I just came across this fascinating post on Roger G3XBM’s blog.
The openings from Europe to Japan – and perhaps Europe to West Coast USA on 50MHz sound candidates for this. Many years ago when I was part of the Square Bashers Expedition Group that activated squares around the UK mostly for 144MHz meteor scatter, we found that we could work well equipped Scandinavian stations such as SM2CKR pretty much any time we wanted. At the time we thought it was ionoscatter, but perhaps not.
I’ve got some reading to do now. Thanks Roger!