High‐energy electron spikes at high latitudes

Over 750 spikes of precipitating electrons with E ≥ 425 kev were observed aboard the low‐altitude polar orbiter Ogo 4 between July 30, 1967, and December 31, 1967. The spikes may be divided into three distinct populations depending on whether they occur at latitudes below, at, or above the local limit of trapping. These spikes are designated type 1, 2, and 3, respectively. Type 3 spikes occur in a narrow latitude band about 3° wide, centered at invariant latitude Λ ≈ 78° at 1000 MLT (magnetic local time) and 68° at 2000 MLT. Their relative frequency of occurrence, intensity, and hardness do not depend significantly on magnetic local time. Type 3 spikes appear to be associated with spikes observed near the magnetopause and the neutral sheet. Type 2 spikes also occur in a latitude band about 3° wide, centered at about 71° at 1000 MLT and 67° at 2200 MLT. Their frequency of occurrence is highly dependent on magnetic local time, a large maximum occurring near 2300 and very few events occurring between 0600 and 1200 MLT. Type 2 spikes appear to be related to island fluxes in the neutral sheet, although they occur on closed field lines and may persist for many hours. Type 1 spikes occur in a wider band of latitudes, from about 62° to 68° near midnight and 66° to 68° near noon. The local‐time dependence of their frequency of occurrence is similar to that of type 2 spikes but less pronounced. Although they are observed on closed field lines, type 1 spikes do not persist for periods longer than about 1 hour, and we conclude that they are produced by strong pitch‐angle scattering from the stably trapped population. The average spectral index (assuming a power‐law spectrum) is 3 to 5, and the median flux (>425 kev) is about 150 el cm −2 sec −1 ster −1 , although type 2 events near midnight tend to be larger and harder. All types tend to be more intense and to occur at lower latitudes when Kp is large.