When the shuttles main engines are fired at lift-off, what causes diamond shaped thrust exhaust?

Question

When the shuttles main engines are fired at lift-off, what causes diamond shaped thrust exhaust?

Answer 1

The "shock wave" effect is because the exhaust gases are traveling at supersonic speed.

Answer 2

At first the flame expands - that gives you the point at the top.  Then lower down, it cools from the outside in.  That gives you the point at the bottom.

Answer 3

I think it has some thing to do with to do with the way the exhaust nozzles flare out and the tighten again.

I am not a rocket scientist so it could be anything.