JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 89, NO. A7, PAGES 5660-5664,JULY 1, 1984 Spectroscopy ofthe EUV(350-1400 Nightglow SUPRIYA CHAKRABARTI, RANDYKIMBLE AND STUART BOWYER Space Sciences Laboratory, University of CaliJbrnia, Berkeley We preõent time averaged spectra of the extreme ultraviolet nightglow in the 350 to1400-• range at 8-A resolution. The uplooking spectrum is dominated by geocoronal and interplanetary resonantly scattered solar lines (HI 1216, HI 1026, OII 834, and He I 584 •). The downlooking spectrum contains OI emission features from the nighttime ionosphere, including several previously unobserved features. In contrastto previously reported observations, no significant N 2 Lyman- Birge-Hopfield emissions are evident in our data,and a 2-o-upper limit of 135 R canbe placed on this radiation for the time of our observations. Observations of the nightglow emission in the extreme ultraviolet can add to our knowledge regarding several important questions, such as the maintenance of the nighttime ionosphere and the morphology and dynamics of the day and night ionosphere. However, few observa- tions of these emissions have been made, and even these have generallybeen made with rather low spectral resolu- tion. In addition to the strong resonantly scattered solar lines of hydrogen and helium (HI 1216, HI 1026, He I 584, and He II 304 •), a few extreme ultraviolet (EUV) nightglow features have been heretoforeidentified. The firsts•ectroscopic observations of the nightglow below 1000 A were made by Kumar et al. [1974] with a 40-/• spectral resolution sounding rocket instrument operatin•g from 600 to 1200•. These authors detected OII 834 A emission, which they attributed to conjugate enhancement. The only other sub-1000 • feature which has been tenta- tively identified is the recombinantion continuum of OI at 911 •, which was suggested by the Naval Research Laborator• group as the source of the signal in their 800 to 1050-A photometer on STP 72-1 [Anderson et al., 19761. Longward of 1000 /•, ionospheric emissions of OI at 1304and 1356/• have been observed at 20-/• resolution with OGO 4 [Gbrardet aL, 1977, and references therein]. In addition, as large as 1 kR of total N2 Lyman-Birge- Hopfield (LBH) band emission has been reported by Huffman etal. [1980], using a 1 to 25-• resolution spec- trometer on board the S3-4 satellite. This surprisingly high N2 intensity has generated considerable interest. Meier and Conway [1983] have folded a synthetic N2 LBH spectrum through the S3-4 instrument's response and have concluded that the observed spectra are indeed con- sistent with N2 LBH emission, but none of several excita- tion mechanisms they considered should explain the reported intensity. Meier and Conway also reexamined STP 72-1 photometer data and detected LBH emission in the northern summer hemisphere having a peak intensity of several hundred Rayleighs. In contrast, Paresce [1981] reported observations made in 1975 looking downward from 215 km which yielded an intensity of only 2 R in the 1350 to 1700-•range. Copyright1984 by the American Geophysical Union. Paper number 4A0564. 0148-0227/84/004A-0564502.00 We report here on observations of nightglow emission in the 300 to 1400-• band with 8-• resolution from the STP 78-1 satellite. These data provide confirmation of some previously suggested identifications, permit the detection of several new features, and present new data on the LBH question. Detailed discussions of the morphol- ogy and theoreticalmodeling of the emissions will be presented in later publications. The spectrometer, describedin detail by Bowyetet al. [1981], consists, essentially, of a 0.5-mm-wide rectangular entrance slit, a concavereflection grating and two position sensitive EUV detectors, both lying on the Rowland cylinder. The insidefirst-orderspectrum is observed, with a reciprocal dispersion of 10.4 •/mm and a resolution of 8 •. About 650 • of the first-order spectrum can beviewed simultaneously. The combination of the slit height and grating diameter with the spin and telemetry rate of the spacecraft providesa triangular working field of view of full width 18 ø by 9ø . The spectrometer was calibrated in the laboratory before flight and was found to have a peak sensitivity of about 10 -• c s -• R -• at 550 •. The absolute accuracy of the calibration is estimated to be _+ 20%. The satellite was placed in a 600-km-altitude circular orbit, with an inclination of 97.7 ø and an orbital period of 96 min. The orbit is sun synchronous, precessing at 1 ø per day with the spacecraft orbit lying essentially in the noon- midnight plane. The spectrometer is housed in the spin- ning wheel of the spacecraft with the spin axis of the wheel perpendicularto the orbital plane. The spectrome- ter line of sight is oriented 120 ø from the spin axis and, as the wheel rotates at 11 rpm, the instrument sweeps out a cone, alternately viewing earth and space, never looking closer than 30 ø to the sun. With this instrument, the average 300 to 1400-• spec- tra of the earth's nightglow at 600 km on the midnight meridian between +_40 magnetic latitude looking down (zenith anglesbetween 120 and 150 ø ) and lookingup (zenith angles between 30 and 80 ø ) have been obtained. Because the EUV spectrometer cannot measure simultane- ously thecomplete 300to 1400-• range, data from two different time periods have been combined. The first part of each spectrum (300-850 •) was obtained byaveraging together all data satisfying the appropriateviewing con- straints in selectedperiods between April 7 and April 9, 1979. A total of 27,250 s of downlooking data and 40,800 s of uplookingdata were included. The secondpart of each spectrum (800-1400 •) was obtained by averaging 5660