ESTIMATES OF SOLAR-WIND VELOCITY-GRADIENTS BETWEEN 0.3 AND 1-AU BASED ON VELOCITY PROBABILITY-DISTRIBUTIONS FROM HELIOS-1 AT PERIHELION AND APHELION

We have estimated the average velocity gradients for the solar wind protons by comparing the velocity probability distributions at 0.3 and at 1 AU Assuming a power law radial dependence, we find, for a full 6-year data set, that the gradients for the lowest velocity ranges are about R0.1 to R0.14 and that the power index decreases steadily with increasing velocity until the slope is near zero for the high-speed solar wind. However, upon examining the solar cycle dependence we find that this trend for the velocity gradient to decrease with increasing velocity is a characteristic primarily of the increasing solar activity and solar maximum period and is almost absent in the solar minimum data. The solar wind above 500 km/s during solar minimum shows an average acceleration similar to the slow wind, about 55 to 85 km/s/AU. On the other hand, winds above 350 km/s from the period of increasing solar activity and solar maximum show essentially no average acceleration beyond 0.3 AU.