Doppler scintillation observations of interplanetary shocks within 0.3 AU

Abstract

Near‐sun spacecraft Doppler scintillation observations have been combined with Solwind coronagraph and Helios 1 plasma measurements to provide more definitive measurements of the evolution and propagation of interplanetary shock waves between the sun and earth orbit than have been available from previous observations. This study shows that substantial deceleration of fast shocks (shock speeds exceeding 1000 km s−1) takes place near the sun and that the amount of deceleration increases with shock speed. This is consistent with the significantly lower and rather narrow range of shock velocities observed by direct spacecraft near earth orbit. When coronal mass ejection (CME) speeds are available for the fast shocks, they are considerably lower than the speeds measured farther out but near the sun. This implies that either the fast shocks first accelerate before decelerating on their way out from the sun (assuming the CME front is identified with the shock) or the CME speeds do not represent and substantially underestimate the shock speeds in the outer corona. If the CME speeds underestimate the shock speeds of the fast shocks, they do not appear to do so for the slow shocks. If the shocks are being driven over distances indicated by the acceleration region or to the point where deceleration begins, then their velocity profiles imply that the slower shocks are being driven farther out than the faster shocks. The analysis of one piston‐driven shock shows that the velocity of the contact surface is about 0.58 that of the shock front velocity.