Long Delayed Echoes

Delay between direct signal and echo on 11. Oct 1928. Notice that several of them were observed simultaneously in Oslo, Norway and at two locations in Eindhoven, Holland. Source: Balthazar van der Pol, Short Wave Echoes and the Aurora Borealis, Letters to Editor, Nature, Dec. 1928.

From my pages at the University of Oslo:
Both of the lists are referred to in the Wikipedia article on the subject.

There are also several blog posts here on these echoes under the tag Long Delayed Echoes, including reports of short MDEs (medium delayed echoes/ magnetospherically ducted echoes).

The graph shown here is one of the most compelling evidences for the reality of long delayed radio echoes ever to have been produced. It convinced most skeptics at the time.

Here's also an interview on Motherboard Vice: "Radio Ghosts Have Haunted the Airwaves for Nearly A Century" (October 2015).