Properties of the mesosphere and lower thermosphere

The variability and systematic variations of the properties of the upper mesosphere and lower thermosphere are probably the least well known aspects of the terrestrial atmosphere. Satellite measurements of this region are very limited and rocket and remote sounding techniques do not provide comprehensive coverage. Progress is being made in theoretical studies of this region, primarily with regard to tidal effects, and some progress is being made in analyzing the relatively sparse experimental data that are available. Turbulence dynamics of the region has been studied by analyzing structure measurements at Kwajalein, wind data from Natal and systematic variations of the turbopause altitude determined from measurements of the diffusive separation of argon. One question that is being raised at this time, and it is appropriate at a time near solar maximum, is the extent of solar activity control of the properties of this region of the atmosphere. The occurrence rates and magnitudes of the turbulent diffusivity in the 70 to 90 km altitude region appear to correlate with solar activity with a time lag, as do also the incidence of aurora and the atomic oxygen green line intensity. Solar cycle dependence has been identified in mean zonal wind speeds in the 65 to 110 km altitude region above Saskatoon and in lower thermosphere temperatures measured at Heiss Island and at St. Santin. Millstone Hill data show that the mean meridional wind changes during a solar cycle. Solar cycle variations have also been detected in the stratosphere and troposphere.