Hydromagnetic Emissions at the Geomagnetic Poles

Abstract

MAGNETIC micro-pulsations of regular appearance in the frequency range 0.2–5 c/s have been studied intensively in recent years. Some of the very regular events show an envelope on an amplitude–time trace that resembles a string of pearls. They have been variously called pearls, Pc 1, hydromagnetic whistlers or hydromagnetic emissions. If these micropulsations are recorded on slowly moving magnetic tape and spectrally analysed with a ‘Sonagraph’ or ‘Rayspan’, using the speed-up technique, the emissions often display distinct structural elements. There are usually repeated series of individual emissions the frequencies of which appear to rise or fall rapidly with time as those shown in the frequency–amplitude–time displays (sonagrams) of Fig. 1.