Fast solar wind after the rapid acceleration

Abstract

We have studied the radial dependence of the velocity of high-latitude fast solar wind in the heliocentric distance range of 0.13-0.9 AU. For this study a new tomographic analysis method which can evaluate uncertainties was developed to obtain velocity distribution maps on two reference spheres at 0.13 and 0.3 AU using interplanetary scintillation (IPS) observations. First of all, it is tested that this tomographic method has enough sensitivity and reliability to investigate the radial dependence of the wind velocity. The analysis was made for the IPS observations during 3 years, from 1995 to 1997, when solar activity was minimum. From this analysis, average velocities of 770-780 km s^-1 were obtained at distances of 0.13-0.3 AU, which were 19 ± 17 km s^-1 lower than those at 0.3-0.9 AU. The results from this work, taken together with measurements of SOHO/LASCO, EISCAT and MERLIN [, 2002], Helios [, 1978], and Ulysses [, 2000], indicate that the fast wind is accelerated almost to its final flow velocity within 20 R_s and a small but not negligible acceleration exists beyond 30 R_s which tends to become smaller at farther heliocentric distances.