Energetic electron variation in the outer radiation zone during early May 1998 magnetic storm

Using NOAA and Akebono observations, we examined variations of the energetic electron flux in the outer radiation zone during the May 2 and 4, 1998 magnetic storm, which was a "two-step" storm. Both a flux dropout and an inward shift of the outer belt MeV electrons were recorded during the main phase of the May 2 magnetic storm. A very big injection of the intermediate energy (30-100 keV) electrons to the heart of the outer radiation zone took place during the main phase of the storm. During the recovery phase of the storm an increase in the MeV electron flux was seen, which surpassed the pre-storm level in one day. Comparison of NOAA and Akebono observations yields that the injected electrons with the energy of similar to 100 keV seeded a subsequent enhancement of the MeV electrons in the outer radiation zone. A more inward shift of the peak position as well as a flux dropout occurred during the main phase of the May 4 magnetic storm. No significant injection of the intermediate energy (30-100 keV) electrons was, however, seen during the main phase of the May 4 magnetic storm. A remarkable increase of the MeV electron flux was seen during the recovery phase of the storm. The pre-existing intermediate electrons seeded the increase of the MeV electrons near the new peak portion. The increase propagated to higher L values with a significant time delay, suggesting an enhanced radial diffusion. (C) 2000 Elsevier Science Ltd. All rights reserved.