Story at Scienceagogo.com:
Noctilucent clouds, also known as night-shining clouds, were first described in 1885, two years after the massive eruption of Krakatoa sent up a plume of ash and debris up to 80 km into Earth's atmosphere. The eruption affected global climate and weather for years and it also may have produced the first noctilucent clouds.Anyone have an idea if this would also propagate VHF or UHF radio communications? I mean, if we can bounce signals off the northern lights...
The effects of Krakatoa eventually faded, but the unusual electric blue clouds remain, nestled into a thin layer of Earth's mesosphere. The clouds, which are visible during the deep twilight, are most often observed during the summer months at latitudes from 50 to 70 degrees north and south, although in recent years they have been seen as far south as Utah and Colorado. Noctilucent clouds are a summertime phenomenon as, paradoxically, the atmosphere at 85 km altitude is coldest in summer, promoting the formation of the ice grains that make up the clouds.
It was 25 years ago that researchers at Poker Flat, Alaska, first noticed that the clouds were highly reflective to radar. This unusual property has long puzzled scientists and Caltech's Paul M. Bellan believes he may now have an explanation. He hypothesizes that the ice grains in noctilucent clouds are coated with a thin film of metal, made of sodium and iron; and the metal film causes radar waves to reflect off ripples in the cloud in a manner analogous to how X-rays reflect from a crystal lattice.