Springtime transitions in mesopause airglow and dynamics: photometer and MF radar observations in the Scandinavian and Canadian sectors

Abstract

Observations from 2 optical ground stations and 3 MF radars at high and mid-latitudes have been combined to describe “springtime transitions” in atomic oxygen and the mesopause wind fields and waves for eight years (1991–1998). The typical signature in the Stockholm (60° N, 20° E) OI 558 nm ‘green-line’ emission involves a rapid (circa) 2-days rise in the nighttime value by factors of 2 or so, with a subsequent decrease by factors of 3–10. There is considerable inter-annual variability in these green-line emissions, and also the hydroxyl airglow (intensities and temperatures) at Bear Lake Observatory (Utah, 42° N, 115° W ), but the 6–8 year means do show a characteristic airglow “springtime transition” (AST) near the end of March. MF radars from Troms ø (70° N, 19° E) , Juliusruh (55° N, 13° E) and Saskatoon (52° N, 107° W) demonstrate springtime reversals in the mean (daily) zonal winds at 85– 95 km , both annually and in 8 year means, at times near the airglow “transitions”. The “tongue” of easterlies (near March 30) is a long-established feature of mesopause dynamics, and clear indications of associated changes in tides and gravity wave fluxes are also presented. The TIME-GCM is also used to investigate the characteristics of the airglow and winds during the interval associated with the AST events. Useful similarities with the observed variations are demonstrated.