The following answer assumes that the ground is wet with beads of water- dew....

First, the brightening around the shadow of the photographer is Glory scattering... (many references on the web explain this phenomenon- the rainbow ring is due to the same phenomenon as rainbows, light enters the droplets and is internally back reflected towards the observer. since the light that reaches the camera is exiting the droplet (index of refraction ~1.5) and going into air (index of refraction 1) the light is separated by wavelength. This separation of color is happening to a lesser and lesser degree as you move towards the center of the circle, but all the 'rainbows' are mixed together. the rainbow formed at radius 'r' is mixed with the 'rainbows formed at radii 'r+/- delta r ' making white light. the only radius at which this isnt the case is at the very edge of the circle of light.

Thats my guess- I cant access APOD right now for some reason so I am going by memory of what I saw on their site this morning....