Large enhancement of the outer belt electrons during magnetic storms

Major magnetic storm produces a big change in relativistic (> a few hundred keV) electron flux in the outer radiation belt. Although this phenomenon is well known, the characteristics of the storm that lead to the increase and the role of substorms in producing such energetic electrons are not well understood. In this paper, we examined the transport and energization of outer belt electrons on the basis of NOAA and Akebono measurements for early November, 1993 magnetic storm. The result demonstrated that the intermediate-energy (30–100 keV) electrons in the plasma sheet were injected to the near-Earth (L = 3–4) region during the storm main phase, and an increase of MeV electrons took place due to the internal acceleration of the source electrons. We have, then, checked the counter example, the January 1997 magnetic storm, in which we saw a strong injection of 30–100 keV electrons but no intensification of relativistic energies. Following processes were requested in intensifying relativistic electrons in the outer radiation belt; i.e. the supply of source electrons with intermediate-energy (30–100 keV) and a possible acceleration of them. Hint of the acceleration would be the strong magnetic activities during the storm recovery phase. We are speculating that substorm-related electric and magnetic field fluctuations and/or waves would accelerate electrons efficiently. Even though the present work is a case study, it demonstrates that the internal acceleration is actually taking place and it leads a large enhancement of the relativistic electrons in the outer radiation belt.