Alfvén wave heating of heavy ions in the expanding solar wind: Hybrid simulations

Abstract

We present hybrid expanding box simulations of the interaction of left-handed Alfvén waves with protons, alpha particles, and a tenuous population of oxygen O5+. The Alfvén waves are initially nonresonant with the ions, and the expansion brings them to the cyclotron resonance with O5+ ions, then with alpha particles, and finally with protons. The simulations show that O5+ ions are efficiently heated in the directions perpendicular to the background magnetic field but are only slightly accelerated. Oxygen scattering has a finite time span and saturates mainly due to the marginal stabilization with respect to the oxygen cyclotron instability generated by the temperature anisotropy. During the scattering, oxygen ions are able to absorb only a limited amount of available fluctuating energy and, for the parameters used in the simulations, their presence has a minimum influence on alpha particles and protons. Copyright 2005 by the American Geophysical Union.