Effect of geomagnetic disturbances and solar wind density on relativistic electrons at geostationary orbit

It is well known that fluxes of relativistic electrons in the Earth's magnetosphere well correlate with solar wind speed but surprisingly they show a weaker correlation with geomagnetic activity indices. For a long time, this result seemed puzzling since geomagnetic disturbances, measured by geomagnetic activity indices, are associated with strong electric fields and low‐frequency waves, which should significantly affect (directly or indirectly) the particle acceleration to high energies. To understand why the relativistic electron fluxes show the relatively weak correlation with geomagnetic disturbances, we investigated statistically the data of relativistic electrons at geostationary orbit for 6 years (1997–2002) and compared these data with solar wind parameters and geomagnetic activity indices. We found that, for the generation of the strong electron fluxes, the combination of two factors is needed: (1) strong geomagnetic disturbances about two days before the following increases in electron fluxes and (2) low solar wind density within these 2‐day intervals between the geomagnetic disturbances and following increases in the electron fluxes. By these conditions, the correlation between the electron fluxes and geomagnetic indices is improved and becomes higher than the correlation between the electron fluxes and solar wind speed. A large majority of the strongest relativistic electron events occurred just during these conditions. This allows us to suggest that not solar wind speed alone but rather geomagnetic activity combined with low solar wind density conditions may be a primary cause for the generation of relativistic electrons.