There was a MOST interesting note on the ARRL propagation report a few years ago.

"Noctilucent Clouds Return - As reported at on June 1, 2009, the first noctilucent clouds (NLC) of the 2009 season were sighted over Russia on May 27. NLCs typically appear about 20 days prior to the summer solstice, increase quickly to a high summer level, and then disappear about 50 days after the summer solstice. These clouds are mostly a high latitude phenomenon, and are believed to be composed of ice crystals. VHF radars see very strong echoes from these clouds, and since they are at mesospheric heights (80 to 90 km), they are also known as polar mesosphere summer echoes (PMSE). These clouds are hypothesized by JE1BMJ and others to be responsible for 6m propagation across high latitudes (for example, from the East Coast of North America to Japan) during the northern hemisphere summer. This mode of propagation has been dubbed Summer Solstice Short-path Propagation (SSSP). Check out page 34 of the February 2009 issue of WorldRadio Online - available free at (this link was not working when I last tried) for a general discussion of PMSE and SSSP and for references in the technical literature. To reiterate, SSSP is still just a theory, but the occurrences of QSOs appear to match the occurrence pattern of PMSE."

Now, there is also evidence from radar returns that microwave DX might be possible using the same noctilucent clouds, possibly allowing superb DX possibilities on 10GHz possibly even with low power. This is a whole area of future ham research just waiting to be exploited. It may be one upside of global warming as these high altitude clouds are now more common than hitherto. I am very surprised there seems to be so little interest in this by microwavers worldwide. If keen microwavers set up beacons on possible paths for mesospheric DX then some quite phenomenal DX might be achieved.

UPDATE 16 June 2017

Today 13 stations in Japan were copied here on 6m JT65. Japanese stations have now been spotted on several mornings. My antenna is just a V2000 omni vertical fed with lossy coax. My RX is just an FT817 - no big antennas or anything special! If I can spot these I am sure others can too.

UPDATE 31 July 2021

Earlier this summer several Japanese stations were spotted on 6m FT8. Again, this was just with the omni vertical and lossy coax. With a better antenna on a mast I could easily gain 10dB which would make a lot of difference on FT8.