Active Magnetospheric Particle Tracer Explorers/Charge Composition Explorer (AMPTE/CCE) magnetometer and Medium Energy Particle Analyzer (MEPA) 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. The second criterion requires that the satellite was close to the onset region of current disruption. Although these strict criteria result in the small number of events, the magnetic and particle flux signatures of the events are considered to be informative concerning the mechanism of substorm onsets. It is found that these tail reconfiguration events are classified into two types: Type I and Type II. In Type I events a current disruption starts in a flux tube that is inward (earthward/equatorward) of the spacecraft, and consequently, the spacecraft is immersed in a hot plasma region expanding from inward (earthward/equatorward). The other type (Type II) 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. This enhancement is inferred to be a local process, rather than a result of a current disruption which has occurred somewhere else, although it is possible that the commencement of the H recovery observed is not exactly simultaneous with a substorm onset. The present results contribute significantly to modeling efforts regarding the triggering mechanims of substorms in the magnetotail. ¿ American Geophysical Union 1992