The spectrum of acoustic gravity waves in the thermosphere observed using the Super Dual Auroral Radar Network 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-region ionization. The interaction of waves in the neutral atmosphere with the ionization is discussed to determine the relationship of the observed spectrum to the gravity wave spectrum. The observations showed that (1) the observed spectrum shows peaks at frequencies that correspond 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 law 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, possibly indicating an energy cascade process from the quasi-monochromatic wave to the power law background.¿ 1997 American Geophysical Union