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

摘要

I investigate the nightly mean emission height and width of theOH* (3–1) layer by comparing nightly mean temperatures measured bythe ground-based spectrometer GRIPS 9 and the Na lidar at ALOMAR. Thedata set contains 42 coincident measurements taken between November 2010 andFebruary 2014, when GRIPS 9 was in operation at the ALOMAR observatory(69.3° N, 16.0° E) in northern Norway. To closely resemble themean temperature measured by GRIPS 9, I weight each nightly mean temperatureprofile measured by the lidar using Gaussian distributions with 40 differentcentre altitudes and 40 different full widths at half maximum. In principle,one can thus determine the altitude and width of an airglow layer by findingthe minimum temperature difference between the two instruments. On mostnights, several combinations of centre altitude and width yield a temperaturedifference of ±2 K. The generally assumed altitude of 87 kmand width of 8 km is never an unambiguous, good solution for any ofthe measurements. Even for a fixed width of  ∼  8.4 km, one cansometimes find several centre altitudes that yield equally good temperatureagreement. Weighted temperatures measured by lidar are not suitable to unambiguouslydetermine the emission height and width of an airglow layer.However, when actual altitude and width data are lacking, a comparison withlidars can provide an estimate of how representative a measured rotationaltemperature is of an assumed altitude and width. I found the rotationaltemperature to represent the temperature at the commonly assumed altitude of87.4 km and width of 8.4 km to within ±16 K, onaverage. This is not a measurement uncertainty.