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D. L. Huestis, P. C. Cosby, T. G. Slanger (SRI International), D. E. Osterbrock, R. Waters (U. C. Observatories, Lick Observatory)
The unsurpassed resolution and sensitivity of the HIRES spectrograph at the 10-m Keck I telescope creates new opportunities for investigation of the emissions of the Earth's atmosphere, providing unanticipated benefits from data collected incidentally by astronomers. In return, aeronomers can assist the astronomy community in disentangling terrestrial and extraterrestrial sources.
Here we report recent analysis of atomic emissions: H(4340, 4861, 6563), N(5198, 5200), O(5577, 6300, 6364, 7774, 8446), Na(5890, 5896), K(7699), and Hg(4047, 4358, 5461) (all wavelengths in {Å}). For the nitrogen lines the Keck/HIRES wavelengths are the best available, and differ from published values by 0.02 {Å}. The stronger emissions are well known, with intensities that typically range from about 1~R (Rayleigh) for N, to 2~R for H(6563), 60~R for Na, 100~R for O(6300, 6364), and 200~R for O(5577). The weaker lines are still conspicuous in the Keck/HIRES spectra: H(4340) about 50~mR, O(8446) 150~mR, Hg 200~mR, H(4861) 250~mR, O(7774) 350~mR, and K 1.5~R.
Previous aeronomy studies suggest production mechanisms, characteristic altitudes, and time-of-night dependences for the atomic emissions. Some, e.g. O(5577), Na, and K, are produced by chemical processes near the mesopause (95 km) that are relatively constant during the night. Others, N, O(6300, 6364, 7774, 8446), are produced by electron-ion recombination in the ionosphere (120-400 km), with intensities that decrease rapidly after twilight. The hydrogen emissions are attributed to solar-excited fluorescence in the geocorona. The mercury emissions are probably scattered city light.
Supported by NSF Astronomy and Aeronomy and NASA Sun-Earth Connection. The W. M. Keck Observatory is operated by the California Institute of Technology and the University of California.
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