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Discussion papers
https://doi.org/10.5194/acp-2018-1047
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2018-1047
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Technical note 10 Dec 2018

Technical note | 10 Dec 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Technical Note: Bimodality in Mesospheric OH Rotational Population Distributions and Implications for Temperature Measurements

Konstantinos S. Kalogerakis Konstantinos S. Kalogerakis
  • Center for Geospace Studies, SRI International, Menlo Park, California, USA

Abstract. Emission from the OH Meinel bands is routinely used to determine rotational temperatures that are considered proxies for the kinetic temperature near the mesopause region. Previous observations determined OH rotational temperatures that show a dependence on the vibrational level, with the temperature rising overall as the OH vibrational quantum number v increases. The source of this trend is not well understood and has generally been attributed to deviations from thermodynamic equilibrium. This Technical Note demonstrates that the existence of bimodal OH rotational population distributions is an inherent feature of rotational relaxation in gases and can provide an explanation for the previously reported temperature trend. The use of only a few lines from rotational transitions involving low rotational quantum numbers to determine rotational temperatures does not account for the bimodality of the OH rotational population distributions and leads to systematic errors overestimating the OH rotational temperature. This Note presents selected examples, discusses the relevant implications, and considers strategies that could lead to more reliable OH rotational temperature determination.

Konstantinos S. Kalogerakis
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Status: open (until 04 Feb 2019)
Status: open (until 04 Feb 2019)
AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment
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Konstantinos S. Kalogerakis
Konstantinos S. Kalogerakis
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Latest update: 20 Jan 2019
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Short summary
Light emission from energetic hydroxyl radical, OH*, is a prominent feature in spectra of the night sky. It is routinely used to determine the temperature of of the atmosphere near 90 km. This Note shows that the common practice of using only a few emission features from low rotational excitation to determine rotational temperatures does not account for the bimodality of the OH population distributions and can lead to large systematic errors.
Light emission from energetic hydroxyl radical, OH*, is a prominent feature in spectra of the...
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