The narrow cusped green flash hangs for a second above the horizon. Some turbulence from the passing ship (at far right) creates patches of deep blues and yellows. The dark ragged sea ‘horizon’ marks the mirage edge. The flash gets no lower. It is the union of a rising sun sliver and a descending one. After this they are gone.
Below, an earlier stage of the sunset, a classical Etruscan Vase or Omega sunset. Air warmed by the sea topped by cooler air has created a mirage of two* images. *Complete mirages always have an odd number of images, here there are only two because the mirage is cut off slightly below the horizon. Back to the mirage below - The descending upper sun is almost met by a second inverted sun rising from the waves.
The suns eventually meet at the mirage ‘fold line’. At, and near, the fold is high vertical magnification and maximum colour dispersion. We see already the enlarged red rims that will eventually merge into a red flash.
At the end, the slivers of the sun’s uppermost limb slide together at the fold line into the final cusped green flash shape. The high vertical magnification stretches the suns' green, and sometimes blue, rims into a vivid flash bright and large enough to be seen by eye.