Ion heating and acceleration in the solar wind

Abstract

We propose a mechanism for Alfvén wave heating and acceleration of heavy ions in high speed solar wind streams that is consistent with the observations. This mechanism is a consequence of the changes experienced by the dispersive properties of the solar wind as it expands from its source region into the interplanetary medium. Assuming that at the origin of the fast solar wind the plasma is isothermal, it is shown that after some distance from the Sun, the Alfvén/cyclotron waves approach the cyclotron resonance of the species with the smallest gyrofrequency heating this species by cyclotron absorption. When the thermal velocity of this species becomes similar to the thermal velocity of the species with the next larger (q/m) ratio (where q is the ion charge and m its mass), the Alfvén branch of the dispersion relation approaches the gyrofrequency of that species, heating mainly this ion component. When the thermal velocities of all the species are similar, the minor heavy ions become disconnected from the proton cyclotron waves and the process stops.