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Sunspots - Causes

field magnetic solar equator

In the second half of the twentieth century a mechanism of sunspot formation was proposed which accounts for much of their observed behavior. To begin with, the Sun does not rotate as a rigid body; the polar regions rotate somewhat more slowly than the equator. (The reason for this is still not known.) Because the solar material is electrically charged, the Sun's overall magnetic field is dragged along with the solar rotation; because the solar rotation is faster at the equator, the field will be dragged faster at the equator than at the poles. Although the overall magnetic field of the Sun is weak (i.e., similar to that of the earth), this differential rotation both distorts and intensifies it over time. The faster-rotating regions of the equator drag the local magnetic field so that the field lines are drawn out into long, thin tubes; the more these tubes are stretched, the more intense the magnetic field within them becomes. As the magnetic tube breaks the surface of the Sun (and returns into it, as all magnetic field lines form closed loops), it forms two spot-like structures. As the field direction is out of the solar surface at one spot and into it at the other, one of these spots will have act as north magnetic pole and the other will act as a south magnetic pole. The global nature of the general solar field is what guarantees that the stretched magnetic tubes will yield leading spots with opposite polarities in opposite hemispheres. A reversal of the Sun's general field between 11–13 year cycles would account for the observed periodic reversal of this order; however, there is no compelling explanation of why the general field should reverse after each 11–13 year solar cycle. Nevertheless, this relatively simple model does provide a beginning basis for understanding sunspots.


Resources

Books

de Pater, Imke, and Jack J. Lissauer. Planetary Sciences. Cambridge, UK: Cambridge University Press, 2001.

Prialnik, D. An Introduction to the Theory of Stellar Structure and Evolution. Cambridge: Cambridge University Press, 2000.


Periodicals

Kerr, Richard A. "Link Between Sunspots, Stratosphere Buoyed." Science 5412 (April 9, 1999): 234-235.

Rast, M.P., et al. "Bright Rings around Sunspots." Nature 401 (October 14, 1999): 678-679.

Solanki, S.K., M. Schüssler, and M. Fligge. "Evolution of the Sun's Large-scale Magnetic Field Since the Maunder Minimum." Nature 408 (November 23, 2000): 445-447.


George W. Collins

KEY TERMS

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Maunder minimum

—A period of time from about 1650–1725 when the Sun did not have sunspots visible with the instruments of the time. This interval is correlated with a period of unusually severe winters in Europe and North America known as the Little Ice Age.

Penumbra

—The lighter, outer region of a sunspot.

Umbra

—The dark, central region of a sunspot.

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