Ion-neutral collision frequency variations in the lower thermosphere from incoherent scatter measurements

Incoherent scatter measurements of ion-neutral collision frequency (vm) in the lower thermosphere at Arecibo (18.4 deg N 67 deg W) are analyzed for the period Aug. 1974 to May 1977. Results indicate a gradual increase of vm from morning to evening in summer by about a factor of two. This behavior reverses in winter when the value decreases from morning to evening. The ion-neutral collision frequency also shows large variability from one observation to next. These results are discussed in relation to static diffusion models which assume constant boundary conditions at the turbopause