or we would call it Polar Stratospheric Clouds or nacreous clouds. This photo was taken by Elin Fumuholt in Norway. They're rare because the stratosphere doesn't hold much moisture and they need to be lit from sunlight below meaning this happens around dawn or dusk.
Thanks to Mary for passing this on. Nacreous is a perfect description, as this image is reminiscent of nacreous iridescence of seashells1