Diminished contribution of ram pressure to Dst during magnetic storms

A statistical determination of the coefficient of the ram pressure term in the Burton et al. equation for Dst has shown that it drops substantially during solar wind conditions that produce magnetic storms. This result is here confirmed and extended. In a separate study based on MHD simulations, the Chapman‐Ferraro current system, which transfers the Dst ‐increasing signal of the ram pressure, has been shown to be replaced by the region 1 current system, which based on geometrical considerations we argue probably makes a smaller contribution to Dst . These two studies suggest that the ram pressure correction to Dst is probably less than that usually assumed during the main phases of magnetic storms. This suggestion is illustrated by an analysis of a storm in which Dst calculated from a kinetic ring current model fits the uncorrected Dst curve better than the pressure‐corrected curve at the peak of the main phase. The work presented here illustrates a new approach to the study of storm‐time solar wind‐magnetosphere coupling.