From my pages at the University of Oslo:
- The Five Most Likely Explanations for Long Delayed Echoes. This list is now being referenced on the RSGB site on this subject.
- 15 Possible Explanations for Long Delayed Echoes
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).