R. Rasinkangas1, V. Sergeev2, G. Kremser1,4, Th.Ulich1, H. Singer3, and A. Korth3
1Department of Physics, University of Oulu,
Linnanmaa, FIN-90570 Oulu, Finland.
2Institute of Physics, University of St. Petersburg, St. Petersburg, 198904 Russia
3NOAA R/E/SE, 325 Broadway, Boulder, CO 80303-3328, USA
4Max-Planck-Institut für Aeronomie, D-37191 Katlenburg-Lindau, Germany
Observations of the magnetic field and energetic particles by the near-equatorial CRRES spacecraft have verified the existence of a distinctive time interval close to the substorm onset during which the following can be observed: (1) The H-component of the magnetic field shows a short-time (about 1 min) depression just prior to the sudden field dipolarization. (2) This depression is accompanied by a more complicated, but well defined signature in the V-component of the magnetic field and (3) by sudden dispersionless increase in the >100 keV proton flux tailward of the satellite. (4) Simultaneous proton flux decrease is sometimes seen at lower energies. (5) The increase in the <100 keV electron flux appears later with the dipolarization front. (6) The event can be accompanied by large D-component variations indicative of simultaneously enhanced field-aligned currents. (7) The conjugated ground-based PiB pulsations start earlier then the H-component depression. The first three points are the signatures of the previously found 'explosive growth phase' events, while the last two points indicate possible relationship with the onset phase of a substorm. We present examples of the events and tentatively propose that formation of substorm current wedge could create the observed magnetic signatures.