The semi-cold approximation in magnetospheric and solar wind physics

Abstract

During the past years, a method has been developed in order to study electromagnetic ion-cyclotron waves in multicomponent plasm·as. The method is reviewed and applied to ring current proton losses, and to the generation of electromagnetic ion-cyclotron waves at the geostacionary altitude. This method is also applied to the preferential acceleration of alpha particles in fast solar wind conditions. It is shown that a small positive drift velocity between Alpha particles and protons can result in an alpha particle acceleration to veloCities well in excess of the proton bulk velocity. During the acceleration process, which is assumed to take place at heliocentric distances less than 0.3 AU, the Alpha particle velocity should exceed the proton velocity; then the gap which exists around the alpha particle gyrofrequency disappears. For proton thermal anisotropies of the order of those observed in fast solar wind, the waves either grow or are not damped excessively, so the waves can exist and might lead to the observed differential speeds. The way in which the alpha particles exceed the proton velocity is not accounted for by this mechanism, but some ideas concerning this initial process are discussed.