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 sol ar wind during Alfvenic periods are analyzed in terms of the two indep endent 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 use d to characterize the anisotropy of the turbulence. Assuming simple an isotropic power law models, the parameters defining the spectra of the two polarizations are determined by fitting the eigenvalues of the va riance matrix on the corresponding eigenvalues evaluated by Bavassano et al. (1982) for Helios 2 data. Then the corresponding form of the co rrelation tenser is obtained. In particular, we found that the spectru m of polarization [1] fluctuations (corresponding, in a weak turbulenc e theory approach, to the Alfven mode) is steeper than the polarizatio n [2] spectrum (corresponding to the magnetosonic modes). While the fo rmer 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 conne ction with other observational and theoretical issues is outlined.