Very Long Baseline Interferometer measurements of turbolence in the inner solar wind

Very Long Baseline Interferometry (VLBI) observations were made of six sources in September and October 1998. The observations were made at a time when the solar elongations of five of those sources ranged from 4fdg 4 to 6fdg8 , so that the lines of sight to these sources passed at distances of closest approach to the Sun of 16.5 to 25.3 Rsun. The solar elongation of the sixth source was 16fdg6 , corresponding to a distance of 62.4 Rsun. The observations were made with the Medicina and Noto antennas of the Istituto di Radioastronomia, the Matera (Italy) antenna of the Agenzia Spaziale Italiana, and the Wettzell (Germany) antenna of the Bundesamt für Kartographie und Geodäsie. On two of the three days of observation (September 24 and October 15, 1998) simultaneous observations were made at 8.380 (X band) and 2.257 (S band) GHz. In the third observing session (September 25, 1998) observations were made at 4.981 GHz (C band). Temporal fluctuations in the interferometer phase, induced by the solar wind, were analysed to yield information on plasma turbulence in the inner heliosphere. The results of this investigation are as follows. (1) The measured interferometer phase power spectra are consistent with a theoretical expression which incorporates independent information on solar wind turbulence. (2) The values for CN2, the normalization constant of the density power spectrum, are reasonably consistent with a previously-determined expression for CN2(r), where r is the heliocentric distance, although the new values for CN2 are systematically high with respect to this prior expression. (3) Our estimates for the speed at which the irregularities move with respect to the antennas, and in the radial direction from the Sun, are in the range of 190-340 km s-1 for heliocentric distances of 16-26 Rsun. These values are in good agreement with observational estimates of the solar wind flow speed in this part of space based on coronagraph observations. The speeds are lower than the sum of the estimated solar wind flow speed and the local Alfvén speed. The potential significance of this result is discussed.