Abstract :
Some of the fadings of the 40-Mc signals from Sputnik I during morning transits are assumed to originate in Faraday rotation in the ionosphere. This has been investigated for the transit of October 12, 1957, 5:30 GMT, for which the orbit was known with reasonable accuracy. The fading records show an increasing fading rate during the last part of the transit. This is contrary to the simple theory based on the assumptions of a flat earth and a homogeneous ionosphere, from which a constant fading rate should be expected. The main reason why simple theory did not apply in this case, is that the satellite travels through the twilight zone from night towards day, i.e. from low to high ionization. The Faraday rotation is calculated by means of data for the orbit, magnetic field maps and predictions of the ionosphere conditions. The result is in good agreement with the observation when a high ionization between the altitude of maximum ionization and the satellite is assumed. The effect of refraction is estimated by means of the perturbation theory. It increases the Faraday rotation from 0 to 19 per cent during the transit. The general conclusion is that, even though the fading behavior of a transit near sunrise is explainable to some extent, it is still difficult to deduce detailed information on the state of the ionosphere from it, especially when the frequency is so low that refraction becomes significant.
