Reading aloud Strictly speaking, a total solar eclipse on Earth is not totally total. Because the sun is much bigger than it appears to us. If the moon covers the visible solar disk, the outer solar atmosphere is revealed: a milky-white radiance that can extend far out into the room up to 20 solar radii. This corona ("crown, wreath") is a million times fainter than the visible solar surface, the photosphere. Therefore, it can only be observed when the moon - or a special device in a telescope - dwarfs the photosphere. As such a Moonshade moved along a narrow strip from the Bering Strait across North America to North Carolina at the solar eclipse of August 7, 1869, Charles A. Young and other American solar scientists made a startling discovery. At a certain wavelength in the green light, the spectrum of the corona showed a bright line, which had to be emitted by a very specific type of atom. But no known element on earth has this property. And even on the sun's surface, such emission lines are undetectable. It must be a hitherto unknown element, therefore, the researchers suspected. A few years later, it even got a name: Coronium. Only in 1940 could the Swedish physicist Bengt Edlén refute this hypothesis. He found out that the green line is derived from iron atoms. But these must have lost 13 of their 26 electrons, so be highly ionized to show the spectral property. The astonishment was perfect: for 13 times ionized iron corona temperatures of over one million degrees are required. And that is paradoxical. How can the corona be more than 200 times hotter than the 5500 degree hot photosphere below? Because heat flows from the approximately 15 million degrees solar center into the cold space, and the greater the distance from the center of the sun, the cooler it gets. Why does the corona therefore have temperatures that are 150, 000 to 300, 000 kilometers below the photosphere?

That the corona is really that hot is proved by x-rays, which are released only there, but not on the sun's surface. So there must be a physical mechanism that heats the corona above the photosphere. But which energy source is responsible for this and how the heat is transported within the corona has been one of the great unresolved questions of solar physics for decades.

Meanwhile, astronomers know quite well the composition of the ionized gas that makes up the corona. They know that the Sun's glory is so low-density that no vacuum created in the lab on Earth can compete with it-only ten grams of matter per million cubic meters. And they found out that the corona is interspersed with a wild jungle of magnetic field lines. Where they do not curve back onto the sun's surface but extend into space as open lines, the corona has huge holes. There it is about 30 percent less dense and two to five times cooler.

But all these findings give no explanation for the heating mechanism. Many speculations have been developed over time to solve the puzzle: that dust grains from space fall on the sun, releasing kinetic energy; that sound waves propagate from the sun's inside via electrically charged particles to the corona and generate shock waves there or that the energy comes from the magnetic field. Much has been said for the last hypothesis. For example, the American-European SOHO (Solar and Heliospheric Observatory) has discovered a myriad of magnetic loops that cover the sun's surface like a carpet, renewing itself every 40 hours. display

"When I saw the data for the first time, I nearly fell off my chair, " recalls Joseph Gurman of NASA's Goddard Space Center, the SOHO American Project Manager. "That could be the mysterious source of energy." The researchers encountered a veritable barrage of explosions at the borderline of the convection cells - the rising gas bubbles from the sun's interior, which cover the sun's surface like a network of 30, 000-kilometer honeycombs. The fireballs blaze for a few minutes and are about the size of the Earth - so much smaller than the huge "flares", which repeatedly hurl large amounts of sun matter into space. Richard Harrison of the Rutherford Appleton Laboratory in Oxfordshire, England, calls the fireballs "solar blinkers" (sunblinkers): "They seem to release magnetic energy and somehow release it into the corona."

Arnold Benz and his colleagues at the Swiss Federal Institute of Technology in Zurich estimate that up to 20, 000 such flashes of lightning per second discharge on the solar surface and that they are based on a kind of magnetic short circuit. These occur when extremely strained magnetic lines collide and break. This allows individual gas bubbles to be heated to a billion degrees and also accelerated. The gas is then quickly distributed in the corona and cools down in the course of a quarter of an hour, the researchers suspect.

"So there is no forest fire, but many thousands of camp fire, " says Philip Scherrer of Stanford University, the alleged heating mechanism of the corona. "But we still do not know how the energy from the magnetic field carpet is transported into the corona." And maybe the Corona Paradox now even reverses to the other extreme. "Meanwhile, we've tracked down more than enough energy sources to heat up the corona, " says Gurman. "Even a thousand times more than we need."

In addition, there has been another contention hypothesis since May: Ron Moore of NASA's Marshall Space Flight Center and his colleagues have discovered "microfilares" with the help of SOHO and the Japanese X-ray satellite Yokoh - solar flares as big as Earth within five minutes release the energy of ten million hydrogen bombs. They constantly sweep across the sun's surface and also heat the corona continuously.

Further observations are needed to expose the heat sources and to measure the energy flows more accurately. For the year 2004, the launch of the international probe Solar-B under the direction of the Japanese Space Agency ISAS is planned. She should devote herself mainly to the study of the corona. Maybe astronomers will finally snatch their secret from the hot outer solar atmosphere.

Rudolf Kippenhahn, === Rüdiger Vaas

© science.de

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