110-113A study is made of diurnal and day-to-day variations of ionospheric absorption on 2·5 MHz measured by Al method at Ahmedabad (23 oN, 72·6 °E; magnetic dip 34 °N) for the year 1973. It is shown that the diurnal variation of absorption follows a law L=L0 cosn where L0 is absorption when the sun is at zenith. The value of the index n is found to vary from 0·4 to 1·05 during the course of a year, its yearly mean value turning out to be 0·70. The maximum of absorption occurs a little later than noon, the delay being about 45 min in winter and about 12 min in summer. Among the day-to-day variations, the increases in absorption are related to the increases in 1-8 Å solar X -ray flux and the decreases to the occurrences of a reflecting low-type sporadic-E layer. The results are discussed in the context of the roles played by the D- and E- layer ionizations and the atmospheric composition
