Proton Cyclotron Heating and Beam Generation in the Solar Wind

We present results from hybrid expanding simulations of the solar wind plasma. We investigate the role of kinetic processes in shaping the proton distribution function along the wind expansion in the prensence of an initial spectrum of Alfvén waves. We find that both wave-particle and wave-wave interactions play a role in the ion evolution, in particular waves interact with protons through ion-cyclotron resonace and non-linear trapping due to the growth of parametric instabilities. Cyclotron interactions control the evolution of the temperature anisotropy providing a perpendicular heating which contrasts the adiabatic cooling caused by the expansion. Ion-acoustic modes driven by parametric effects produce a velocity beam in the particle distribution function. We discuss and compare our results with direct solar wind observations between 0.3 and 1 AU, and we find that the resulting proton distribution functions are in reasonable agreement with Helios data.