Cyclotron-resonant diffusion regulating the core and beam of solar wind proton distributions

Abstract

Ion diffusion as predicted by quasi‐linear theory has been compared with in‐situ solar wind proton measurements. It is found that the observed phase‐space‐density contours match very well those corresponding to the time‐asymptotic plateau generated by proton diffusion in cyclotron‐wave resonance. Observations show that the perpendicular temperature of the beam distribution is of the same order as its parallel one. A perpendicular heating mechanism is needed to balance the radial tendency for adiabatic cooling. Outward and inward propagating cyclotron waves may together be able to control the thermal anisotropy of the core distribution. However, there are hardly any cyclotron waves, which could resonate with a proton beam having a drift velocity equal to or greater than the Alfvén speed. Therefore, we consider also outward‐propagating waves, with both left and right hand polarization, on a second dispersion branch existing in a cold plasma with electrons, protons and alpha particles. These waves can resonate with the beam protons. The resulting diffusion can indeed explain the shape of the beam distribution. A time‐dependent kinetic code, in two‐dimensional velocity space, has been developed to integrate the quasi‐linear diffusion equation. An initial shuttle‐like distribution function is shown to develop into a distribution having a core and a beam. The beam is found to drift at the Alfvén speed and be less anisotropic than the core. The radial evolution of the beam density in the model is found to be consistent with the observations.