Attempts have been made to estimate the height distribution of the electron collision frequency in the lower thermosphere from simultaneous observations of the frequency dependence of both real (ionogram data) and imaginary (A1 absorption data) parts of the height-integrated complex refractive index. The method is essentially a process of trial and error in which the observed experimental values are matched with those derived from the proposed models. Good agreement is observed between the derived values of collision frequencies and those inferred from purely gas kinetic considerations. Further, the collision frequencies at E region heights are found to increase considerably with solar activity level. The collision frequencies at F region heights, on the other hand, undergo considerable day to night variation, nighttime values being consistently higher than those during the daytime.
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