Scientists Predict Big Solar Cycle
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Dec. 21, 2006: Evidence is mounting: the next solar cycle is going to be a big one.
Right: An erupting solar prominence photographed by the Solar and Heliospheric Observatory (SOHO). [More]
Their forecast is based on historical records of geomagnetic storms.
Hathaway explains: "When a gust of solar wind hits Earth's magnetic field, the impact causes the magnetic field to shake. If it shakes hard enough, we call it a geomagnetic storm." In the extreme, these storms cause power outages and make compass needles swing in the wrong direction. Auroras are a beautiful side-effect.
Hathaway and Wilson looked at records of geomagnetic activity stretching back almost 150 years and noticed something useful:. "The amount of geomagnetic activity now tells us what the solar cycle is going to be like 6 to 8 years in the future," says Hathaway. A picture is worth a thousand words:
Above: Peaks in geomagnetic activity (red) foretell solar maxima (black) more than six years in advance. 
In the plot, above, black curves are solar cycles; the amplitude is the sunspot number. Red curves are geomagnetic indices, specifically the Inter-hour Variability Index or IHV. "These indices are derived from magnetometer data recorded at two points on opposite sides of Earth: one in England and another in Australia. IHV data have been taken every day since 1868," says Hathaway.
Cross correlating sunspot number vs. IHV, they found that the IHV predicts the amplitude of the solar cycle 6-plus years in advance with a 94% correlation coefficient.
"We don't know why this works," says Hathaway. The underlying physics is a mystery. "But it does work."
Left: Hathaway and Wilson's prediction for the amplitude of Solar Cycle 24. 
Astronomers have been counting sunspots since the days of Galileo, watching solar activity rise and fall every 11 years. Curiously, four of the five biggest cycles on record have come in the past 50 years. "Cycle 24 should fit right into that pattern," says Hathaway.
These results are just the latest signs pointing to a big Cycle 24. Most compelling of all, believes Hathaway, is the work of Mausumi Dikpati and colleagues at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado. "They have combined observations of the sun’s 'Great Conveyor Belt' with a sophisticated computer model of the sun’s inner dynamo to produce a physics-based prediction of the next solar cycle." In short, it's going to be intense. Details may be found in the Science@NASA story Solar Storm Warning.
"It all hangs together," says Hathaway. Stay tuned for solar activity.
Author: Dr. Tony Phillips | Production Editor: Dr. Tony Phillips | Credit: Science@NASA
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Hathaway points out that there are actually two types of geomagnetic activity:
"Only the first type has predictive value," says Hathaway. "Storms caused by solar wind streams come and go in a regular pattern that foretells the solar cycle. Storms caused by flares and CMEs don't have this property." To improve their results, Hathaway and Wilson used a technique developed by Joan Feynman to remove storms caused by flares and CMEs from their data.
Reference: Hathaway, D. H. and Wilson, R. M. 2006, "Geomagnetic activity indicates large amplitude for sunspot cycle 24", Geophys. Res. Lett. in press.
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