Spectrum of the Extreme Ultraviolet Nightglow as Measured by EURD

After 2 years of operation onboard the Spanish satellite MINISAT, EURDhas achieved an unprecedented success in the observation of the terrestrialnightglow in the EUV, covering a range of ∼ 350-1100 A EURD hasprovided a total of more than 543 hours ofintegration in the long wavelength spectrometer and more than898 hours in the short wavelength one, allowing the achievement of the most detailed atlas of the terrestrial EUV nightglow ever obtained. We present here the spectra obtained, together with the identification of the lines, some of themdetected for the first time in the nightglow. These spectrarepresent an improvement in sensitivity of several orders of magnitude with respect to previous observations. It has beenpossible, for the first time, to identify the complete Lymanseries of atomic hydrogen, resolving up to Lyman-e. It hasalso been possible to identify the helium Lyman-β line at 537 A andto detect other lines of the blended Lyman series of helium, at 515 and522 A. The spectra clearly show the presenceof the OII lines at 617, 644, 673, and 718 A, previouslyobserved in the dayglow but seen here for the first time in thenightglow. In addition to the recombination continuum of the atomicoxygen at 911 A, two features of OI have been detected in thenightglow: the 3s′ 3Do transition at 989 A, previouslyobserved by Chakrabarti (1984) and the 2p34s3So transition at 1040 A, partially overlapped with Lyman β, but clearly distinguishable from it. This feature has been seen for the first timein the terrestrial nightglow. The radiative recombination continuum of atomic oxygen at 911 A, that wasabsent in the observations of the first year of operation of EURD,is now clearly visible. The reasons of the absence of the OIfeature during the first year of operation are still unknown.Anderson et al. (1976) also noticed a strong variation with time of this 911 A emission.