Measurement of the Ionospheric Absorption on 2·5 mc./s. at Ahmedabad

Abstract

Measurements of ionospheric absorption were carried out at Ahmedabad (latitude 23°0'N., longitude 72°6'E.) on 2·5 mc./s., using vertical pulsed transmission during the period August 1957 to July 1958. The strength of the transmitted signal was kept constant and the intensities of the vertically reflected pulses were cut down by the use of a passive attenuator so as to give a constant intensity of signal on the oscilloscope screen of the receiving circuit.

It was found that the mean monthly values of the absorption plotted against cos χ for each month from August 1957 to July 1958 obeyed a relation of the type log ρ α cosnχ. The value of ‘n’ for individual months ranges from 0·64 to 0·89 and the mean value is 0·73. To eliminate the effects of the seasonal changes in the noon zenith distance of the sun, the values of absorption for cos χ = 1 were obtained by extrapolation. These extrapolated values show a fairly close correlation with the sunspot number.

Generally the maximum absorption is reached some time after the local noon, suggesting relaxation time for the D region. Absorption larger than that expected by the cos χ law is observed in the late evening hours. This is attributed to a contribution from the deviative type of attenuation in the E layer.