Evolution of an Alfvén Wave–driven Proton Beam in the Expanding Solar Wind

Abstract We investigate the self-consistent formation and long-term evolution of proton beams in the expanding solar wind using an ensemble of one-dimensional hybrid expanding box simulations. Initial conditions are chosen to represent a range of plasma states observed by the Helios spacecraft at 0.3 au, including an amplitude-modulated Alfvén wave that nonlinearly drives a proton beam aligned with the magnetic field. We compare simulation results with solar wind data out to 1.5 au and show that our model reproduces key observed features of proton beams on average, such as the radial evolution of the drift and the relative core-to-beam density ratio. These findings support the theory that the observed evolution of the proton beam drift in the solar wind is determined by kinetic instabilities. More broadly, our results indicate that the interplay between nonlinear Alfvén wave dynamics, expansion effects, and kinetic instabilities plays a fundamental role in solar wind dynamics, with implications for interpreting solar wind heating rate estimates.