Can you see the aeroplane vapour trail in the middle of the picture? It was dark. The plane was on the sunny side of the cloud by this point. I have seen this sort of thing before when a shadow of the vapour trail was projected onto the thin layer of cloud below but there is none such here. I don't think this can be a shadow because I know that shadows can't project onto thin air. So I'm guessing that the vapour trail is reflecting the duller scattered light from the cloud.
