A new class of proton cyclotron echoes observed on topside sounder ionograms is presented; these echoes occur at frequencies above the electron plasma frequency and at the third and fourth harmonics of the electron cyclotron frequency, 3ƒH and 4ƒH. Some features are noted such as the dependence on low dip angle values. It is suggested that the 3ƒH and 4ƒH proton cyclotron echoes may be generated by wave-particle interaction between the electrostatic waves and the protons that are both energized by the dipole antenna on Alouette II, ISIS I, or ISIS II.
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