Noctilucent clouds occur when ice crystals are high enough in the atmosphere to be visible in both day-lit and dark parts of the earth. The sun reflects off the clouds and causes an apparent glow to observers where the sun is mostly set. Check it:
The BBC has an article describing the NASA spacecraft, AIM, which is designed specifically to study such clouds.
The BBC has an article describing the NASA spacecraft, AIM, which is designed specifically to study such clouds.