A Kinetic Model of the Preferential Acceleration and Heating of Coronal Hole Minor Ions

Heavy ions in the fast solar wind have long been observed to flow faster and have higher temperatures than the proton population. Measurements of minor ion speeds and temperatures in polar coronal holes by the UVCS instrument on SOHO have shown that the preferential heating and acceleration leading to these properties occurs in the collisionless region very close to the Sun. We have recently presented a new analysis of the resonant cyclotron wave-particle interaction which can take the form of a second-order Fermi acceleration acting on thermal ions in a low-beta plasma. This Fermi mechanism is not available to protons, so it inherently provides a preferential energization. Here, we report on the results of incorporating this Fermi mechanism into a coronal hole model, where we follow the kinetic evolution of an Oxygen+5 distribution in response to this interaction in combination with the forces of gravity, charge-separation electric field, wave ponderomotive force and the mirror force in the decreasing magnetic field. Matching the model results with the observations yields information on the required resonant wave intensities and spectra. Implications for models of resonant wave generation through a turbulent cascade or other kinetic processes will be discussed. Prospects for resonant energization of the other ion species in the fast solar wind will also be addressed. This work was supported primarily by the NASA Living With a Star TR&T Program through grant NNG04GN64G to the University of New Hampshire.