My theory on shuttle illumination late in dark ascent.

Did anybody capture good views of the 'ring of fire' effect (with
apologies to Eric Flint) just prior to MECO? Since the ring collapsed
at MECO I'm presuming it was the shock wave of the engine plume and
the upper atmosphere. Even though shot backwards at 3000 meters/sec,
the combustion productions were impacting ambient upper atmosphere at
about that same speed because the stack was nearly at orbital velocity
(8000 meters/sec).

The light was not the plume -- LH2/LO2 flame is essentially invisible.

Why it should be green is a bigger question -- I wonder about color
imbalance
in the low light level situation. Ground observers were seeing orange and
yellow-orange colors, not green -- and that's the proper wavelength for the
source of the light, atomic oxygen recombination immediately following O2
dissociation from molecular impacts.

Illumination at ET sep, on the other hand, was white -- the color of flares
from RCS jets.