Helium ion acceleration and heating by Alfven/cyclotron fluctuations in the solar wind

The interaction between alpha particles and a given magnetic power Spectrum of Alfven/cyclotron fluctuations at propagation parallel to the background magnetic field B-o is examined in a model of a homogeneous, collisionless, magnetized plasma. Exact solutions of the linear Vlasov dispersion equation and second-order theory are used to calculate the wave-particle scattering rates for alpha/proton momentum exchange as well as for proton and alpha heating and anisotropy change. It is well established that alpha velocities are not observed to oscillate when the alpha/proton relative flow speed parallel to B-o matches the phase speed of long-wavelength Alfven waves, the "surfing" speed. Second-order theory predicts that wave-particle scattering is dominated by the relatively short wavelength, relatively low phase speed Alfven/cyclotron fluctuations which have a strong cyclotron resonance with the alphas. Thus the theory predicts that at sufficiently high proton beta the average alpha/proton speed should be observed to be substantially smaller than the surfing speed associated with long-wavelength fluctuations.