This letter addresses in detail the structure of the gap near the helium cyclotron frequency observed within the statistical distribution of the frequencies of electromagnetic cyclotron waves encountered at synchronous altitudes. It is shown that, at most, two-thirds of the gap (the high frequency portion) results from linear dispersion effects. It is suggested that at least one-third of the gap (the low frequency portion) results from nonlinear, off-resonant absorption of the waves by means of near cyclotron resonance with singly charged helium ions.
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