Will you accept speculation?
First- I would bet it ISN'T a 'ring'-it is a cylinder viewed end on...
second, the passage of the shuttle through the thin atmosphere creates a higher density cylindrical shell of atmosphere immediately behind it...
the plasma from the engines excites the atmosphere behind it making it glow, but the denser cylindrical shell behind the shuttle glows preferentially brighter, naturally...
That is my 2 cents- dont quote me on it...