Below:
Shadow bands from higher up atmospheric density fluctuations are projected onto lower cloud. Usually they are seen on the ground but here an intermediate level cloud layer is acting as a translucent screen.

Corona or Iridescence?

Both are produced by diffraction from individual cloud droplets.   Scattered waves radiate outwards from all points on the drop surface. Where they overlap they interfere. Waves in-phase reinforce and waves out of phase are destroyed.

When all the drops are of nearly the same size the overlapping patterns from individual drops are not blurred out and there is a ringed corona. When the size changes from place to place in a cloud the pattern and colours become more irregular. This gives iridescence.