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

TL;DR

This study shows that using temperature comparisons between lidars and spectrometers cannot unambiguously determine the emission altitude and width of the airglow layer, highlighting limitations in current methods.

Contribution

It demonstrates the ambiguity in determining airglow emission height and width solely from temperature comparisons, challenging common assumptions and methods.

Findings

Multiple altitude and width combinations yield similar temperature matches.

The assumed 87 km altitude and 8 km width are not uniquely supported by data.

Temperature comparisons can estimate how representative a temperature measurement is of a specific altitude.

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.