3.5 Magnetic field

Magnetic fields on the Sun were first measured in sunspots by Hale (1908). The magnetic nature of the solar cycle became apparent once these observations extended over more than a single cycle (Hale et al., 1919Jump To The Next Citation Point). Hale et al. (1919Jump To The Next Citation Point) provided the first description of “Hale’s Polarity Laws” for sunspots:

“…the preceding and following spots of binary groups, with few exceptions, are of opposite polarity, and that the corresponding spots of such groups in the Northern and Southern hemispheres are also of opposite sign. Furthermore, the spots of the present cycle are opposite in polarity to those of the last cycle.”

Hale’s Polarity Laws are illustrated in Figure 12View Image.

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Figure 12: Hale’s Polarity Laws. A magnetogram from sunspot cycle 22 (1989 August 2) is shown on the left with yellow denoting positive polarity and blue denoting negative polarity. A corresponding magnetogram from sunspot cycle 23 (2000 June 26) is shown on the right. Leading spots in one hemisphere have opposite magnetic polarity to those in the other hemisphere and the polarities flip from one cycle to the next.

In addition to Hale’s Polarity Laws for sunspots, it was found that the Sun’s polar fields reverse as well. Babcock (1959) noted that the polar fields reversed at about the time of sunspot cycle maximum. The Sun’s south polar field reversed in mid-1957 while its north polar field reversed in late-1958. The maximum for cycle 19 occurred in late-1957. The polar fields are thus out of phase with the sunspot cycle – polar fields are at their peak near sunspot minimum. This is also indicated by the presence of polar faculae – small bright round patches seen in the polar regions in white light observations of the Sun – whose number also peak at about the time of sunspot minimum (Sheeley Jr, 1991).

Systematic, daily observations of the Sun’s magnetic field over the visible solar disk were initiated at the Kitt Peak National Observatory in the early 1970s. Synoptic maps from these measurements are nearly continuous from early-1975 through mid-2003. Shortly thereafter similar (and higher resolution) data became available from the National Solar Observatory’s Synoptic Optical Long-term Investigations of the Sun (SOLIS) facility (Keller, 1998). Gaps between these two datasets and within the SOLIS dataset can be filled with data from the Michelson Doppler Imager (MDI) on the Solar and Heliospheric Observatory (SOHO) mission (Scherrer et al., 1995). These synoptic maps are presented in an animation here.

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Figure 13: avi-Movie (26984 KB) A full-disk magnetogram from NSO/KP used in constructing magnetic synoptic maps over the last two sunspot cycles. Yellows represent magnetic field directed outward. Blues represent magnetic field directed inward.

The radial magnetic field averaged over longitude for each solar rotation is shown in Figure 14View Image. This “Magnetic Butterfly Diagram” exhibits Hale’s Polarity Laws and the polar field reversals as well as “Joy’s Law” (Hale et al., 1919):

“The following spot of the pair tends to appear farther from the equator than the preceding spot, and the higher the latitude, the greater is the inclination of the axis to the equator.”

Joy’s Law and Hale’s Polarity Laws are apparent in the “butterfly wings.” The equatorial sides of these wings are dominated by the lower latitude, preceding spot polarities while the poleward sides are dominated by the higher latitude (due to Joy’s Law), following spot polarities. These polarities are opposite in opposite hemispheres and from one cycle to the next (Hale’s Law). This figure also shows that the higher latitude fields are transported toward the poles where they eventually reverse the polar field at about the time of sunspot cycle maximum.

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Figure 14: A Magnetic Butterfly Diagram constructed from the longitudinally averaged radial magnetic field obtained from instruments on Kitt Peak and SOHO. This illustrates Hale’s Polarity Laws, Joy’s Law, polar field reversals, and the transport of higher latitude magnetic field elements toward the poles.

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