Read out The spaceship SOHO unveils the secrets of our star. Why is the sun vibrating? Where do the devastating magnetic storms come from? The new solar observatory in the forecourt of the earth has provided some answers and new insights in the first two years of its use. The seething gas masses on the sun still hide many secrets. So astronomers still do not know what causes the eleven-year cycle of activity - its most striking feature is the periodic appearance of dark spots on the solar surface. Equally open is the question of the origin of the solar wind, which penetrates the solar system as a stream of electrically charged particles. Particularly puzzling is the mechanism that heats the corona to well over a million degrees.

SOHO, the "Solar and Heliospheric Observatory", has been trying to solve such problems for two years. The solar observatory was developed under the direction of the European Space Agency ESA and is operated together with the American Nasa. On December 2, 1995 launched the approximately two -ton spacecraft from the Cape Canaveral spaceport in Florida in space. Two months later she had reached her final position, the so-called Lagrange point L1.

This is the best possible location for observing the sun. About 1.5 million kilometers away from the earth in the direction of the sun, the attraction forces of the two celestial bodies cancel each other out. So SOHO - lashed by invisible threads over the earth - can orbit the sun with our planet without ever obscuring the open view. From this exposed position it is possible for the first time to observe the sun completely undisturbed for years.

SOHO has eleven scientific instruments on board. They register the UV radiation of the sun, magnetic fields, temperatures, matter and energy flows within the corona and in their transition area to the solar surface. Other instruments investigate the energy distribution in the solar wind and determine its chemical composition. In three SOHO experiments, vibrations are measured that cause the entire gas sphere of the sun to shake nonstop, similar to a battered bell. display

These rhythmic vibrations are caused by sound waves that pass through the sun on curved paths of different lengths and are reflected again and again on their surface. Each sound wave has a certain frequency - usually in the range of a few millihertz. Their superposition resembles a - inaudible - "song of the sun".

Helio seismology, the study of solar vibrations, allows a look deep below the familiar face of our home star. Information about the internal structure of the sun can be obtained from the frequencies, the distances and the transit times of individual sound waves.

To do so, the SOHO measuring instruments register movements of the sun's surface to a few meters with the help of the Doppler effect: Depending on whether the gas masses rise or fall in one area of ​​the sun, the spectrum of the light emitted there is shifted to slightly higher or lower frequencies. The "Michelson Doppler Imager" aboard SOHO determines the vertical movement of nearly one million points on the solar surface once per minute. Computer simulations determine the distribution of temperature and density inside the sun.

The results were confirmed by models of astrophysicists from the construction of the sun. They also show that in the boundary area between the convection zone - the outer area of ​​the solar globe - and the underlying radiation zone, the gases in violent turbulence mix with each other. In addition, the convection and radiation zones rotate at different speeds. About 200, 000 kilometers below the surface this leads to strong shear forces. Together with the turbulence, they could be responsible for the emergence of the solar magnetic field.

Magnetic fields probably play a crucial role in many processes in the sun, including heating up the corona. For decades, astronomers were puzzled: why is the corona almost a thousand times hotter than the sun's surface, which is around 5800 degrees Celsius? There is no heat source in the corona itself, a heat flow from the underlying cooler areas is not possible according to the laws of physics. So there must be another mechanism that transfers energy into the corona.

The SOHO instruments have now taken the scientists on their trail. They show that the sun's surface is covered by a carpet of countless magnetic loops - arcuate magnetic fields, each starting from a positive magnetic pole on the surface and ending at a negative pole. Usually a few days after their formation, these short-lived structures break open, their open ends then move freely around. If two ends meet with different magnetic field directions, a short-circuit occurs which releases huge amounts of energy into the solar atmosphere.

Magnetic fields are also probably involved in the formation of the solar wind. For a long time, astronomers have known that the solar wind is made up of two parts of different energy content: slower particles move around at around 400 kilometers per second, while faster ones make it twice as fast.

Observations of the "Ultraviolet Coronograph Spectrometer" by SOHO show that the slower part of the solar wind comes from the equatorial region, where matter flows along the magnetic field lines into space. The more energetic particles, on the other hand, have their origins in polar regions. There, the magnetic field lines are open and act as an accelerator. They propel electrically charged particles that evaporate from the sun to unprecedented levels at very high speeds. Also near the sun's poles, British researchers recently discovered giant cyclones using the SOHO spectrometer. Each one of them is almost as big as the earth, inside the solar tornadoes romp winds at speeds up to 500, 000 kilometers per hour.

Again and again so-called coronal mass ejections throw billions of tons of matter into space. These massive explosions are responsible for magnetic storms over the earth that can disrupt radio communications and cause power outages. Electrons trapped in Earth's magnetic shield are released by magnetic storms. As high-energy "killer electrons" they can damage the electronics of satellites and endanger astronauts during spacewalks. So far, attempts have been made to predict magnetic storms from solar "flares" that are easier to observe from Earth. These are strongly heated areas of the solar atmosphere. The success rate of such predictions is rather meager: Only about every third flare attracts a magnetic storm. SOHO offers the opportunity to directly observe the coronal mass ejections and thus promises much more reliable forecasts.

Last year, the probe registered nine such explosions within seven months. Each time a magnetic storm raged over the earth a few days later. Astrophysicists hope to be able to use computing models to predict the strength of such SOHO data in the future.

So far, what is open is what causes the eleven-year cycle of the sun. It could be related to large plasma streams below the solar surface that SOHO has trapped. Flows of ionized gases flow around the polar regions of the Sun at a depth of around 40, 000 miles. They are strongly reminiscent of terrestrial jet currents: fast vertical winds that blow around the globe on meandering streamlines. Other plasma flows resemble the trade winds on earth. They often occur together with patches of sunspots, as it will be around the year 2000. Then the scientists hope to discover the secret of the solar cycle as well.

=== Ralf Butscher

science.de

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