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.

A Monte Carlo simulation of magnetic field line tracing in the solar wind

“…We have computed the diffusion coefficient in the more general case when all the correlation lengths are different, as a function of δB/B 0 , and where l z / l y equals either 1 or 10, and l x / l y varies from 1 to 8. Such a range of correlation lengths is consistent with that found from the analysis of the solar wind data (Matthaeus et al, 1990;Carbone et al, 1995). Indeed, several studies of solar wind turbulence indicate a strong anisotropy in the correlation lengths, although further data analysis is needed to better constrain the values of l x , l y , and l z .…”

Small Scale Processes in the Solar Wind

“…These hybrid Vlasov-Maxwell simulations in 1D-3V phase space configuration (one dimension in physical space and three dimensions in velocity space) (Valentini et al, 2008); Valentini & Veltri, 2009)) were focused on a physical situation where Magnetohydrodynamics turbulence evolves to a state where a significant amount of energy is stored in longitudinal wavevector modes (slab turbulence) (Carbone et al, 1995;Dobrowolny et al, 1980;. The turbulent energy cascade is triggered by nonlinear wave-wave interaction of large scale ion-cyclotron (IC) waves.…”

Anisotropic MHD Turbulence in the Interstellar Medium and Solar Wind

“…This remark is also applicable to observations of anisotropic turbulence in the solar wind which are based so far entirely on single spacecraft observations [48]. Despite this limitation, by analyzing Helios 2 data, Carbone et al [27] have obtained significant quantitative information on the three-dimensional structure of anisotropic turbulence in the solar wind.…”

“…However, it is also possible that the spectra may have a turbulent origin, with turbulence eventually steepening to produce shocks. As in the ISM, anisotropy is a persistent feature of solar wind turbulence and manifests itself in several in situ observations as more power perpendicular than parallel to the local magnetic field [26][27][28].…”