A model for the three‐dimensional magnetic field correlation spectra of low‐frequency solar wind fluctuations during Alfvénic periods

Abstract

Using statistical homogeneity, magnetic field fluctuations in the solar wind during Alfvénic periods are analyzed in terms of the two independent polarizations allowed for each wave vector k. It is shown that the energy spectra of the two polarizations can be related both to the correlation tensor and to the variance matrix, which is generally used to characterize the anisotropy of the turbulence. Assuming simple anisotropic power law models, the parameters defining the spectra of the two polarizations are determined by fitting the eigenvalues of the variance matrix on the corresponding eigenvalues evaluated by Bavassano et al. (1982) for Helios 2 data. Then the corresponding form of the correlation tensor is obtained. In particular, we found that the spectrum of polarization [1] fluctuations (corresponding, in a weak turbulence theory approach, to the Alfvén mode) is steeper than the polarization [2] spectrum (corresponding to the magnetosonic modes). While the former spectrum is dominated by wave vectors parallel to B0, the latter is strongly flattened on the plane containing the radial and the mean magnetic field B0 directions. A discussion of these results in connection with other observational and theoretical issues is outlined.