Background and summary


The first results from the Swedish 1-m Solar Telescope on La Palma will be published as a Letter in Nature on November 14, 2002. These results are based on images and movies of sunspots showing smaller details than ever seen before. Of particular interest is the discovery of dark cores in bright filaments of sunspot penumbrae. In the same volume of Nature, there will appear also a News and Views commentary, written by John H Thomas.


After almost 400 years of scientific study, sunspots remain mysterious. We know that they are concentrations of strong magnetic fields. They are cooler than the surrounding solar surface because the magnetic field partly blocks upwelling hot gas from the solar interior. Thus they look dark on the solar disk. But how are they formed? How can they be so stable that they may last for weeks? And what is the reason for the solar activity cycle that causes sunspots to reach maximum numbers every eleventh year on average?

The penumbral mystery

A sufficiently large sunspot consists of a dark umbra, which is the coolest part of a sunspot, surrounded by a brighter penumbra. The penumbra appears to consist of a thin, very long filaments that have remained unresolved by solar telescopes until now. The penumbra is known to be a dynamic structure which is in constant motion and shows gas flows along the filaments. It has been suggested that the penumbra has an important role in keeping the sunspot together. But the mechanisms that drive these flows in the penumbra and the very reason for its existence are unknown.

Why seeing small solar details is important.

The key to observational studies of sunspots is the spatial resolution. An unresolved penumbra filament will simply look as a fuzzy thin thread that cannot provide enough information to allow us to explain its origin. It is only when we can see some structure within these filaments that we can search for explainations and use models to verify these. The observations presented in Nature suggest that filaments are dark in the center which is a distinct property that will be important in constraining future interpretations and models. Outside sunspots, magnetic field often appears concentrated into such small areas that they appear as points in most solar telescopes. Again, without being able to see structure within such concentrations, it is very difficult to be certain that current theories and models are correct. Perhaps we have ignored some important physics in these models? The Swedish solar telescope is starting to resolve also such structures and will therefore be able to constrain theoretical interpretations. It has been known for many years that fundamental processes in the solar atmosphere take place on scales smaller than 100 km. Such small structures have not been resolved on the sun until now.

The most highly resolving solar telescope ever built.

In theory, the ability to resolve small details increases with the diameter of the telescope: A 1 meter telescope should be able to see details that are two times smaller than can be seen with a 1/2 meter telescope. In reality, this is not at all possible. The main obstacle is the turbulent mixture of cold and warm air of our atmosphere which blurs and deforms the images - an effect astronomers call seeing. (Good seeing = stable air and sharp images, bad seeing = turbulent air and fuzzy images.) Great efforts have been made to find the places on Earth with the best seeing. Solar astronomers have particular problems since they (for natural reasons!) have to do their observations during daytime when the sun heats the ground. The Roque de los Muchachos on the Canary Island of La Palma is one of these superb sites found and presently the best known for solar observations.

But even finding the best site on the Earth is not enough! Even at these sites, the blurring from the atmosphere is so serious that it is hardly meaning ful to build solar telescopes larger than about 1/2 meter in diameter in order to increase the resolution. What is needed is something called adaptive optics. Adaptive optics is a relatively new technology that can compensate the blurring effects from the atmosphere. It is used with the Swedish solar telescope and its succesful functioning is necessary in order for this telescope to reach its diffraction limit. The adaptive mirror actually changes shape 1000 times a second in order to adjust for the rapidly changing blurring of the image. Finally, we are using techniques to further sharpen the images after they have been captured by electronic cameras. In the best images the resolution is close to 0.1 arcseconds. This is a factor of 1200 better than 20/20 vision. It is a combination of the excellent site, the simple and high-quality optical system of the telescope and the adaptive optics system that makes this telescope the most highly resolving solar telescope ever built.

A breakthrough in observational solar physics.

Achieving a spatial resolution of 0.1 arcsecond has been a repeatedly expressed goal in solar physics for over 20 years. The Swedish solar telescope is the first to achieve that resolution. The demonstration that a large telescope solar equipped with adaptive optics and located on an excellent site can reach this resolution represents a breakthrough in observational solar physics.

The dark cores

A striking feature in the first images of sunspots is the existence of dark cores within bright penumbral filaments. This is an unexpected discovery. For now we can only speculate what this means.

Implications of the discovery

From the technical side, the new observations demonstrate that we can now finally examine the solar surface at a resolution better than 100 km. At first sight the discovery of the dark cores and the other new structures may seem to only further increase the mysteries of the sunspots. There is hope, however, that these new features will increase the understanding of the penumbrae because they put up critical tests which every theoretical model must pass.

The Sun is a cosmic laboratory

The interplay between observations and theory will eventually lead to an understanding of the sunspot phenomenon. This would be important not only for solar physics but also for other astrophysics. The Sun is the only astrophysical object that is close enough to allow us to study the interaction of plasma (ionized gas) with magnetic field in great detail. Since the conditions in the solar atmosphere and elsewhere in the universe are so extreme, it is not possible to create similar environments in a laboratory. Observations of the magnetic field in the solar atmosphere are therefore of great importance to astrophysics in general.

What does it mean for us?

For us Earth-dwellers, sunspots are important as a manifestion of the solar activity that also causes 'space weather' which has an impact on our technological environment (satellites, telecommunications, and power transmission). The role of the changing sun for our climate is something that is hotly debated. Whatever the importance of this, the roots of the solar activity is in this tiny magnetic structures.


The island of La Palma is not to be confused with the city of Las Palmas, the capital of another Canary Island: Gran Canaria.

The Swedish 1-m Solar Telescope is managed by the Institute for Solar Physics of the Royal Swedish Academy of Sciences. The scientific staff of the institute is located in AlbaNova University Centre in Stockholm. The director of the institute is Professor Göran Scharmer who also led the construction of the new telescope.

Time-stamp: <2013-04-19 11:07:57 mats>