Resonant heating and acceleration of ions in coronal holes driven by cyclotron resonant spectra

Abstract

Recent observations and models suggest that the resonant absorption of ion cyclotron waves heats and accelerates the ions in the solar wind. Velocity distributions of minor ions derived from SOHO Ultraviolet Cronagraph Spectrometer (UVCS) observations in coronal holes indicate that the minor ion temperature anisotropy is T⟂/T∣∣ > 10 and that outflow speeds are higher than those of the solar wind protons. Here one‐dimensional hybrid simulations of initially homogeneous, collisionless plasmas are used to study a model of coronal plasmas including kinetic protons, a tenuous component of oxygen ions, and massless fluid electrons. Spectra of ion cyclotron resonant Alfvén waves are imposed on the system to study the resultant heating of both ion species. We investigate the effects of various power spectra of the form f−1 or f−5/3 and vary the input frequency range. We find that the ion heating strongly depends on the power contained in the frequency range of the input spectrum that can resonate with the ions. The minor O5+ ions are easily heated and become anisotropic due to various forms of the spectra. The protons remain nearly isotropic and are weakly heated in most cases in this study. We investigate the self‐consistent fluctuation spectrum generated by the response of the ions and the non‐Maxwellian features in the velocity distribution.