The magnetic nature of sunspots

In 1908, Georges Ellery Hale used the Zeeman effect with a modified spectrohelioscope to establish that sunspots have strong, concentrated magnetic fields.

sunspot image 1
Diagram 1, illustrates the magnetically-induced Zeeman splitting in the spectrum of a sunspot.
sunspot image 2
Diagram 2, illustrates Hale's polarity laws, which present evidence for the existence of a well-organized large-scale magnetic field in the solar interior that cyclically changes polarity every 11 years or so.

Subsequent work demonstrated a strong tendency for east-west alignment of magnetic polarities in sunspots, with mirror symmetry across the solar equator; and that the polarity in each hemisphere switched orientation from one sunspot cycle to the next. This systematic property of sunspot magnetic fields is now commonly referred to as the "Hale–Nicholson law", or in many cases simply "Hale's law." The pressure provided by such strong magnetic fields 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 show:

  1. the same magnetic polarity pattern in each solar hemisphere,
  2. the opposite polarity patterns between the North and South solar hemispheres, and
  3. a reversal of polarity patterns 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 stood the test of time and are since known as Hale's polarity Laws. Their physical origin is 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.

Bibliography:
G.E. Hale, F. Ellerman, S.B. Nicholson, and A.H.Joy, The Astrophysical Journal, vol. 49, pps. 153–178.