Telluric current micropulsations at the auroral zone

Abstract

This report presents analysis of individual events and statistical results derived from several years of continuous Pc 1 recording at College, using the telluric current technique. Histograms of occurrence distributions versus Kₚ and versus pulse period, together with sonagrams, justify dividing Pc 1 into two main classes. Class 1 activity has pulse periods mainly shorter than 3 sec, is usually structured, has a pronounced mid-day maximum in occurrence, not much seasonal variation in occurrence, and has little occurrence relationship to Kₚ except for a tendency to be absent when Kₚ is large. Class 2 Pc 1 activity has periods usually longer than 2 sec, is usually unstructured, has a very pronounced afternoon maximum in occurrence at College, a seasonal maximum in summer, and has a positive correlation with Kₚ. Class 1 activity may originate mainly in closed field line regions and Class 2 in open field line regions. Class 1 mid-frequencies tend to rise as Kₚ rises and tend to be higher at night than in the daytime. Average Pc 1 amplitudes maximize in daytime at College as do the occurrences. Thus there is an extreme daytime maximum in summed amplitudes, indicating that the Pc 1 incident energy has a pronounced daytime maximum at College. College sonagrams of Pc 1 events show a great variety of frequency-time forms. Rising frequency periodic structures are most common, but falling frequencies are often seen sometimes superimposed on rising events. At times narrow band “necklaces” are clearly seen. Long duration structured events were observed with durations up to 18 hours. The long events tended to occur only at times of rather low Kₚ.