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T.G. Slanger, P.C. Cosby, D.L. Huestis (SRI International)
Sky spectra from the Keck telescopes have been used in recent years to obtain the best existing spectra of the terrestrial nightglow. Many new features have been discovered from data obtained in the 1993-1997 time period, which corresponds to solar minimum. Astronomers know that at solar maximum the terrestrial background often increases significantly, although this is rarely quantified.
New spectra obtained from Keck I/HIRES and Keck II/ESI in 2000 show that additional features appear in the night sky. These can be attributed to the unpredictable Appleton Equatorial Anomaly, or fountain effect, that on some nights near solar maximum greatly enhances the ionospheric electron density and atmospheric emissions at locations about 10-30 degrees from the magnetic equator. The new emissions include a large range of atomic oxygen lines from cascading Rydberg transitions, and molecular oxygen emission near 7700 Å which arises as a consequence of enhanced ionospheric O(1D) production. Here we report significant night-to-night and hour-to-hour variations in the relative intensities of night sky emission features.
Previous investigators, using instruments with inadequate sensitivity and resolution, have tended to attribute equatorial nightglow emissions to weak aurorae. We carried out a search of the enhanced spectra for evidence of the characteristic auroral emissions, specifically those from N2 and N2+. We are able to say that even at the high sensitivity of the HIRES and ESI systems, there are no traces of these emissions, which require higher energy electrons than are available in the nighttime equatorial ionosphere.
We would like to thank T. A. Barlow for providing us with the sky spectra. The data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and NASA. This work was supported by a grant from the NSF Aeronomy program.
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The author(s) of this abstract have provided an email address for comments about the abstract: tom.slanger@sri.com