Over Orange County, California within the last 1/2 hour, one or two solo fighters are circuiting at over 20,000 feet with afterburners on. They have come from the east, then turn south and back east again. There are high thin clouds, broken to overcast, depending upon the sky quadrant, which make spotting difficult. It is midnight here, and even though I cannot see them, I can follow the sound with my ears. This has happened at least three times now. I know that sound from attending plenty of air shows. Otherwise the skies in the Los Angeles area are totally empty of aircraft.