Explained by the Bad Astronomer.
Another idea posted by board member neufer was that this was from a detonation charge in the missile’s third stage. There are ports, openings in the sides of the third stage. Those ports are sealed for the flight until the right time, when they’re blown open by explosive charges. This allows the fuel to escape very rapidly, extinguishing the thrust at a precise time to allow for accurate targeting of the warhead.
At this point, the missile is above most of the Earth’s atmosphere, essentially in space. So when that gas is suddenly released from the stage expands, it blows away from the missile in a sphere. Not only that, the release is so rapid it would expand like a spherical shell — which would look like a ring from the ground (the same way a soap bubble looks like a ring). And not only that, but the expanding gas would be moving very rapidly relative to the ground since the missile would’ve been moving rapidly at this point in the flight.
Image credit: Frame processed by Phil Plait from the video by Kanoa Withington/CFHT.
