The activity index of Pc1 geomagnetic pulsations and its use in geophysical studies

An index N describing the occurrence frequency of Pc1 geomagnetic pulsation s ("pearls") is proposed for Pc1 data analysis. We chose a 15-min time inte rval, in the course of which these pulsations were observed during a period of not less than 5 min as a unit of measurement of the Pc1 activity; the N index can be represented as a daily sum of such intervals. The N index is developed on the basis of continuous observations of Pc1 pulsations at Boro k observatory (Phi = 54.05 degrees N, Lambda = 113.44 degrees E) performed from 1957 through 1992. Results of studies of the variations in the Pc1 pul sation activity with season and solar cycle, as well as the Pc1 relation to magnetic storms and the sector structure of the interplanetary magnetic fi eld, are presented as an illustration of an application of the N index.