A Parallel-Propagating Alfvénic Ion-Beam Instability in the High-Beta Solar Wind

Abstract

We investigate the conditions under which parallel-propagating Alfvén/ion-cyclotron waves are driven unstable by an isotropic ($T_{\perp \alpha }=T_{\parallel \alpha }$) population of alpha particles drifting parallel to the magnetic field at an average speed $U_{\alpha }$ with respect to the protons. We derive an approximate analytic condition for the minimum value of $U_{\alpha }$ needed to excite this instability and refine this result using numerical solutions to the hot-plasma dispersion relation. When the alpha-particle number density is $\simeq 5$ of the proton number density and the two species have similar thermal speeds, the instability requires that ${\beta }_{\mathrm{p}}\gtrsim 1$, where ${\beta }_{\mathrm{p}}$ is the ratio of the proton pressure to the magnetic pressure. For $1\lesssim {\beta }_{\mathrm{p}}\lesssim 12$, the minimum $U_{\alpha }$ needed to excite this instability ranges from $0.7v_{\mathrm{A}}$ to $0.9v_{\mathrm{A}}$, where $v_{\mathrm{A}}$ is the Alfvén speed. This threshold is smaller than the threshold of $\simeq 1.2v_{\mathrm{A}}$ for the parallel magnetosonic instability, which was previously thought to have the lowest threshold of the alpha-particle beam instabilities at ${\beta }_{\mathrm{p}}\gtrsim 0.5$. We discuss the role of the parallel Alfvénic drift instability for the evolution of the alpha-particle drift speed in the solar wind. We also analyze measurements from the \emph{Wind} spacecraft's Faraday cups and show that the $U_{\alpha }$ values measured in solar-wind streams with $T_{\perp \alpha }\approx T_{\parallel \alpha }$ are approximately bounded from above by the threshold of the parallel Alfvénic instability.

8 pages, 7 figures

References in corpus (1)

• The Dispersion Relations and Instability Thresholds of Oblique Plasma Modes in the Presence of an Ion Beam

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