An image of the sunspot taken in blue light where the spatial resolution is highest. The image has been colourised for aesthetic reasons. The sunspot consists of the central dark umbra surrounded by the filamentary penumbra.
Credit: The Royal Swedish Academy of Sciences, Vasco M.J. Henriques
Map of velocities in the sunspot as measured using the Doppler effect. Blue implies that the gas is moving towards us - this corresponds to an upward motion on the solar surface. Red implies that the gas is moving away from us - this corresponds to a downward motion on the solar surface. The net-like pattern surrounding the sunspot is solar granulation. The field of view corresponds to 24000 km on the Sun.
Credit: The Royal Swedish Academy of Sciences, G.B. Scharmer, V.M.J. Henriques, D. Kiselman, J. de la Cruz Rodriguez
SHROPSHIRE: New telescope observations have revealed downward flows of gas in the outer regions of a sunspot for the first time, solving a problem that had puzzled astronomers for over a century.
These results, published in the current issue of Science, provide observational evidence of an overturning circulation within sunspots, a feature that had been predicted by computer simulations but never directly observed.
"There is now harmony between theory and observations," said lead author Göran Scharmer from the Institute for Solar Physics in Stockholm, Sweden. "We understand the mechanism that transports energy to the surface - a 102-year-old astrophysical mystery!"
The Sun's famous pockmarks
Sunspots are the most recognisable feature on any textbook image of the Sun. These dark freckles on the Sun's surface are cooler regions with typical temperatures of 3,000-4,000 degrees Celsius, compared to the usual solar temperatures of around 5,500 degrees Celsius.
Although the precise physical mechanisms are yet to be fully understood, sunspots are thought to result from magnetic flux tubes in the Sun's convective zone puncturing the Sun's surface. This inhibits the convection of gas, reducing the energy flux from the Sun's interior and hence lowering the surface temperature.
A sunspot can be divided into two parts: the central umbra (the darkest region) and the surrounding lighter penumbra. In 1909, radial outflows were detected in the penumbrae of sunspots, a phenomenon known as the 'Evershed flow' after its discoverer, John Evershed.
However, this flow alone does not provide the energy needed to account for a sunspot's radiative output - only an overturning circulation could give a full explanation.
Sunspot circulation
Göran Scharmer and colleagues discovered such a circulation using the Swedish 1-m Solar Telescope (SST), the most highly resolving solar telescope available for such studies.
Results were obtained by analysing the Sun's radiation spectrum, which contains patterns known as 'spectral lines', caused by atoms changing energy when they absorb or emit electrons.
By using a particular spectral line that forms in the deepest observable layers of the solar atmosphere, this team of scientists was able to probe for convective flows.
After careful compensation for various effects such as image degradation and stray light from the Earth's atmosphere, convective downflows were successfully detected inside the sunspot's penumbra.
The science of sunspots
These findings indicate that the outer regions of sunspots are made up of columns of gas, generated by an overturning circulation. In other words, the magnetic 'filaments' that make up penumbrae have a convective nature.
By estimating the magnitude of the convective downflows and comparing with simulations, the researchers found that the circulation was strong enough to explain the radiative energy output of the penumbra.
It was further shown that the convective flows are related to strong, nearly horizontal radial outflows. The previously discovered Evershed flow therefore represents the horizontal component of the overturning circulation.
