Physics
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufmsa33a0257h&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #SA33A-0257
Physics
0310 Airglow And Aurora, 0358 Thermosphere: Energy Deposition (3369), 3359 Radiative Processes
Scientific paper
777 and 845 nm emissions from the 3p-3s multiplets of atomic oxygen are commonly observed at non-auroral latitudes in the terrestrial nightglow. By studying the relative strengths of these emissions we can learn something about the mechanisms that produce them and what they can teach us about the atmosphere. Recently [1] we have used intensity-calibrated sky spectra from the Keck telescopes to investigate the relative strengths of a wide range of O-atom Rydberg lines and have confirmed that electron-ion radiative recombination is a primary source of excitation for both the triplet and quintet systems. Following the intensity of the 777 and 845 nm lines during the night, we find that for most of the night the quintet 777 nm line is consistently stronger than the triplet 845 nm line, with a nearly constant intensity ratio I(777)/I(845) near 2.3, although both intensities fall rapidly as the night progresses. However, late in the night the 845 nm intensity levels off, while the 777 nm intensity continues to fall, and the I(777)/I(845) ratio plunges by a factor of 5-10. We interpret these observations as indicating that the O-atom quintet states are still being excited by the same mechanism as earlier in the night, i.e. radiative recombination, but some triplet states are also being excited by an additional mechanism. Such a mechanism has been proposed before [2-6] but not previously observed directly in the terrestrial nightglow. The oxygen triplet 3d-2p transition at 102.576 nm is in close coincidence with the solar hydrogen Lyman-β line at 102.572 nm. Radiative transport in the hydrogen geocorona will deliver Lyman-β intensity into the Earth's shadow and will produce triplet O(3d 3D) high in the atmosphere, even prior to direct solar illumination. The result is observable in a radiative cascade sequence 3d-3p(1129 nm) → 3p- 3s(845 nm) → 3s-2p(130 nm). A similar effect is observed in the H-α emission, which is also excited by Lyman-β absorption. This process is an example of Bowen fluorescence [2]. Because the oxygen ground state is a triplet, only triplet transitions can be generated in this manner, excluding quintet 3p- 3s 777 nm production. Therefore, when the radiative recombination process dies away as charged particles are neutralized, Lyman-β pumping continues. We note that at low spectral resolution, the 777 nm multiplet is strongly contaminated by OH Meinel-band emission. Better indicators of quintet O-atom production are the strong 3d-3p lines near 927 nm. Support for this work comes from NSF Atmospheric Sciences (Aeronomy). [1] T.G. Slanger, D.L. Huestis, P.C. Cosby, and R.R. Meier, JGR 109, A10309 (2004). [2] I.S. Bowen, PASP 59, 196-198 (1947). [3] I.S. Shklovskii, Ast. Zhur. 34, 127-130 (1957). [4] J.C. Brandt, J.C., ApJ 130, 228-240 (1959). [5] J.W. Chamberlain, Physics of the Aurora and Airglow (Academic Press, 1961). [6] R.R. Meier, J.D.E. Anderson, L.J. Paxton, and R.P. McCoy, JGR 92(A8), 8767-8773 (1987)
Cosby Philip C.
Huestis David L.
Sharpee Brian D.
Slanger Tom G.
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