A mechanism for the production of certain types of very-low-frequency emissions

It has been shown that some types of VLF emissions (hooks, risers, and falling tones) may be the result of cyclotron emission by energetic electrons trapped in the earth's magnetic field. Observational evidence suggests that a relatively intense electromagnetic whistler-mode wave may be necessary to trigger each such emission. An interaction mechanism is proposed by which certain electrons may couple with a wave already propagating along the same line of force. A gyroresonance is established which organizes the electrons so that cyclotron radiation can be coherent. A change in phase relationship occurs for those electrons that have a longitudinal velocity component such that the Doppler-shifted frequency of the whistler wave seen by the electron is equal or nearly equal to the electron gyrofrequency. This condition is met by electrons moving in a direction opposite to the whistler wave and is satisfied only over a certain fraction of the line of force. Observations also show instances of a sudden change in slope of the rising tail of a hook or other VLF emission at the crossing of the spectrums of the emission and a whistler. This change is interpreted as the effect of the encounter of a whistler with the electron stream that changes both energy and pitch angle of the emitting electrons.