THE SOLAR-WIND DENSITY SPECTRUM NEAR THE SUN - RESULTS FROM VOYAGER RADIO MEASUREMENTS

Spacecraft radio propagation data are analyzed to estimate the solar wind density fluctuation power spectrum across five decades of spatial wave number. The data were from radio science observations made during superior conjunction of the Voyager 1 and Voyager 2 spacecrafts in 1979 and 1980. These data were analyzed to yield the wave structure function, a statistic which is related to, but obtained more readily than, the spectrum itself. Structure functions were obtained from a total of 28 observations spanning the heliocentric distance range 7-22 R(S). Each structure function was a composite of two shorter segments estimated, respectively, from phase scintillation and spectral broadening analyses. The composite structure functions tend to be steep (approximately Kolmogorov) at large scales (10(3)-10(6) km) and nearly always show some flattening at smaller scales (10-100 km). The inflection between the steep and flatter regions is abrupt and occurs at scales of 100-300 km. Most of the structure functions also shown some turnover at the very smallest scales. These results are in excellent agreement with the spectral shape model proposed by Coles and Harmon [1989]. A few "transients" were seen in the Voyager data which were characterized by an overall increase in power at large scales and a steepening of the structure function at small scales. These transients were similar in appearance to transients reported earlier from Arecibo spectral broadening observations.