Measurements of Ionospheric Refraction at 80 MHz Using Discrete Radio Sources

About 2000 measurements of source positions obtained at 80 MHz with the Culgoora radioheliograph near the peak of the solar activity cycle in 1968-71 have been used to determine the regular ionospheric lefraction. Diurnal, seasonal and zenith-angle dependencies of the refraction are deduced and found to be qualitatively consistent with the known gradients in peak electron density. The variability of the refraction from one measurement to the next is found to depend upon the zenith angle and season and to be mainly due to ionospheric disturbances whose presence is not usually revealed by measurements of the F2 critical frequency. A quantitative comparison of the measured values of angular refraction with the F2 critical frequency and its north-south and east-west gradients demonstrates the validity of the theoretical expressions for regular refraction developed by Komesaroff (1960) and yields a value of 190± 12 km for the equivalent thickness of the nocturnal ionosphere. A comparison of the present measurement of the thickness parameter with measurements of the same quantity made during the minimum of solar activity shows that the shape of the F2 layer is not dependent on the solar activity cycle and hence is not a sensitive measure of the gas temperature.