This is a fogbow, informally known as a white rainbow. I got to spend about an hour working with it last week, which was phenomenal. When I’ve worked with fogbows before, I’ve had to work much faster.
I could nerd out on the physics of these, but the short version is that they are a type of rainbow. The difference is that their light is scattered by especially tiny water droplets — likely just one-tenth of a millimeter in diameter — which causes the colored bands of the rainbow to overlap producing one white band.
In a regular rainbow, the droplets are at least three to ten times larger. You can tell how large the water droplets are by the colors you see. In rainbows with the largest droplets, you see exceptionally bright reds and yellows, but hardly any blue. With smaller droplets, about three-tenths of a millimeter or so, you see much more blue than red.
The long period of time that I had to work with this fogbow allowed me to try different vantage points and wait for some swallows to fly through it.
And if you had turned around and faced the other direction, this is what you would have seen:
Note the gorgeous halo around the rising sun.
If you’re interested in the physics of natural light, The Field Guide to Natural Phenomena is one of my favorite books on the topic. It’s not the most detailed, but it covers the science without being too complicated to understand.