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Volume 18, issue 9 | Copyright
Atmos. Chem. Phys., 18, 6691-6697, 2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 14 May 2018

Research article | 14 May 2018

The airglow layer emission altitude cannot be determined unambiguously from temperature comparison with lidars

Tim Dunker Tim Dunker
  • 1Department of Physics and Technology, UiT The Arctic University of Norway, Postboks 6050 Langnes, Tromsø, Norway
  • anow at: National laboratory, Justervesenet, Postboks 170, 2027 Kjeller, Norway

Abstract. I investigate the nightly mean emission height and width of the OH* (3–1) layer by comparing nightly mean temperatures measured by the ground-based spectrometer GRIPS 9 and the Na lidar at ALOMAR. The data set contains 42 coincident measurements taken between November 2010 and February 2014, when GRIPS 9 was in operation at the ALOMAR observatory (69.3°N, 16.0°E) in northern Norway. To closely resemble the mean temperature measured by GRIPS 9, I weight each nightly mean temperature profile measured by the lidar using Gaussian distributions with 40 different centre altitudes and 40 different full widths at half maximum. In principle, one can thus determine the altitude and width of an airglow layer by finding the minimum temperature difference between the two instruments. On most nights, several combinations of centre altitude and width yield a temperature difference of ±2K. The generally assumed altitude of 87km and width of 8km is never an unambiguous, good solution for any of the measurements. Even for a fixed width of  ∼ 8.4km, one can sometimes find several centre altitudes that yield equally good temperature agreement. Weighted temperatures measured by lidar are not suitable to unambiguously determine the emission height and width of an airglow layer. However, when actual altitude and width data are lacking, a comparison with lidars can provide an estimate of how representative a measured rotational temperature is of an assumed altitude and width. I found the rotational temperature to represent the temperature at the commonly assumed altitude of 87.4km and width of 8.4km to within ±16K, on average. This is not a measurement uncertainty.

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Short summary
Often, the emission height of the mesospheric hydroxyl layer has been inferred from a comparison of temperature measured by ground-based lidars and hydroxyl spectrometers. I use temperatures measured by two independent instruments to show that such comparisons usually lead to ambiguous height determinations, especially if a variable layer width is taken into account. Even though this dataset is from a single location, the results apply to all airglow layers at any location.
Often, the emission height of the mesospheric hydroxyl layer has been inferred from a comparison...