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In the parts of the solar corona and solar wind that experience the fewestCoulomb collisions, the component proton, electron, and heavy ion populationsare not in thermal equilibrium with one another. Observed differences intemperatures, outflow speeds, and velocity distribution anisotropies are usefulconstraints on proposed explanations for how the plasma is heated andaccelerated. This paper presents new predictions of the rates of collisionlessheating for each particle species, in which the energy input is assumed to comefrom magnetohydrodynamic (MHD) turbulence. We first created an empiricaldescription of the radial evolution of Alfven, fast-mode, and slow-mode MHDwaves. This model provides the total wave power in each mode as a function ofdistance along an expanding flux tube in the high-speed solar wind. Next wesolved a set of cascade advection-diffusion equations that give the time-steadywavenumber spectra at each distance. An approximate term for nonlinear couplingbetween the Alfven and fast-mode fluctuations is included. For reasonablechoices of the parameters, our model contains enough energy transfer from thefast mode to the Alfven mode to excite the high-frequency ion cyclotronresonance. This resonance is efficient at heating protons and other ions in thedirection perpendicular to the background magnetic field, and our modelpredicts heating rates for these species that agree well with bothspectroscopic and in situ measurements. Nonetheless, the high-frequency wavescomprise only a small part of the total Alfvenic fluctuation spectrum, whichremains highly two-dimensional as is observed in interplanetary space.

Proton, Electron, and Ion Heating in the Fast Solar Wind from Nonlinear Coupling Between Alfvenic and Fast-Mode Turbulence