Long-delayed radio echoes
J. Hals first observed long-delayed radio echoes in 1927. During the following half-century, scientists have been studying this perplexing problem, but it has been the amateurs who have accumulated the bulk of the data. Over 100 reports exist where echoes of radio transmissions were received seconds later at the original transmitting station. Since light travels 186,000 miles per second, any simple radio-wave reflector would have to be well beyond the moon's orbit.
A wide variety of natural phenomena (interplanetary matter) and even artificial devices (alien space probes) have been postulated to explain the long delays. Muldrew's article is first of all an excellent summary of the long and fascinating history of this effect. His bibliography is extensive and apparently nearly complete.
Muldrew next examines the various ionospheric mechanisms that might cause long delays. The ionosphere is a complex structure with ducts in which radio signals can get trapped. Delays of a second or so might be due to such trapping but the longer delays require some other explanation. Muldrew favors a rather complex interaction between signals from separate transmitters that (theoretically at least) can create a long-lived electrostatic wave that travels in the ionosphere -- a sort of natural memory device. The coded signals could then be read out much later when the proper natural conditions developed. Delays of up to 40 seconds might be possible with this "ionospheric memory."
(Muldrew, D.B.; "Generation of Long Delay Echoes," Journal of Geophysical Research," 84:5199, 1979.)
References. More information on these curius echoes is located at GER1 in our Catalog: Rare Halos, Mirages. For a description of this book, visit: here.