External impact on wind in the mesosphere / lower thermosphere

Horizontal winds in the mesosphere/lower thermosphere (MLT) over East Siberia (observatory Badary near Irkutsk) have been regularly measured using the spaced-receiver technique since 1975. The database for more than 20 years (1974-1996) allows us to gain an insight into the atmospheric coupling by studying the external impact on MLT winds from below (stratospheric warmings) and from above (geomagnetic storms caused by magnetospheric disturbances) with quite sufficient statistical reliability. Effects of stratospheric warmings and strong geomagnetic storms on prevailing winds and semidiurnal tidal amplitudes were estimated by the superposed epoch method. The coupling from below was demonstrated to depend on the type (intensity) of stratospheric warmings and on the phase of quasi-biennial oscillations (QBO phase) in the equatorial stratosphere at 30 hPa. The response of MLT winds to the external impact differs for the 21st and 22nd solar cycles. The effect of geomagnetic storms (Ap>100) manifests itself in the eastward prevailing wind decrease and semidiurnal zonal tidal amplitude increase.