What causes this?
 

What causes the rings in the exhaust?

Photos: SABCA F-16AM Fighting Falcon Aircraft Pictures | Airliners.net

The afterburner has multiple stages in it. If that's not why, I can't tell ya.

A cool sight indeed,

for an answer, another site:

Aerospaceweb.org | Ask Us - Shock Diamonds and Mach Disks

Thanks! Interesting read.

yeah, I was gonna say, its shockwaves, but thanks for the cool article! A much better answer than mine...