232-235Results of a study of AI method ionospheric absorption on 2.3 MHz at a midlatitude station, De Bilt (52.1°N, 5.2°E), for the years 1975-80 to find the seasonal and solar cycle variations of the index n of co sχ for diurnal variation of absorption (Ldb), delay time () in its daily maximum, and absorption at constant solar zenith angle (SZA, χ)are presented. These are compared with the results of such variations obtained at a low latitude station, Ahmedabad (23°N, 72.6°E), which is well outside the winter anomaly zone. It is found that there is, in general, an agreement in the seasonal variation of n at the two places, but not so in the case of . The time-delay at Ahmedabad is generally larger in winter than in summer varying almost in opposition to n while it has a sharp peak in winter and a shallow trough in summer at De Bilt, thus having no definite relation with n. But reasonably good anticorrelation between n and at midlatitude is also obtained if the values of these parameters for winter months are not taken into account. The range of seasonal changes in n and seems to decrease with increase in solar activity and the yearly smoothed values of rather than n also indicate a fall at the midlatitude. The absorption at constant SZA taken together for all months varies according to a linear relation with solar activity at Ahmedabad and at De Bilt; however, the linearity at the latter place is masked by the anomalous increase of absorption in winter. A curious feature noticed was that the absorption during Apr-Oct 1979 fell below that observed during the same months in 1978 although the solar activity in 1979 continued to rise. The above results have been discussed at appropriate contexts in the light of the ionizing radiations, changes in gas composition and the loss rate of ionization
