Some studies of geomagnetic micropulsations

Oscillations of the geomagnetic field with periods between 1 second and a few minutes have been studied by means of large ground loops at Hobart, Adelaide, Camden, and Townsville, Australia. Records have been made both on charts and on slowly moving magnetic tape; the tape recordings have been analyzed for frequency components on a ‘Sonagraph’ sound spectroscope. The period of the continuous oscillations known as PC's shows a diurnal variation and a marked ependence on latitude, shorter periods occurring at lower latitudes. The damped trains of oscillations, occurring predominantly around local midnight, known as Pt's are followed by magnetic bays with an average delay of about 15 minutes. During storms, ‘sweepers’—oscillations with a progressive change of frequency and sometimes with aharmonic overtones—are observed.