Ion Beam Instabilities during Solar Flare Energy Release

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 ³He 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_∥/T_⊥=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 (Ω_cp) and a left-handed, lower frequency mode.  We also find highly oblique modes with frequencies below Ω_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 Alfvén wave, will preferentially heat ³He, making it a good candidate for the driver of the abundance enhancements commonly observed for this species in impulsive events.