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P.C. Cosby, D.L. Huestis, T.G. Slanger (SRI International)
Over the last three years, detailed studies of the sky spectra from the W. M. Keck (I) 10-meter telescope and the HIRES echelle spectrometer have been carried out. Many atomic and molecular emissions have been observed in the nightglow for the first time. For O2 in particular, an entire new range of molecular bands in the Atmospheric Band system has been found, following initial identifications by D.E. Osterbrock and collaborators. At the beginning co-added spectra were used, so as to optimize the identification of weak spectral features [Osterbrock et al. PASP, 110, 1499 (1998)]. We are now using individual 50-minute spectra, making it possible to use time as a variable.
Of spectral lines normally associated with light pollution, by far the most prominent in Keck/HIRES sky spectra are those of neon, of which 36 have so far been identified, in the spectral range 5800-8100 Å. They exhibit a great deal of variability with time, but during a given night the variation is not random. At the most intense the summed intensity of the neon lines is on the order of 30 rayleighs (R), with the strong 5852 Åline having an intensity of ~ 2 R. At other times, the 5852 Åline is weaker by a factor of 50. The mercury line at 5461 Åis typically weaker than the Ne [5852] line, and at its maximum does not exceed 1 R.
Particularly noteworthy is the fact that the neon and mercury lines are not correlated in time, as would be expected if they originate with a city source. Urban mercury radiation normally outshines that from neon by two orders of magnitude (cf. Lick Observatory observations). The highest neon intensities are observed when the telescope is pointed in a southerly direction, and no significant neon emission is observed at the azimuth of the closest city, Hilo. Approximately one-hundred 50-minute sky spectra, taken in 1993-1997, have been evaluated for this study. Continuing analysis reveals that argon and xenon lines also appear in some spectra; noteworthy is the fact that emission from the three atoms is uncorrelated in time.
This work has been supported by the NSF Aeronomy program.