Radial gradient of solar wind velocity from 1 to 5 AU

Solar wind velocity measurements made by Pioneers 10 and 11 are compared to investigate radial variations in the velocity at heliocentric distances of 1 to 5 AU. Two hundred days of corresponding Pioneer 10 and 11 data are plotted, the velocity profiles for 25-day segments are compared, and the same general pattern of peaks and troughs is found in the corresponding profiles. A comparison of the relative smoothness of the profiles clearly shows that velocity amplitudes in the solar wind stream structure decrease dramatically with increasing radial distance from the sun, although the rate of decrease is not as clear. It is hypothesized that stream-stream interactions play a dominant part in inhibiting the classical radial expansion process in the solar wind and produce scattering centers which prevent the observation of a significant galactic cosmic ray gradient in this region of space.