Solar- Terrestrial Predictions in regard to Ionospheric Absorption of Radio Waves

85-91The AI-ionospheric radiowave absorption, LdB, measured on three different frequencies, viz. 1.8, 2.2 and 2.5 MHz at Ahmedabad (Iat., 23ᵒN; long., 72.6 E; mag. dip 34°N) during the 9-yr period (1972-1980) of the solar cycle 20-21 is studied for finding its relationship with solar activity at different times of the day or solar zenith angles (SZA) for each month of the year. It is found that the cos -index, n, for diurnal variation of absorption does not undergo much change with solar activity and it remains at a mean value of about 0.8 with a scatter of up to ±0.3 depending on the season. The variation of absorption is more sensitive to solar activity at higher SZA both in respect of the time of the day and the season. The smoothed 12-monthly running average of LdB and R: show that the former increases by a factor of about √2 when the latter goes up to 110, beyond which the rate of increase of absorption slows down, finally stopping to increase any further. Two examples are given to illustrate the method of prediction of diurnal variation of absorption in different seasons for a specified solar activity, and application of these in practical radio communication. The predicted values are found to be in good agreement with the observed values of absorption

Seasonal & Solar Control of Ionospheric Absorption at a Low Latitude Station

171-175Results of ionospheric absorption measured by Ai-method on internationally accepted standard frequency 2.2 MHz at Ahmedabad (23°N, 72.6°E; mag. dip 34°N) are presented for a period of over seven years (1972-1978)including solar minimum epoch in 1976.The diurnal index (n) of cos is found to be in anticorrelation with the time delay () in the diurnal maximum of absorption. The absorption at fixed values of cos shows better agreement with the sunspot number as well as S10.7 radio flux than its noontime (1200 hrs) value. Absorption at constant solar zenith angle () normalized to minimum solar activity shows two maxima in equinoxes corresponding to the atmospheric pressure maxima at 85-90 km. No significant correlation of n and is found with the index of solar activity. These are explained in terms of changes in the intensity of solar ultraviolet ionizing radiation and the atmospheric gas composition in the lower ionosphere with solar activity and the season