Measurements of the intensity and spectrum of electrons at 1000-kilometer altitude and high latitudes

Abstract

The intensity and spectrum of electrons at 1000-km altitude were measured by detectors on the SUI satellite Injun I at magnetic latitudes between about 45° and 80°. A preliminary report is given here for the period July and August 1961; further observations are continuing. Trapped electrons with energy above 40 kev were found between magnetic latitudes 45° and about 75° in typical intensities of 105 to 106 particles (cm2 sec ster)–1, with peak intensity measured at 4×106 particles (cm2 sec ster)−1. The intensity above 75° was less than 102 particles (cm2 sec ster)–1. Occasionally the electron intensity reached a broad peak around 50° and a second peak near the auroral zone. The intensity in the second region often varied by an order of magnitude in times of a few seconds or distances of a few tens of kilometers. If we assumed that the differential electron spectrum followed a power law of the usual form dN = constant E−γ dE, then, for both trapped and dumped electrons in the energy range 40 to 120 kev, γ varied significantly between 0 and 2 in the low-latitude region below 60°, and between 0 and 7 at latitudes above 60°. The spectrum was generally softer at higher latitudes. On several occasions in the daytime, electrons with energy above 1 kev were observed in fluxes of the order of 10 ergs (cm2 sec ster)−1 at such angles to the magnetic field vector that they must have penetrated to auroral altitudes of 200±200 km. Since they could produce weak auroras, it is concluded that auroras can occur in the daytime. Since they were detected at 1000-km altitude, it is concluded that an acceleration mechanism in the ionosphere is not always necessary to produce an aurora in the same hemisphere. The measurements are compared with other satellite, rocket, and balloon measurements. It is tentatively concluded that most of the counts of the type 302 shielded Geiger tube in Explorer VII were due to penetrating particles, very likely electrons with energy of several Mev in peak intensity of order 103 particles (cm2 sec)−1. The significance of the latitude distribution of the low-energy electron fluxes is discussed.