Polar Mesospheric Clouds

From NASA.Gov here:

The first observations of these "night-shining" clouds by a satellite named "AIM" which means Aeronomy of Ice in the Mesosphere, occurred above 70 degrees north latitude on May 25. People on the ground began seeing the clouds on June 6 over Northern Europe. AIM is the first satellite mission dedicated to the study of these unusual clouds.

These mystifying clouds are called Polar Mesospheric Clouds, or PMCs, when they are viewed from space and referred to as "night-shining" clouds or Noctilucent Clouds, when viewed by observers on Earth. The clouds form in an upper layer of the Earth’s atmosphere called the mesosphere during the Northern Hemisphere’s summer season which began in mid-May and extends through the end of August and are being seen by AIM’s instruments more frequently as the season progresses. They are also seen in the high latitudes during the summer months in the Southern Hemisphere.