Sunspots

Sunspots, cooler regions on the visible disc (photosphere) of the Sun that appear dark by contrast. Sunspots have typical temperatures of around 4,000 K (7,200° F), compared with 6,000 K (10,800° F) for the surrounding photosphere. Historically, sunspots have been known since at least the time of the Chinese Han Dynasty in 200 bc. Scientific observations began soon after the first astronomical application of the telescope in 1609 by Galileo, Christoph Scheiner, and others. Observations by Heinrich Schwabe in the early 19th century revealed the more-or-less regular cycle of sunspot activity, with maximum activity occurring every 11 years.

The most widely accepted explanation for the appearance of sunspots is that offered by H. W. Babcock and R. B. Leighton in the 1960s. Being comprised of gas, the Sun shows differential rotation, with the equatorial regions rotating faster than the poles. This results in the solar magnetic field becoming wrapped around itself: loops of magnetic flux from the interior are forced out through the photosphere, disrupting the normal pattern of convection. Gas trapped at the surface continues to cool further than if it were able to descend back into the Sun, leading to the development of sunspots.

In each 11-year cycle, sunspots first appear at higher solar latitudes, then move gradually closer to the equator. Spots often form in groups, with regions of opposed magnetic polarity separated in longitude. The arrangement of polarity between leader and following spots is reversed between the northern and southern solar hemispheres. In the next cycle, polarities flip over, so the entire magnetic cycle of the Sun takes about 22 years to repeat.

Sunspots range in size from small pores 1,000 km (600 mi) in diameter and lasting only a few hours, to huge groups spanning up to 100,000 km (60,000 mi) with lifetimes of weeks or months. Individual large spots show a dark central umbra, surrounded by a lighter, greyish penumbra. Bright clouds of hydrogen (faculae) may overlie sunspot-forming regions for several months. The Sun’s overall radiation is about 0.1 per cent higher at sunspot maximum relative to minimum, due to emissions from solar flares and faculae. This small variability has negligible effect on Earth temperature: flare and other activity occurring in the inner solar atmosphere above sunspot groups can have significant effects, however, in producing magnetic storms on the Earth.