March 3, 2010

The dark specks are sunspots, which are cooler than the surface of the sun.  Photo: Courtesy NASA

Ask The Professor

What are sunspots, and do they affect the Earth's weather?

This month’s expert, Kenneth R. Lang, professor of astronomy, responds:

To most of us, the sun looks like a perfect, white-hot globe, round, smooth and without a blemish. But detailed scrutiny shows that it is often pitted with dark spots, called sunspots. A pair or group of sunspots appear from inside the sun, and can remain visible for weeks before disappearing back into the solar interior.

Large sunspots are as big as the Earth and contain magnetic fields that are thousands of times stronger than the Earth’s magnetic field. Sunspots occasionally grow large enough to be seen with the naked eye through fog or haze, or sometimes at sunrise or sunset, when the sun’s usual brightness is heavily dimmed. But normally, you cannot look directly at the sun without severely damaging your eyes, and most sunspots are too small to be seen without telescopes, which were introduced in the early 17th century. Still, ancient Chinese records indicate that large sunspots have been observed with the unaided eye for nearly 2,000 years.

When he pointed the newly invented telescope at the sun in 1610, Galileo noticed that sunspots change in size and shape, and sunspots near the sun’s equator move across the solar disk more rapidly than those nearer to the solar poles, with an apparent rotation period ranging from 27 to 30 days. This meant that different parts of the sun’s visible disk rotate about its axis at different speeds, which could only happen in a gaseous body like the sun. A solid body such as the Earth rotates with the same speed, regardless of latitude, which is why the day is 24 hours long wherever you are on the planet.

Because they are relatively cool, sunspots appear dark, in contrast to their bright surroundings. A sunspot might have a temperature of 3,500 kelvin (5,840.3 degrees Fahrenheit), for example, instead of the 5,780 kelvin (9,944.3 degrees Fahrenheit) of adjacent regions. The powerful magnetic fields in sunspots act as a filter or valve, choking off the heat and energy flowing outward from the solar interior and cooling the dark spots.

But appearances can be deceiving. A sunspot is almost 10 times hotter than the temperature of boiling water at 373 kelvin (211.7 degrees Fahrenheit), and although sunspots appear dark in comparison to the nearby hotter gas, they still radiate light.

The magnetic fields in each individual sunspot either point into or out of the sun, with a single magnetic polarity, but sunspots usually travel in connected pairs. Sunspots are linked by magnetic loops that extend upward into the overlying, and transparent, solar atmosphere. The region in, around and above sunspot pairs, or groups of pairs, is called an active region, and is the seat of powerful activity on the sun.

The number and positions of sunspots, and the active regions that envelop them, vary over an 11-year period, known as the sunspot cycle or the cycle of solar magnetic activity. Explosive activity on the sun, such as solar flares or coronal mass ejections, is more frequent and of greater intensity at the maximum of this cycle than at its minimum.

Sunspots are not by themselves directly related to the Earth’s weather, but the active regions in which they are immersed do result in violent explosions that produce space weather. The solar flares and coronal mass ejections emit powerful radiation and hurl energetic particles into interplanetary space, producing gusts and squalls in the perpetual solar winds blowing from the sun. We are shielded from this space weather by the Earth’s atmosphere and magnetic fields, which keep us safe. But out in deep space there is no place to hide, and both humans and satellites are vulnerable.

When solar explosions are directed at the Earth, they can produce geomagnetic storms, high above our atmosphere, and alter the terrestrial ionosphere, affecting radio communications. 

The ionosphere lies well above the ground-hugging troposphere, in which all of Earth’s weather occurs, and there is no indication that solar activity or sunspots affect weather down here near the ground, even though some have thought it did. From about 1645 to 1710, no sunspot activity was observed, and that coincided with part of the Little Ice Age, which ebbed and flowed from 1300 to 1850 in Europe and other northern latitudes. Some thought that there might be a connection, but that has been discounted, even though it still turns up in the popular press.


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