Remarkable new images of Earth's highest and most mysterious clouds have been captured by a Nasa spacecraft.
Noctilucent, or "night-shining", clouds appear as thin bands in twilight skies, some 80km (50miles) above the surface.
The AIM probe has now returned the first truly global pictures of these phenomena which appear to be increasing in frequency and extent.
Scientists say their observations show how the clouds alter rapidly, hour by hour and day by day.
They hope their studies will reveal the key triggers to the clouds' formation and why these triggers appear to be undergoing long-term change.