No. 39: May-Jun 1985
LDEs (Long-Delayed Radio Echoes) have been known for over 50 years. Radio hams, in particular, will on rare occasions hear their transmissions repeated several seconds later. Various theories have been proposed, including: roundthe-world propagation; trapping in ionospheric ducts; reflections from distant plasma clouds; and beam-plasma interactions. R.J. Vidmar and F.W. Crawford, at Stanford, have been studying the LDE problem experimentally and theoretically and conclude that we still don't know which of the proposed mechanisms are valid.
(Vidmar, F.R., and Crawford, F.W.; "Long-Delayed Radio Echoes: Mechanisms and Observations," Journal of Geophysical Research, 90:1523, 1985.)