Zeeman Effect in Sunspots

Measurement of the light from sunspots, obtained by masking off the light from parts of the Sun not in the sunspot, indicate significant Zeeman splitting of the spectral lines (see the adjacent figure, the right panel, and this animation).

Magnetic Effects and Sunspots

Thus, sunspots are associated with strong magnetic fields, typically one thousand times stronger than the average solar magnetic field according to the Zeeman splitting. Furthermore, it is observed that

Sunspots usually come in pairs and one member of the pair tends to have a magnetic field polarity that is opposite that of the other. That is, one sunspot of the pair behaves magnetically like the north pole of a bar magnet and the other behaves magnetically like the south pole of a bar magnet (see the figure below).

During a given sunspot cycle, the leading sunspots in groups in the northern hemisphere of the Sun all tend to have the same polarity, while the same is true of sunspots in the southern hemisphere, except that the common polarity is reversed (see the figure below).

During the next sunspot cycle, the regularities noted in the previous point reverse themselves: the polarity of the leading spots in each hemisphere is opposite from what it was in the previous cycle.

The Twenty-two Year Magnetic Cycle

The preceding discussion of Zeeman splitting for sunspots indicates that the solar magnetic field has a twenty-two year cycle, exactly twice that of the sunspot cycle, because the polarity of the field returns to its original value every two sunspot cycles. Thus, the fundamental period governing solar activity is actually the twenty-two year magnetic cycle, and the sunspot cycle (which is exactly half that) is just a special manifestation of the magnetic cycle. The magnetic field plays an important role in all aspects of the active Sun (sunspots, prominences, flares, the solar wind, and the nature of the corona), so the twenty-two year magnetic cycle is central to the periodicity of the active Sun.