The role of substorms in the generation of magnetic storms

Abstract

A typical magnetic storm is characterized by an initial phase, a main phase and a recovery phase. The initial phase is caused by increased solar wind dynamic pressure, the main phase by injection into and energization of particles in the radiation belts, and the recovery phase by charge exchange loss of these particles. The effect of the Radiation belt particles is roughly equivalent to that of a geocentric ring of current of radius 3-4 Re which decreases the horizontal component of the earth's surface field by as much as 500 nT. The magnitude of this effect is generally indexed by the Dst index. The Dessler-Parker-Sckopke relation shows that in the absence of other effects Dst is directly proportional to the total energy of the radiation belt particles. Thus if Dst is corrected for other effects its time rate of change is a direct measure of the rate at which energy is flowing into or out of the ring current. Various models have been developed to explain the rate of energy input via solar wind coupling and the rate of loss by charge exchange. Statistical studies have parameterized these models and the resulting differential equation solved to predict the time history of Dst as a function of solar wind coupling. It is noteworthy that these models are remarkably accurate, and that they do not involve any measure of substorm activity such as the AL index. In fact, when the rectified solar wind electric field is used to predict both Dst and AL, the prediction residuals for these two indices are completely uncorrelated. This result suggests that the effect of particles injected and energized by the expansion phase of substorms is undetectable in the pressure corrected Dst index. It thus seems more likely that it is the global convection electric field that injects and energizes ring current particles. Thus, despite the fact that substorms occur throughout the main phase development of the ring current, the inductive electric field of the expansion phase is not likely to be the primary source of ring current particles or energy. However, it almost certainly plays a role in trapping the particles transported by the global convection field.