On the generation of proton beams in fast solar wind in the presence of obliquely propagating Alfvén waves

Abstract

Over the past few decades, satellite investigations have revealed that the solar wind contains a plethora of distribution function signatures (proton beam, anisotropic cores) requiring models departing from the conventional fluid theory and necessitating the inclusion of wave‐particle interactions. Numerous theoretical models and explanations have been proposed, but several questions concerning time and heliocentric evolution as well as the formation of beam components and anisotropic cores remain unanswered. We propose a simple solution for the generation of beam components, as a result of physical trapping due to obliquely propagating Alfvén modes, based on a previously reported dynamical system describing the wave interaction with a single ion in the absence of dissipation mechanisms. We have found that beams with significant densities (7%–8%) and with velocities of the order of the Alfvén speed can be generated. The beams' properties, including density, velocity, and temperature, are dependent on the wave propagation angle. The results are in qualitative agreement with the observations and could provide an explanation for the evolution of nonthermal distribution functions observed in fast solar wind and other space plasmas regimes.