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

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Abstract
The interaction between alpha particles and a given magnetic power spectrum of Alfvén/cyclotron fluctuations at propagation parallel to the background magnetic field Bo 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 Bo matches the phase speed of long‐wavelength Alfvén waves, the “surfing” speed. Second‐order theory predicts that wave‐particle scattering is dominated by the relatively short wavelength, relatively low phase speed Alfvén/cyclotron fluctuations which have a strong cyclotron resonance with the alphas. Thus the theory predicts that at sufficiently high proton β the average alpha/proton speed should be observed to be substantially smaller than the surfing speed associated with long‐wavelength fluctuations.

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