Turbulence‐driven Solar Wind Heating and Energization of Pickup Protons in the Outer Heliosphere

Abstract

We improve and extend the dominant turbulence model of Isenberg et al., which describes the turbulent heating of the distant solar wind by the dissipation of wave energy generated by the isotropization of interstellar pickup protons. We first modify some of the detailed expressions in Isenberg et al. to fully incorporate the effects of wave dispersion in the pitch-angle scattering of the newly ionized pickup protons. The corrected pickup proton distributions have a more involved shape, but the energy given up to the waves is only slightly smaller than that found in Isenberg et al. We then treat the effects of second-order Fermi acceleration of the pickup protons, which occurs through the same resonant cyclotron interaction that controls the pitch-angle scattering. In principle, the pickup proton energization is an energy sink that was not included in Isenberg et al. and so could modify the model results. In addition, since this turbulent-evolution model makes specific predictions of the intensity of resonant waves as a function of heliocentric radius, our quantitative results for the pickup proton evolution are on a firmer footing than possible in previous models. We find that neither of these modifications produces significant changes in the solar wind heating predicted by Isenberg et al. Thus, the results of the dominant-turbulence model appear to be robust in that they are not sensitive to the incorporation of second-order effects.