On the Efficiency of Nonresonant Ion Heating by Coronal Alfvén Waves

Abstract

Nonresonant wave-particle interactions are studied within the framework of quasi-linear theory for the reduced velocity distribution functions of coronal ions. Our parametric study shows that in collisionless low-β plasma, Alfvén waves at low-frequencies (with ω<<Ωi, where Ωi is ion gyrofrequency) can heat ions perpendicularly to the direction of the mean magnetic field. Consequently, a temperature anisotropy can be achieved by the ions, whereby the heavy species are heated more strongly, by a factor of the mass ratio, than the protons. Yet in the lower corona, such wave-induced features will be destroyed by collisions that are still strong there. Although the coronal plasma β is small, Alfvén waves may efficiently heat the nonresonant ions only if their energy is relatively large. The heuristic values of wave energy needed for strong heating are, however, much larger than the ones assumed in previous models and obtained from recent observations. Nevertheless, heating of the ions by low-frequency Alfvén waves can contribute to raising the temperature of the lower solar transition region.