The dark spots of the Sun

The solar telescope GREGOR enables scientists to measure magnetic fields and flows on the Sun with unprecedented accuracy. The first results are now available. At times of strong magnetic activity, the Sun offers spectacular sights of violent eruptions, evolving sunspots and strong magnetic fields. But there are also times at which it seems as though nothing is happening on the Sun surface with its almost regular pattern of grains referred to as granules. These ‘quiet’ regions also contain magnetic fields but they are very weak and therefore difficult to measure. Scientists at Kiepenheuer Institute for Solar Physics work with the GREGOR solar telescope, inaugurated in 2012, to analyse sunspots and their finely chased structure with an accuracy never seen before. This and other first GREGOR results will be presented in nine articles of a special edition of Astronomy & Astrophysics.

Picture of a sunspot taken with GREGOR. Thanks to the high spatial resolution, the rich microstructure is clearly visible. Bright ribbons referred to as “light bridges” cross the sunspot, and recent discoveries suggest how heat transfer occurs inside these bridges. The picture shows an area of the Sun surface sized 18,000km per 18,000km. Accordingly, the length of the side margin of the picture roughly exceeds by 50% the Earth’s diameter (ca. 12,700km).

With the help of GREGOR it is now possible to perform measurements in the deepest atmospheric layers of the Sun with very high spatial and spectral resolutions. These measurements in the sunspot penumbra show that everywhere the magnetic field has a strength of at least 500 Gauß. This discovery makes it now possible to discard a specific model proposed for the penumbra. On the other hand, this measurement confirms another model.

The new GREGOR telescope with an aperture of 1.5m is located at the base of Mount Teide, Tenerife, at an altitude of 2,400m.

The GREGOR telescope is the second building from the right. It is located on the Teide Observatory site on Tenerife that hosts a large number of telescopes for observing the Sun and the night sky.

Sunspots are much more than the simple „dark spots on the Sun“ that Galilei discovered over 400 years ago. Quite to the contrary, they are characterized by rich structures that can be studied in detail only with the help of a telescope such as GREGOR. This was the undertaking of researchers at Kiepenheuer Institute and the results are astonishing: The bright ribbons, known as light bridges, that traverse some of the sunspots have their own, very small-scaled pattern. This discovery suggests plasma movements that nobody expected to exist in this form. Two other specialist articles published in Astronomy and Astrohphysics deal with the fine structure of the magnetic field whose tilt causes the outer ring of the sunspots to be characterized by radial filaments. New data from GREGOR helped the scientists at Kiepenheuer Institute to look at the question of how the magnetic field lines are positioned in the external areas of sunspots and of the minimum strength required in the weakest magnetic fields there. These new measurements can be used to falsify existing model representations and to confirm other models.
Apart from spots, the Sun surface is like a pot with bubbling boiling water. Hot plasma rises from the depth, cools down and sinks down again. The result is a pattern of bright bubbles with dark borders, referred to as granules. Even this ‘quiet’ Sun has a weak magnetic field but it is difficult to measure, not only because of its weakness but also because it is very small-scale. With the help of GREGOR it has now become possible to perform these measurements with unprecedented sensitivity. This is the subject of some of the specialist articles published in the special issue of Astronomy and Astrophysics.

GREGOR is Europe’s largest solar telescope and one of the most efficient telescopes worldwide. Not only is it equipped with a large primary mirror but also with adaptive optics technology that compensates in real-time for any disturbing influences from turbulences in the earth atmosphere. Four years ago, GREGOR was officially inaugurated. After a phase of technical observations, the first scientific measuring campaigns started in 2014.

The GREGOR solar telescope is part of the Teide Observatory on Tenerife. It is operated under the leadership of the Kiepenheuer Institute for Solar Physics in Freiburg.  Partners are the Max Planck Institute for Solar System Research based in Göttingen and the Leibniz Institute for Astrophysics in Potsdam (with 20% each).

 

Original publications: 

R. Schlichenmaier et al.
Active region fine structure observed at 0.08 arcsec resolution,
Astronomy & Astrophysics, 30. November 2016,
DOI: dx.doi.org/10.1051/0004-6361/201628561

J.M. Borrero et al.:
Deep probing of the photospheric sunspot penumbra: no evidence for magnetic field-free gaps,
Astronomy & Astrophysics, 30. November 2016,
DOI: dx.doi.org/10.1051/0004-6361/201628313

M. Franz et al.:
Magnetic fields of opposite polarity in sunspot penumbrae,

Astronomy & Astrophysics, 30. November 2016,
DOI: dx.doi.org/10.1051/0004-6361/201628407

A. Lagg et al.:
Probing deep photospheric layers of the quiet Sun with high magnetic sensitivity,
Astronomy & Astrophysics, 30. November 2016,
DOI: dx.doi.org/10.1051/0004-6361/201628489

J. Joshi et al.:
Upper chromospheric magnetic fields of a sunspot penumbra: observations of fine structure,

Astronomy & Astrophysics, 30. November 2016,
DOI: dx.doi.org/10.1051/0004-6361/201629214

M. Sobotka et al.:
Slipping reconnection in a solar flare observed in high resolution with the GREGOR solar telescope,
Astronomy & Astrophysics, 30. November 2016,
DOI: http://dx.doi.org/10.1051/0004-6361/201527966

M. Verma et al.:
Horizontal flow fields in and around a small active region. The transition period between flux emergence and decay,
Astronomy & Astrophysics, 30. November 2016,
DOI: dx.doi.org/10.1051/0004-6361/201628380

M. J. Martínez González et al.:
Inference of magnetic fields in the very quiet Sun,
Astronomy & Astrophysics, 30. November 2016,
DOI: http://dx.doi.org/10.1051/0004-6361/201628449

T. Felipe et al.:
Three-dimensional structure of a sunspot light bridge,
Astronomy & Astrophysics, 30. November 2016,
DOI: http://dx.doi.org/10.1051/0004-6361/201629586

 

Contact:
Prof. Dr. Wolfgang Schmidt
wolfgang.schmidt@leibniz-kis.de
Ph. 0761-3198-162

 

Links:
A&A Editorial
Text on A&A website
GREGOR-Seite auf KIS-Homepage