Energy Cascade Rate In Compressible Fast And Slow Solar Wind Turbulence

TL;DR

This study extends the estimation of energy cascade rates in solar wind turbulence by incorporating plasma compressibility, revealing significant differences from incompressible models and highlighting the role of compressible fluctuations.

Contribution

It introduces a method to estimate energy cascade rates in compressible MHD turbulence using in-situ data, advancing beyond previous incompressible approximations.

Findings

Cascade rate is amplified in slow solar wind.

Broader inertial range observed due to weaker fluctuations.

Cascade rate scales with turbulent Mach number.

Abstract

Estimation of the energy cascade rate in the inertial range of solar wind turbulence has been done so far mostly within the incompressible magnetohydrodynamics (MHD) theory. Here, we go beyond that approximation to include plasma compressibility using a reduced form of a recently derived exact law for compressible, isothermal MHD turbulence. Using in-situ data from the THEMIS/ARTEMIS spacecraft in the fast and slow solar wind, we investigate in detail the role of the compressible fluctuations in modifying the energy cascade rate with respect to the prediction of the incompressible MHD model. In particular, we found that the energy cascade rate: i) is amplified particularly in the slow solar wind; ii) exhibits weaker fluctuations in spatial scales, which leads to a broader inertial range than the previous reported ones; iii) has a power law scaling with the turbulent Mach number; iv) has a lower level of spatial anisotropy. Other features of solar wind turbulence are discussed along with their comparison with previous studies that used incompressible or heuristic (non exact) compressible MHD models.