I think the pre-MECO ring of fire is the result of our perspective from
observing from on board the shuttle -- and the actual 3-D shape of the
glowing region is a hollow cone with the shuttle at its apex. The cone,
widening with distance from the shuttle, is the region of exhaust gases from
the main engines, and the boundary of the cone is where the interaction with
the thin upper atmosphere is creating this glow. It is at some angular
distance from the centerline, perhaps, because the shuttle's own physical
shockwave is protecting the very tip of this cone from getting the full
blast from the ambient air at 55 miles altitude. From the side -- for
observers on Earth, the visible light is strongest where the lines of sight
pass through the greatest density of glowing plasma -- along the top and
bottom edges as viewed from a distance. Hence these observers see the same
3-D shape in side profile, and it looks like a letter 'V'.