Substorm variations in the magnitude of the magnetic field: AMPTE/CCE observations

Abstract

By using AMPTE/CCE energetic particle data, 167 ion injections that are essentially dispersionless over a nominal energy range of 25 keV to 285 keV have been cataloged. The ion, electron, and magnetic field signatures of these events are the kind normally associated with substorms in the near‐Earth magnetotail. We have examined the total magnetic field strength variations over the course of these events to identify the dependence on dipole latitude. The change in is poorly correlated with magnetic latitude. We interpret this result as indicating that during periods of substorm activity, the latitudinal position of the current sheet varies significantly within the 32° wedge centered on the dipole equator traversed by CCE. On the other hand, the changes in are well correlated with the local field properties themselves. This study therefore indicates that even in the near‐Earth magnetotail out to 8.8 RE (CCE apogee), the local field measurements are a better guide to the determination of one's position relative to the current sheet during a substorm than the magnetic latitude. Statistical relationships between Δ and both and |Bx| (in GSM coordinates) are consistent with the changes in that one would expect from a disruption/diversion of the cross‐tail current. The radial gradient of the reduction of the cross‐tail current during substorms appears to be negative, although the evidence is not conclusive. Finally, the local time variation of the statistical relationships is consistent with Birkeland current segments of the substorm current wedge which have a large extent in local time.