ON THE SPECTRUM OF THERMOSPHERIC GRAVITY-WAVES OBSERVED BY THE SUPER-DUAL-AURORAL-RADAR-NETWORK

The spectrum of acoustic gravity waves in the thermosphere observed us ing the Super Dual Auroral Radar Netowrk HF radar network is presented and various features are discussed. HF radars are sensitive to medium -scale gravity waves in the thermosphere that interact with the F-regi on ionization. The interaction of waves in the neutral atmosphere with the ionization is discussed to determine the relationship of the obse rved spectrum to the gravity wave spectrum. The observations showed th at (1) the observed spectrum shows peaks at frequencies that correspon d to quasi-monochromatic wave packets observed in the time series; (2) the spectrum often shows multiple peaks at harmonic frequencies; and (3) the spectrum nearly always shows a background level with a power l aw decrease having a slope of -5/3. It was also found that the overall level of the spectrum was elevated when gravity waves were present, p ossibly indicating an energy cascade process from the quasi-monochroma tic wave to the power law background.