Radial variation of the solar wind speed between 1 and 15 AU

Pioneer 10 and 11 solar wind speeds measured between 1.4 and 15.2 AU are compared with IMP 6, 7, and 8 solar wind speeds measured at 1 AU for six radial alignment periods between 1973 and 1978. The speed‐time profiles measured beyond 1 AU show a decreasing correlation with the 1‐AU profiles as the heliocentric distance increases. At 15‐AU distance the speed‐time profile is modified to such an extent that it is no longer correlated to any significant degree with the 1‐AU speed‐time profile. The linear correlation coefficient decreases from 0.83 at 1.8 AU to 0.06 at 14.8 AU. However, a stream structure is still evident at a distance as far out as 15 AU. Among various mechanisms which can modify the speed‐time profile are evolution of the streams during outward expansion and interactions between adjacent streams. The mean solar wind speed appears to vary little with heliocentric distance. The change in the mean solar wind speed between 1 and 15 AU is probably no greater than ±25 km/s. Both temporal changes and speed variations with heliographic latitude are too large to allow a more accurate determination. Speed variations become smaller with increasing radial distance from the sun. Both the highest and the lowest speeds disappear as the radial distance increases. An exponential fit to the ratios of the standard deviation of speeds at 1 AU to the standard deviation of speeds measured farther out at Pioneer 10 or 11 from all the alignments gives an e‐folding distance of 11±3 AU. If we assume a standard deviation at 1 AU of 100 km/s (the average measured standard deviation at 1 AU), this fit predicts a standard deviation of 5 km/s in the solar wind speeds at 33 AU. Therefore at distances from the sun greater than 30–40 AU the solar wind behavior may closely resemble that of a radially expanding constant speed plasma.