ON STORM WEAKENING DURING SUBSTORM EXPANSION PHASE

Iyemori and Rao recently presented evidence that the strength of a mag netic storm, as measured by -Dst, weakens, or its rate of growth slows , during the substorm expansion phase. Yet the expansion phase is know n to inject energetic particles into the ring current, which should st rengthen the storm. We propose to reconcile these apparently contradic tory results by combining the virial theorem and a principle of energy partitioning between energy storage elements in a system with dissipa tion. As applied to the unloading description of the substorm expansio n phase, the virial theorem states that -Dst is proportional to the su m of the total magnetic energy and twice the total kinetic energy in t he magnetosphere including the tail. Thus if expansion phase involves converting magnetic energy stored in the tail into kinetic energy stor ed in the ring current, a drop in -Dst during expansion phase requires that less than half the drop in magnetic energy goes into the ring cu rrent, the rest going into the ionosphere. Indeed Weiss et al., have e stimated that the energy dissipated in the ionosphere during expansion phase is twice that injected into the ring current. This conclusion i s also consistent with the mentioned energy partitioning principle, wh ich requires that more energy be dissipated than transferred between s torage elements. While Iyemori and Rao's observations seem to contradi ct the hypothesis that storms consist at least in part of a sum of sub storms, this mode of description might nonetheless be preserved by inc luding the substorm's growth phase contribution. Then the change in st orm strength measured from before the growth phase to after the expans ion phase is positive, even though the expansion phase alone makes a n egative contribution.