Whistler-Triggered Periodic Very-Low-Frequency Emissions

From new experimental data on the relationship between whistlers and periodic VLF emissions (VLF ionospheric noise) it is found that both the dispersive and nondispersive types of periodic noise are of common origin. In the cases examined, a sequence of emissions is initiated, or ‘triggered,’ by a whistler, and the period between emissions is the same as the whistler-mode echoing period at some frequency within the range of the emission. To explain these results it is proposed that the generation, or triggering, of the emissions is controlled by packets of electromagnetic waves echoing in the whistler mode. In this triggering hypothesis it is supposed that the wave packets act to organize temporarily the particles in existing streams of charge so that their radiation is coherent. The resultant temporary increase in total radiation is seen on the ground as a short burst of noise. An earlier explanation of non-dispersive periodic noise based on a postulated small bunch of particles that oscillates between its mirror points in the earth's magnetic field is found to be incompatible with the data.