The next sunspot cycle will be a year late and as much as 50% stronger than the last one, according to a forecast released Monday by scientists from NASA and the National Science Foundation.
Such predictions are vital because the solar storms associated with the sunspots not only endanger humans in space, but can slow satellites in orbit, disrupt communications, interfere with Global Positioning Systems and bring down power grids.
The most recent cycle, which peaked in 2001, was relatively weak with few significant disruptions reported. A major solar storm associated with the sunspot cycle on March 13, 1989, brought down the power grid in Quebec, darkening much of the Canadian province for nine hours.
The latest forecast was made using a sophisticated computer program developed by solar scientist Mausumi Dikpati of the National Center for Atmospheric Research in Boulder, Colo, and her colleagues.
The program is based on mapping of subsurface plasma flows discovered by NASA's Solar and Heliospheric Observatory.
The satellite observatory uses sound waves in the sun's interior to reveal the details of its structure, much as a doctor uses ultrasound to get images of internal organs.
Dikpati found that the solar cycle was powered by massive rivers of electromagnetic plasma flowing near the sun's surface from its equator to the pole and back again.
The flow is like a massive conveyor belt, carrying large quantities of plasma -- as well as isolated magnetic fields, or eddies -- from the equator to the pole and back over a 22-year period, about twice the 11-year period of sunspot cycles.
Dikpati's findings, released Monday, were published in the journal Geophysical Research Letters.
Sunspots -- first observed by Galileo in 1610 -- are dark areas on the sun's surface that are about 2,700 degrees Fahrenheit cooler than surrounding areas. They are known to be caused by the eruption of magnetic fields through the sun's surface, disrupting plasma and creating cool spots.
How they are generated and what controls their periodicity has been a matter of dispute since the cycle was discovered 150 years ago.
The magnetic eddies that break through the surface release enormous amounts of energy, sending sheets of ionized particles and ultraviolet radiation toward Earth.
Heat from the ultraviolet emissions causes the Earth's atmosphere to balloon slightly, increasing the drag on satellites in low-Earth orbit, including the International Space Station.
Forecasting the strength of sunspots is important to satellite operators and other businesses, said Joseph Kunches of the National Oceanic and Atmospheric Administration's Space Environment Center in Boulder.
Until now, such forecasts could be made only by extrapolating from past events, and those forecasts have not been very accurate.
Dikpati and her colleagues "give us something better than we have ever had before," he said.
NASA's solar observatory found that the river of plasma flows poleward near the sun's surface, then dives about 125,000 miles toward the center of the sun before surfacing again at the equator, creating a massive loop.
But the loop becomes distorted because the sun's equator and poles spin at different speeds.
The fast spin at the equator tugs the plasma sideways, putting massive kinks into the loop.
That injects energy and causes magnetic eddies to break through the surface, forming sunspots.
The sunspots form about 25 to 30 degrees north of the equator.
Dikpati and her colleagues used characteristics of one sunspot cycle to predict the next.
Looking backward over records from the last 12 sunspot cycles, she said, the team has been able to predict the timing and magnitude of each successive cycle with 98% accuracy.
They predict that the next sunspot cycle, called cycle 24, will begin in late 2007 or early 2008 and will produce sunspots across an area slightly larger than 2.5% of the sun's surface.
The cycle is likely to reach its peak about 2012.