Observations of plasma sheet dynamics earthward of the onset region with the Geotail spacecraft

During the onset of an auroral substorm at about 0406 UT on August 14, 1996, the position of the Geotail spacecraft was located very near the corresponding region of instability in the equatorial plane as determined with a global model of the geomagnetic fields which provided the mapping from the auroral ionosphere. The Earth Camera on board the Polar spacecraft was used to acquire the global auroral images. The magnetic fields in the ring current were sufficiently stretched by the equatorial current sheet such that the radial position of the Geotail spacecraft at 10.2 R E was earthward of the onset region. Earthward flows at speeds in the range of 200 km/s for a period of several minutes were coincident with the initial brightening of a localized region of the previously existing auroral oval. This auroral luminosity subsequently declined within several minutes without a well‐developed expansive phase; that is, a “pseudo‐onset” occurred. Partial dipolarization of the magnetic fields occurred with this onset as well as large‐scale magnetic fluctuations which continued through the brightening of another spot on the auroral oval at 0413 UT. These fluctuations ceased with the end of a well‐developed expansive phase at about 0424 UT, when the auroral luminosities entered their recovery phase. At the beginning of the short expansive phase at 0420 UT the magnetometer on GOES 8 at a radial distance of 6.6 R E recorded a major dipolarization of the magnetic fields as an earthward moving injection front of hot plasma passed across the Geotail position. The present observations of the onset phenomena earthward of the instability region are combined with previously reported measurements of plasmas and magnetic fields at distances tailward of the instability in the ring current in order to begin sketches of the relationship among several dynamical features of the onset region.