Influence of the substorm current wedge on the Dst index

One of the major questions confronting researchers studying the nature of t he solar-terrestrial interaction centers around whether or not the substorm expansive phase has any causal effect on the growth of the storm time ring current. This question is often addressed by using the Dst index as a prox y for the storm time ring current and inspecting the main phase growth of D st in the context of the substorm expansive phases which occur in the same time frame as the ring current growth. In the past it has been assumed that the magnetic effects of the substorm current wedge have little influence o n the Dst index because the current wedge is an asymmetric current system, while Dst is supposed to reflect changes in the symmetric component of the ring current. In this paper we shall show that the substorm current wedge c an have a significant effect on the present Dst index, primarily as a conse quence of the fact that only four stations are presently used to formulate the index. Calculations are made assuming the instantaneous magnitude of th e wedge current is constant at 1 MA. Hourly values of Dst may be as much as 50 degrees smaller than those presented here because of variation of the w edge current over the hour. We shall show how the effect of the current wed ge depends on the UT of the expansive phase onset, the angular extent of th e current wedge, and the locale of the closure current in the magnetosphere . The fact that the substorm current wedge is a conjugate phenomenon has an important influence on the magnitude of the expansive phase effect in the Dst index.