A search for solar wind velocity changes between 0.7 and 1 AU

Abstract

Simultaneous observations of the solar wind velocity as measured at the Pioneer 9 and Ogo 5 spacecraft during five solar rotations in 1968 and 1969 are presented. During this time, Pioneer 9 was traveling in toward the sun to approximately 0.7 AU while the earth orbiter Ogo 5 was spending long periods in the interplanetary medium. A comparison of the 3-hour averages of solar wind velocity obtained at both spacecraft indicates that the same basic solar wind velocity structure was seen at both spacecraft. There was no statistically significant dependence of average velocity on radial distance from the sun. The amplitude of variations about the average velocity apparently decreased with increasing distance from the sun; some but probably not all of this decreased variation is consistent with the exchange of momentum between high-velocity and low-velocity streams. The correlation coefficient of pairs of velocity averages computed from a corotation model decreases with increasing distance between the two spacecraft.