Mechanisms of the outer radiation belt electron flux variation during magnetic storms

We have investigated variations of the energetic electron flux (> 0.4 MeV) and the magnetic field in the outer radiation belt obtained from the Standard DOse Monitor (SDOM) and the MAgnetoMeter (MAM) of the Space Environment Data Acquisition equipment (SEDA) onboard Tsubasa (Mission Demonstration Test Satellite (MDS)-1). Since Tsubasa operates in geostationary transfer orbit (GTO) with an orbital period of 10 hours and an inclination of 28.5 degrees, it has provided a rare opportunity for directly observing near-equatorial radiation belt plasma particles and the magnetic field during magnetic storms. The decreases of the energetic electron flux during the main phase of the magnetic storms, and the subsequent recoveries and enhancements during the recovery phase in the outer radiation belt are linked respectively to typical variations of the magnetic field. At the moment that the outer radiation belt flux sharply drops during the main phase of the 17 April 2002 magnetic storm, the butterfly distribution is observed at L=5 and the magnetic equator where the magnitude of magnetic field is much smaller than the IGRF model. Calculating the drift motions of the energetic electrons in the Tyganenko 2001 magnetospheric magnetic field model, shows that the drift-shell splitting mechanism could generate the butterfly distribution due to loss of the near-equatorially mirroring electrons through dayside magnetopause boundary. We evaluate roles and contributions of the other possible mechanisms to explain the flux decreases. We discuss the three-dimensional field configuration in the magnetopause to compare with the low earth orbital observation of the outer radiation belt flux.