A comparative study of four‐field and fully compressible magnetohydrodynamic turbulence in the solar wind

Abstract

A four‐field system of equations has been recently derived from the compressible magnetohydrodynamic (MHD) equations to describe turbulence in the solar wind. These equations apply to a plasma permeated by a spatially varying mean magnetic field when the plasma beta is of the order unity or less. In the presence of spatial inhomogeneities, the four‐field equations predict pressure fluctuations of the order of the Mach number of the turbulence, as observed by Helios 1 and 2. It is shown that inhomogeneities that occur only in the form of pressure‐balanced structures cannot account for density fluctuations as large as the Mach number of the turbulence. Numerical predictions from the four‐field and compressible MHD equations are compared using the same initial condition. The comparison shows that the predictions of the four‐field model are qualitatively consistent with the predictions of the compressible MHD model and suggests that the four‐field model is a viable model of compressible turbulence in the solar wind.