The Effect of Large Amplitude Alfven Waves in the Solar Wind on Relativistic Electron Fluxes at Geostationary Orbit

*B. Hausman[1] ,G. Rostoker [1],H.-J. Kim [1],W. Choe [1]
F. C. Michel [1],Y. Kamide [1]
Solar-Terrestrial Environment Laboratory, Nagoya University[1]

For practical reasons, there has been considerable interest in recent years with the sudden appearance at geostationary orbit of enhanced fluxes of relativistic electrons. How these electrons come to be found at geostationary orbit is somewhat of a mystery and several different mechanisms have been put forward proposing in situ acceleration of these electrons in or around geostationary orbit. In this paper we suggest that, at least on occasion, these rapid increases in flux merely reflect changes in the tail-like character of the earth's magnetic field when an observing spacecraft makes measurements at different positions on the radial gradients in particle fluxes associated with the trapping boundaries for the specific energies being detected. We find that the solar wind dynamic pressure affects both changes and the absolute values of the measured electron fluxes. We also find that unusually high fluxes of relativistic electrons can arise at geostationary orbit without any magnetic storm involvement. Such cases are accompanied by large amplitude Alfven waves in the interplanetary medium, of the type associated with so-called HILDCAA events in the auroral oval. We discuss the impact of these solar wind Alfven waves on the ability of the magnetosphere to go through the energy storage-release cycle that is associated with magnetospheric substorm expansive phase activity.