"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.

I agree there is a bow shock -- but the ring apears
constant in angular size until MECO when
it rather suddenly collapses to what is
probably the remnant bow shock. But
now that it's monday morning
the professional plasma folks will come to the
office and read their email!! grin