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

Дата и время публикации : 2013-06-11T14:08:35Z

Авторы публикации и институты :
Daniel Verscharen
Sofiane Bourouaine
Benjamin D. G. Chandran
Bennett A. Maruca

Ссылка на журнал-издание: Astrophys. J. 773, 8, 2013
Коментарии к cтатье: 8 pages, 7 figures
Первичная категория: physics.space-ph

Все категории : physics.space-ph, astro-ph.SR, physics.plasm-ph

Краткий обзор статьи: We investigate the conditions under which parallel-propagating Alfv’en/ion-cyclotron waves are driven unstable by an isotropic ($T_{perp alpha} = T_{parallelalpha}$) 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 $beta_{rm p} gtrsim 1$, where $beta_{rm p}$ is the ratio of the proton pressure to the magnetic pressure. For $1lesssim beta_{mathrm p}lesssim 12$, 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 Alfv’en 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 Alfv’enic 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 $T_{perp alpha}approx T_{parallelalpha}$ are approximately bounded from above by the threshold of the parallel Alfv’enic instability.

Category: Physics