WAVE STRUCTURES IN LOWER THERMOSPHERE DENSITY FROM SATELLITE ELECTROSTATIC TRIAXIAL ACCELEROMETER MEASUREMENTS

High-resolution density data between 170 and 220 km from the SETA (Sat ellite Electrostatic Triaxial Accelerometer) Experiment during the sum mers of 1982 and 1983 are analyzed spectrally to investigate the natur e of horizontal density structures ranging in scale from about 150 to 2500 km. The lower thermosphere is found to be frequently characterize d by wavelike structures corresponding to peak-to-peak density variati ons of order 10-40%. Orbit-by-orbit spectral analyses of densities nor malized to 200 km of altitude are performed, and these are combined to construct average periodograms for magnetically quiet and active peri ods. Integration of these curves provides the relative percentage of t otal density variance attributable to various horizontal scales; for i nstance, 10% for lambda(H) < 750 km and 70% for 1500 < lambda(H) < 250 0 km. As geomagnetic activity increases from quiet (K-p < 2) to active (K-p > 4) 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 unrelate d to wave activity, but ''direct'' waves generated by auroral activity , which are known to travel long distances, probably also make importa nt contributions at these wavelengths. The magnetic activity enhanceme nt at shorter scales is also significant, especially at low latitudes, since waves with these horizontal wavelengths are not expected on the oretical grounds to propagate very far within the thermosphere away fr om an auroral source. Moreover, the shorter scale waves regularly occu r during geomagnetically quiet times when an amoral source mechanism i s unlikely.