Solar wind and magnetospheric conditions leading to the abrupt loss of outer radiation belt electrons

摘要

The trapped radiation belt electron population is maintained through a competition between multiple source and loss processes occurring within the magnetosphere and driven by the solar wind. In this research we have concentrated on the solar wind and the magnetospheric conditions that lead to the loss of electrons through abrupt energetic electron flux dropouts. We have focused on times when there is only a moderate level of geomagnetic activity, since the magnetospheric response during these conditions is expected to be far less complex than during large geomagnetic storms. We have found that under certain circumstances the radiation belt electrons are remarkably sensitive to the onset of southward IMF and to solar wind dynamic pressure increases. The onset of southward IMF is found to be sufficient to cause the flux dropouts, while increases in solar wind pressure are not necessary but are likely to enhance the loss when they occur in conjunction with southward IMF, as is often the case. It is not clear if an increase in solar wind pressure in the absence of southward IMF is sufficient to cause a flux dropout. The radiation belt fluxes can decrease by more than an order of magnitude with the onset of only minor geomagnetic activity. The level of solar wind forcing (as estimated by the epsilon parameter) and of geomagnetic activity (as estimated by AE, Dst, and the local magnetic field inclination at geosynchronous orbit) responsible for the flux loss is intermediate between lower levels of activity that create localized, adiabatic variations in the flux and large geomagnetic storms that result in both loss and acceleration. The dropout events examined here occurred after one or more days of quiet geomagnetic conditions, which we suggest preconditioned the magnetosphere to be highly sensitive to the onset of new activity. Although it is not known which specific conditions within the magnetosphere lead to this extreme sensitivity of the relativistic electrons, the time periods identified here are ones where the electron loss processes appear to operate in relative isolation of the acceleration processes.