Observed dynamical coupling through tidal wave propagation in the mesosphere and lower thermosphere at midlatitudes

The incoherent scatter radar at Millstone Hill (42.6 degrees N, 71.5 degrees W) and the nearby meteor wind radar at Durham have been used in coordinated experiments to study the characteristics of the semidiurnal tide that dominates the dynamics of the upper mesosphere and lower thermosphere at midlatitudes. Observations of horizontal neutral winds during 10-16 August 1994 at Millstone Hill have provided the first opportunity to derive neutral wind data at altitudes as low as 94 km, overlapping the Durham meteor wind observations with good agreement. Comparisons of the results with those obtained during 20-30 January 1993 suggest that the seasonal phase transition of the semidiurnal tide is smaller in the lower thermosphere than in the upper mesosphere. The Millstone Hill meridional wind measurements were also extended to 250 km altitude and reveal that the semidiurnal tide reaches a maximum amplitude of 70 m s(-1) in a relatively narrow layer with a peak altitude of 110 km. The dynamo currents induced by the observed winds and electric fields agree with observed magnetic field fluctuations, indicating overall consistency of the observations. (C) 1997 Elsevier Science Ltd.