A MODEL FOR THE 3-DIMENSIONAL MAGNETIC-FIELD CORRELATION SPECTRA OF LOW-FREQUENCY SOLAR-WIND FLUCTUATIONS DURING ALFVENIC PERIODS

Using statistical. homogeneity, magnetic field fluctuations in the solar wind during Alfvenic 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 tenser 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 tenser is obtained. In particular, we found that the spectrum of polarization [1] fluctuations (corresponding, in a weak turbulence theory approach, to the Alfven mode) is steeper than the polarization [2] spectrum (corresponding to the magnetosonic modes). While the former spectrum is dominated by wave vectors parallel to B-0, the latter is strongly flattened on the plane containing the radial and the mean magnetic field B-0 directions. A discussion of these results in connection with other observational and theoretical issues is outlined.