Abstract :
The paper describes a continuation of the investigations of Best, Budden, Ratcliffe and Wilkes on the ionospheric propagation of radio waves of frequency 16 kc/s emitted from the Post Office sender GBR at Rugby, and observed at Cambridge, 90 km away. The present experiments over the period March, 1948, to October, 1949, have confirmed the tentative conclusions of the earlier work, and revealed some new and interesting effects. Most of the observations were made on the abnormal component of the downcoming wave, using the method previously developed to isolate the downcoming wave and to make separate measurements on its amplitude and phase. On a few occasions simultaneous observations were made on two frequencies transmitted over comparable distances. The observations showed that reflection took place from a height of about 72 km by day and 87 km at night, and that the daily cycle of phase change was related to a real change in the height of reflection. The apparent height of reflection usually varied in a regular and predictable way with the zenith angle of the sun. The total change of height in passing from midday to midnight was 16.8 km in summer and 13.5 km in winter. There was a marked seasonal variation of the amplitude of the downcoming wave, the variation of the day-time amplitude being different from that of the night-time amplitude. The conversion coefficient was 0.13 on a summer day and 0.26 on a winter day, whereas at night it was 0.56 in summer and 0.37 in winter. During the summer months the amplitude was closely controlled by the sun, but it was not related in any simple way to the zenith angle. The daily cycles of amplitude change and phase change appeared to take place independently. The sunrise effect on the amplitude occurred about one hour before the effect on the phase, the latter occurring about 9min after ground sunrise. Some subsidiary variations of the phase and amplitude, which were regularly observed, were explicable on the assumption that r- eflection took place from two levels in the upper atmosphere. There was no clear evidence of waves reflected twice from the ionosphere. During and after great magnetic storms the diurnal variation of the height of reflection was markedly abnormal, the abnormality lasting for as long as 10 days after the end of the magnetic disturbance. Anomalous behaviour of a less violent nature was frequently observed both by day and by night. Some of these anomalies could be directly associated with magnetic disturbances, but in general there was no simple connection between the two phenomena.
