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

The Dst index has been conventionally used as a measure of the storm intens ity, which ideally assumes that the associated ground magnetic disturbance is caused by the ring current. The present study examines the contribution of the tail current to Dst, focusing on the occurrence of geosynchronous di polarization close to the Dst minimum, in other words, the start of the sto rm recovery phase. The Sym-H (referred to as Dst((Sy-H)) hereafter) index r ather than the conventional Dst index is used because of its higher time re solution (1 min). For the June 1998 storm event, dipolarization started at two GOES satellites and the Geotail satellite in the near-Earth tail when D st((Sym-H)) reached its minimum. This result indicates that the source curr ent was located outside of geosynchronous orbit, and therefore the recovery of Dst((Sym-H)) can be attributed to the reduction of the tail current rat her than the decay of the ring current. A statistical study based on 59 sto rm events (79 GOES events) confirms the tendency for geosynchronous magneti c field to dipolarize at the Dst((Sym-H)) minimum. It is therefore highly l ikely that the Dst((Sym-H)) minimum is misidentified as the start of the ri ng current (storm) decay at a time when the ring current may actually be in tensifying owing to substorm-associated injection, From the magnitude of th e Dst((Sym-H)) recovery during the interval of geosynchronous dipolarizatio n, the contribution of the tail current to Dst((Sym-H)) at the Dst((Sym-H)) minimum is estimated to be 20-25%. However, the contribution of the tail c urrent may be even larger because the tail current may not return to preint ensification levels and may continue to contribute to Dst(sym-fn after dipo larization. The trigger of dipolarization (substorm) and the subsequent rec overy of Dst((Sym-H)) tend to take place in the course of the reduction of the southward interplanetary magnetic field (IMF) B-Z. It is therefore sugg ested that the ring current (storm) recovers after the substorm since the m agnetospheric convection weakens because of weaker southward IMF B-Z.