24- and 12-h oscillations during a time of strong eastward wind in the upper mesosphere at 31° and 45° N

Abstract

Horizontal mean winds and wind oscillations with periods of 48, 24, and 12 h are derived from MF radar observations of the upper mesosphere (75–100 km) over Northern and Southern Japan around the December solstice (7 November 1996–7 February 1997). At the onset of the winter jet the eastward wind rapidly increases from around 10 m⧹s to values beyond 60 m⧹s, while the zonal amplitude of the strong diurnal oscillation at Southern Japan suddenly decreases. This finding is supported by ground-based magnetometer observations at Kanoya (31°N) which is near to the MF radar site. The diurnal oscillation of the geomagnetic declination is highly correlated with the diurnal oscillation of the zonal wind, and also shows the strong decrease of amplitude at the onset of the winter jet. The phase profile of the diurnal oscillation of zonal wind is suggestive of wave reflection at 90 km. These observations indicate that the propagation of the diurnal tide (or tidally modulated gravity wave flux) from the mesosphere into the dynamo region is probably disturbed by the sudden enhancement of the eastward wind. On the other hand, the meridional amplitude of the diurnal oscillation remains strong at all heights during the time of strong eastward wind, and the phase profile could be interpreted by a tide with upward energy propagation. A quite similar effect of the winter jet is also seen in the amplitude and phase profile of the dominant semidiurnal oscillation at Northern Japan.