Storm-Substorm Relationship: Anticorrelated Variations of the Tail-Current and
Ring-Current Intensities

Shin-ichi Ohtani (JHU/APL, Laurel, MD), Pontus C:son Brandt, Donard G. Mitchell 


The present study addresses the role of magnetospheric substorms in the
storm-time ring current development, for which the storm events of August 2000
and October 2001 are examined. It is found that geosynchronous magnetic field
dipolarized at the start of the recovery phase (as defined by Sym-H) for each
event, and other substorm features such as the development of an auroral bulge
were also detected. The simultaneous increases in the ground and geosynchronous
H components indicate that the source current was located outside of
geosynchronous orbit, rather than between the Earth and geosynchronous altitude
as expected for the ring current. Thus the start of the apparent recovery of the
Sym-H index is not due to the decay of the ring current, but it can be
attributed to the substorm-associated reduction of the tail current intensity.
Energetic neutral atom (ENA) images taken during the events by the High Energy
Neutral Analyzer (HENA) onboard the IMAGE satellite are available for these
events, from which the reduction of the ground H magnetic component owing to the
ring current, DENA, can be estimated. It is found that during the substorm
expansion phase, DENA indeed decreases (the total ring-current energy
increases), whereas Sym-H recovers. That is, the reduction of the tail current
during the expansion phase overcompensates the intensification of the ring
current. This is the first direct confirmation that the substorm plays a
positive role in the ring current development. The study makes a more detailed
comparison between substorm-associated changes of DENA and Sym-H, and it
discusses the result in terms of particle transport and betatron effects.

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Presented at the August 2003 AGU Chapman Conference, Helsinki, Finland