Shock waves in the solar wind and geomagnetic storms

Certain geomagnetic storms exhibit, in addition to the usual initial sudden positive impulse, a subsequent sudden negative impulse. The former is normally ascribed to a shock wave in the interplanetary medium, and it has recently been suggested that the latter may be ascribed to a reverse shock convected away from the sun by the solar wind. If the velocity of efflux of gas from a source is supersonic (with respect to the source), if the velocity is instantaneously increased, and if certain subsidiary conditions are met, a pair of shock waves will be produced which propagate away from the source. The ‘fast’ shock propagates away from the contact surface in the ambient gas (which was ejected from the source before the change in efflux velocity); the ‘slow’ shock propagates away from the contact surface in the driver gas but has an outward velocity when this velocity is measured relative to the source. This problem (which may be identified with a classical problem considered by Riemann) is discussed in its relation to the production of pairs of shock waves by the enhanced solar wind produced by a solar flare. The equations giving the relationship between the velocities of the shock waves and those of the ambient and driver gases become very simple in the strong-shock approximation. It is shown that the propagation times of the positive and negative impulses of the July 11, 1959, magnetic storm may be explained satisfactorily on the basis of this theory.