Two subtypes of type Pi micropulsations

Two Pi subtypes were recognized in a study of three years of continuous magnetic tape and chart recordings made at College, Alaska. Impulsive broadband bursts occurring near local geomagnetic midnight are associated with impulsive ionospheric cosmic noise absorption events, and thus with impulsive bursts of charged particles, which seem to come down from the tail region of the magnetosphere. Another Pi subtype is nonimpulsive, narrower in frequency range, and seems to be generated mainly within the auroral electrojets. Simple events of the latter subtype were inaugurated by at least one impulsive Pi burst. ‘Elementary’ Pi storms thus have two distinct components.