What do the components of a shockwave's visual artefacts represent?

Question

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?

You see a mirage, that is an image deformed by the change of refractive index, due to the huge change in air pressure / temperature / density. See shadowgraph. As the index changes, the phase of the light is altered, creating interferometric shadows. You can compare this with the effect visible over a hot road surface. — mins, Jun 06 '16 at 05:59
I'm not sure this question is about aviation, also given @mins' comment above. — Federico, Jun 06 '16 at 06:33
Related (not a duplicate): Is the visual artifact seen over the wing from different air densities, a reflection or something else? — mins, Jun 06 '16 at 07:06
This phenomenon is actually often used to visualize shocks in wind tunnels and experiments, see Schlieren photography And since we have an 'aerodynamics' tag, I don't think this is off-topic. — ROIMaison, Jun 06 '16 at 07:39
@mins It's not clear from your answer whether what I am seeing is the wing, with a narrow line of distortion in the image caused by the change in refractive index, or a shadowgram of that refractive distortion projected onto the wing by the (nearly) overhead sun. — Daniele Procida, Jun 06 '16 at 16:52
The wing is illuminated 1/ By the scattered ambient light; 2/ By the ambient light altered when crossing the shockwave refractive volume: Refraction creates a shadow more or less visible by diverting a part of the light elsewhere; 3/ The direct light from the Sun (if any) which creates another (stronger) shadow. Then you see this illuminated wing through the shockwave. The rays are again curved / folded by the index changes, so the portion of the wing may not match exactly the surrounding details (that's not visible on the image)... — mins, Jun 06 '16 at 17:23
... About shadow creation: Can air make shadows? --- and Why do the edges of glass blocks create a shadow?. See also Schlieren imaging pointed by @ROIMaison (video) — mins, Jun 06 '16 at 17:38

score 2 · Answer 1 · answered Apr 01 '17 at 08:00

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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...

answered Apr 01 '17 at 08:00

xxavier

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In this video, the shock wave produced by a big explosion is clearly recorded by high-speed cameras... http://www.wimp.com/invisible-worlds-explosion/ – xxavier Apr 01 '17 at 13:00

What do the components of a shockwave's visual artefacts represent?

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