Where I'm from the light in the attached image is called "godlight" - i.e. a single sunbeam from a cloudy sky that illuminates the subject. I don't know what the native English speaking call it?
You've probably guessed the next question. How to do this in a very believable manner? I have comped in the right sky in my image.
I have also "double-exposed" my image - i.e. I have a layer on top of my normal-exposed image that is 2 stops darker. In other words - I probably need to construct the right layermask. OR any other way that you'd recommend - a curves layer that lights in a believable manner with a layermask?