Two diagrams from the 1919 paper by G.E. Hale, F. Ellerman, S.B. Nicholson, and A.H. Joy (in The Astrophysical Journal, vol. 49, pps. 153-178). The left diagram illustrates the magnetically-induced Zeeman splitting in the spectrum of a sunspot. The diagram on the right illustrates what is now known as Hale's polarity laws., which present evidence for the existence of a well-organized large-scale magnetic field in the solar interior, which cyclically changes polarity every 11 years or so.
The study of the solar cycle was finally put on a firm physical footing by the epoch-making work of Georges Ellery Hale and collaborators, in the opening decades of the twentieth century. In 1907-1908, by measuring the Zeeman splitting in magnetically sensitive lines in the spectra of sunspots and detecting the polarization of the split spectral components, Hale provided the first unambiguous and quantitative demonstration that sunspots are the seats of strong magnetic fields (see also slide 4 and slide 5 of the HAO slide set The Sun: a Pictorial Introduction). It was subsequently realized that the pressure provided by such strong magnetic field would also lead naturally to the lower temperatures observed within the sunspots, as compared to the photosphere.
In the following decade Hale and collaborators went on to show that large sunspots pairs almost always (1) show the same magnetic polarity pattern in each solar hemisphere, (2) show opposite polarity patterns between the North and South solar hemispheres, and (3) these polarity patterns are reversed from one sunspot cycle to the next, indicating that the physical magnetic cycle has a period of twice the sunspot cycle period. These empirical observations have sttod the test of time and are since known as Hale's polarity Laws. Their physical origin is now now known to originate with the operation of a large scale hydromagnetic dynamo within the solar interior, although the details of the process are far from adequately understood. Because the sun's dynamo generated magnetic field is ultimately responsible for all manifestations of solar activity (flares, coronal mass ejections, etc.), to this day solar dynamo modeling remains a very active area of research in solar physics.
-Written by paulchar@ucar.edu.