May 10, 2012
Text: Thomas Buhrke
The institute sits amid tranquil meadows and fields, and a stroll through the entrance hall highlights the long tradition of solar system research. From the Helios space probes of the 1970s to Ulysses and Cluster to the modern solar observatories known as SOHO and STEREO – Max Planck researchers have been participating in all of these missions. Over the last ten years, probably the largest group of solar physicists in Europe has come together here. One of their main objects of study is the corona. “It is the interface between our star and its heliosphere, that is, the sphere of influence of the solar wind, in which our Earth is also embedded,” says Director Sami Solanki.
The scientists consider the Sun as a holistic system in order to understand its corona: One group is concerned with the interior of our Sun, where the roots of the activities visible from the outside ultimately lie. In 2009, the balloon- borne telescope Sunrise studied the surface of the Sun with an accuracy never achieved before. Both observers and theoreticians research the corona, and Solanki himself investigates the impact of the Sun’s activity on the Earth’s climate.
Astronomers have long been aware that the temperature on the surface of the Sun is around 5,500 degrees Celsius. The surface is the part of the hot, bubbling ball of gas that we can see with the naked eye. Eighty years ago, scientists began to investigate the corona – the very thin outer atmosphere of the Sun – more closely. To their surprise, they found the temperatures there to be several million degrees. At first glance, this seems as physically impossible as the attempt to get water to boil on a hotplate at a temperature of 50 degrees. But this is what happens on the Sun.
Gas, at a temperature of one million degrees, emits radiation mainly in the ultraviolet and X-ray ranges. The corona’s light, which can be seen during a solar eclipse, is only a weak glow. Telescopes have to be positioned in space, since our atmosphere absorbs the short-wavelength UV and X-radiation. The US-European observatory SOHO is positioned 1.5 million kilometers from Earth and keeps the Sun continuously in its sight. The images recorded by the various instruments are automated to such a degree that it is possible to view them practically in real time via the Internet.
The solar observers in Katlenburg- Lindau are particularly proud of the SUMER spectrometer (Solar Ultraviolet Measurements of Emitted Radiation); they designed and built most of it and it has performed tireless service since 1996. SUMER disperses the sunlight into its spectral colors, albeit not in the range of visible light, but deep in the ultraviolet, as this is where the corona can be studied particularly well.
“SUMER has played its part in the investigation of many details of the corona’s heating mechanism, because important gas parameters, such as temperature, density and velocity, can be derived from the spectrally dispersed UV light,” says Max Planck researcher Werner Curdt. The experts now agree that the Sun’s magnetic field heats the corona. The only question is how.
The magnetic field is generated around 200,000 kilometers below the surface. In contrast to Earth, where it emerges mainly at the two poles, the surface of the Sun is permeated everywhere with field lines emanating and reentering. The magnetic fields are particularly strong in the dark sunspots. Pairs of these sunspots form the footpoints of a bridge-shaped bundle of field lines emanating from the surface. Two spots thus mark the north and south poles, respectively, of a local magnetic field.