Relativistic Electron Variability in Earth's Magnetosphere

The trapped electron flux in Earth's magnetosphere is highly dynamic, varying over many orders of magnitude on time scales ranging from minutes to days to years. In this research, we have concentrated on determining the solar wind and magnetospheric conditions that lead to abrupt energetic electron dropouts and enhancements, and on the local-time dependence, radial dependence, and energy dependence of the response of the energetic particles to these conditions. This research has not been limited to time intervals of strong geomagnetic storms, since considerable variability in the radiation belt populations is observed during weak storms or even during non-storm times. We have found that abrupt flux dropouts occur through a combination of solar wind driving and a preconditioning of the magnetosphere. The dropouts occur coincident with the onset of moderate geomagnetic activity following a period of prolonged quiet conditions. Furthermore, we have found that there is a strong preference for the dropouts initially to occur in the dusk sector of the magnetosphere due to an asymmetric distortion of the magnetic field. Although the dropouts initially result from adiabatic motion of the electrons and not due to actual loss, it is often the case that actual loss from the magnetosphere does eventually occur prior to the recovery of the magnetic field. The relative importance of the enhanced ring current, tail-like magnetic field stretching, and magnetopause compression for causing the abrupt particle dropouts is discussed.