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June 2006

The long wait

HAO researchers have issued a forecast for the next solar cycle. But they won't know until about 2010 whether it is accurate.

Sasha Madronich
HAO scientists Mausumi Dikpati (left), Peter Gilman, and Giuliana de Toma examine results from the Predictive Flux-transport Dynamo Model. The backward C shapes are the modeled solar convection zone. (Photo by Carlye Calvin, UCAR.)

Sometime in the next four or five years, Mausumi Dikpati and her colleagues in HAO will know whether they have succeeded in a groundbreaking attempt to predict the next solar cycle. For now, they can only continue with their research and keep their fingers crossed.

The forecast, based on an innovative computer model that tracks the influence of past sunspots on solar cycles, calls for the coming cycle to be 30-50% stronger than the last one. It will also begin as much as a year late.

If the forecast is correct, it will represent a significant scientific breakthrough because researchers have not yet been able to explain precisely why sunspot activity peaks and wanes on a cycle of 10 to 12 years. They also have not been able to predict the timing of the cycle with great accuracy. Several other research teams have issued competing forecasts—but those forecasts are based on statistics or empirical correlations, rather than on a physical model.

An accurate forecast would have significant benefits for society. Solar storms, which are more common when the solar cycle reaches maximum, can disrupt communications and power systems and affect the orbits of satellites.

Satellite operators try to time their missions to take advantage of weak solar activity because solar peaks heat the outer atmosphere and increase the drag on orbiting satellites.

"This is a significant breakthrough with important applications, especially for satellite-dependent sectors of society," says HAO's Peter Gilman, who worked with Mausumi on the forecast. "Mausumi's model points the way to a much deeper understanding of the magnetism of the Sun, and how often solar storms of the future will impact Earth and human activity."

HAO's Giuliana de Toma also worked on the forecast. The scientists published their prediction of the next solar cycle, known as cycle 24, in Geophysical Research Letters in March.

Several experts are offering up competing predictions. A group led by Leif Svalgaard of ETK (a Houston-based consulting firm) predicts the weakest peak in over a century. Using a different technique, David Hathaway and Robert Wilson of NASA are calling for the strongest cycle since the 1950s, which is closer to the HAO prediction.

Mausumi isn't worried. She notes that the first part of the forecast has already been verified: the present solar cycle has gotten off to a delayed start. Moreover, the model has performed almost flawlessly in recreating the strength of past solar cycles.

"It is not a tense situation for me," she says. "I am confident about the model calculation."

The problem with statistics

Solar magnetic activity has piqued scientists' curiosity ever since Galileo first observed sunspots in 1610. Sunspots, dark regions of concentrated magnetic fields, are associated with solar storms that can buffet Earth's atmosphere with charged particles, especially during the peak of the solar cycle. By analyzing the long-term sunspot record, scientists have tried to estimate the strength and timing of upcoming solar cycles based on statistical relationships between one cycle and the next. Among the most important correlations:

  • Strong cycles tend to follow short ones.
  • Strong cycles generally reach their peaks more quickly than weak ones.
  • High minima are typically followed by high maxima.

But these statistical techniques are not foolproof. They rely on correlations but are not rooted in physical understanding. And they greatly overpredicted the strength of the last solar cycle, cycle 23, calling for a maximum sunspot number of about 160 when the actual number was just 121.

When the prediction for cycle 23 proved to be wrong, Mausumi was as surprised as other solar researchers. Yet she and her colleagues were already working on a model that can now reproduce the character of the last peak well. According to that model, each solar maximum is linked not just to the previous cycle, but to the last several. As Mausumi explains, "The seeds for cycle 23 came from cycles 20, 21, and 22."

Solar
This graphic shows how magnetic fields are recycled to produce sunspots within the solar convection zone (the top 30% of the solar interior, shown in white, surrounding the radiative core, in orange). Because the sun rotates faster at the equator than the poles, the north-south (poloidal) magnetic field (a) gets twisted into an east-west (toroidal) field (b). Pockets of enhanced toroidal field rise to the surface, twisting in the process, and emerge to create sunspots (c, upper right). Magnetic flux emerges and spreads outward as the spots decay. Panels (d) and (e) show the conveyor belt of plasma flow (yellow) carrying the surface magnetic flux toward the poles—reversing the polar field—and eventually downward and back toward the equator. New sunspots eventually form in the poloidal field (f), which is now reversed from that in (a). (Figure by Mausumi Dikpati, Peter Gilman, and Giuliana de Toma.)

A conveyor belt of sunspots

Mausumi began modeling solar cycles for her doctoral dissertation at the Tata Institute in Bangalore, India. After graduating in 1996 and joining HAO as a postdoctoral fellow, she and HAO colleague Paul Charbonneau (now at the University of Montreal) completed the first version of their groundbreaking solar-dynamo model. Mausumi spent years improving the model, known as the Predictive Flux-transport Dynamo Model, with Paul, Peter, Giuliana, Keith MacGregor, Matthias Rempel, and other colleagues in HAO.

The model simulates a current of plasma, or electrified gas, that circulates between the Sun's equator and its poles over a period of about 17 to 22 years. The plasma is believed to act like a sort of conveyer belt, transporting imprints of sunspots that occurred during the previous two sunspot cycles.

The model is based on research indicating that the sunspot process begins with tightly concentrated magnetic field lines in the solar convection zone (the outermost layer of the Sun's interior). The field lines 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 only about 1-2 meters per second (2-4 miles per hour). The increasingly concentrated fields become stretched and twisted by the internal rotation rate of the Sun, which is faster at the equator than at the poles. As the fields approach the equator, they become 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.

One of the innovative aspects of the Predictive Flux-transport Dynamo Model is its three-dimensional perspective. It tracks motion not only on the Sun's surface but throughout the depth of its convection zone, which extends nearly a third of the way toward the solar core.

Using observations

The HAO model relies heavily on data from helioseismology, a relatively new discipline of tracking sound waves reverberating inside the Sun to reveal details about the interior, much as a doctor might use an ultrasound to see inside a patient. The helioseismology data allowed the HAO team to infer the leisurely pace of the subsurface flow and the sunspot seeds it carries. Variations in the speed of this flow appear to determine the length of time between cycles, while the presence or absence of magnetic remnants from the last three cycles helps determine the strength of the next one.

Sasha Madronich
X-ray flares (visible in the upper right), coronal mass ejections, and other phenomena that occur at solar maximum can wreak havoc on satellite-
dependent industries. (Image courtesy NASA.)

Giuliana is focusing on importing data into the model from two sources: magnetic flux measurements from the National Solar Observatory at Kitt Peak in Arizona, which is available for the last three solar cycles, and the historical sunspot area record which goes back to 1874, when the Royal Greenwich Observatory in London started a program to monitor sunspots. By using both sources and averaging the data over roughly half-year increments, the team has been able to extrapolate back in time and test the model against the last eight solar cycles. The results have been impressive: the model has simulated the strength of those cycles with more than 98% accuracy, including the unusually active 1957–58 maximum.

Outside researchers have been impressed. For the HAO model to capture the 1957–58 maximum is "quite amazing," says NASA's David Hathaway. "It says volumes about the validity of Mausumi's model."

For now, all eyes are beginning to turn to the Sun's midlatitudes, where the first sunspots of the new cycle will appear. Cycle 23 is in its final phase, with sunspots occurring near the equator. Solar cycles usually overlap by a year or two, so the first spots of cycle 24 could appear in the 25–30° latitude band at any time.

Mausumi is already looking further to the future. She and her colleagues are working on a prediction of cycle 25. They are also conducting other research, such as simulating the northern and southern hemispheres of the Sun separately.

"There is always more work to do," she says. "The Sun is a fascinating place, and we still have a lot to learn."

• by David Hosansky and Bob Henson


In this issue...

The long wait

New book helps water utility managers grapple with climate change

Katy Schmoll, COMET win awards in May

Climate change meets the arts

Random Profile: Meral Demirtas

UCAR Child Care Center accreditation

Delphi Question: Protecting polyamorous individuals

Just One Look


 

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