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Appleton Lecture on Ionospheric Studies

"High-power-radar studies of the ionosphere" was the title of the 1Oth Appleton Lecture given by Dr J. V. Evans of the Lincoln Laboratory of the Massachusetts Institute of Technology, at the Institution of Electrical Engineers, on 4 December, 1974. The Appleton Lecture was inaugurated to commemorate the life and work of Sir Edward Appleton. This year's lecture was particularly appropriate because Appleton's most important contribution to the advancement of electrical science was his research in the field of radio propagation. Some 50 years ago, the first experiments were conducted that provided unequivocal evidence for the existence of an electrically charged region in the upper atmosphere capable of reflecting radio waves. Appleton and Barnett, in England, studied the signals from a distant BBC transmitter whose frequency could be swept at a controlled rate. They observed the beating between the direct and reflected rays, and thereby were able to deduce that the height of the reflection was about 90 km. In the United States, Breit and Tuve employed a transmitter capable of emitting short pulses together with a receiver connected to an oscillograph to take pictures of the delayed returns reflected near vertical incidence. They observed echoes from several heights between 100 and 200 km. Some years later, Watson-Watt coined the term "ionosphere" for this reflecting region. These early experiments led to the development of instruments, known as vertical-incidence sounders or ionosondes, to perform routine sounding of the reflection characteristics of the ionosphere (typically, over the frequency range 1-16 MHz). During World War 11, these instruments were widely