NCAR model may explain sunspot activity and predict solar storms
Scientists have long wondered why sunspot activity tends to ebb and flow in cycles of about 11 years. Now a research team led by HAO's Mausumi Dikpati may have found the key: a current of plasma that circulates between the Sun's equator and its poles.
The finding could help society better predict sunspots, which are regions of concentrated magnetic fields quite often associated with powerful solar storms that can buffet Earth's atmosphere.
The team published its findings in February in The Astrophysical Journal. It concluded that the current of plasma, or electrified gas, acts as a sort of conveyor belt by slowly transporting remnant magnetic signatures of old cycles' sunspots from the Sun's surface to the interior, where they lead to a new generation of magnetic fields for producing sunspots. Using this theory, the team created a model that successfully accounts for the 11-year duration of the solar cycle as well as such mysterious events as the reversal of the Sun's -magnetic north and south poles that occurs during the maximum of each sunspot cycle.
"In our model, we can show how physical processes relate the surface signatures of solar magnetic fields from old cycles to that of the new cycle," says Mausumi, the article's lead author. "Our results are consistent with solar features, including the cycle."
Mausumi is working with HAO's Giuliana de Toma, Peter Gilman, and Oran White, as well as with Charles Arge (CU and NOAA).
Scientists for years have known about the current of plasma, or the meridional flow, which moves at about 20 meters (66 feet) per second near the surface. But they had not previously connected it to sunspot activity.
According to the HAO model, the tightly concentrated magnetic field lines that form sunspots near the Sun's equator imprint the moving plasma with a type of magnetic signature. As the plasma nears the poles, it is deflected toward the base of the solar convection zone, which is about 200,000 kilometers (124,000 miles) beneath the surface of the Sun. The denser plasma at that depth and a strong radial shear further concentrate the magnetic fields, and converts them into spot-producing fields.
On its return, the plasma appears to move deep below the Sun's surface at a speed of about one meter (three feet) per second or slower. As it nears the equator, the spot-producing fields amplify, -eventually causing these strong magnetic fields to rise up, tear through the Sun's surface, and create new sunspots.
Since the plasma flows toward the equator, this theory explains why sunspots appear mostly in the Sun's mid-latitudes early in the solar cycle and then gradually become more common near the equator. The model shows why the Sun's radial fields (produced from the decaying sunspots) move away from the equator. It's because the fields are influenced by the surface plasma flow that moves toward the poles.
Mausumi and her colleagues tested their model with data from the most recent solar cycle, known as cycle 23, which was unusual in that the magnetic reversal of the Sun's north and south poles took place slowly.
The team put meridional flow measurements from NASA's Solar and Heliospheric Observatory into the model. The model successfully reproduced the solar cycle, showing that a near-surface plasma flow slowdown and temporary reversal back toward the equator correlated with the delay in the magnetic changes in the poles. The team plans to further test the model with observations that have been taken over the past 20 years.
The research may represent a breakthrough in helping society better prepare for solar activity. That's because the model indicates the meridional flow, which takes about 17-22 years to move from the equator to the poles and then beneath the surface, contains imprints of sunspots that go back at least two solar cycles. By analyzing these past solar cycles, researchers may be able to forecast sunspot activity about two solar cycles, or 22 years, into the future. Should such forecasts prove accurate, society could anticipate solar storms, which sometimes disrupt communications and power systems and expose astronauts to high amounts of radiation.
In a few months, the NCAR team is planning to release a prediction of solar cycle 24, which is likely to begin about 2007 to 2008.
The research also may have implications for understanding "G stars," which are stars that have similar properties to the Sun. Observations have shown that the faster G stars rotate, the more starspots they produce. This may indicate that the plasma flow on such stars is speeded up, thereby transporting starspots more quickly, creating faster -magnetic cycles, and making these faster-rotating G stars more active than the Sun.
"In all G stars, a similar kind of oscillatory dynamo may be operating," Mausumi said.
NASA's "Living with a Star" -program helped fund the research. ĚDavid Hosansky