IN-SITU LOCAL SHOCK SPEED AND TRANSIT SHOCK SPEED

A useful index for estimating the transit speeds was derived by analyz ing interplanetary shock observations. This index is the ratio of the in situ local shock speed and the transit speed; it is 0.6-0.9 for mos t observed shocks. The local shock speed and the transit speed calcula ted for the results of the magnetohydrodynamic simulation show good ag reement with the observations. The relation expressed by the index is well explained by a simplified propagation model assuming a blast wave . For several shocks the ratio is approximately 1.2, implying that the se shocks accelerated during propagation in slow-speed solar wind. Thi s ratio is similar to that for the background solar wind acceleration.