The SI +-SI − pair and interplanetary forward-reverse shock ensembles

Abstract

On the basis of arguments given by Gold regarding the source of the storm sudden commencement, it follows that the sudden impulse is due to the arrival at the magnetosphere of an interplanetary shock wave. From the general inadmissibility of rarefaction shocks together with the fast decrease in the horizontal component of the Earth's field associated with the SI −, it is argued that the source of this disturbance is a shock wave pointing backward to the Sun and being simultaneously convected outward by the flow of the solar wind. Since the SI − often follows an SI + by several hours, the two shocks are taken to be part of a complex structure in which the two waves are separated by a contact discontinuity, the whole being convected outward by the solar wind. Such shock structures are known to occur in the laboratory under suitable starting conditions and also take place as an aftermath of nuclear explosions. The conditions leading to this situation follow from the over-expansion in an expanding blast wave; they can also result from interaction of an advancing blast wave with a density discontinuity in the solar wind or even from a gradual increase in the velocity of the piston gas pushing out from a solar disturbance. Likely conditions leading to the shock pair are explored, and a procedure is given for examining graphically the change in gas states responsible for the phenomena. For that case where the SI − occurs without being preceded by a positive impulse (SI +), it is argued that the generation of the pair took place by the expansion of the flow of fresh gas from the Sun and the forward wave dissipated prior to the time of the observation in accord with theoretical requirements on expanding shocked flow.