Science Spin September 2007

Solving the sunspot riddle

A funny thing happened on the Sun recently. Two sunspots appeared. That is remarkable in itself, as the Sun, between sunspot cycles, is in a quiet mode. But what was even more interesting was their magnetic signature or polarity. Sunspots usually appear in pairs and, for the previous decade or so the northernmost or leading one in the Sun's southern hemisphere always had a north magnetic signature. The new ones, seen in late July and mid-August, had the opposite magnetic signature.

To solar astronomers that can mean that the old solar cycle, number 23, is formally ended and a new one, number 24, is beginning. If this is true, over the next five years or more the numbers of sunspots will increase, then, over another five and a half years or so, will wane as the Sun goes quiet again.

A Sun with lots of sunspots is a stormy one which can hurl billion ton clouds of charged particles at us. These eruptions, called coronal mass ejections, have the power to disrupt radio communications on earth. Communications satellites are regular victims and astronauts in the Space Station must scurry to a specially shielded part of their orbiter if one is detected. They have even been known to knock out the electricity supply to major cities. On a brighter note, they also generate beautiful displays of the Northern Lights, or Aurora Borealis, similar to the fantastic displays of November 2003 which lit up the skies over Ireland.

The engine which drives the sunspot cycle has been a mystery for decades. Theories have been promoted, then discredited as new discoveries update knowledge of our nearest star. It's now generally accepted that sunspots are the visible ends of tubes of highly magnetic plasma, or flux which break through the Sun's surface in the forms of loops. But where does the magnetism come from? And why does it change polarity with each cycle? Even the Sun's north and south poles flip over in this process.

Theory

Dr Mausumi Dikpati, from the High Altitude Observatory at the National Council for Atmospheric Research in Boulder, Colorado, thinks she has all the answers. Not only does her theory explain how the sunspot cycle works, she claims it can also be used to predict the sun's behaviour up to twenty years in advance. Many of the components of her Flux Transport Dynamo Theory have been outlined before by other researchers but Dr Dikpati is the one of the first people to convincingly put it all together in a unified fashion.

One of the cornerstones of this theory is the fact that the interior of the sun rotates at a different speed to the outer convective layer. The shear layer where these two rotating shells meet is called the techocline and for some time scientists have theorised that this is the location of the Sun's dynamo and not its turbulent outer layers as had been previously thought. Another keystone of the theory is the discovery that the Sun has currents, somewhat analagous to the Earth's Gulf Stream. For her model Dikpati seized upon what astrophysicists call the meridional flow, a 20 metres per second leisurely migration of surface plasma from the Sun's equator north and south to its poles.

A convenient time at which to simplistically examine how this theory works is to take the Sun's northern hemisphere at the peak of the Sunspot cycle. Dozens of sunspots have blossomed and faded on the surface. As they decay their magnetic residue is carried northwards by the meridional flow. As the magnetic flux passes through the area of the Sun's north pole, it cancels out the polar charge of the previous cycle giving it the signature previously held by the south pole. (A mirror image of this process is also occurring in the southern hemisphere).

Just as major ocean currents form conveyor belts linking the surface with the seabed, so does the meridional flow link the surface with the tachocline, the junction between the core and the convective layer which move at different speeds. But here the Sun is upwards of a million times denser and the meridional flow barely inches along. It will take up to twenty years before they reach the latitudes where sunspots normally erupt.

Just as a child makes a snake out of a blob of plastecine by rolling it under its hand on the table, the tachocline winds up the magnetic fields into long snaking coiled flux tubes. The more they are coiled, the more their magnetic charge is amplified until, by the time they reach the sunspot latitudes, the charge has become so great it makes the tubes bouyant and they float upwards through thousands of miles of the convective layer. When they reach the surface a loop of flux tube protrudes but all we see are the two holes representing the bases of the loop. These are new sunspots some two cycles on from where we first took up the story. As they decay their magnetism is again transported northwards by the meridional flow and the cycle repeats itself.

Dr Dikpati visualises the process as a sort of magnetic memory tape looping through the sun, a tape which effectively programs future sunspot cycles. Each cycle is generated by the magnetic residue of an earlier cycle, a residue which provides each cycle with it's unique magnetic signature, or polarity, but can also influence its strength. Of course there are variables to be taken into account, variables such the strength and speed of the meridional flow, a factor which does vary from cycle to cycle.

Not only does her model explain the engine by which sunspots are created, it explains how the cycle occurs and how both the polarity of the Sun's poles, and individual sunspots, are altered with each cycle. It also goes some way towards explaining how the Sun manages to conserve its magnetic material, despite blowing billions of tons of it into space every year.

Forecasts

Earlier this year Dr Dikipati put her head on the block and said her sunspot model forecasts a very energetic forthcoming Cycle 24 which could be between 30 and 50% stronger than Cycle 23 just ended. To do this forecast she reached back to earlier cycles. Other forecasters, who use what are often called precurser methods, look at elements of the cycle just gone.

One such forecast was made by Miruna Popescu, a researcher at Armagh Observatory. She looked at the energy given off by coronal mass ejections towards the end of Cycle 23 just gone. She theorised that if they are very energetic, then there will be less magnetic material available for the next cycle.

Indeed the last two years of Cycle of 23 were noteworthy for a series of large coronal mass ejections on the Sun which hurled multi-billion ton clouds of magnetic material into space. Some of them were the largest CMEs ever recorded. She suggests that this reduces the amount of magnetism available for the upcoming Cycle 24. Other forecasters have even suggested that the Sun may be entering into a long quiet period, similar to the famous Maunder Minimum of the 17th Century when no sunspots whatever were recorded for many years.

Miruna is not yet laying bets that she's right. "Mine is an emperical method and one can never be really sure," she explained. For the moment she's quite lukewarm about Dr Dikpati's theory although she is impressed about the breadth of her model. For example, while most other theories simply look at what goes on on the surface, Dr Dikpati is probing deep into the Sun's interior.

"She's using some very good data sets. If it works it will be quite a breakthrough," she said. However, if the recent sunspots really do signal the start of Cycle Number 24, Dikpati's predictions are already wrong, said Miruna. "She predicted the new cycle would start in late 2007 or early 2008," Popescu points out.

But, Dr Dikpati says she has confidence in her forecasts because none of the previous models were complete dynamo based schemes.

"They all considered the least active portion, or minima, of the previous cycle past minima because in a minima polar field becomes strongest so they considered the polar field of the past minima and then they tried to predict the next cycle's maxima but there is no strong physical grounds for that."

"My scheme says features of the last minima cannot be the determining factor because you need two cycles," Dr Dikpati said recently. "To predict cycle 24 I need the polar field pattern of cycle 22 more than 23 or 21. There is a memory effect. The polar field of the previous minima is strongest just 5.5 years laier; but the sun's memory effect is 17 to 21 years."