On a recent flight, I was able to observe visual artefacts over the wing (Boeing 767-300 in cruise).
You can see them in the image below; they look like creases in a surface. They moved forwards and backwards over the wing. Later in the flight they extended to the end of the wing; at this point, they extended no further than a few metres.
I've added red and yellow arrows pointing to their ends - as you can see, one of them appears to fork.
Adjusting brightness and contrast makes the artefacts more visible:
I understand that this artefact is caused by a shockwave produced by supersonic airflow over the forward part of the wing.
I'd be interested to know: what exactly is it that I am seeing?
Is the artefact projected onto the surface of the wing, like a shadow?
Is the artefact a refractive distortion caused by different air densities?
In this image, there appear to be two creases, one behind the other - what do they represent?
What does the fork at the right-hand end of one of the 'creases' represent?
What do the darker and lighter portions of the 'creases' represent?