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

The Dst index has been conventionally used as a measure of the storm intensity, 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 dipolarization close to the Dst minimum, in other words, the start of the storm recovery phase. The Sym‐H (referred to as Dst ( Sym‐H ) hereafter) index rather than the conventional Dst index is used because of its higher time resolution (1 min). For the June 1998 storm event, dipolarization started at two GOES satellites and the Geotail satellite in the near‐Earth tail when Dst ( Sym‐H ) reached its minimum. This result indicates that the source current was located outside of geosynchronous orbit, and therefore the recovery of Dst ( Sym‐H ) can be attributed to the reduction of the tail current rather than the decay of the ring current. A statistical study based on 59 storm events (79 GOES events) confirms the tendency for geosynchronous magnetic field to dipolarize at the Dst ( Sym‐H ) minimum. It is therefore highly likely that the Dst ( Sym‐H ) minimum is misidentified as the start of the ring current (storm) decay at a time when the ring current may actually be intensifying owing to substorm‐associated injection. From the magnitude of the Dst ( Sym‐H ) recovery during the interval of geosynchronous dipolarization, 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 current may be even larger because the tail current may not return to preintensification levels and may continue to contribute to Dst ( Sym‐H ) after dipolarization. The trigger of dipolarization (substorm) and the subsequent recovery 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 suggested that the ring current (storm) recovers after the substorm since the magnetospheric convection weakens because of weaker southward IMF B Z .