From DNA:
“These clouds exist literally on the edge of space,” said James Russell, principal investigator for NASA’s Aeronomy of Ice in the Mesosphere (AIM) satellite, adding that the clouds form only in a very narrow band a little more than 50 miles (80km) above Earth’s surface. According to a report in National Geographic News, once seen mostly in the Arctic, night-shining clouds are now appearing more frequently at lower latitudes. Scientists suspect that the increase in night-shining clouds may be due to climate change. Even as surface temperatures rise, the upper atmosphere is getting colder due to the buildup of carbon dioxide, creating perfect conditions for cloud formation, according to experts…
High-altitude night-shining clouds are similar in structure to lower-level clouds – a fact that is “startling,” according to AIM deputy principal investigator Scott Bailey, of Virginia Polytechnic Institute and State University. “That’s because the two types of clouds form under such radically different conditions,” Bailey said. AIM’s data on night-shining clouds have told scientists a lot about the upper atmosphere. “The processes that control these clouds are very likely similar to the ones that control clouds down near the surface of Earth,” said Bailey…
In addition, more night-shining clouds tend to light up the skies during times when the sun is quiet, according to Daniel Marsh of the National Center for Atmospheric Research in Boulder, Colorado. “That’s because when solar activity is most intense, ultraviolet radiation breaks up the air” water molecules and prevents the clouds from forming,” Marsh said. Volcanoes also inject water vapor into the upper atmosphere, which can lead to night-shining clouds.