Anticorrelation between the ring current intensity and the Dst index

Ohtani et al. [2001] previously found that geosynchronous magnetic field dipolarizes at the start of the storm recovery phase as measured by the Dst (Sym-H) index, suggesting that energetic particles are injected into the ring current even though Dst is being recovering. They inferred that this apparent contradiction can be resolved if the recovery of Dst is actually caused by the reduction of the tail current rather than the ring current. From the magnitude of the Dst recovery during the interval of geosynchronous dipolarization, the contribution of the tail current to Dst was estimated to be at least 20-25%. The result suggests that the Dst minimum is misidentified as the start of the ring current (storm) decay at a time when the ring current may actually be intensifying due to substorm-associated injection. The present study seeks to test this idea by referring to the flux of energetic neutral atoms (ENAs) measured by the IMAGE/HENA instrument. The instrument measures ENA fluxes in the energy range of 10 to 200 keV, which covers the core part of the ring current energy density. By cross-examining ENA fluxes, geosynchronous dipolarization, and Dst variations, the present study will discuss the Dst decay in terms of the two current systems, that is, the ring current and the tail current. Reference: Ohtani, S., M. Nose', G. Rostoker, H. Singer, A. T. Y. Lui, and M. Nakamura, Storm-substorm relationship: Contribution of the tail current to Dst, J. Geophys. Res., 2001 (in press).