Performing at the 2007 Mirimar Marine Corp Air Station in San Diego, California. From flickr: Superbug Superfast.
The white halo is formed by condensed water droplets which are thought to result from a drop in air pressure around the aircraft. It appears that the transonic fighter jet is flying close to, but not exceeding, Mach 1, it is not breaking the sound barrier.
On flickr you can see a lot more pictures of the sonic envelop: Pushing The Sonic Envelop.
Here is a video of the same phenomenon:
F-18 Super Hornet – MCAS Miramar Air Show 2008 from Rapier55 on Vimeo.