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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 110, A08304, doi:10.1029/2004JA010855, 2005

Generation of metastable helium and the 1083 nm emission in the upper thermosphere

L. S. Waldrop

Department of Electrical and Computer Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, USA

R. B. Kerr

School of Engineering, Dartmouth College, Hanover, New Hampshire, USA

S. A. González

National Astronomy and Ionosphere Center, Cornell University, Arecibo Observatory, Arecibo, Puerto Rico

M. P. Sulzer

National Astronomy and Ionosphere Center, Cornell University, Arecibo Observatory, Arecibo, Puerto Rico

J. Noto

Scientific Solutions, Inc., Chelmsford, Massachusetts, USA

F. Kamalabadi

Department of Electrical and Computer Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, USA

Abstract

Models of metastable helium, He(2³S), production in the upper thermosphere and lower exosphere show that creation by recombination of He⁺ may be nonnegligible relative to the photoelectron impact on He(1¹S) source. Owing to large ground state He abundance in the winter and to photoelectrons from an illuminated conjugate thermosphere, the strongest 1083 nm intensities (arising from He(2³S) solar resonance) occur during the winter at Arecibo. During late spring and early fall, however, recombination of the nighttime He⁺ layer begins to compete as a source of metastable He, and the contribution to the early twilight 1083 nm airglow brightness from recombinant He(2³S) reaches more than 10%, regardless of solar activity levels, and becomes increasingly dominant for solar zenith angles greater than 105°. A hot metastable component from recombination potentially renders ambiguous interpretation of the 1083 nm spectral profile in terms of exospheric temperature. The presence of such a population may explain reported observations of 1083 nm line widths that increase with shadow height, implying twilight temperatures much hotter than those expected of a thermalized neutral population.

Received 20 October 2004; accepted 16 March 2005; published 11 August 2005.

Keywords: metastable helium; helium ion recombination; thermosphere photochemistry.

Index Terms: 0355 Atmospheric Composition and Structure: Thermosphere: composition and chemistry; 0310 Atmospheric Composition and Structure: Airglow and aurora; 0322 Atmospheric Composition and Structure: Constituent sources and sinks; 0335 Atmospheric Composition and Structure: Ion chemistry of the atmosphere (2419, 2427).

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Citation: Waldrop, L. S., R. B. Kerr, S. A. González, M. P. Sulzer, J. Noto, and F. Kamalabadi (2005), Generation of metastable helium and the 1083 nm emission in the upper thermosphere, J. Geophys. Res., 110, A08304, doi:10.1029/2004JA010855.

Copyright 2005 by the American Geophysical Union.