An extremely intense electron flux at 1000-kilometer altitude in the auroral zone

The State University of Iowa radiation research satellite Injun I passed through an extremely large electron flux at 1000-km altitude and high latitudes around 2238 UT on September 25, 1961. The peak flux was 400 ergs (cm2 sec ster)−1 due to electrons with energy above 1 kev and 108 particles (cm2 sec ster)−1 with energy above 40 kev. The electron flux varied greatly both in intensity and in spectral character during the satellite pass. For example, the intensity changed by a factor of about a million in a time of about 30 seconds or a distance of several hundred kilometers. The electron spectrum changed so greatly and so rapidly that, if we assume a differential power-law spectrum applied, values of the exponent ranged between about 4 and more than 14. At this same time, the electron flux was measured with similar detectors on Explorer XII in the heart of the outer zone on the same magnetic shell but at a different longitude. Since the electron intensity and spectrum measured by Explorer XII were normal, the anomalous flux seen by Injun was confined in longitude. The Injun observations indicate that the flux was essentially isotropic over the upper hemisphere. The energy flux is sufficient to cause a very bright aurora, and the measurements are tabulated for comparison with auroral theories. The measurements prove that an acceleration mechanism in the ionosphere is not always necessary to produce an aurora.