Magnetospheric and ionospheric response to a substorm: Geotail HEP-LD and Polar PIXIE observations

The High Energy Particle - Low energy particle Detector experiment (HEP-LD) on board the Geotail spacecraft and the Polar Ionospheric X-ray Imaging Ex periment (PIXIE) on board the Polar satellite have been used to examine a s ubstorm event. On December 10, 1996, around 1700 UT, a substorm event with two onsets took place. The event occurred during a weak magnetic storm that started on December 9. Several of the classical substorm features were obs erved during the event: reconnection and neutral-line formation in the near -Earth geomagnetic tail, injection of energetic particles at geosynchronous orbit, and particle precipitation into the ionosphere. Magnetic field line mapping of the energetic precipitation area into the geomagnetic tail show s that the substorm development on ground is closely correlated with topolo gical changes in the near-Earth tail. In the first onset, mainly soft elect rons are involved, with only a transient energetic precipitation delayed re lative to the onset. The second onset about 30 min later includes both soft and energetic electrons. The source regions of both onsets are found to be located near the earthward edge of the plasma sheet, while the source regi on of the transient energetic precipitation during the first onset is in th e distant tail. Magnetic reconnection occurs sporadically and burst-like be fore the onsets. Both onsets appear to be triggered by northward turning of the interplanetary magnetic field. The study also demonstrates that the co ncept of pseudobreakups should be used with care unless global observations are available.