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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union

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Atmos. Meas. Tech., 10, 265-271, 2017
http://www.atmos-meas-tech.net/10/265/2017/
doi:10.5194/amt-10-265-2017
© Author(s) 2017. This work is distributed
under the Creative Commons Attribution 3.0 License.
Research article
24 Jan 2017
Evidence of a significant rotational non-LTE effect in the CO2 4.3 µm PFS-MEX limb spectra
Alexander A. Kutepov1,2, Ladislav Rezac3, and Artem G. Feofilov4 1Space Weather Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD, USA
2Physics Department, The Catholic University of America, Washington, D.C., USA
3Department of Planets and Comets, Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
4Laboratoire de Météorologie Dynamique, IPSL/CNRS, UMR8539, Ecole Polytechnique, Paris, France
Abstract. Since January 2004, the planetary Fourier spectrometer (PFS) on board the Mars Express satellite has been recording near-infrared limb spectra of high quality up to the tangent altitudes ≈ 150 km, with potential information on density and thermal structure of the upper Martian atmosphere. We present first results of our modeling of the PFS short wavelength channel (SWC) daytime limb spectra for the altitude region above 90 km. We applied a ro-vibrational non-LTE model based on the stellar astrophysics technique of accelerated lambda iteration (ALI) to solve the multi-species and multi-level CO2 problem in the Martian atmosphere. We show that the long-standing discrepancy between observed and calculated spectra in the cores and wings of 4.3 µm region is explained by the non-thermal rotational distribution of molecules in the upper vibrational states 10011 and 10012 of the CO2 main isotope second hot (SH) bands above 90 km altitude. The redistribution of SH band intensities from band branch cores into their wings is caused (a) by intensive production of the CO2 molecules in rotational states with j > 30 due to the absorption of solar radiation in optically thin wings of 2.7 µm bands and (b) by a short radiative lifetime of excited molecules, which is insufficient at altitudes above 90 km for collisions to maintain rotation of excited molecules thermalized. Implications for developing operational algorithms for massive processing of PFS and other instrument limb observations are discussed.

Citation: Kutepov, A. A., Rezac, L., and Feofilov, A. G.: Evidence of a significant rotational non-LTE effect in the CO2 4.3 µm PFS-MEX limb spectra, Atmos. Meas. Tech., 10, 265-271, doi:10.5194/amt-10-265-2017, 2017.
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Short summary
We show that the long-standing discrepancy between observed and calculated spectra of the PFS/MEx in the cores and wings of 4.3-micron region is explained by the non-thermal rotational distribution of molecules in the upper vibrational states of second hot (SH) CO2 bands above 90 km altitude. We discuss the implications that accounting for this effect has for developing operational algorithms aimed at massive processing of PFS and other instrument limb observations.
We show that the long-standing discrepancy between observed and calculated spectra of the...
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