Preferential perpendicular acceleration of heavy ionospheric ions by interactions with electrostatic hydrogen cyclotron waves

Abstract

The energization of heavy ionospheric ions by electrostatic hydrogen cyclotron (EHC) waves is examined. In particular, the problem of preferential perpendicular acceleration of heavy ions above 5000 km along auroral field lines is examined critically. Observations have indicated that above this altitude the occurrences of ion beams and EHC waves are highly correlated and that the occurrence rates are high. The EHC waves are found to be coherent. The energization process is examined in the light of interactions of a single coherent wave with the ions. The interactions can be stochastic or nonstochastic, depending on the parameters of the wave and of the background H+ ions supporting the wave. With electric field wave amplitudes of a few tens of mV/m, it is shown that the interaction can give perpendicular energies of the order of keV for O+ ions. He+ ions are shown to gain perpendicular energies smaller than O+ ions. It is indicated that upward flowing H+ ions cannot gain any appreciable amount of perpendicular energy. For a given electric field wave amplitude Eo, the extent of energization critically depends on the average perpendicular energy (temperature) W⊥B of the background H+ ion plasma. Examples are worked out for values of W⊥B ranging from 1 to 200 eV. The resulting energizations and pitch angles for O+ and He+ ions are compared with observations from satellites. The heating times for the ions are found to be of the order of 2 s at an altitude of 5000 km.