That's no warp engine, according to Andrew Probert:
Aft saucer module of Enterprise-D
Probert: They were just supposed to be more windows. Probably lounges of some sort, if I recall, but again, what you're seeing are windows that are broken up. I think that there were larger windows initially.
Doesn't matter, he's not an authority on what is what, as long as it's not mentioned on screen it's just a glowy thing on the saucer that could be a warp drive.
In what way is Andrew Probert not an authority on what is what, on the Enterprise