World-Wide Characteristics of Geomagnetic Micropulsations

A detailed investigation is carried out on geomagnetic micropulsations recorded by rapid-run magnetograms during the International Geophysical Year from 17 observatories as widely distributed geographically as possible. A number of well-defined cases (14 Pt's and 3 Pc's ) are investigated in detail. Diurnal and world-wide characteristics are derived and the equivalent overhead current systems which may give rise to the micropulsations are constructed. Many of the examples of Pt's can be divided into two groups: viz. Pt− which is a Pt associated with a negative bay in the auroral zone and Pt+ which is a Pt either associated with a positive bay or one which appears with no bay in the auroral zone. Although both groups of Pt's, with their associated bays, have different characteristics, there are many points of similarity. Both classes have more or less coherent, synchronous wave forms over a wide area, i.e. they have the same period at different stations, where they are either nearly in or out of phase. It is found that Pc's consist of two different wave bands, both occurring during the daylight hours. The first band contains shorter period Pc's (15-30s) with maximum amplitudes of a few gammas in moderate latitudes. They are not synchronous over wide areas. The second band contains longer period Pc's (30-90s) with maximum amplitudes in the polar regions, amounting, on occasions, to as much as a few tens of gammas. They are well synchronized round the world. Attention is drawn to the occurrence of long period, continuous, pulsations (called LPc's) which have a longer period than Pc's and usually appear simultaneously with Pc's in polar regions. The characteristics of micropulsations found in this investigation may throw some light on their origin. In this regard, a tentative discussion is given of hydromagnetic oscillations in the outer atmosphere which are excited by solar corpuscular streams.