11.   The Sun's Magnetic Cycle

   During the time when Gauss was studying the Earth's magnetic field, elsewhere in Germany a serious amateur astronomer named Heinrich Schwabe (Shwah-bay), a pharmacist by trade, was searching for a new unknown planet. That planet, tentatively named "Vulcan," was expected to be inside the orbit of Mercury, so close to the Sun that its presence became evident only when it passed between us and the Sun, a dark spot crawling across the solar disk. However, the Sun also had dark "sunspots" of its own, and to tell these apart from a new planet, Schwabe kept track of them as well.

Variation of the observed "sunspot number"

   Vulcan doesn't exist--it was never seen in any total eclipse. However, after a decade and more of diligent observations, Schwabe found something that had eluded all astronomers of the preceding two centuries--since sunspots were first reported by Galileo and Christopher Scheiner. He found that the number of sunspots rose and fell in a nearly regular cycle, lasting about 11 years. Interestingly, it soon turned out that big "magnetic storms" when the magnetic field was disturbed (typically by up to 1%) seemed to occur most frequently during the years with the most sunspots.

   The evidence for sunspot magnetism was their emitted light. Glowing gases emit light in narrowly defined wavelengths (i.e. colors), a different set for each substance. In 1897, however, Pieter Zeeman found that when such light was emitted from the region of a strong magnetic field, the emission split into slightly different wavelengths, with a separation that increased with the strength of the field. The colors of the light emitted from sunspots were "split up" in just this way.

   The method was later improved by Babcock and others, allowing astronomers to observe not only the magnetic field of sunspots but also the weak fields near the Sun's poles. It turned out that the Sun has a polar field somewhat like the Earth's, but that it reverses its polarity during each 11-year cycle.

   Sunspots have also led us to a better understanding of the Earth's own magnetic field. The face of the Sun consists of ionized hot gas ("plasma"), hot enough to conduct electricity. Sunspot fields were evidently produced by electric currents, and it was well known that such currents could be generated by a "dynamo process," by the motion of an electric conductor (e.g. the flow of solar plasma) through a magnetic field.

   In 1919 Sir Joseph Larmor proposed that the fields of sunspots were due to such dynamo currents. He suggested that a closed chain of cause-and-effect existed, in which the field created by these currents was also the field which made them possible, the field in which the plasma's motion generated the required currents. Many features of sunspots remain a mystery, but Larmor's idea opened an era of new understanding of magnetic processes in the Earth's core.

Solar Activity and Geomagnetic Storms: The First 40 Years   E.W. Cliver, Eos, 6 December 1994

Next Stop:   12. The Dynamo Process