Signatures of Alfven-cyclotron wave-ion scattering: Advanced Composition Explorer (ACE) solar wind observations

[1] Theory and simulations predict that Alfven-cyclotron fluctuations of sufficiently short wavelengths and at propagation approximately parallel or antiparallel to the background magnetic field B-o in collisionless plasmas scatter ions so as to increase their energies in the directions perpendicular to B-o. Linear Vlasov theory for a homogeneous, isotropic plasma of electrons, protons, and alpha particles predicts that alpha particle cyclotron resonance properties and ion cyclotron damping of such fluctuations are sensitive functions of the alpha/proton relative velocity. If a strong ion resonance corresponds to strong ion pitch-angle scattering, linear theory implies observable signatures in the anisotropies of solar wind alphas and protons. Statistical analyses of proton and alpha particle anisotropies measured in the solar wind near 1 AU by the plasma and magnetic field instruments on the Advanced Composition Explorer ( ACE) spacecraft are reported which are largely consistent with such signatures.