Read out The solar matter is a good electrical conductor. It consists of plasma - so much heated matter, that the electrons have detached themselves from the atomic nuclei and freely roam around between them. Such electrical currents generate magnetic fields. Popular is the idea that the sun, as in a bicycle dynamo, turns rotational energy into electromagnetic energy. Magnetic fields are the driving force of many phenomena on the sun. For example, they can let gigantic eruptions, the protuberances, hover over the sun's surface for several minutes as 100, 000-kilometer fire arches. Although the solar physicists have already found out a lot about the magnetic phenomena of our home star, many basic things are still unclear, especially with regard to the origin and long-term behavior of the magnetic fields. It is not even sure if one should speak of many individual fields or better of only one field.

The SOHO solar research probe has measured the oscillation patterns of the sun over long periods of time and found indications of a turbulent boundary layer between the radiation zone and the convection zone, the tachocline. In the radiation zone, the energy generated by nuclear fusion in the solar center is transported outwards by photons. About 500, 000 kilometers (0.8 sun radii) from the center of the sun, this radiation zone ends and passes into the convection zone.

Here the temperature is already so low that the atomic nuclei - especially those of the heavier elements - can capture some of the free electrons. Therefore, the sun-matter is twenty times less radiolucent than in deeper layers. This makes the heat flow so difficult that the sun needs a new transport mechanism for its energy dissipation: hot gas bubbles are formed, which rise upwards. This process is called convection.

The tachocline between the radiation and convection zones could be the mysterious solar dynamo. Because the electric currents that are generated by the turbulent, charged gas build up a powerful magnetic field. "I do not want to say that we have discovered the solar dynamo, " says Bernhard Fleck of the European Space Agency ESA, the European project leader of SOHO. "But we have good evidence that something is going on here." Ad

Other solar researchers are more skeptical. "There is not even a proof that a sun dynamo exists at all, " says Douglas Gough of Cambridge University. "Maybe the magnetic field on the solar surface is a relic from the time of origin of the sun? This original field could simply decay over time, and what we observe today is just the remnants of it. "The magnetic dynamics of the sun are still puzzling. Weak magnetic fields are pulled along by moving matter - they literally "swim" in the plasma. Strong magnetic fields can hinder the movement of the plasma. Magnetic fields, which are directed in the convection zone from north to south, are stretched and pulled by the uneven rotation of the sun. They absorb rotational energy from the sun and are constantly amplified.

Where a tube of field lines pierces the photosphere and reaches out into space, sunspots are created. There the magnetic field is ten thousand times stronger than the earthly one that aligns our compass needles.

Sunspots occur preferably in pairs, oriented in east-west direction. Both spots are magnetically differently polarized. For example, in the northern hemisphere in all pairs the eastern spot is a magnetic north pole, then the western is a south pole. In the southern hemisphere, on the other hand, the polarity is reversed: the eastern spot is the south pole, the west spot is the north pole.

The magnetic spectacle of the sun goes even further: in the middle of the 19th century, it was discovered that the frequency of sunspots fluctuates at eleven-year intervals. When the number of spots reaches a maximum, the sun is particularly active. The next sunspot maximum is expected in 2001. When a cycle comes to an end and the spots become sparse, the first spots of the next cycle are already appearing. You just got the reverse magnetic polarity of the previous cycle. Based on the example above, the Ostfleck South Pole would now be in the Northern Hemisphere, the North Pole - and vice versa. So the magnetism of the sun is not repeated in an 11-year cycle, but in a 22-year cycle. Why this happens is another mystery that the sun poses to its researchers.

=== Rüdiger Vaas, Rudolf Kippenhahn

© science.de

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