Very low frequency emission periods and whistler-mode group delays

VLF emissions that appear at regular intervals are called periodic emissions. Periodic emissions were first reported by Dinger [1957], who likened the sound of these emissions to squeaky wagon wheels, and spectrums of periodic emissions have been published by Galtet [1959], Pope and Campbell [1960], Brice [1962], and Helliwell [1963]. Spectrums of conjugate periodic emissions, in which the emissions were observed alternately in opposite hemispheres, were published by Lokken et al. [1961]. Pope and Campbell [1960] and Dowden [1962a, b] suggested that periodic emissions could be caused by mirroring bunches of particles, the emission period being the mirror period of the bunch.