On the correlation between small scale density fluctuations and speed in the solar wind Original Research Article

The correlation between the level of small scale density fluctuations and the solar wind speed was recently found by Asai et al. (1998) from the tomography analysis of interplanetary scintillation measurements. The physical origin of this correlation is considered within an assumption that density fluctuations are generated locally by nonlinear interactions of Alfven waves. The radial evolution of density turbulence power spectrum is investigated as well as the speed dependence of the levels of Alfven waves and of density turbulence. It is shown that the combination of these two effects can result in sufficiently strong dependence of the fractional level of small scale density fluctuations on the solar wind speed. The fractional levels of density and of magnetic field fluctuations increase weakly with increasing of heliocentric distance due to the radial evolution of the turbulence power spectra. The results of the considered theoretical model are in a reasonably good agreement with the observational data of Asai et al. (1998) if invariance of the solar wind mass flux, but not the momentum flux, is assumed.