R. E. Hibbins

Utilizing riometry to observe gravity waves in the sunlit mesosphere

The novel use of imaging riometers to observe mesospheric gravity waves is described. Imaging riometers respond to changes in the absorption of cosmic radio noise in the ionospheric D-region which enables them to detect the compression and rarefaction of the atmosphere at ∼90 km altitude generated by the passage of gravity waves. A considerable advantage of this method is that, unlike conventional techniques which rely on imaging faint optical emissions from the airglow layer at ∼87 km altitude, riometers remain operative under daylit, moonlit or cloudy conditions. This is particularly important for research into gravity wave forcing of mesospheric temperature at polar latitudes in summer when continuous 24-hour daylight prevails. An example in which the same wave event is characterized in co-located airglow imager and imaging riometer shows good agreement between the two instruments.

Quasi‐biennial modulation of the semidiurnal tide in the upper mesosphere above Halley, Antarctica

We present an analysis of a long-term archive of horizontal wind data derived from meteor wind observations from a SuperDARN radar at Halley, Antarctica (76S, 27W). Systematic differences between the 12-hour component in the meridional wind and the climatological mean are observed showing evidence of a quasi-biennial modulation of the high-latitude semidiurnal tide in the upper mesosphere. The amplitude of the observed tides is enhanced when the equatorial stratospheric quasibiennial oscillation above 10 hPa is westerly. This enhancement is greatest in the summertime tidal amplitudes when the zonal wavenumber one (S = 1) nonmigrating component dominates the semidiurnal wind field, and is coincident with an enhancement of the summertime planetary wave activity in the upper mesosphere. These observations strongly support the hypothesis that the S = 1 component of the semidiurnal tide observed at high latitudes is due to a non-linear interaction between the migrating S = 2 semidiurnal tide and quasi-stationary S = 1 planetary waves.