Velocity fluctuations in the interplanetary scintillation pattern

The skewness of the temporal cross-correlation function of radio scintillation observed simultaneously at spaced antennas is considered as characteristic of random motions in the turbulent solar wind. The results of the skewness measurements using radio telescopes of Solar-Terrestrial Environment Laboratory at 327 MHZ and European Incoherent Scatter facilities at 932 MHZ in the range of heliocentric distances 0.1-0.3 AU were presented by Misawa and Kojima [1996]. The measured skewness was sufficiently higher for the fast solar wind than for the slow wind inside 0.2 AU. We considered the skewness theoretically, taking into account the possible physical reasons of velocity fluctuations in drifting scintillation patterns. It is shown that the skewness is proportional to typical relative variance of velocities parallel to regular drift as in the case of frozen-in irregularities and in the case of wave-like irregularities. The skewness parameter is not sensitive to the random velocities transversal to the average pattern velocity. The theoretical estimates are compared to the observational data. It is shown that the observed skewness is caused mainly by the bulk velocity spread in the region outside heliocentric distance 0.2 AU. The enhanced skewness between 0.1 and 0.2 AU can be explained by the additional effects of fast magnetosonic waves. The attributes of the assumed fast magnetosonic waves are pronounced more strongly in the high-latitude observations of the fast solar wind streams than in low-latitude observations of the slow solar wind.