Anisotropy and velocity of small-scale irregularities in the region of solar-wind acceleration

Abstract

We present the amplitude and frequency fluctuations of radio signals detected simultaneously at ground stations at Evpatoriya and Ussuri$\overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\smile}$}}{l}$sk during experiments in which radio signals with wavelengths of 5, 8, and 32 cm emitted from the Venera-15 and Venera-16 spacecraft were transmitted through the solar plasma environment. Relations are obtained between the rates of amplitude scintillations (determined from the width of temporal spectra) and frequency scintillations (determined from the delay of the fluctuations between the ground stations), on the one hand, and the velocities of the solar wind and of wave-like density disturbances and the anisotropy of irregularities, on the other hand. Analysis shows that the transition of the solar-wind flow to a supersonic regime takes place at heliocentric distances of 10–15 solar radii (R _s ) and that the flow becomes supersonic at distances exceeding 15R _s . Solar-wind density inhomogeneities with scales of about 50 km are radially extended. For heliocentric distances from 10 to 20R _s , the degree of extension is modest and, as a rule, does not exceed a factor of two.