Nightside energetic particle decreases at the synchronous orbit

More than 100 cases of major decreases of proton and electron fluxes at the synchronous orbit have been observed and interpreted as a movement of the energetic particle trapping boundary earthward of 6.6 earth radii before the substorm expansion phase. These events are observed only between 1700 and 0800 LT and are consistent with the existence of a westward magnetospheric electric field of a few tenths of a millivolt per meter before substorm expansion. Most substorm particle events seen on the nightside do not exhibit this behavior, presumably because the trapping boundary moves inside 6.6 earth radii only during major events. Such events show evidence of particle acceleration, which probably was concentrated at higher L values, near the instantaneous location of the trapping boundary.