Solar Activity Cycle Solved?

Have astronomers figured out what drives the Sun's electromagnetic activity cycle? That question seems to be answered in the following Sky and Telescope article describing some recent work done in this area. Excerpt:

March 8, 2006 Scientists at the National Center for Atmospheric Research in Boulder, Colorado, have declared a breakthough in understanding our Sun's 11-year activity cycle. And they are using their new model to make predictions: that the next solar cycle will be 30 to 50 percent stronger than the one now ending, and that it will begin 6 to 12 months late.

Team leader Mausumi Dikpati described the group's findings at a March 6th NASA press conference. "For the first time we can predict the strength of the 11-year solar activity cycle based on the underlying solar physics," she said. The breakthrough came from recognizing a slow, large-scale circulation pattern involving much of the Sun's bulk. In addition to other, smaller-scale motions, solar material flows in a huge, slow "conveyor belt" from near the Sun's equator toward the poles, then sinks down to the bottom of the solar convection zone 200,000 kilometers (124,000 miles) deep. There it flows back toward the equator by a mere 1 meter per second, then rises back up to complete the loop.

Even though sunspots don't last long on the surface, when they break up they leave magnetic traces embedded in the near-surface material. When this material sinks down to the bottom of the convention zone, the Sun's differential rotation at that great depth stretches the remnant magnetic fields east-west, thereby strengthening the fields (drawing on the Sun's rotational energy). When the newly strengthened embedded fields get carried up again, they become the seeds for a new solar cycle.

The scientists based their model on helioseismology studies of motions in the Sun's interior, along with records of the last 12 solar cycles going back to 1880. Their model successfully simulates the timing and strength of the last eight cycles. "So we have confidence in predicting the future," Dikpati said. The last sunspot cycle peaked in 2001–2002; we're now just about at sunspot minimum. The team predicts that the first spots of the next cycle won't appear until late 2007 or early 2008.

Solar activity — sunspots, flares, coronal mass ejections, and other violent magnetic events on the Sun's surface — has many effects on and around Earth. Solar outbursts interfere with radio communications, cause auroras to light the night and electricity to surge in power lines, threaten satellites and astronauts with high-energy protons, and degrade the accuracy of GPS systems. Therefore, accurate solar forecasts are highly valued.

A few years ago, during a heavy outburst of solar activity that accompanied the last peak, the U.S. and Canada saw several communications satellites zapped by the particles described in the last paragraph. The circuits were fried, which led to massive radio and television blackouts. That period also saw the phenomenon of the Northern Lights to be observed at latitudes much further to the south than usual. So to be able to predict the Sun's activity would, if nothing else, give us the opportunity to position our communications satellites on the night side to avoid having them get their wiring scrambled again.

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