Initial signatures of magnetic field and energetic particle fluxes at tail reconfiguration: Explosive growth phase

Active Magnetospheric Particle Tracer Explorers/Charge Composition Explorer magnetometer and Medium Energy Particle Analyzer data are used to examine the initial signatures of tail field reconfiguration observed in the near-Earth magnetotail (< 9 RE). Sixteen events are selected preliminarily from 9 months (January-September 1985) of magnetometer data according to two criteria, that is, an unambiguous commencement of tail field reconfiguration and a sharp recovery of the north-south (H) component. It is found that these tail reconfiguration events are classified into two types. In Type 1 events a current disruption starts in a flux tube that is inward of the spacecraft, and consequently, the spacecraft is immersed in a hot plasma region expanding from inward (earthward/equatorward). The other type (Type 2) is characterized by a distinctive interval (explosive growth phase) just prior to the local commencement of tail reconfiguration. The duration of this interval is typically 1 min, much shorter than that of the so-called growth phase. During this interval the north-south magnetic (H) component is depressed sharply, and the flux of energetic ions increases outward (tailward/poleward) of the spacecraft, suggesting that the cross-tail current is explosively enhanced. It is also found that the radial magnetic (V) component changes with a distinctive phase relationship relative to the north-south component, which can also be explained in terms of the explosive enhancement in the cross-tail current intensity just prior to the current disruption. The present results contribute significantly to modeling efforts regarding the triggering mechanism of substorms in the magnetotail