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HF chirps: Eigenmode trapping in density depletions
Author(s) -
McAdams K. L.,
Ergun R. E.,
LaBelle J.
Publication year - 2000
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/1999gl003655
Subject(s) - chirp , physics , trapping , normal mode , perpendicular , computational physics , mode (computer interface) , atomic physics , magnetic field , optics , acoustics , laser , quantum mechanics , biology , geometry , ecology , vibration , mathematics , computer science , operating system
Short‐duration, narrow‐bandwidth, HF chirps occur at slowly drifting frequencies above f pe when f pe > f ce [ 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.