I believe these are shock waves in the supersonic exhaust -
And that might explain why military jet engines are so incredibly *loud*.
They act like some sort of turbulent transonic whistle.
I wonder why there is a *series* of bring rings in the exhaust (no, there
is nothing special about mach 2, 3, etc., all the interesting things occur
at mach 1)
Perhaps the column of hot gas, because its density is lower than the
surrounding air, behaves as a sort of waveguide. If a shock front
attaches to the edge of the engine outlet, it would appear as a 3D cone,
and the tip of this cone perhaps would create a bright spot. But the
shock front would then proceed along, reflect from the inner "surface" of
the exhaust column, and re-focus into another cone with another bright
spot. This process might continue, and this would create an evenly-spaced
row of dots. But the spacing is not uniform.