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.