Deceleration of relative streaming between proton components among nonlinear low‐frequency Alfvén waves

Abstract

Proton distributions in fast solar winds often have a beam component with a differential streaming speed near the local Alfvén speed. The Alfvén speed and differential streaming speed decrease with increasing distance from the Sun. Thus the beam decelerates, especially within 1 AU where β (which is ratio of plasma to magnetic pressure) can be significantly smaller than unity. We present 2 1/2‐dimensional hybrid numerical simulation results of the evolution of particle proton components streaming relative to each other for moderate relative beam densities (up to 50%) for initially isotropic distributions with mostly equal beam and main proton temperatures and small plasma β(=0.2). Electrons are treated as a fluid. We consider cases without and with initial nonlinear low‐frequency (nearly dispersionless) shear Alfvén waves propagating in the direction of the beam. Without initial waves, a strong linear beam instability can occur for streaming speeds above the Alfvén speed generating oblique proton‐proton cyclotron waves through both cyclotron and Landau resonances. The initial beam speed can decelerate and saturate at speeds below the Alfvén speed. When nonlinear Alfvén waves are included in simulations, we find that the deceleration rates are enhanced. Deceleration is especially strong for initial super‐Alfvénic speeds where we interpret the results with initial waves to be due to a wave amplification of the linear beam instability.