Ionospheric Source for Low-Frequency Broadband Electromagnetic Signatures

Abstract

A linear eigenmode analysis shows that small-scale static electric field structures commonly found in the auroral ionosphere are natural generators of low-frequency ( $\ensuremath{\omega}\ensuremath{\le}{\ensuremath{\Omega}}_{i}$, where ${\ensuremath{\Omega}}_{i}$ is the ion cyclotron frequency) electromagnetic waves with broadbanded frequency spectra. These waves can account for the observed upward Poynting flux and distinctive polarization signatures in the ionosphere. The ratio of wave electric to magnetic field strength can be much larger than the Alfv\'en velocity. The local Doppler shift is important for facilitating resonant energization of ions and electrons.