High-resolution density data between 170 and 220 km from the SETA (Satellite Electrostatic Triaxial Accelerometer) Experiment during the summers of 1982 and 1983 are analyzed spectrally to investigate the nature of horizontal density structures ranging in scale from about 150 to 2500 km. The lower thermosphere is found to be frequently characterized by wavelike structures corresponding to peak-to-peak density variations of order 10--40%. Orbit-by-orbit spectral analyses of densities normalized to 200 km of altitude are performed, and these are combined to construct average periodgrams for magnetically quiet and active periods. Integration of these curves provides the relative percentage of total density variance attributable to various horizontal scales; for instance, 10% for &lgr;H4) levels, the spectral energy increases by a factor of 2 for wavelengths in the range 150 to 500 km and by a factor of 4 at longer wavelengths (1000--2500 km). In part the spectral energy enhancement at the longer wavelengths reflects mesoscale density variations unrelated to wave activity, but ''direct'' waves generated by auroral activity, which are known to travel long distances, probably also make important contributions at these wavelengths. The magnetic activity enhancement at shorter scales is also significant, especially at low latitudes, since waves with these horizontal wavelengths are not expected on theoretical grounds to propagate very far within the thermosphere away from an auroral source. Moreover, the shorter scale waves regularly occur during geomagnetically quiet times when an auroral source mechanism is unlikely. ¿ American Geophysical Union 1995