Diffusion plateaus in the velocity distributions of fast solar wind protons

In a collisionless plasma, such as the fast solar wind, wave‐particle interactions play the decisive role in determining the shape of particle velocity distribution functions (VDFs). In this paper we provide observational evidence for cyclotron‐resonant interaction of ion cyclotron Alfvén waves, which propagate outward from the Sun along the interplanetary magnetic field, with fast solar wind protons. According to quasi‐linear theory, the protons thereby diffuse in velocity space, a process leading to the formation of plateaus in the VDF. This diffusion plateau formation naturally explains the observed thermal anisotropies in the core of the proton VDFs. In this respect, we investigated a large number of data from several distinct fast solar wind streams between 0.3 and 1 AU. All measurements were made on Helios 2 during the solar minimum in 1976 and 1977. The proton VDFs as provided by the plasma instrument are modeled by a superposition of multiple Gaussians, such that the plasma dispersion relation for parallel propagating cyclotron waves can readily be solved numerically. Thus the details of the proton VDFs are well represented and are reflected in the dispersion relation, which makes our analysis as self‐consistent as possible. Proton thermal effects on the wave dispersion relation are naturally taken into account.