What do the components of a shockwave's visual artefacts represent? 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?
What you see are changes in the refractive index of the air, caused by the different pressure and temperature at different places. Sometimes, with the right conditions, there may be zones of transient condensation of water vapor, that add to the effect...