Astronomers say they have solved the mystery of why supergranules–enormous cells of turbulent, charged gas that pepper the sun’s visible surface–appear to move across the sun faster than the sun rotates.
Data taken by the SOHO (Solar and Heliospheric Observatory) spacecraft reveals the motion as an optical illusion. The supergranules undulate, and it’s the wave they form that races ahead of the sun’s rotation.
Researchers liken the phenomenon to people in a stadium doing the wave. Each person jumps up and then sits back down, creating a wave that moves across the stadium even though no one is actually moving beyond their seat. Similarly, the bobbing of supergranules “is just a pattern of activity that is moving across the solar surface in waves,” says SOHO astronomer Tom Duvall of NASA’s Goddard Space Flight Center in Greenbelt, Md.
Duvall and his collaborators, who report their work in the Jan. 2 Nature, relied on SOHO’s Michelson Doppler Imager to make their measurements. The device measures the velocity of material at the solar surface so that scientists can discern the movement and structure of gases both on the surface and far beneath.
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