Resonant acceleration of alpha particles by ion cyclotron waves in the solar wind

Abstract

Preferential acceleration of alpha particles interacting with left‐hand polarized ion cyclotron waves is studied. It is shown that a small positive drift velocity between alpha particles and protons can lead to alpha particle 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 drift velocity should exceed the proton bulk velocity, and then the gap which exists around the alpha particle gyrofrequency should disappear. It is also shown that for proton thermal anisotropies of the order of those observed in fast solar wind, the waves either grow or are not damped excessively, so that the waves can exist and might thus lead to the observed differential speeds. However, the way in which the alpha particles exceed the proton velocity remains unexplained.

Beam effect on electromagnetic ion-cyclotron waves with general loss – cone distribution function in an anisotropic plasma-particle aspect analysis

“…In the linear theory these waves have been studied both numerically and analytically (Gomberoff and Elgueta, 1991) in multi-component plasmas. The findings of the investigation may be of importance to the coronal heating and acceleration of solar wind by electromagnetic ion-cyclotron waves (e.g.…”

“…The non-drifting ions, consisting of resonant and non-resonant particles, participate in the energy exchange with the wave. The cold plasma dispersion relation approximation is also a good approximation to the warm plasma dispersion relation, provided that plasma β<1 (Gomberoff and Elgueta, 1991). Thus, the non-drifting ions are treated as anisotropic and the wave growth is possible.…”

“…The dispersion relation for electromagnetic ion-cyclotron waves propagating in the direction of the external magnetic field, in a system consisting of electrons, proton core and proton beam is given in Gomberoff and Elgueta (1991); Gomberoff (2003). If the beam density relative to core density is small, the dispersion relation reduces to a cold plasma dispersion relation, as we have adopted.…”

“…Thus, the non-drifting ions are treated as anisotropic and the wave growth is possible. For the large ion beam density, one must consider the dispersion effects of the beam, which does change the dispersion relation (Gomberoff and Elgueta, 1991;Gomberoff, 2003). Only when the drift velocity becomes very large, do the EMIC and the beam contribution to the dispersion relation decouple, but then the drifting ions are not influenced by the original cyclotron wave.…”

“…In past the studies on EMIC instability, including H + e , O + and their anisotropies, have been performed by various authors (Gomberoff and Neira, Published by Copernicus GmbH on behalf of the European Geosciences Union. 558 G. Ahirwar et al: Beam effect on electromagnetic ion-cyclotron waves 1983; Gomberoff and Elgueta, 1991;Gomberoff et al, 1996;Thorne and Horne, 1997;Horne and Thorne, 1997). In the present analysis, we have considered only H + e to examine the behavior of EMIC waves for the auroral acceleration process.…”