Relations between geomagnetic micropulsations and magnetotail field changes

The magnetic field in the tail region observed by the Imp 1 satellite was compared with geomagnetic-micropulsation activities detected by ground-based observations at Ralston, Alberta, Canada. The tail field strength was found to decrease, typically by one to a few gammas, in simultaneity with the onset of a Pi 2 micropulsation activity detected on the ground. Several tens of such examples were obtained during continuous rapid-run micropulsation observations for about a week. During the same time interval, four Pc 3 micropulsation events took place with clearly identifiable onsets—an unusual feature for Pc 3. The tail field followed a characteristic pattern of evolution in each of the four events. In a period of about one-half hour to one and one-half hours before a Pc 3 onset the tail field strength gradually built up, and with the onset it abruptly decreased by several gammas. Whereas a sudden onset-time decrease in the tail field strength is common to Pi 2 and Pc 3, Pc 3 activity is found to diminish in sympathy with a gradual tail field decrease over a time of about twenty minutes or more, provided that the tail field decrease exceeds about ten gammas.