Background and summary
Introduction
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.
Sunspots
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.
Notes
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.
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