Deceleration of streaming alpha particles interacting with waves and imbedded rotational discontinuities

Abstract

Alphas (He42+) and other ions in the interplanetary medium show a tendency to stream near or below the local Alfvén speed (VA) relative to the main component of protons. Because VA decreases with increasing distance from the Sun, forces must exist to slow the heavier ions with increasing distance. We have conducted hybrid simulations in a plasma with particle protons and alphas and with a quasineutralizing electron fluid. Simulation runs with other streaming minor ions were also performed. In our simulations, a group of Alfvén waves steepen and generate imbedded rotational discontinuities (RDs) and compressional waves. We examine cases of almost steady waveforms and RDs and ones with evolving waveforms and RDs due to a significantly nonuniform background. When alphas stream with the waves and imbedded RDs faster than the protons, they decelerate more rapidly from higher speeds and heat. We have concluded that alphas do not remain at one streaming speed due to nonlinear Lorentz forces from the wave compressional component and the presence of collisionless dissipation, which dissipates this component so that bulk alpha kinetic energy is ultimately deposited into alpha thermal energy. Imbedded RDs play no significant role in the overall deceleration of alphas. Protons heat similarly to cases without alphas and are slightly accelerated so that the total ion momentum along B0 is nearly conserved. For small streaming speeds (≲0.5 Alfvén speeds), the deceleration rate can be relatively small because the loss of streaming energy competes with the gain in wave kinetic energy required by large‐amplitude Alfvén waves. Less proton and more alpha heating is also found since alphas can resonate with the left‐handed portion of oblique waves. Starting from rest, alphas and protons can develop a small differential flow in which Lorentz and pressure forces become balanced. The simulation behavior of alphas for streaming speeds near the Alfvén speed is fairly consistent with solar wind observations in high‐speed streams. Turbulent Alfvénic fluctuations do have a small compressional component and so might be responsible for the observed deceleration and heating of alphas. Simulations with other streaming minor ions also gave deceleration, suggesting that the behavior of a wide range of solar wind minor ions might be explained by the same processes that affect alphas.