I remember seeing these while flying in a Helio Courier as a kid. It was more like a rainbow that completed a full circle. We did not get that close to the popcorn clouds themselves ... but as you get closer and closer to the cloud the full-circle rainbow turns into what Edward posted and you see your shadow is right in the middle. If you enter the clouds you will fly 'into the glory.' There's even a book with that title. The person who took this picture was sitting a bit forward of the wings since the camera perspective should be directly in the center of the glory.
I've always wondered about the bright spot that surrounds your shadow, even over the ground. There is probably a physical/optical explanation for that, but I've never heard anyone speak of it, just something I notice all the time when seeing my shadow from the air.