Observations of lunar tides in the mesosphere and lower thermosphere at Arctic and middle latitudes

Meteor radars have been used to measure the horizontal winds in the mesosphere and lower thermosphere over Castle Eaton ( 52 degrees N) in the UK and over Esrange ( 68 degrees N) in Arctic Sweden. We consider a 16-year data set covering the interval 1988 - 2004 for the UK and a 6-year data set covering the interval 1999 - 2005 for the Arctic. The signature of the 12.42-h (M-2) lunar tide has been identified at both locations. The lunar tide is observed to reach amplitudes as large as 11 ms(-1). The Arctic radar has an interferometer and so allows investigation of the vertical structure of the lunar tide. At both locations the tide has maximum amplitudes in winter with a second autumnal maximum. The amplitude is found to increase with height over the 80 - 100 km height range observed. Vertical wavelengths are very variable, ranging from about 15 km in summer to more than 60 km in winter. Comparisons with the Vial and Forbes ( 1994) model reveals generally good agreement, except in the case of the summer vertical wavelengths which are observed to be significantly shorter than predicted.