(Above) Composite image of multiple solar flares on the sun, (below) Dr Choudhuri with his students Piyali Chatterjee (left) and Jie Jiang

In about five years, the sun will turn hyperactive — as it usually does every 11 years, and physicist Arnab Rai Choudhuri is trying to predict the level of its ferocity. On his computer at the Indian Institute of Science (IISc), Bangalore, Choudhuri has simulated the outer layers of the sun — the flow of blazing hot matter, and the twists and turns of the solar magnetic field — to produce the first forecast from India about the next solar cycle.

Working with doctoral students Piyali Chatterjee of IISc and Jie Jiang of the National Astronomical Observatories in Beijing, China, Choudhuri has predicted that solar cycle 24, expected to peak in mid-2012, will be one of the mildest in decades — 35 per cent weaker than the previous cycle 23 now in its tail end.

Every 11 years, the sun enters the active phase of its cycle marked by an increased number of sun spots and gigantic explosions on the surface; these are called solar flares and they eject enormous blobs of hot solar material into interplanetary space. These solar storms bombard earth with energetic photons and charged particles that can jolt the planet’s ionosphere and geomagnetic field and threaten satellites, telecommunications and even power grids.

The IISc forecast for solar cycle 24 is based on a model of the solar dynamo — Surya — recently developed by Choudhuri’s team to simulate the solar magnetic field whose twists and turns give rise to sun spots, the dark, relatively cooler regions on the surface of the sun. Solar cycle intensity is measured in terms of the maximum number of sun spots. The more the sun spots, the greater is the chance of solar flares.

“Computer simulations of the outer layers of the sun are now sophisticated enough for attempts to predict the activity of future solar cycles,” said Choudhuri, a professor of physics. “Models today allow us to switch from forecasts based purely on empirical observations of past solar activity to forecasts based on the actual dynamo processes in the solar interior,” Choudhuri told KnowHow.

In a paper published in the journal Physical Review Letters, Choudhuri and his colleagues say their simulations of the solar dynamo tally with observations of solar activity during cycles 21, 22 and 23, and predict a weaker cycle 24.

But the Bangalore physicist finds himself battling his own former student in what has turned out into a scientific duel over the behaviour of the sun. The forecast by the IISc-Chinese team challenges an earlier prediction by scientists at the National Centre for Atmospheric Research (NCAR) in Boulder, Colorado, who believe that cycle 24 will be one of the strongest yet.

Physicists Mausumi Dikpati and Peter Gilman at the NCAR last year used their own independent model of the solar dynamo to predict that cycle 24 would be 30 to 50 per cent stronger than the last one. The NCAR researchers say they have confidence in their forecast because in tests, their model has simulated the strengths of the past eight solar cycles with more than 98 per cent accuracy.

Dikpati said the disagreements in predictions by the two models stem from differences in the way each model factors in the solar magnetic field in the simulation of the dynamo.

“We’ve used our model to successfully predict the late onset of cycle 24,” said Dikpati. “Recent solar observations suggest that the cycle will begin only in early 2008 — something we had predicted three years ago,” she told KnowHow.

Disagreements over forecasts of solar cycles are not surprising. An international panel of experts appeared last month to be split over the intensity of cycle 24. In a forecast issued by the US National Oceanic and Atmospheric Administration on April 25 in coordination with the experts, half the panel predicted a moderately strong cycle, while the other half predicted a moderately weak cycle.

Scientists have issued cycle predictions only twice in the past. In 1989, a panel of experts met to predict cycle 22 which peaked in the same year. And in September 1996, scientists met again to predict cycle 23 — six months after the cycle began. Both groups were better at predicting the timing of the peak than the intensity, said Douglas Biesecker, who chaired the panel that met in April this year.

Some solar physicists have suggested that the solar dynamo is a chaotic system and predictions are impossible. But Choudhuri and other physicists who’re trying to simulate the solar magnetic field believe the dynamo has predictive components and a random — or unpredictable — component.

“This random component emerges from turbulence in the convection zone in the solar interior,” Choudhuri said. The IISc’s Surya model takes into account actual observational data on the magnetic field strength at the solar polar regions before the beginning of the cycle to compensate for the random components.

The polar magnetic field approach, said Choudhuri, allows the model to capture both the predictive and the random aspects to yield forecasts. He believes that factors that led the NCAR to successfully predict a late onset of cycle 24 do not influence the intensity of the cycle.

“Let’s wait and see which prediction is closer to reality,” Choudhuri said.