Decrease of keV electron and ion fluxes in the dayside magnetosphere during the early phase of magnetospheric disturbances

It is shown that a decrease of the keV particle fluxes in the dayside magnetosphere near the geosynchronous orbit is characteristic of the first several hours of magnetospheric disturbances. After some hours newly injected plasma from the nightside reaches the 'evacuated' regions of the dayside magnetosphere and strong flux increases are observed. The 'evacuation' of the dayside magnetosphere is interpreted in terms of a change in the convection pattern associated with an increase of the large scale electric field at the onset of the disturbance. The model presented is capable of accommodating all characteristics of the observational data, such as the temporal and spatial distributions, energy and pitch angle characteristics, and differences between electrons and protons.