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S.R. Spangler, D.W. Kavars (Univ. of Iowa), M. Bondi, F. Mantovani (Ist. di Radioastronomia del CNR, Bologna)
Very Long Baseline Interferometer (VLBI) observations of a compact radio source viewed through the solar wind show fluctuations in the interferometer phase. These phase fluctuations, on time scales of seconds to tens of seconds, are caused by turbulent fluctuations in the solar wind plasma density. The power spectrum of phase fluctuations contains information on the spatial power spectrum of density fluctuations and the speed at which the irregularities move relative to the interferometer. We present VLBI phase scintillation measurements made on October 15, 1998 with an interferometer consisting of antennas at Medicina, Noto, and Matera, Italy. Simultaneous observations were made at frequencies of 8.374 and 2.257 GHz with the Mark III recording system. Observations of the sources 1334-127 and 3C279 were made over a nine hour period. The closest approaches of the lines of sight to the Sun were 22.6 and 25.6 Rsun for 1334 and 3C279, respectively. Our principal results are as follows. (1) The mean value of the coefficient of the density power spectrum CN2 is within a factor of two of the prediction of a previously-established relationship for the heliocentric distance range 10-60 Rsun. (2) The solar wind speed at 22-26 Rsun is 200-300 km/sec. This value is in good agreement with independent estimates of the flow speed in this part of space, but is less than the sum of the flow speed and Alfven speed, as would have been expected. This research was supported at the University of Iowa by grants ATM96-16721 and 99-86887 from the National Science Foundation.