For more than a century, researchers have been puzzled by the nature of the filaments known as the penumbra in sunspots. Understanding the structure of the penumbra, which surrounds a sunspot’s dark core, may help scientists explain the energy flow below the surface of sunspots. The research appears in the 10 June issue of the journal Science.
“The observations mean that we can now explain the physics and filamentary structure of sunspot penumbrae and their source of energy,” said lead author Göran Scharmer, professor at the Royal Swedish Academy of Sciences and Stockholm University.
Sunspots are regions of the sun that appear dark because they are cooler than the surrounding solar surface. Using data from the Swedish 1-meter Solar Telescope, the researchers were able to probe the deepest layers of the solar atmosphere at one sunspot to discover signs of small-scale upward and downward gas flows. They subsequently compared the new telescope data to earlier computer simulations of sunspot convection.
Velocity map of gas flows in the sunspot. Blue implies that the gas is moving towards us, corresponding to an upward motion on the solar surface. Red implies that the gas is moving away from us, corresponding to a downward motion on the solar surface. The field of view corresponds to 24,000 km on the Sun. | Image credit: The Royal Swedish Academy of Sciences, G.B. Scharmer, V.M.J. Henriques, D. Kiselman, J. de la Cruz Rodriguez; © Science/AAAS
The team found evidence for convective flows: Hot gases rising up from the interior, moving outward, cooling, and then sinking down at the sides of filaments. These flows are responsible for the energy transported to the Sun’s surface, they concluded, as well as the structuring of penumbra filaments.
“The key ingredients that shape these structures are an energy flux from the interior of the Sun, convection that must transfer this energy upwards, and radiative cooling of the convective flows at the surface layer,” said Scharmer.
Simulations show that convection opens up “gaps” in the penumbra by spreading out the Sun’s magnetic field horizontally. This spreading of the magnetic field, as well as the convective movement of gases, creates the penumbra filaments observed by the researchers.
“The physics of all this is not fundamentally different from that in the darkest parts of sunspots or even regions outside sunspots — the main difference is the strength and inclination of the magnetic field,” said Scharmer.
The results also confirm an earlier proposal that the strong horizontal flows first observed more than 100 years ago in sunspot penumbrae are a direct consequence of the convection.
2 June 2011