Preferential Perpendicular Heating of Coronal Hole Minor Ions by the Fermi Mechanism

Abstract

We present a kinetic mechanism for the preferential perpendicular heating of minor ions in coronal holes, which may be responsible for the consistent observation in fast solar wind that the heavy ions are hotter and flow faster than the proton population. The basis of this mechanism is the familiar resonant cyclotron interaction between ions and parallel-propagating ion cyclotron waves. The preferential effects naturally arise from the ability of minor ions to simultaneously resonate with several modes in a spectrum of inward- and outward-propagating waves, while protons can encounter only a single resonance for a given particle parallel speed. The single resonance defines a marginally stable proton distribution which represents a limit on the wave dissipation and heating attainable by the proton population. Minor ions have no such limitation and may continue to be heated as long as the resonant wave power is maintained. The multiply resonant interaction available to minor ions is equivalent to a second-order Fermi acceleration of these thermal particles. In this paper, we derive the quasi-linear expressions for this interaction and present illustrative results for the evolution of an O⁺⁵ ion distribution in a spatially homogeneous system under various assumptions on the resonant wave spectra. These results demonstrate strong and continuous perpendicular heating to temperatures comparable to those seen in coronal holes.