November 11, 2010
The Sun is a turbulent place. Hot plasma is constantly in motion, wells up from the hot interior of our star, cools and sinks down again. In the photosphere, the visible surface of the Sun, these bubbling flows create net-like structures a few thousand kilometres in diameter. Underneath the bright, hot regions the plasma bubbles to the surface. In the dark, cooler rims it recedes again (see Figure 1). Scientists refer to these patterns as granulation.
Since the kinetic energy of these flows of hot plasma can transform into magnetic energy, the currents within the Sun are closely linked to the star’s magnetic properties. One expression of this magnetic nature are the dark sunspots that can even exceed the Earth in size. However, the Sun’s magnetic field also consists of much smaller structures. Tiny, bright points between the granules are an indicator of these fields. Within the bright points strong magnetic fields squeeze the bubbling plasma outward so that it is possible to look deeper within the Sun’s interior. Due to the higher temperatures there, these bright points appear brighter than their surroundings.
Even though several solar telescopes are able to visualize these bright points, their physical properties such as the magnetic field strength could not be identified until now. "Without this knowledge one of the most important requirements for a fundamental understanding of the magnetic processes within the Sun was missing", explains Prof. Dr. Sami K. Solanki, Managing Director of the MPS. "It was like trying to study an anthill without being able to discern and describe the individual ants".
Only the solar observatory Sunrise built and operated by an international team headed by the MPS unites the crucial properties: a high spatial resolution of about 100 kilometres and precise instrumentation for measuring physical parameters such as the magnetic field. During the observatory’s flight in June of last year, a balloon filled with helium carried Sunrise up to a height of 37 kilometres into the stratosphere. There, Sunrise had left the largest part of Earth’s atmosphere behind -- and thus the perturbing influence of air turbulence. With a main mirror of one meter in diameter, the Sunrise telescope is the largest solar telescope ever to have left the Earth. In addition, unlike earthbound telescopes Sunrise was also able to study the Sun’s ultraviolet (UV) radiation that is essentially absorbed by the Earth’s atmosphere and doesn’t reach the ground.
"Sunrise’s instrumentation reached a resolution of 100 kilometres", explains Solanki. This can be compared to discerning a one-euro coin from a distance of 30 kilometres. Because of this high resolution, it was possible to characterize the bright points within the photosphere for the first time. With up to 1.8 kilogauss the magnetic field in these regions is approximately 3,000 times as strong as the Earth’s magnetic field. The temperature is approximately 1,000 degrees higher than in the areas surrounding the bright points. "Additional theoretical calculations imply that these hot magnetic structures correspond to individual magnetic flux tubes", says Dr. Andreas Lagg from MPS. They are the building blocks of the solar magnetic field. In the photosphere, their magnetic field lines lie perpendicular to the Sun’s surface -- like tubes.
Apart from that, the scientists were able to determine the brightness of the bright points for the first time -- also in the UV. It is approximately five times higher than in the surrounding area. "With this information it is possible to estimate the contribution of the bright points to the total radiation variability of the Sun", says Tino Riethmuller from MPS. Since the Earth’s atmosphere absorbs the UV radiation almost completely, this part of the Sun’s light plays an important role in the heating of the upper air layers. This knowledge is therefore crucial for climate research, since it holds the key to discerning the manmade part of global warming from the Sun’s influences. The variation of the brightness within the photosphere, that Sunrise makes visible like no other mission before, also shows the temperature profile at the Sun’s surface.
In June 2009 the solar observatory Sunrise started from Esrange Space Center in Kiruna (Sweden) on its 130 hour flight around the North Pole. The mission was led by the MPS. Apart from a solar telescope and refined mechanisms for image stabilization, the observatory also carried two instruments that measured the solar magnetic fields. SuFI (Sunrise Filter Imager) was developed under the lead of the MPS, ImaX (Imaging Magnetograph eXperiment) was created in a Spanish consortium headed by the Instituto de Astrofisica de Canarias. Further partners were the Kiepenheuer Institute for Solar Physics in Freiburg (Germany) and the High Altitude Observatory in Boulder, Colorado.