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 fo 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 fo range of 4/o//o~ 2x 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.