Thermal Expansion of Ionospheric Layer and Temporary Morning Disappearance of Radio Signals

Abstract

IT is usually assumed that under normal conditions of the ionosphere, if a radio signal is received after ground sunrise, it is likely to maintain its strength until absorption in the ionospheric region becomes pronounced during the day. It has often been found, however, during our observations of fading, that the reception of signals on shorter wave-bands completely ceases for about an hour or more after ground sunrise. The present communication is to show that thermal expansion of the F₂-region may be sufficient to overcome the enhancement of ionization caused by increase in solar altitude with advance of the day. To quote a few typical examples out of many occasions, the signals transmitted from All India Radio, Delhi, on December 28, 1946, on 19-m. band were received in Benares (678 km.) until 0711 hr. I.S.T., when they disappeared for about an hour. A similar disappearance of signal on the same waveband was observed on April 11, 1947, at 0750 hr. I.S.T., lasting for about two hours.