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S.R. Spangler, P.S. Kortenkamp, R.L. Mutel (Univ. of Iowa)
We report measurements of plasma turbulence in the inner solar wind, within approximately 32 R\odot of the Sun. The measurements were made with the Very Long Baseline Array (VLBA) of the National Radio Astronomy Observatory at frequencies of 2.3, 5.0, and 8.4 GHz. The technique analyses interferometer phase fluctuations (phase scintillations) due to turbulent density irregularities along the line of sight. The new observations were of the sources 3C273 and 3C279, and were made on September 30, 1999. The power spectrum of the phase fluctuations contains information on the constant which determines the level of the density fluctuations, CN2, and the speed at which the irregularities move relative to the antennas, V0. The technique is most sensitive to irregularities with sizes of order the interferometer baseline. Our results are as follows. (1) The heliocentric distance dependence of CN2 is consistent with an empirically-developed expression in the literature, although our measurements confirm the previously reported, substantial dispersion in values at the same heliocentric distance. (2) The observations on one line of sight (3C273) yield a value of V0 which is consistent with the bulk flow speed of the solar wind, rather than the bulk speed plus the Alfvén speed. However, the other line of sight yields a propagation speed which is higher and consistent with the sum of the bulk and Alfvén speeds. (3) Changes of about a factor of four in the path-averaged value of CN2 were observed over a timescale of several hours. These changes could be due to denser plasma structures occulting the line of sight, or more turbulent regions intersecting the line of sight to the radio source. This research was supported by grant ATM99-86887 from the National Science Foundation.
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Bulletin of the American Astronomical Society, 35 #3
© 2003. The American Astronomical Soceity.