Ion Beam Instabilities during Solar Flare Energy Release

TL;DR

This study investigates ion beam instabilities during solar flares using linear plasma theory, revealing unstable wave modes that influence particle scattering and heating, especially of $^3$He, affecting abundance enhancements.

Contribution

It introduces a detailed analysis of ion beam-driven wave instabilities during solar flares using the ALPS solver, highlighting their role in particle scattering and element enrichment.

Findings

Identified two unstable parallel-propagating modes near the proton cyclotron frequency.

Discovered highly oblique, lower-frequency unstable modes at higher beam energies.

Found that these modes contribute to proton scattering and preferential heating of $^3$He.

Abstract

The linear stability of waves driven by ion beams produced during solar flare energy release are explored to assess their role in driving abundance enhancements in minority species such as $^3$He and in controlling, through pitch-angle scattering, proton/alpha confinement during energy release. The Arbitrary Linear Plasma Solver (ALPS) is used to solve the linear dispersion relation for a population of energetic, reconnection-accelerated protons streaming into a cold background plasma. We assume equal densities of the two populations, using an anisotropic ($T_\parallel/T_\perp = 10$), one-sided kappa distribution for the energetic streaming population and a cold Maxwellian for the background. We find two unstable modes with parallel propagation: a right-handed wave with a frequency of the order of the proton cyclotron frequency ($\Omega_{cp}$) and a left-handed, lower frequency mode. We also find highly oblique modes with frequencies below $\Omega_{cp}$ that are unstable for higher beam energies. Through resonant interactions, all three modes will contribute to the scattering of the high-energy protons, thereby limiting their transport out of the flare-acceleration region. The higher-frequency oblique mode, which can be characterized as a kinetic Alfv\'en wave, will preferentially heat $^3$He, making it a good candidate for the driver of the abundance enhancements commonly observed for this species in impulsive events.