LDE's (long-delay echos) have been reported since the early days of radio. There are numerous references to the mystery in pre-WW2 QST and RADIO magazines. Time delays ranging from a few seconds to over an hour have been reported, and cannot be explained as any earth-bound phenomenon.
Reports of LDE's have sometimes fallen into the same category as UFO's and crop circles, but there seems to be enough evidence from reports to convince some scientists and engineers that this is not some kind of hoax.
The following is the summary of an article written in Japanese to the monthly DX magazine Five Nine, June, August, and September editions 2004, published in Tokyo.
The main points are as follows; it is considered that Long Delayed Echoes (LDEs) actually exist, and that is neither hoax nor mystery and it will be obvious that LDEs is one of physical phenomena whatever the time length of delaying will be unusual. And also it would be reasonable to consider a hypothetical ionosphere in interplanetary space.
http://park1.wakwak.com/~ja7ao/lde/ldese.htm