Fast Magnetosonic Waves Driven by Ring-like Proton Velocity Distributions
Abstract: Fast magnetosonic waves are enhanced waves at frequencies close to the proton cyclotron frequency and its harmonics (up to the lower hybrid frequency) observed near the geomagnetic equator in the terrestrial magnetosphere. They can pitch-angle scatter as well as energize radiation belt electrons. The waves arise from the ion Bernstein instability driven by ring-like proton velocity distributions with a positive slope with respect to the perpendicular velocity (∂f(v_perp)/∂v_perp>0). The unstable waves are essentially ion Bernstein waves but occur near the intersections of the cold-plasma dispersion relation for fast magnetosonic waves and the multiple dispersion branches of the ion Bernstein modes when the plasma is dominated by a cool background. Linear instability analyses and corresponding two-dimensional electromagnetic particle-in-cell (PIC) simulations are performed to study the ion Bernstein instability driven by various ring-like proton velocity distributions. The growth rate patterns are very different when the proton distribution varies from a ring to an isotropic shell. In addition, a ring distribution can simultaneously drive the Aflvén cyclotron instability and excite electromagnetic ion cyclotron (EMIC) waves. The PIC simulations revealed that, despite their generally smaller linear growth rates, EMIC waves saturate at higher levels than fast magnetosonic waves unless the proton distribution is sufficiently wide in pitch angle space and close to an isotropic shell. Moreover, EMIC waves mainly lead to pitch angle scattering of the protons while fast magnetosonic waves can cause significant energy scattering. The results also have implications for the pickup ions in the heliosphere.