Atmospheric scientist Gary Thomas of the University of Colorado has seen thousands of noctilucent cloud (NLC) photos, and he ranks this one among the best. "It's lovely," he says. "And it shows just how high these clouds really are--at the very edge of space."
He estimates the electric-blue band was 83 km above Earth's surface, higher than 99.999% of our planet's atmosphere. The sky at that altitude is space-black. It is the realm of meteors, high-energy auroras and decaying satellites.
People first noticed NLCs at the end of the 19th century after the 1883 eruption of Krakatoa. The Indonesian supervolcano hurled plumes of ash more than 50 km high in Earth's atmosphere. This produced spectacular sunsets and, for a while, turned twilight sky watching into a worldwide pastime. One evening in July 1885, Robert Leslie of Southampton, England, saw wispy blue filaments in the darkening sky. He published his observations in the journal Nature and is now credited with the discovery of noctilucent clouds.
Scientists of the 19th century figured the clouds were some curious manifestation of volcanic ash. Yet long after Krakatoa's ash settled, NLCs remained.
"It's a puzzle," says Thomas. "Noctilucent clouds have not only persisted, but also spread." In the beginning, the clouds were confined to latitudes above 50o; you had to go to places like Scandinavia, Siberia and Scotland to see them. In recent years, however, they have been sighted from mid-latitudes such as Washington, Oregon, Turkey and Iran.READ THE ENTIRE ARTICLE HERE.
(From a link I found on Space Weather which redirects to an article at science.nasa.gov.)