It is a sign that the sy is clear in the direction of the setting sun (south-west). That is the direction from which our bad weather comes in the form of depressions which are always very cloudy.
No cloud in the SW means no depression so for the next 12 hours or so the weather should stay fine.
We're just outside Glasgow and got some weird pictures. It was lovely! I actually only stumbled across this site via google while looking for an explanation!
Agree with previous poster who says that it's not the Auraura. I've seen that and this wasn't that.