe (PSP) Observations
• The Parametric Decay Instability of Alfven waves in Turbulent Plasmas and the Applications in the Solar Wind
• On the Existence of Fast Modes in Compressible Magnetohydrodynamic Turbulence

Cited by in corpus (3)

• Anomalous compressible mode generation by global frame projections of pure Alfven mode
• Compressible Turbulence in the Near-Sun Solar Wind: Parker Solar Probe's First Eight Perihelia
• On the Interpretation of the Scalings of Density Fluctuations from In-situ Solar Wind Observations: Insights from 3D Turbulence Simulations