Periodic Variation of Solar Magnetic Fields

    Sunspot activity exhibits a characteristic of periodic variation. At the beginning of each solar cycle, sunspots emerge in mid-latitude zones of solar surface, and then migrate towards lower latitude zones. At the end of the cycle sunspots normally emerge close to the equator (within ±10°). This recurrent behavior of sunspots gives rise to the 'butterfly diagram' of sunspots as shown in Figure 1

Figure 1 Butterfly diagram of sunspots

    Based on long-term observations of sunspots, G. E. Hale found that the evolution of solar magnetic field also has some regularities, which are known as Hale's polarity law: (1) in a solar cycle, the leading sunspots of active regions on one hemisphere have the same magnetic polarity while on the other hemisphere they show just the opposite polarity; (2) The bipolar sunspot groups in the two (northern and southern) hemispheres have opposite magnetic orientation; (3) The magnetic orientation of bipolar groups reverses from one cycle to the next. Taking into account this polarity reversal, the solar magnetic field has a period of about 22 years, also known as the Hale cycle.

Figure 2 Butterfly diagram of solar magnetic field

    Observations using the Zeeman Effect show that prototypical sunspots come in pairs with opposite magnetic polarity (see the right panel of the above figure). Their number varies according to the approximately 11-year solar cycle. From cycle to cycle, the polarities of leading and following (with respect to the solar rotation) sunspots change from north/south to south/north and back. George Ellery Hale first linked magnetic fields and sunspots in 1908. Hale suggested that the sunspot cycle period is 22 years, covering two periods of increased and decreased sunspot numbers, accompanied by polar reversals of the solar magnetic dipole field.





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