Ionograms produced by the Alouette I topside sounder frequently show well-developed series of cyclotron harmonics. Their frequencies have been determined from A (amplitude) scans with an accuracy of ~0.02 Mc/s for the sweep range of 1 to 6 Mc/s. In all cases examined, the frequencies of all of the members of the harmonic series are, within the experimental accuracy, integral multiples of the cyclotron frequency derived from the best present estimates of the earth's magnetic field strength at the satellite height. This result is discussed in the light of recent laboratory results and theoretical studies.
