Anisotropy of small-scale inhomogeneities in the region of solar-wind acceleration

Abstract

A model is proposed to explain observational data on the scattering of radio signals, which indicate that small-scale plasma-density inhomogeneities in the region of solar-wind acceleration are strongly elongated in the radial direction, with the degree of elongation sharply decreasing at heliocentric distances of about six solar radii. The evolution of the energy spectra of the fluctuations of the magnetic field and plasma density is studied assuming that the plasma-density fluctuations are generated locally by nonlinear interactions of high-frequency Alfven waves, and that the gradients of the mean plasma parameters are smooth. The growth rates of the main nonlinear processes are estimated. The strong elongation of the inhomogeneities first arises when the Alfven waves travel through the chromosphere-corona transition layer, then survives to considerable distances from the Sun because the associated nonlinear relaxation processes are fairly slow. Estimates of the degree of elongation of the inhomogeneities and the characteristic distance for changes in the angular wave spectra are in good agreement with radio propagation data.