Waves in the lower thermosphere: Recent experimental investigations

Abstract

The main experimental methods recently used to investigate the dynamics of the lower thermosphere (80–120 km) are: rocket releases, meteor radar, incoherent scatter and radio echo fading, Some progress has been made, by analyzing the wind and temperature profiles so obtained as a superimposition of waves and other components (steady motion, turbulence). Three kinds of atmospheric waves have been studied: 1. (1) Gravity waves, of periods 10 min–10 hr, were identified by meteor radar and incoherent scatter. Those having a period of many hours were found to exhibit a great variability in time, allowing them to be distinguished from tidal harmonics. 2. (2) Tides are detected by all the experimental methods mentioned above; but only those having a height resolution of one or a few kilometers allow them to be analyzed as a superposition of tidal ‘modes’. Low-order modes of the semi-diurnal tide ( S 2 2, S 2 4) have been identified, but there exists some indications of the existence of higher-order modes. Evidence is found of the existence of the fundamental propagating mode S 1 1 and of evanescent modes of the diurnal tide. The ter-diurnal tide was identified in a few eases. 3. (3) Planetary waves, of periodic 2–8 days or more, were only detected during a few long-duration records. However, a cooperative recording program including many European stations is now in progress, and some preliminary results about their horizontal propagation are now available.