"Ian Stirling" wrote
I don't think it's exhaust gasses - as it's quite visible even after the
tank has seperated.
I think we're seeing the bow shock, which is an increasingly narrow cone
as mach number (is it strictly correct to call it that?) increases.

Ian, see this view:
http://www.nasa.gov/images/content/1...116_et_sep.jpg

I don't see any trace of a trailing shock wave -- with the SSMEs off,
the only glow is along leading-edge structure and on the
orbiter's nose throuster plumes.