Statistical study of long-term dropouts of relativistic electron fluxes in the geosynchronous orbit

The enhancement and loss mechanisms of relativistic electron (known as killer electron) fluxes in the radiation belt have always been hotspots in the research area of space physics and space weather. In this paper, we study a special phenomenon that the relativistic electron flux in the geosynchronous orbit decreased to the background level and lasted for more than 3 days from 2000 to 2016. Firstly, we study the distribution of total 62 events in about 1.5 solar cycles with the sunspot number, the results show that fewer events occurred during the declining phase of a solar cycle. However, in the solar maximum and minimum, the occurrence of events did not seem to be directly related to the sunspot number. Then, in order to analyze and discuss the objective law and generation mechanism of the long-term dropouts of relativistic electrons, a statistical analysis on the beginning, continuation, and end periods of these 62 events is performed. The results show that before the event, the significant increase in solar wind dynamic pressure and density resulting in the inward movement of magnetopause, the maintenance of plasmapause at high L-shell for a long time, the southward IMF B-z component and the magnetic storm process reduced the relativistic electron flux to the background level through adiabatic and non-adiabatic physical mechanisms. When the relativistic electron reached the background flux level, weaker solar wind conditions and geomagnetic activities cannot provide enough sources for relativistic electrons; although sometimes there were some geomagnetic storms, they were small or medium geomagnetic storms. Under these circumstances, the physical mechanisms that led to the flux loss or enhancement during geomagnetic storms could keep a dynamic balance, so the relativistic electron still kept at background level. If long-term substorm activities, high-intensity ULF (Ultra-Low Frequency) wave activities or significant increase in solar wind occurred, these processes could provide enough seed electrons and acceleration mechanisms, so that the relativistic electron flux broke the dropout and presented a significant enhancement.