Anticorrelation between the ring current intensity and the Dst index

Ohtani, S, C:son Brandt, P, Mitchell, D G


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] IMAGE/HENA [Image]
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).