Sunspots, Cycle of

Sunspots are areas of the sun that appear relatively dark when viewed by astronomers using special techniques. These regions are approximately 2500°F cooler than the rest of the photosphere, which gives them their darker appearance. Sunspots have strong magnetic activity that inhibits normal convection, resulting in the lower surface temperatures. Seventy-five percent of sunspots [Page 1211]appear in groupings of two or more. They range in size from the smallest, called pores, which are less than 100 miles in diameter, to those that are more than 100,000 miles in diameter.

Sunspots have a fairly regular cycle of activity. The German astronomer Heinrich Schwab identified a 10-year cycle of activity while studying sun-spots during the years 1826 and 1843. Rudolf Wolf, a Swiss amateur astronomer, used old astronomical records going back as far as 1700 to establish a sunspot cycle database and, in 1948, refined the sunspot cycle to 11.2 years.

A period of maximum activity, in which more than 100 spots may be visible, is referred to as the sunspot maximum. The sunspot minimum, the period of lowest activity, can have few or no spots visible for weeks at a time. At the initiation of the sunspot cycle, sunspots are more frequent at higher latitudes, generally appearing at 40°. As the sun-spot maximum approaches, usually within 4 years after the initiation of the cycle, the sunspots will have moved toward the equator with the majority lying at 15° latitude. Near 5° latitude they begin to fade out with new spots forming at the higher latitudes. This movement is referred to as Sporer's law. However, the activity cycles will vary. For example, sunspot records show a period of 70 years, beginning in 1647 when activity was minimal. This era was called the Maunder Minimum, after the British astronomer E. W. Maunder, who was credited with the discovery.

The earth regularly reverses its magnetic polarity, and this change induces changes in the sun's magnetic field. The reversal occurs every 11 years. Using this information, George Ellery Hale suggested that the sunspot cycle is actually 22 years and encompasses the two polar reversals of the earth's magnetic field. Horace W. Babcock proposed a qualitative model for the dynamics of the solar outer layers. The Babcock model explains the behavior described by Sporer's law, as well as other effects, as being due to magnetic fields that are manipulated by the sun's rotation. Thus, the sun-spot cycle may actually be a 22-year cycle, which includes two sunspot maximums and two polarity flips of the earth.

Sunspots are correlated with various events. Intense magnetic activity, such as coronal loops and reconnection, occurs with high sunspot activity. Large sunspot groupings produce magnetically active regions from which most solar flares and coronal mass ejections originate. Magnetic storms and auroral displays are more frequent and intense during periods of maximum sunspot activity. The Maunder Minimum clearly corresponded with an era of low temperatures and longer-than-normal winters. Correlations to other events, including such things as cycles of the stock market and baseball statistics, although hypothesized, have not been demonstrated.

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