It's the Wave, Stupid!

Caption: Supergranules, the network cells ("dimples") seen in the Doppler image and movie above, have puzzled solar scientists for two and a half decades - the pattern appears to rotate faster (a few percent) than the gas which they are made up of! Using advanced analysis of SOHO/MDI data, a team of scientists at Stanford University believe they have the explanation: The Sun is doing The Wave!

When people in a stadium do the wave, nobody actually moves in the direction of the wave - they just jump up and sit down. In the same way, individual supergranule cells don't really move faster than the solar surface; it is just a pattern of activity moving across the solar surface in waves, according to Tom Duvall of Goddard Space Flight Center, stationed at the W.W. Hansen Experimental Physics Laboratory (HEPL) at Stanford.

The artist's conception shows this for a band of supergranules rotating faster across the Sun's surface than other features. In reality this occurs, in a much more chaotic manner, everywhere on the Sun's surface, but is simplified here to highlight the phenomenon.

The scientist's conception shows a much more realistic scenario. This sequence is based on the analysis of the original data (noise with a power spectrum resembling that of the observations). The underlying average motion of the gas has been taken away - as if the camera is following the solar rotation. If you look closely, you will see individual bright points appear and disappear, moving in all directions; up, down, left and right. However, seen as a whole, the changing pattern appears to move from left to right. This illusion is generated by a random superposition of waves that are propagating predominantly from left to right.

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Picture/movie credits:

SOHO/MDI (ESA & NASA), NASA (artist's conception animation), Laurent Gizon (scientist's conception animation).