On the role of wave-particle interactions in the evolution of solar wind ion distribution functions

We investigate the role of kinetic effects in the solar wind expansion using 1-D numerical hybrid simulations. The analysis of proton distribution functions in the solar wind shows a non-adiabatic evolution and suggests that several kinetic processes are at work during the expansion. From simulation studies wave-particle and wave-wave interactions, as cyclotron heating and non-linear trapping due to parametric instabilities, are found to play an important role on constraining the proton temperature anisotropy and generating secondary velocity beams. We report results from hybrid comoving simulations that self-consistently retain and describe these processes. We find that cyclotron interactions control the evolution of the proton temperature anisotropy with distance providing a perpendicular heating which contrasts the adiabatic cooling caused by the expansion. At the same time ion-acoustic modes driven by parametric effects produce a velocity beam in the proton distribution function. The resulting proton distribution functions are reasonable agreement with those observed in situ.