Title: The airglow layer emission altitude cannot be determined unambiguously from temperature comparison with lidars,
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 between November 2010 and February 2014, when GRIPS 9
was in operation at the ALOMAR observatory (69.3$^\circ$N, 16.0$^\circ$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 $\pm$2 K. The generally assumed
altitude of 87 km and width of 8 km is never an unambiguous, good solution for
any of the measurements. Even for a fixed width of $\sim$8.4 km, one can
sometimes find several centre altitudes that yield equally good temperature
agreement. Weighted temperatures measured by lidar are not suitable to
determine unambiguously 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
87.4 km and width of 8.4 km to within $\pm$16 K, on average. This is not a
measurement uncertainty.