Opposition "Surge" on the A-Ring

The Opposition Effect exists because of two contributing factors: one is due to the fact that the shadows of Ring particles directly opposite the Sun from Cassini - the Region of Opposition - fall completely behind the particles as seen from the spacecraft. These shadows are thus not visible to the spacecraft: all ring particle surfaces visible to the spacecraft in this image are in Sunlight and therefore bright. Much farther away from the Region of Opposition, the Ring particle shadows become more visible and the scene becomes less bright. The brightness falls off in a circular fashion around the Opposition Point. The main factor to the Opposition Surge in this image is an optical phenomenon called "coherent backscatter": here, the electromagnetic signal from the rays of scattered Sunlight making its way back to the spacecraft is enhanced near the Region of Opposition because, instead of canceling, the electric and magnetic fields comprising the scattered radiation fluctuate in unison.