Abstract
The extreme ultraviolet telescope on the Apollo-Soyuz mission observed the equatorial aurora from an altitude of 220 km on four separate occasions in July 1975, in quiet geomagnetic conditions (Ap = 6). In all cases signals well above ambient background in the 50–150, 114–150, 170–600, and 500–780 Å bands were recorded as the spacecraft moved across the equator and the instrument viewed the atmosphere below it. The observed emissions are confined to a band roughly 10 to 20° in width with the peak emission occuring in the range ≃−15° to +2° magnetic latitude. No enhancement on the 1350–1550 Å channel was noted. The observed signals are interpreted as recombination radiation of energetic helium and, possibly, oxygen ions originating in the terrestrial ring current.
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