HF chirps: Eigenmode trapping in density depletions

Short‐duration, narrow‐bandwidth, HF chirps occur at slowly drifting frequencies above fpe when fpe > fce [McAdams and LaBelle, 1999]. We put forth a model of the HF chirp emissions as quasi‐trapped eigenmodes in a density depletion perpendicular to the ambient magnetic field. The escaping waves retain the frequency structure of the eigenmodes and are observed as HF chirps. We show that this model is quantitatively consistent with observed characteristics of HF chirps, most notably the frequency spacing of the chirp emissions (0.1–4 kHz), the fact that they are equally spaced independent of mode number, and the number of modes predicted for a given density cavity. Several mechanisms for the escape of the waves from the cavity are suggested.