Breakthrough Research to Improve Forecasts of Sunspot Cycle

Using a new computer model of the Sun, scientists have begun work on a groundbreaking forecast of the next cycle of sunspots. Mausumi Dikpati of the National Center for Atmospheric Research (NCAR) announced new research leading to an improved forecast of cycle 24 at the annual meeting of the American Astronomical Society (AAS) in Denver. Predicting features of the solar cycle may help society anticipate sunspots and associated solar storms, which can disrupt communications and power systems and expose astronauts to high amounts of radiation.

The forecast draws on research by scientists at NCAR’s High Altitude Observatory indicating that the evolution of sunspots is caused by a current of plasma, or electrified gas, that circulates between the Sun’s equator and its poles over a number of years. The forecasters believe the next solar cycle will begin in 2007 to 2008 if the plasma circulation, which has slowed down during the present solar cycle, continues to decelerate. That would mean cycle 24 would begin about a half-year later than if the cycles followed the standard 11-year span.

“We will spend the next several months incorporating additional plasma flow data into our model to determine the rising pattern of cycle 24,” explains Dikpati, a leader of the research team. “Our focus will be on when the cycle is likely to reach maximum and cause geomagnetic storms in Earth’s atmosphere.”

The next sunspot cycle is referred to as cycle 24 because of a numbering system that dates back to the eighteenth century. Solar scientists have tracked these cycles for some time but have not been able to model them with sufficient accuracy to make long-term predictions.

The team’s computer model, known as the Predictive Flux-transport Dynamo Model, 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 toward the end of each solar cycle.

The research may represent a breakthrough in helping society better prepare for solar storms. It focuses on the meridional flow pattern of plasma, which circulates between the equator and the poles over a period of about 17 to 22 years and is believed to transport imprints of sunspots that occurred over the previous two sunspot cycles. By analyzing these past solar cycles, scientists hope eventually to forecast sunspot activity about two solar cycles, or 22 years, into the future.

The work also may have implications for understanding stars that have similar properties to the Sun. Observations have shown that the faster such G stars rotate, the more disturbances they experience. This may indicate that the plasma flow on such stars is sped up, thereby transporting sunspots more quickly and creating more stellar storms. “In all G stars, a similar dynamo may be operating,” Dikpati says.

The model incorporates the current of plasma, which acts as a sort of conveyor belt of sunspots. The sunspot process begins with tightly concentrated magnetic field lines in the solar convection zone (the outermost layer of the Sun’s interior). They rise to the surface at low latitudes and form bipolar sunspots, which are regions of concentrated magnetic fields. When these sunspots decay, they imprint the moving plasma with a type of magnetic signature. As the plasma nears the poles, it sinks about 200,000 kilometers (124,000 miles) back to the convection zone and starts returning toward the equator at a speed of about one meter (three feet) per second or slower. The increasingly concentrated fields become stretched and twisted by the internal rotation of the Sun as they near the equator, gradually becoming less stable than the surrounding plasma. This eventually causes coiled-up magnetic field lines to rise up, tear through the Sun’s surface, and create new sunspots.

Since the plasma flows toward the equator, the theory explains why sunspots appear mostly in the Sun’s midlatitudes early in the solar cycle and then gradually become more common near the equator. Sunspots also become increasingly powerful with the progress of the solar cycle because the continuous shearing of the imprints of the magnetic fields by the denser plasma beneath the surface of the Sun increases the strength of the spot-producing magnetic fields.

In addition to Dikpati, the team includes NCAR scientists Giuliana de Toma, Peter Gilman, and Oran White, as well as Nick Arge of the University of Colorado and the National Oceanic and Atmospheric Administration. The NCAR team has received funding from the National Science Foundation and a NASA Living with a Star grant for its research. The National Center for Atmospheric Research and UCAR Office of Programs are operated by UCAR under the sponsorship of the National Science Foundation and other agencies.