Stimulated electromagnetic emissions during pump frequency sweep through fourth electron cyclotron harmonic

Abstract

The frequency of a high-power HF radio wave incident on the ionosphere was swept, using a computer-controlled transmitter signal, in <10 s within a 60-kHz-wide frequency band approximately centered on the fourth harmonic of the electron cyclotron frequency. Measurements of the spectral behavior of stimulated electromagnetic emissions (SEE) across this harmonic after preconditioning could thereby be made with unprecedented resolution, speed, and ionospheric stability. Comparison of local electron cyclotron frequency estimations based on the experimental data reveals discrepancies between certain downshifted maximum models and the empirical broad upshifted maximum (BUM) feature formula ΔfBUM =f0 -nfcc. Weak emissions related to the BUM were discovered below the nominal BUM cutoff frequency. Finally, we observed that the intensity of certain SEE components differed depending on the whether the pump frequency sweep was ascending or descending. Copyright 2002 by the American Geophysical Union.