The 16-day planetary wave in the mesosphere and lower thermosphere

A meteor radar located at Sheffield in the UK has been used to measure wind oscillations with periods in the range 10-28 days in the mesosphere/lower- thermosphere region at 53.5 degrees N, 3.9 degrees W from January 1990 to A ugust 1994. The data reveal a motion field in which wave activity occurs ov er a range of frequencies and in episodes generally lasting for less than t wo months. A seasonal cycle is apparent in which the largest observed ampli tudes are as high as 14 ms(-1) and are observed from January to mid-April. A minimum in activity occurs in late June to early July. A second, smaller, maximum follows in late summer/autumn where amplitudes reach up to 7-10 ms (-1). Considerable interannual variability is apparent but wave activity is observed in the summers of all the years examined, albeit at very small am plitudes near mid summer. This behaviour suggests that the equatorial winds in the mesopause region do not completely prevent inter-hemispheric ductin g of the wave from the winter hemisphere, or that it is generated in situ.