Read on Inexorably, the current solar cycle is nearing its peak: Almost every day, the sun now throws glowing matter into space in spectacular explosions. Nasa researchers expect the maximum for mid-year, with strong activity expected to continue until the end of 2001. The impressive eruptions - on 18 January, for example, the sun excited with a huge eruption in the form of an eight, on January 31 she blew a kind of smoke ring into space - can cause more aurorae on Earth, disturb the radio traffic or even lead to major power outages - as in 1989 in the Canadian province of Quebec, at the last solar maximum.

According to NASA physicist David Hathaway of the Marshall Space Flight Center, the number of sunspots will peak around mid-2000. The number of sunspots is the measure of the activity of the sun, since the sun flares and other eruptions are mostly related to the strong magnetic fields of the spots. "The latest calculations suggest that the number of spots will be slightly lower than initially expected, " said Hathaway, "probably about the same magnitude as the last two maxima in 1989 and 1978, above the average of all observed solar cycles since mid-summer of the 18th century. "At present, the sunspot number is over 200. On average, Hathaway expects about 150 spots during the maximum. Their number, however, is subject to strong fluctuations.

The sun shows various types of eruptions, which are summarized under the term "protuberances": The so-called sun flares ("flares") are eruptions on the surface of the sun. Within a few minutes, matter is heated to several million degrees. A flare releases as much energy as the explosion of one billion megatons of TNT. This energy is released in the form of hard electromagnetic radiation in the gamma and X-ray range, as well as ejecting protons and electrons, which take only minutes to reach the earth.

Normally, the flares occur in sunspots, along lines where the magnetic field changes polarity. Spaceweather.com reports that two large speckle groups with a complex magnetic field are hoping for mid-range flares (class M) in the near future. The flares are classified according to their brightness in the X-ray light: The largest belong to the class X and are ten times as energetic as the middle solar torches of class M. Ten times less energy radiate the class C flares. display

A second type of protuberance is "coronary mass ejection" (CME's). These are large bubbles of ionized gas that have their own magnetic field. The sun emits such a bubble for several hours. The CME's are often associated with sun flares, but not always. Both are caused by changes in the local magnetic field. At the solar minimum, there is about one CME per week, while the maximum is two to three a day.

On February 17, the solar observatory Soho observed a CME coming directly to Earth. As the cloud approached the earth, it seemed like a growing yard around the sun. Since CMEs move much slower than the particles from the flares, they take two to three days to reach the earth. On the 19th of February the time had come: the cloud, which was twice as fast as the normal solar wind and therefore a so-called shock wave in front of itself, hit the magnetic field of the earth. Since the cloud's field had a strong south facing component, it had the potential to partially cancel the Earth's magnetic field. This would have allowed the charged particles to break through the Earth's radiation belts and possibly cause auroras in mid-latitudes. This time, however, this did not happen: Shortly after the collision, the field of cloud turned around.

On the 23rd of February the next particle storm hit the earth: this time a weak spot in the sun's magnetic field was responsible. It created a hole in the corona, the solar atmosphere, through which the charged particles of the solar wind could escape twice as fast as usual. The hole, which has now disappeared on the back of the sun, rotates once every twenty seven days around the sun's axis. About once a month, the hole, which has existed for seven months, points directly to the earth and can also cause polar lights, satellite interference and the like due to the increased solar wind.

It is only gradually becoming possible to predict the "space weather" -thus disturbances of the earth's magnetic field due to increased radiation from the sun. The space observatories Soho, Advanced Composition Explorer (ACE) and Yohkoh are the main contributors. Soho and ACE are 1.5 million kilometers from Earth in orbit around the Sun, where they need a year to orbit just as Earth does. As a result, they always stand exactly between the earth and the sun and can observe the central star without interruption. On March 15 NASA wants to send the satellite image into an elliptical orbit around the Earth. Image is designed to measure the response of the Earth's magnetic field to solar activity, thereby also enabling a better prediction of space weather.

"The more our society becomes dependent on satellite television, satellite navigation, cell phones, and other space-age blessings, the more important it is to predict geomagnetic and solar storms - possibly as important as determining the chances of rainfall the next day, " NASA writes, "Unfortunately, the space weather forecast is 40 to 50 years behind the usual weather report." Image should finally give a larger overview of operations in the magnetosphere of the earth and measure how the shield reacts to the particle rush from the sun. "Just think how the geosynchronous weather satellites have improved meteorologists' position, " says Dennis Gallagher of Nasa's Marshall Space Flight Center. "Nowadays, anyone can spot a hurricane on a satellite image. If it is now possible to view images of ringstrings, plasma phospae, and magnetopause in real time, our subject will also change dramatically. "

Ute Kehse

science.de

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