Scientific Objectives

The picture shows a large sunspot group observed with the Vacuum Tower Telescope (VTT) at the Teide Observatory on Tenerife on Sept. 4, 1998. Sunspots are huge concentrations of magnetic flux with a field strength of up to 0.5 Tesla (5 kG). Note the richness of filiganeous, radially oriented structure in the penumbrae and in the light bridges crossing the dark centers (umbrae). The black circle corresponds to the size of the Earth.
The picture shows solar granulation observed with the VTT (top) and results of a two-dimensional simulation of the convection in the photosphere (inset below). The cellular pattern is caused by convective cells near the solar surface, where hot gas moves upwards in the centers of the bright granules, cools at the top and moves back into the Sun in darker intergranular lanes. A typical granule is about 1000 km across. The numerical simulation represents contours of equal temperature in a vertical cut of the solar atmosphere.
Two-dimensional time dependent numerical simulation of a small magnetic flux concentration in a vertical cut of the solar atmosphere. The colours represent temperature (scale on top). The small white arrows show the velocity pattern of the plasma. The bundle of nearly vertical lines represents the magnetic field lines. The thick, horizontal line represents the layer from which most of the observed solar radiation emerges (tau = 1). This structure is quite small. A representative field concentration is the small bright dot at the center of the line marked "A ---- A" in Fig. 4 above. Their study requires the resolving and light collecting power of GREGOR.

High resolution solar observations

The prime scientific goal of GREGOR is high precision measurements of the solar magnetic field. Magnetic activity of the Sun plays a dominant role in virtually all processes in the solar atmosphere. It is responsible for the energy balance of the outer atmosphere, it causes the activity cycle and the concomitant variability of the solar luminosity and it produces most of the sometimes spectacular visible phenomena, like sunspots, prominences, flares and coronal mass ejections. 

From theoretical and numerical computations it is known that much of the interaction between the solar plasma and the magnetic field occurs on very small spatial scales of about 70 km on the Sun, corresponding to an angle of 0.1 arcsec. It is therefore important to have a large enough telescope which can resolve such small details. In addition, a large aperture is needed to achieve the photometric accuracy and sensitivity needed for a quantitative physical understanding of the solar magnetic field.  

Specific Science Objectives

Emergence, evolution and disappearance of magnetic flux

Magnetic flux appears at the solar surface as dipoles with a variety of sizes, from large spots to small magnetic elements. The total flux of the Sun is replaced within 2 or 3 days. Since the magnetic flux does not constantly increase, a mechanism for flux disappearance must exist. The corresponding processes occur on the scale of the smallest magnetic elements.

Energy budget of sunspots

The strong magnetic field of a sunspot blocks the convective energy transport to the solar surface. This blocking effect qualitatively explains the presence of cool sunspots, but the sunspot temperature is much higher as one would expect from complete suppression of convective energy transport. Small-scale phenomena, like umbral dots or penumbral grains are likely to provide the observed heat flux in a sunspot.

Chromosphere

The bright points at the boundaries and the interior of the supergranulation cells play a key role for the heating and the dynamics of the chromosphere. The size of and the wave motion in these structures need to be measured with high photometric precision and sufficient spatial resolution.