Electrostatic broadband ELF wave emission by Alfvén wave breaking

Abstract

The linear and nonlinear kinetic properties of electrostatic oblique waves below the lower hybrid frequency are investigated. For propagation angles the waves are damped by either parallel electron Landau or ion cyclotron damping. For T i / T e ≫ 1 the waves are only weakly damped and can propagate. These waves are called slow ion cyclotron (SIC) and slow ion acoustic (SIA) Waves. A fluid‐kinetic model, comprised of hot linear kinetic ions and cold nonlinear fluid electrons, is proposed to describe a nonlinear wave breaking process of small‐scale Alfvén waves resulting in broadband extremely low‐frequency (ELF) wave emission. Numerical solutions of the fluid‐kinetic model are compared to the electric and magnetic fields of solitary kinetic Alfvén waves and broadband ELF waves observed by the Freja satellite within a hot ion environment. The agreement in waveform morphology and amplitude between the fluid‐kinetic simulations and the observed waves provides support for the theory that observed SIA waves are the result of a nonlinear emission process from SIC waves.