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The fine structure of the auroral kilometric radiation is explained as a nonlinear interaction between the locally generated cyclotron emission created in an electron plasma cavity and the cavity boundary. Specifically, it is demonstrated that broadband cyclotron emission can form discrete tones after numerous interactions with an oscillating boundary that has quasi-monochromatic wavelike motion. This boundary will Doppler shift most radio photons in a stochastic way, creating a diffuse background radio component. However, some radio photons will have regular, stable frequencies that form discrete tones that overlie on the diffuse background emission. The discrete tones are created in situations where the photon travel time in the cavity is an integer multiple of the boundary oscillation period, this effect establishing a resonance between the boundary oscillations and photon. A set of coupled nonlinear equations is used to describe the radio photon/cavity boundary interaction. Effects of wave growth in the local density cavity and boundary reflections are also included. As the character of the cavity slowly changes, the resonance condition also changes in an adiabatic way, shifting the resonance to different frequencies. The net result is the creation of drifting radio tones which are comparable to those actually observed on radio spectrograms of the auroral kilometric radiation.