Instabilities Driven by the Drift and Temperature Anisotropy of Alpha Particles in the Solar Wind

TL;DR

This paper derives and validates analytical thresholds for instabilities driven by alpha particle drift and temperature anisotropy in the solar wind, highlighting how these factors lower the conditions needed for wave excitation.

Contribution

It provides new analytical expressions for instability thresholds considering alpha particle properties, validated with numerical solutions, enhancing understanding of solar wind plasma behavior.

Findings

Temperature anisotropy lowers the alpha-beam speed needed for instability.

Differential flow reduces the required temperature anisotropy for wave excitation.

Analytical thresholds agree with numerical dispersion relation solutions.

Abstract

We investigate the conditions under which parallel-propagating Alfv\'en/ion-cyclotron (A/IC) waves and fast-magnetosonic/whistler (FM/W) waves are driven unstable by the differential flow and temperature anisotropy of alpha particles in the solar wind. We focus on the limit in which $w_{\parallel \alpha} \gtrsim 0.25 v_{\mathrm A}$, where $w_{\parallel \alpha} $ is the parallel alpha-particle thermal speed and $v_{\mathrm A}$ is the Alfv\'en speed. We derive analytic expressions for the instability thresholds of these waves, which show, e.g., how the minimum unstable alpha-particle beam speed depends upon $w_{\parallel \alpha}/v_{\mathrm A}$, the degree of alpha-particle temperature anisotropy, and the alpha-to-proton temperature ratio. We validate our analytical results using numerical solutions to the full hot-plasma dispersion relation. Consistent with previous work, we find that temperature anisotropy allows A/IC waves and FM/W waves to become unstable at significantly lower values of the alpha-particle beam speed $U_\alpha$ than in the isotropic-temperature case. Likewise, differential flow lowers the minimum temperature anisotropy needed to excite A/IC or FM/W waves relative to the case in which $U_\alpha =0$. We discuss the relevance of our results to alpha particles in the solar wind near 1 AU.