Hale and Sunspots

George Ellery Hale was one of the 20th century’s greatest astronomers — his research gave astronomers their first peek inside our nearest star.

This is for June 29 — the anniversary of the birth in 1868 of American astronomer George Ellery Hale.

George Ellery Hale at his Mount Wilson office, circa 1905

Hale’s work revolved around sunspots — dark regions on the sun — often thousands of kilometers in diameter. Like storms on Earth, sunspots can grow and intensify. There tends to be a “leader” and “follower” in each cluster of spots — the leader is simply the spot that goes first, as the sun’s rotation moves the sunspot cluster across the solar disk. The number of spots on the sun is observed to increase and decrease in an 11-year cycle.

In 1908, Hale used an instrument of his own design to conclude that sunspots contain magnetic fields over a thousand times stronger than that of the Earth! He found that, during any given sunspot cycle, all leading sunspots in the sun’s northern hemisphere had the same magnetic polarity — while all leading spots in the sun’s southern hemisphere had the opposite polarity.

During the next sunspot cycle, the polarity of the leading spots reversed in each hemisphere. They reversed again 11 years later. Thus, Hale showed that a 22-year magnetic cycle provides a context for the 11-year sunspot cycle.

This insight continues to yield valuable information about the star nearest Earth.

The American astronomer and entrepreneur George Ellery Hale published a paper in 1908 entitled “On the Probable Existence of a Magnetic Field in Sun-spots.” He had compared sunspot spectra with the spectra of luminous gases subjected to a powerful magnetic field in the laboratory. It had been known years before Hale’s work that sunspot spectra occasionally showed spectral lines that were broadened or split into two, and sometimes more, components. This phenomenon is known as the Zeeman effect, after work done by Dutch scientist Pieter Zeeman 1865 1943, and for which he won the 1902 Nobel Prize. Zeeman observed that in the presence of a strong magnetic field, spectral lines emitted by light from gases in a laboratory were split into two, narrow, juxtaposed lines. The distance between the line components increased with the strength of the magnetic field.

Light is emitted by charged atomic particles electrons that are in motion. Magnetic fields, however, can exert a force upon electrical charges when they are in motion. Magnetic fields therefore affect the electrons responsible for emitting light, and hence they change the radiation emitted by atoms. Hale was the first to succeed in showing that the light from the individual components of these lines exhibited the polarization predicted by the Zeeman effect. The cause of the splitting was magnetic fields.

In 1908, George Ellery Hale observed the Zeeman effect in the spectrum of sunspots and found evidence for strong magnetic fields. The magnetic fields observed in sunspots range from 100 to nearly 4000 gauss over a thousand times greater than Earth’s magnetic field 0.7 gauss. The general solar magnetic field, where there are no sunspots, is only a few gauss.

Whenever sunspots are observed in pairs or groups, one of the spots usually has a north-seeking polarity while the other has the opposite polarity. In addition, during a given sunspot cycle, which lasts approximately 11 years, the leading spots of pairs in the northern hemisphere all tend to have the same polarity, while those in the southern hemisphere all tend to have the opposite polarity. During the next sunspot cycle, however, the polarity of the leading spots is reversed in each hemisphere.

For example, if during one sunspot cycle, the leading sunspots in the northern hemisphere have a north seeking polarity, the leading spots in the southern hemisphere would have a south-seeking polarity. During the next sunspot cycle, the situation is reversed. Leading sunspots in the northern hemisphere have a south-seeking polarity and those in the south have a north-seeking polarity.

Thus, the magnetic cycle a north or south seeking predominance does not repeat itself for two 11-year maxima. The Sun’s magnetic cycle is therefore said to last 22 years.

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