Pi 1–2 magnetic field pulsations on dayside cleft field lines

Abstract

TYPE Pi 1–2 magnetic field pulsations, T = 1–150 s, have a broad-band, noise-like character and a tendency to occur during active aurorae and magnetospheric substorms^1,2. It is well-known that type Pi 1–2 magnetic field pulsations observed in the night–morning sector during magnetospheric substorms maximise in amplitudes along the auroral oval^1,2. But the relation of the Pi 1–2 activity to the day sector of the auroral oval has not been clear. This relation is of particular interest because the day sector of the auroral oval seems to coincide approximately with the feet of the dayside cleft field lines; the precise relation between the dayside cleft and the dayside auroral oval, however, has not yet been established (S. I. Akasofu, personal communication). Dayside cleft field lines usually terminate between the geomagnetic latitudes of 75° and 80°. We show here that intense Pi 1–2 activity appeared at College on two occasions when the cleft came down to the College latitude, 64.7° N, at times when College was in the midday sector. Further, it is shown that an intense flux of particle precipitation into the dayside ionosphere occurred on the second occasion, 4 August 1972.