Spectrum of the Extreme Ultraviolet Nightglow as Measured by Eurd

After 2 years of operation onboard the Spanish satellite MINISAT, EURD has achieved an unprecedented success in the observation of the terrestrial nightglow in the EUV, covering a range of ∼ 350 − 1100 Å. EURD has provided a total of more than 543 hours of integration in the long wavelength spectrometer and more than 898 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 them detected for the first time in the nightglow. These spectra represent an improvement in sensitivity of several orders of magnitude with respect to previous observations. It has been possible, for the first time, to identify the complete Lyman series of atomic hydrogen, resolving up to Lyman. It has also been possible to identify the helium Lyman-β line at 537 Å and to detect other lines of the blended Lyman series of helium, at 515 and 522 Å. The spectra clearly show the presence of the OII lines at 617, 644, 673, and 718 Å, previously observed in the dayglow but seen here for the first time in the nightglow. In addition to the recombination continuum of the atomic oxygen at 911 Å, two features of OI have been detected in the nightglow: the 3s′ Do transition at 989 Å, previously observed by Chakrabarti (1984) and the 2p34s3So transition at 1040 Å, partially overlapped with Lyman β, but clearly distinguishable from it. This feature has been seen for the first time in the terrestrial nightglow. The radiative recombination continuum of atomic oxygen at 911 Å, that was absent in the observations of the first year of operation of EURD, is now clearly visible. The reasons of the absence of the OI feature during the first year of operation are still unknown. Anderson et al. (1976) also noticed a strong variation with time of this 911 Å emission.