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

Theory and simulations predict that Alfvén‐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.