NARROW CYCLOTRON HARMONIC ABSORPTION RESONANCES OF STIMULATED ELECTROMAGNETIC EMISSION IN THE IONOSPHERE

We present measurements of stimulated electromagnetic emission (SEE) excited in the ionosphere by a powerful high-frequency ordinary mode radio wave vertically injected from a ground-based transmitter, at frequencies f0 near high harmonics of the ionospheric electron cyclotron frequency f(c). A prominent lower-sideband SEE feature was found to be absent in an extremely narrow f0 range of DELTA-f0/f0 almost-equal-to 2 x 10(-5). This absorption resonance width allows an estimate of f(c) with an accuracy of a few tens of Hz, corresponding to a magnetic field magnitude accuracy of about 1 nT.