CHINESE astronomers recorded them as far back at 800 BC. Aristotle thought they were physically impossible. And Galileo’s observations of them, with his newly invented telescope, led to accusations of heresy by the Inquisition in the 1630s. For centuries, astronomers have recorded sunspots blossoming like exotic dark flowers on the sun’s surface and then fading into obscurity after a few days. And while we know that sunspot activity waxes and wanes in cycles lasting around 11 years, what drives this unceasing pattern has remained a mystery.
Now researchers at the High Altitude Observatory at the US National Center for Atmospheric Research (NCAR) in Boulder, Colorado, think they have found the answer. The key, they believe, is a current of gas that circulates between the sun’s equator and its poles, dragging huge portions of magnetic field with it. Incorporating this flow into their latest computer model of the sun, Mausumi Dikpati and her colleagues at NCAR can account for sunspot behaviour. What’s more, Dikpati and her colleagues believe they may soon be able to forecast sunspot activity more than 20 years into the future.
Sunspots are more than a mere curiosity, they are windows on the sun’s moods. Groups of sunspots lasting a few days appear close to the equator at times when giant solar storms hurl massive eruptions of charged particles into space. These eruptions can play havoc with communications on Earth as the burst of particles passes by. In 2002 a huge solar eruption damaged Japan’s Nozomi space probe on its way to Mars, temporarily cutting its communications with Earth.
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When the sun is calm, fewer sunspots pop up, but these quiet times, too, can have an effect on Earth. During a period called the Maunder minimum, starting in 1645, no sunspots were recorded for 70 years and the world suffered a prolonged spell of cold weather – a mini ice age, in fact. This has led some climate modellers to think that our weather may be more closely linked to the sun’s activity than we think.
The sun reaches the height of its tantrums every 11 years or so, following a cycle that appears to be in sync with the maxima and minima of sunspot activity. This period isn’t consistent, however. Sometimes the interval between sunspot maxima is as short as nine years, other times it is as long as 14 years. Until now, no one has been able to explain this variation. All we know for sure is that sunspots are a manifestation of the sun’s complex magnetic field.
Like Earth, the sun has magnetic north and south poles, though its average magnetic field is hundreds of times stronger than Earth’s. Yet observations made as far back as 1908 by American astronomer George Hale showed that the magnetic field at sunspots is some 3000 times stronger still. This concentration of the magnetic field arises from the way the sun rotates. It is not a rigid sphere but a ball of plasma – gas too hot for the atoms to hold together, so they break up into negatively charged electrons and positively charged ions. This allows different latitudes of the sun to spin at different rates. For instance, while the equator takes 26 days to make a complete revolution, regions closer to the poles take several days longer.
The moving charges in a plasma create magnetic fields, just as a current flowing in a wire does. The differential rotation leads to distortion in the magnetic field, eventually causing the magnetic field lines to wrap around themselves in twists and coils. The effect is strongest 200,000 kilometres below the surface, at the boundary between the sun’s inner layer, called the radiative zone, and a surrounding layer called the convection zone (see Diagram).
We already know that as the field lines twist around each other, the magnetic field becomes stronger. Eventually it becomes unstable and part of the coiled-up field unravels, sending a loop of magnetic field flying up through the sun’s interior like a hosepipe full of water. Where the two points of the loop punch through the sun’s outer shell (the photosphere), sunspots form. At these sites, magnetic field lines are packed tightly together, which is what gives sunspots their intense magnetism.
Sunspots do not just pop up at random locations on the photosphere. At the start of the solar cycle, sunspots appear near to two lines of latitude 30 degrees either side of the equator. As the cycle progresses, these belts where sunspots emerge creep closer to the equator. But why they do this nobody knows, and there are other mysteries, too. Why are some solar cycles more intense than others? And what happens to the magnetic fields brought to the surface by sunspots?
These are questions that Dikpati and her colleagues think they can now answer. The key, they say, lies in a slow but steady movement of plasma on the sun’s surface, from the equator towards the poles, called the meridional flow. Until now, nobody thought the meridional flow could have much to do with sunspots, because with a modest speed of 20 metres per second it is far slower than other flows on the sun. What’s more, the meridional flow travels from the equator to the poles, while the bands in which sunspots appear and fade shift in the opposite direction. Despite this, Dikpati and her colleagues think it cannot be ignored. “Our model includes meridional circulation for the first time,” she says.
In a flowing plasma, the interaction between the moving, charged particles that make up the plasma and the magnetic field they generate results in the magnetic field lines being “frozen” into the plasma: as the plasma flows, the field flows with it. According to the NCAR simulation, the sunspots’ magnetism becomes embedded in the plasma making up the meridional flow. Even after a sunspot has faded, it leaves a magnetic imprint that is carried towards the poles in the plasma of the meridional flow. During the course of its journey, the plasma cools and becomes denser. By the time it reaches the poles it is so dense it sinks into the solar interior, where it is compressed even further by the high pressure inside the sun. This further concentrates the magnetic field imprinted on it by the sunspots.
No one has been able to observe what happens next, but Dikpati and her colleagues suspect that it descends as far as the base of the convection zone and then creeps slowly back from the pole to the equator. According to her calculations, the dense viscous plasma drags along the base of the convection zone at a mere 1 metre per second, slowing down even further near the equator, where the flows in the northern and southern hemispheres meet.
It is this return mechanism, Dikpati believes, that causes the belts of sunspot activity to shift towards the equator as the cycle progresses. As the plasma moves along the base of the convection zone, it crosses the coiled up magnetic field lines that circle the sun. This induces an electric current in the slow-moving plasma, which in turn amplifies the equator-bound magnetic field. Eventually the magnetic field becomes so strong that it destabilises and rises up to produce sunspots again. In effect the returning meridional flow deep below the surface of the sun acts like a conveyor belt, carrying a magnetic memory of sunspots’ magnetic fields which then “seed” future generations of sunspots. More sunspots appear at the equator because the meridional flow there is slower, giving the magnetic field longer to wind up.
Dikpati and her team reported their results in a paper published last month in Astrophysical Journal (vol 601, p 1136). What has given them confidence in their calculations is that their model shows a relationship between the speed of the meridional flow and the 11-year duration of the solar cycle in a way that no other has done before. “The faster the meridional flow, the shorter the solar cycle, and vice versa,” says Dikpati. And when they fed their model with meridional flow measurements made by the space-based SOHO solar observatory over the past eight years, it reproduced the main features of the last solar cycle including several previously inexplicable ones.
One of these hitherto mysterious features concerns the way in which the magnetic north and south poles recently switched places. Towards the end of each solar cycle, the sun’s magnetic field briefly disappears, before reappearing with north and south poles reversed. The peculiarity of the last solar cycle was that the flip happened 18 months later than usual. The NCAR team believes that the meridional flow can explain this.
We know from observations of the magnetic field of the sun that when its magnetic north pole lies north of the equator, all the sunspots in that hemisphere act like a magnetic south pole. And the reverse goes for the southern hemisphere. A record of the sunspots’ polarity remains imprinted in the meridional flow as it travels towards the poles, which have the opposite polarity. So when this part of the flow reaches the pole, the magnetic field imprinted in it starts to cancel out some of the field at the pole, and eventually gives the pole the opposite polarity. So why the delay at the end of the last cycle? When Dikpati’s team re-examined meridional flow measurements made by SOHO, they discovered that the flow had slackened, so it took longer for it to cancel out the fields at the pole.
Dikpati’s ideas have found favour with other solar researchers, though not everyone is yet convinced that the case is proved. At NASA’s Marshall Space Flight Center in Huntsville, Alabama, a team led by sunspot expert David Hathaway drew together measurements made at three observatories over the past 128 years. They looked at how both the position and strength of the sunspots change with time, and then compared these measurements with the NCAR model. Their results not only confirm the meridional flow idea, they also support the view that the flow acts as a magnetic memory (Astrophysical Journal, vol 589, p665). Hathaway’s team also found that a disruptive solar cycle with frequent giant sunspots sows the seeds of another disruptive cycle 20 years later. “It’s the premier model at the moment,” says Hathaway.
Eugene Parker of the University of Chicago in Illinois, one of the first physicists to produce a model describing solar activity, is more cautious. He concedes that meridional flow may be the answer, but suspects it will be a long time before we know for sure. “You are dealing with something like meteorology on the sun,” he says. He compares the problem to attempting to develop a full model of the Earth’s atmosphere. “Progress in 10 years will be substantial, but that doesn’t mean we’ll have the problem solved.”
Parker points to the difficulties of simulating flow at the base of the convection zone, where the plasma may be up to a million times as dense as it is at the surface. Steve Tobias, an astrophysicist at the University of Leeds, UK, is even more sceptical. “It is hard to see how you can get a strong enough meridional flow to push sunspots towards the equator,” he says.
Whatever the doubts, Dikpati’s model does appear to be better than any other so far at explaining features of sunspot activity. And she believes it could have forecasting powers too, because of the way the meridional flow holds a magnetic imprint of past sunspot activity. Her team’s simulations suggest that the memory of sunspots two cycles ago could be reflected in activity today – and that today’s meridional flow may be able to predict the sun’s behaviour 20 years from now. If this turns out to be so, such forecasts could be used to help scientists decide if it is safe to launch satellites or plan space walks with little risk of giant eruptions from the sun.
But for confident forecasts to be possible, Dikpati and her team at NCAR will have to demonstrate that their model can describe an entire solar cycle. So far, SOHO has only provided eight years’ worth of complete meridional flow measurements. To do the job properly, Dikpati says she will need at least 17 years’ data – though she hopes not to wait that long. The team is planning to extract meridional flow speeds from observations made over the past 20 years at the Mount Wilson Observatory in California, and put these into the model in the next few months. If these are as successful as the earlier calculations, we may finally be on the verge of understanding some of the sun’s most prominent yet mysterious features.