Hybrid simulations of wave propagation and ion heating in the solar wind using a 1D expanding box model
P. C. Liewer1, M. Velli2, B. E. Goldstein1
1Jet Propulusion Laboratory, Pasadena, CA
2University of Florence, Italy

We present simulations of parallel propagating Alfven waves in the expanding solar wind and their interactions with protons, alpha particles and minor ions, using an expanding box hybrid (kinetic ions, fluid electrons) simulation code. The expanding box hybrid model takes into account the basic properties of the background solar wind flow, i.e., the spherical expansion and the consequent decrease in magnetic field and cyclotron frequency with increasing distance from the Sun. Use of the hybrid model yields a fully self-consistent treatment of the resonant cyclotron wave-particle interaction for all ions in the simulation. We present results for the evolution of a spectrum of circularly polarized Alfven wave propagating radially. Initially, most of the wave energy is below the cyclotron frequency for both the alpha particles and protons. As the wind expands, the wave frequencies decrease more slowly than the cyclotron frequencies and the waves come into cyclotron resonance. When only protons are present, significant perpendicular heating is observed as the dominant wave frequencies approach the proton cyclotron frequency. When both alphas and protons are present, the alphas, which come into resonance first, are observed to be preferentially heated and accelerated. In both cases, the cyclotron damping leads to a steepening of the wave spectrum. For some simulations with protons, alphas and minor ions, we find that the minor ion temperture are proportional to their mass as observed in fast solar wind. The conditons under which this result is obtained will be discussed.