Direct observations in the dusk hours of the characteristics of the storm time ring current particles during the beginning of magnetic storms

The characteristic features of the initial enhancement of the storm time ring current particles in the evening hours are consistent with flow patterns resulting from a combination of inward convection, gradient drift, and corotation, which carries plasma sheet protons into low L values near midnight and the higher-energy proton component into the plasmasphere and through the evening hours. Data from four magnetic storms during the early life of S³-A (Explorer 45) when the local time of apogee was in the afternoon and evening hours show that protons with lower magnetic moments penetrate deeper into the magnetosphere until a lower limit, determined by the corotation and gradient drift forces, is reached. Such particle motions produce the stable energy-dependent inner boundary of the ring current protons inside the plasmapause in the dusk sector and also provide the mechanism for energy injection into the ring current region. From the analyses of the pitch angle distributions it is evident that charge exchange and wave-particle interactions are not the dominant causes of this inner boundary. Observations of plasma sheet protons and electrons at altitudes just beyond the measured plasmapause are also consistent with the resulting flow patterns.