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Sunspots

Sunspots
The sunspot group seen on 22 Sept 2000, the largest for 9 years, covered about 1/500th of the solar surface, 12 times the surface area of the EarthImage: SOHO (ESA & NASA)
Sunspots are a phenomenon that has been known about for at least several thousands of years. There is evidence that the Greeks knew of them at least by the 4th century BC and the Chinese had already made systematic observations 2000 years ago. Early observers would have been able to view the Sun when its brilliance was filtered by the atmosphere near the horizon; this would reveal dark patches on the surface. Another common method was to 'project’ the Sun using a pin-hole camera or a camera obscura; something the first Astronomer Royal John Flamsteed may have done.

Despite these early observations, it was only after the invention of the telescope, in 1609, that any real study of sunspots was possible. In 1610 (though he was not the first) the astronomer and mathematician Galileo Galilei made one of the first serious studies of sunspots. However, the quality of telescopic optics was still very primitive and it was some years before any conclusive theories were established. Daily observations were started at the Zurich Observatory in 1749 and, with the addition of other observatories, continuous observations began in 1849.

Although we have known of the existence of sunspots for many centuries, our understanding of them has been far less certain. Since they moved across the surface of the Sun, some thought they might be small planets in orbit; sceptics felt they were simply imperfections within the telescope optics themselves.

The theory behind sunspots
The generally accepted theory – proposed by H. Babcock in 1961 – suggests that they are caused by changes in the Sun’s magnetic field. Because the rotation period of the Sun is faster at the equator than towards the poles, a 'differential rotation’ is created. Babcock argued that this caused the magnetic field to become increasingly 'wound up’ as the Sun rotates, stretching the magnetic field between the poles and the equator. This stretching causes tubes or tunnels to form in the magnetic field. The loops rise and break the surface, preventing convection of the superheated gases underneath. The result of this is the creation of areas of lower temperature which are visible as dark spots. However, they are still in the region of 4,000 ºC. By comparison, the surface temperature is around 6,000 ºC.

Coronal loops
Coronal loops being created along the magnetic field above a sunspot groupImage: TRACE/NASA
Their duration varies greatly. Some last for just a few hours, the longest (in 1943) about six months. The 'orbits’ of the longer lasting spots can be plotted since they can survive several revolutions around the Sun. Their size also varies enormously. For a spot to be visible without magnification it would have to be approximately twice the size of the Earth or 1/2000th of the Sun’s diameter. By contrast the largest group on record, from April 1947, would have needed about 141 'Earths’ to cover it.

The number of sunspots follows a cyclical period of about 11 years, something that was first noted by Heinrich Schwabe in 1843. This year (2000) is a peak of the cycle, a period known as the Solar Maximum. This period of high activity will drop steeply over the next few years and begin building again in around five years time. There are two ways of plotting this cycle. One is to simply count the number of spots and by plotting the numbers against a time scale a periodicity can be determined. Another is by means of the so called 'Butterfly Diagram’. At the beginning of a new sunspot cycle sunspots appear mainly close to the Sun's north and south poles. As the sunspot cycle progresses, more sunspots appear closer to the Sun's equator.

Sunspots are usually seen in groups of two. One set will have positive or north magnetic field while the other set will have negative or south magnetic field. All but the smallest spots show a dark central portion called the umbra with a lighter outer area called the penumbra. Perhaps the biggest 'unknown’ in our understanding of sunspots is the fact that their magnetic field flips between cycles. After Solar Maximum there is a short period of overlap where some spots of the old cycle have a polarity that is opposite to that of the new ones.

Sunspot groups
The sunspot number is calculated by first counting the number of sunspot groups and then the number of individual sunspots. The 'sunspot number’ is achieved by adding the sum of the number of individual sunspots and ten times the number of groups. Since most sunspot groups have, on average, about ten spots, this formula for counting sunspots gives a reliable figure, even when the observing conditions are less than ideal and smaller spots are hard to see.

Although the 11 year cycle has been consistent in modern times, there was a period, approximately between 1645 and 1715, when there were virtually no spots at all. This period is known as the 'Maunder Minimum’, after the British astronomer who discovered it from the records in 1890. This might itself be a part of a cyclical period though it is impossible to be sure since we have been recording them for a relatively short period of time.

Scientists have not found any measurable connection between the number of sunspots on the Sun or any other measure of the solar cycle and the day-to-day variation in the weather on Earth. However, some scientists make do make a link between climate-change on Earth and state of the solar cycle.


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