Sunspots are getting weaker for years. (c) Southwest Research Institute
Reading The sun seems to have fallen into a kind of hibernation. Numerous observations suggest that solar activity is getting weaker and weaker - even though the sun's eleven-year cycle of activity is expected to peak next or next year. "This is very unusual and unexpected, " said Frank Hill of the National Solar Observatory at a conference in New Mexico. The next solar cycle with the number 25 could possibly turn out completely, it was said at the conference. This is shown by independent measurements from inside the sun, from its surface and from its atmosphere, the hot corona.

Normally, the solar activity fluctuates in an eleven-year rhythm. At the height of the cycle, the sun's surface is littered with dark spots. At these black spots, the sun's magnetic field is especially strong. Frequently, the sunspots emit glowing prominences and sun flares, sometimes they hurl massive plasma bubbles into space.

In recent years, however, the spots have become scarce. The sun experienced an extensive minimum activity from 2007 to 2010. Activity only picked up again last year, but forecasts suggest that the upcoming maximum, expected in 2012 or 2013, will be rather weak. Cycle number 25 could become even more modest. "The start could be delayed by a few years, or the cycle could be over, " says Hill.

There are several signs of this trend. Thus, in the interior of the sun certain currents should already show themselves, which usher in the new cycle. But there is no sign of it, Hill reported in New Mexico. The sunspot's magnetic field has also been weakening for years. If the trend continues, the magnetic field strength in the next cycle is insufficient to produce visible sunspots. display

Another anomaly shows up in the corona of the sun. This hot shell is punctuated by slightly cooler regions, the so-called coronal holes. Normally these holes travel to the poles of the sun during the solar minimum, but this march has been delayed. The corona still contains magnetic remnants of the previous cycle number 23. "Perhaps the current cycle is too weak to displace these residues. That would be a big dilemma for the theorists, "says Richard Altrock of the Air Force Research Laboratory. "No one knows how the sun will behave then."

Richard Altrock (Air Force Research Laboratory): American Astronomical Society Solar Physics Division Meeting 2011 Frank Hill (National Solar Observatory) et al. American Astronomical Society Solar Physics Division Meeting 2011 William Livingston (National Solar Observatory) et al. American Astronomical Society Solar Physics Division Meeting 2011 science.de - Ute Kehse

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