An unusual long-lived relativistic electron enhancement event excited by sequential CMEs

An unusual long-lived intense relativistic electron enhancement event from July to August 2004 is examined using data from Fengyun-1, POES, GOES, ACE, the Cluster Mission, and geomagnetic indices. During the initial 6days of this event, the observed fluxes in the outer zone enhanced continuously, and their maximum increased from 2.1x10(2)cm(-2)sr(-1)s(-1) to 3.5x10(4)cm(-2)sr(-1)s(-1), the region of enhanced fluxes extended from L=3.5-6.5 to L=2.5-6.5, and the flux peak location shifted inward from L4.2 to L3.3. During the following 7days, without any locational movement, the flux peak increased slowly and exceeded the prestorm fluxes by about 4 orders of magnitude. Subsequently, the decay rate of relativistic electrons is so slow that the peak remains over 10(4)cm(-2)sr(-1)s(-1) for about 30days. The drift resonance between ULF waves, which arose from high-speed solar wind and frequent impulses of solar wind dynamic pressure, and energetic electrons injected by substorms could be an important acceleration mechanism in this event. The local acceleration by whistler mode chorus could be another mechanism contributing to this enhancement. The plasmaspheric response to the interplanetary disturbances reveals that the enhanced outer zone is divided into two portions by the plasmapause. Accordingly, the slow loss rate in the plasmasphere due to hiss primarily contributed to the long-lived characteristic of this event. This event reveals that the outer zone population behaviors are dominated by the interplanetary variations together with the responses of geomagnetic field and plasmasphere to these variations.