On storm weakening during substorm expansion phase

Abstract

Abstract. Iyemori and Rao recently presented evidence that the strength of a magnetic storm, as measured by -Dst, weakens, or its rate of growth slows, during the substorm expansion phase. Yet the expansion phase is known to inject energetic particles into the ring current, which should strengthen the storm. We propose to reconcile these apparently contradictory results by combining the virial theorem and a principle of energy partitioning between energy storage elements in a system with dissipation. As applied to the unloading description of the substorm expansion phase, the virial theorem states that -Dst is proportional to the sum of the total magnetic energy and twice the total kinetic energy in the magnetosphere including the tail. Thus if expansion phase involves converting magnetic energy stored in the tail into kinetic energy stored 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 current, the rest going into the ionosphere. Indeed Weiss et al., have estimated that the energy dissipated in the ionosphere during expansion phase is twice that injected into the ring current. This conclusion is also consistent with the mentioned energy partitioning principle, which requires that more energy be dissipated than transferred between storage elements. While Iyemori and Rao's observations seem to contradict the hypothesis that storms consist at least in part of a sum of substorms, this mode of description might nonetheless be preserved by including the substorm's growthphase contribution. Then the change in storm strength measured from before the growth phase to after the expansion phase is positive, even though the expansion phase alone makes a negative contribution.

Periodic magnetospheric substorms: Multiple space‐based and ground‐based instrumental observations

“…The observed variations of SYM-H may imply that SYM-H (or Dst) is also effected by other magnetospheric currents beside the ring current. It has been suggested that the increase in SYM-H (or Dst) after substorm onsets is related to a reduction in the cross-tail current [Iyemori and Rao, 1996;Siscoe and Petschek, 1997].…”

Storm‐substorm relationship: Contribution of the tail current to Dst

“…The result of Iyemori and Rao [1996] therefore appears to be counterintuitive. Siscoe and Petschek [1997] sought to reconcile this apparent discrepancy in terms of the virial theorem, which predicts that Dst actually decreases if less than half of the tail magnetic energy is transferred to the ring current, with the rest of energy dissipated in the ionosphere. Thus the reduction of the tail current (i.e., tail current disruption) at storm time should be crucial for under-standing the result of Iyemori and Rao [1996] and, more generally, the physical meaning of the Dst index.…”

“…Recent modeling studies have found that the contribution of the tail current to Dst can be as much as 20% [Friedrich et al, 1999;Turner et al, 2000]. It is possible that the reduction of the tail current intensity, tail current disruption, at substorm onset causes a recovery of Dst, and the associated redistribution of the magnetic energy may explain the result of Iyemori and Rao [1996], as suggested by Siscoe and Petschek [ 1997].…”

The THEMIS Mission

“…During its course, auroral arcs intensify, move poleward and break up into smaller formations (Akasofu 1976). Substorms are ubiquitous at all solar phases and appear within all types of magnetospheric responses to solar wind input: Embedded within large storms they influence storm development (Daglis et al 2000) and geo-effectiveness (Siscoe and Petschek 1997). They bind the beginning and end phases of magnetospheric convection bays (Sergeev et al 1996a) and are closely related to pseudo-breakups (Aikio et al 1999).…”