Turbulent Inner Life of a Sunspot Uncovered
Sunspots are not simple blemishes on the sun's face. These dark patches of concentrated magnetic activity are dynamic, variegated features that solar physicists have long tried to understand in detail.
A team of researchers studied this 2010 spot, with a larger diameter than Earth, to unravel a new clue to the inner workings of sunspots. The group, led by Göran Scharmer of Stockholm University and the Institute for Solar Physics of the Royal Swedish Academy of Sciences, measured the movement of plasma in the sunspot using the Swedish 1-meter Solar Telescope on the Canary Islands in Spain. They published their findings online June 2 in Science.
Scharmer and his colleagues found that plasma within the penumbra, the filamentary region of the sunspot surrounding the dark central umbra, is circulating vertically, rising or falling at various locations with velocities of roughly one kilometer per second, or more than 3,000 kilometers per hour. (To use the human eye as an analogue, the umbra is the pupil and the penumbra is the iris.) That rise and fall is evidence for convection in the penumbra, a phenomenon that had been predicted by computer simulations of sunspot dynamics but that the researchers say had not been observationally confirmed. The convection occurs as hot plasma rises from below, radiates away its heat, and sinks into the sun again as it cools.
That convection appears to be closely related to the Evershed flow, a plasma current that streams horizontally outward from the inner portion of the penumbra at several kilometers per second. The phenomenon is named after John Evershed, the British astronomer who discovered it in 1909. According to the new study, the Evershed effect is simply the outward, horizontal component of the newly detected vertical convection in the sunspot's penumbra.