Observations of Twilight and Sunlit Aurora

The 3914-Å band of [Formula: see text] has been observed in the zenith at twilight by a photoelectric spectrometer; simultaneous measurements of the 5577-Å line were made as a check on auroral activity. A small twilight enhancement was probably observed in the absence of aurora in 1960, but it could not be detected in 1961. After subtraction of this background, the auroral measurements showed [Formula: see text] densities of about 1000 ions/cm3 at 160 km, with a sharp decrease at lower altitudes; some displays, presumably located near 100 km, showed no twilight effect at all. It is suggested that [Formula: see text] ions are rapidly removed in the 100-km region by either charge–exchange or ion–atom interchange reactions which convert them into NO+ or some other ion. The ions in the twilight airglow are therefore probably located at 550 to 620 km, being produced there by solar extreme ultraviolet. However, this process requires a much higher N2 concentration at such heights than is normally supposed. This may be produced at times of high solar activity, and especially during magnetic storms.