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Are the mysterious cycles of sunspots dangerous for us?

Galileo and some of his contemporaries left careful records of their telescopic observations of sunspots – dark patches on the surface of the sun, the largest of which can be larger than the whole earth. Then in 1844 a German apothecary reported the unexpected discovery that the number of sunspots seen on the sun waxes and wanes with a period of about 11 years.

Initially nobody considered sunspots as anything more than an odd curiosity. However, by the end of the nineteenth century, scientists started gathering more and more data that sunspots affect us in strange ways that seemed to defy all known laws of physics. In 1859 Richard Carrington, while watching a sunspot, accidentally saw a powerful explosion above it, which was followed a few hours later by a geomagnetic storm – a sudden change in the earth’s magnetic field. Such explosions – known as solar flares – occur more often around the peak of the sunspot cycle when there are many sunspots. One of the benign effects of a large flare is the beautiful aurora seen around the earth’s poles. However, flares can have other disastrous consequences. A large flare in 1989 caused a major electrical blackout in Quebec affecting six million people.

Interestingly, Carrington’s flare of 1859, the first flare observed by any human being, has remained the most powerful flare so far observed by anybody. It is estimated that this flare was three times as powerful as the 1989 flare that caused the Quebec blackout. The world was technologically a much less developed place in 1859. If a flare of the same strength as Carrington’s 1859 flare unleashes its full fury on the earth today, it will simply cause havoc – disrupting electrical networks, radio transmission, high-altitude air flights and satellites, various communication channels – with damages running into many billions of dollars.

There are two natural cycles – the day-night cycle and the cycle of seasons – around which many human activities are organized. As our society becomes technologically more advanced, the 11-year cycle of sunspots is emerging as the third most important cycle affecting our lives, although we have been aware of its existence for less than two centuries. We have more solar disturbances when this cycle is at its peak. For about a century after its discovery, the 11-year sunspot cycle was a complete mystery to scientists. Nobody had any clue as to why the sun has spots and why spots have this cycle of 11 years.

A first breakthrough came in 1908 when Hale found that sunspots are regions of strong magnetic field – about 5000 times stronger than the magnetic field around the earth’s magnetic poles. Incidentally, this was the first discovery of a magnetic field in an astronomical object and was eventually to revolutionize astronomy, with subsequent discoveries that nearly all astronomical objects have magnetic fields.  Hale’s discovery also made it clear that the 11-year sunspot cycle is the sun’s magnetic cycle.

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Sunspot 1-20-11, by Jason Major. CC BY-NC-SA 2.0 via Flickr.

Matter inside the sun exists in the plasma state – often called the fourth state of matter – in which electrons break out of atoms. Major developments in plasma physics within the last few decades at last enabled us to systematically address the questions of why sunspots exist and what causes their 11-year cycle. In 1955 Eugene Parker theoretically proposed a plasma process known as the dynamo process capable of generating magnetic fields within astronomical objects. Parker also came up with the first theoretical model of the 11-year cycle. It is only within the last 10 years or so that it has been possible to build sufficiently realistic and detailed theoretical dynamo models of the 11-year sunspot cycle.

Until about half a century ago, scientists believed that our solar system basically consisted of empty space around the sun through which planets were moving. The sun is surrounded by a million-degree hot corona – much hotter than the sun’s surface with a temperature of ‘only’ about 6000 K. Eugene Parker, in another of his seminal papers in 1958, showed that this corona will drive a wind of hot plasma from the sun – the solar wind – to blow through the entire solar system.  Since the earth is immersed in this solar wind – and not surrounded by empty space as suspected earlier – the sun can affect the earth in complicated ways. Magnetic fields created by the dynamo process inside the sun can float up above the sun’s surface, producing beautiful magnetic arcades. By applying the basic principles of plasma physics, scientists have figured out that violent explosions can occur within these arcades, hurling huge chunks of plasma from the sun that can be carried to the earth by the solar wind.

The 11-year sunspot cycle is only approximately cyclic. Some cycles are stronger and some are weaker. Some are slightly longer than 11 years and some are shorter.  During the seventeenth century, several sunspot cycles went missing and sunspots were not seen for about 70 years. There is evidence that Europe went through an unusually cold spell during this epoch. Was this a coincidence or did the missing sunspots have something to do with the cold climate? There is increasing evidence that sunspots affect the earth’s climate, though we do not yet understand how this happens.

Can we predict the strength of a sunspot cycle before its onset? The sunspot minimum around 2006–2009 was the first sunspot minimum when sufficiently sophisticated theoretical dynamo models of the sunspot cycle existed and whether these models could predict the upcoming cycle correctly became a challenge for these young theoretical models. We are now at the peak of the present sunspot cycle and its strength agrees remarkably with what my students and I predicted in 2007 from our dynamo model. This is the first such successful prediction from a theoretical model in the history of our subject. But is it merely a lucky accident that our prediction has been successful this time? If our methodology is used to predict more sunspot cycles in the future, will this success be repeated?

Headline image credit: A spectacular coronal mass ejection, by Steve Jurvetson. CC-BY-2.0 via Flickr.

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