MeV electron-flux enhancement events at the geosynchronous orbit in April-May 1998

The time and energy response characteristics of the relativistic (MeV) electronflux enhancement events at the geosynchronous orbit are analyzed and compared to the interplanetary disturbances and magnetic storms by using the high resolving data for the satellite GOES-9 electron fluxes and the spacecraft ACE solar-wind parameters in April-May 1998. The results show that there was the daily variation for the MeV electron flux at the geosynchronous orbit, the maximum flux at about the noon and minimum flux at about the midnight. In two great events starting at the geosynchronous orbit on April 22 and May 5, 1998, the rising time scales of the noon maximum flux up to the peak fluxes for electrons of the energy >2MeV are for about 4 days and 1 day, respectively. The continued times of the noon maximum fluxes (greater than background level) are 13 days (from April 22 to May 4) and 16 days (from May 5 to 20), respectively. The energetic ranges for the MeV electron-flux enhancement events are not same exactly. The rising phases for two great events in April-May correspond to the recovery phases of storms and are closely associated with the solar-wind dynamic pressure pulse, the high-speed flow pulse, and the negative B_z component of the interplanetary magnetic field (IMF).