Weakly Compressible MHD Turbulence in the Solar Wind: Scaling of Anisotropic Magnetic Fluctuation Spectra

Physics – Plasma Physics

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2149 Mhd Waves And Turbulence (2752, 6050, 7836), 2164 Solar Wind Plasma

Scientific paper

The weakly compressible MHD turbulence model was motivated originally by the need to obtain a reduced description of MHD turbulence in solar wind plasmas characterized by plasma beta of order unity or less. The theory leads to the formulation of a four-field system of equations that incorporate naturally the effect of background spatial inhomogeneities in the magnetic field and the plasma pressure, and has been shown to account successfully for the scaling of density fluctuations with the Mach number of the turbulence observed by Helios (ApJ, 494, 409, 1998). In this paper, we present a new application of this model to account for anisotropic magnetic fluctuation spectra (the so-called variance anisotropy) observed by ACE. For a model of local pressure-driven interchange turbulence, using the Invariance Principle approach (developed by J. W. Connor and J. B. Taylor in fusion plasma physics), we calculate explicitly the scaling of magnetic field fluctuations with plasma beta and other background plasma parameters. The Invariance Principle is simple but powerful and can be stated as follows: if the system of equations governing plasma turbulence is invariant under a scale transformation, then the fluctuation spectra calculated from these equations must exhibit the same invariance. The theory predicts precise scaling laws for the magnetic fluctuation spectra parallel and perpendicular to the background magnetic field. We calculate the beta scaling of the variance anisotropy for electrostatic as well as electromagnetic pressure-driven turbulence in the solar wind, and demonstrate that they bracket well recent ACE observations. This research is supported by DOE and NASA.

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