Observations of POGO Ion Chamber Experiment in the outer radiation zone

The ion chamber on OGO-C, D (POGO) measures the total ionization produced by radiation able to penetrate 0.2 g/cm² of iron (Ep > 10 Mev, Ee > 1 Mev). The boundary of the outer radiation zone is observed at invariant latitude of 67°–70° at 423–1525 km altitude, and on at least 34% of passes a narrow spike of intensity is observed a few degrees below the boundary. The spikes are distributed uniformly in local time, but correlate strongly with Kp. Spikes above the boundary occur preferentially on the dawn side. When perigee fell in the outer zone, the point conjugate to the observation point was below 100-km altitude at longitudes 300°–50° E. During the period of March 22 through April 1, 1966, the ionization was very high at such points. We interpret this as being due to intense injection of 1–10 Mev electrons at Λ = 57°–62°, and we deduce that this is different from the lower injection that repopulates the outer zone in about half a longitudinal drift period.