Tidal oscillations of the Arctic upper mesosphere and lower thermosphere in winter

The upper mesosphere and lower thermosphere of the northern polar cap was investigated using Fabry‐Perot interferometer measurements of the horizontal wind velocity in the lower thermosphere, meridian scanning photometer observations of atomic oxygen green line brightness in the lower thermosphere, and Michelson interferometer records of the infrared Meinel OH bands brightness and rotational temperature in the upper mesosphere. Tidal airglow oscillations in the absence of local solar heating were studied using a superposed epoch analysis of observations obtained around three new‐Moon intervals (November 9–22, 1993, December 7–20, 1993, and January 4–17, 1994) in Eureka (80°N). Tidal harmonics were uncovered with an iterated least chi‐squared fit, and their existence in the data tested using a goodness‐of‐fit probability against a null hypothesis of no oscillations. No single dominant tide was found at all times and altitudes of the winter Arctic upper mesosphere and lower thermosphere. The horizontal wind velocity of the lower thermosphere exhibited all the first three harmonics of the tide, with the largest‐amplitude oscillations shown by the diurnal component of the meridional wave speed. The atmospheric layer near the mesopause that contributes to the OH airglow emission experienced terdiurnal oscillations throughout the winter season. Theoretical polarization relations for evanescent tides, considerations of energy density propagation with altitude, and a day‐by‐day analysis of the zonal and meridional wind speed and temperature all indicate that the terdiurnal tide observed on November was an evanescent zonally symmetric tide. Other data indicate propagating tides, migrating tides, or a mixture of both.