On the propagation and origin of interplanetary shock waves

Abstract

During the STIP intervals III and IV (March 15 – May 15, and October 15 – December 15, 1977, respectively), 36 shocks were seen by Helios 1 and 2, the IMP's and Voyager 1 and 2. Through a detailed study of the multiple spacecraft observations of the plasma and magnetic field data, and a study of the arrival time of shock events at each spacecraft, these shocks are classified into three classes of corotating shocks and five classes of transient shocks. The 36 shocks are found to originate from 16 events, where ten of the 16 shock events are corotating events and the other six are transient events. Only three of the 16 shock events were observed by all five spacecraft in interplanetary space. Occasionally, the angular separation of the spacecraft is less than 5 degrees in heliocentric longitude and or latitude, but the shock events are not necessarily seen at all the spacecraft. It is suggested that shock waves in interplanetary space are not continuously propagating. A physical process due to interactions of a weak interplanetary shock with a nonuniform ambient solar wind is suggested. This interaction will result in the geometry of an interplanetary shock surface being highly distorted. In addition a substantial portion of the shock surface has degenerated into a disturbance which does not satisfy the Rankine-Hugonoit shock jump conditions. A one-dimensional MHD simulation of a shock interacting with a contact discontinuity shows degeneration of the shock into a nonlinear disturbance which will steepen into a shock again when the ambient fluid becomes uniform.