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Polar cap observations of mesospheric and lower thermospheric 4‐hour waves in temperature

Lower thermospheric and upper mesospheric rotational temperatures have been derived from ground‐based measurements of the O2(0–1) and OH(6–2) nightglow emissions over Resolute Bay, Canada (74.68°N, 94.90°W) and OH(4–2) nightglow emission over Kiruna, Sweden (67.90°N, 21.10°E). From measurements taken during two nights in November, 2001, we have observed a dominant and coherent 4‐hr oscillation in both the O2 and OH airglow emission rates and rotational temperatures. The phases for the two oscillation events remain almost constant at each location, indicating that these oscillations may not be caused by transient passage of instantaneous gravity waves. More importantly, there is little phase difference in universal time between the 4‐hr oscillations in temperature at Resolute and Kiruna. The small phase difference gives a strong indication that the oscillation maybe related to a zonally symmetric tide, since any tidal waves in temperature with a nonzero zonal wavenumber are very weak at high latitudes. Waves in both events have large vertical wavelengths (76 and 175 km). The 4‐hr wave shows almost no latitudinal variation from Resolute to Kiruna.