19. Hale's sunspot polarity law

This slide shows two solar magnetograms taken in the descending phases of cycles 21 and 22. White and black correspond respectively to positive and negative magnetic polarities (i.e., normal magnetic field component pointing toward and away from the observer). The images have been rescaled and the color scale adjusted to emphasize polarity changes (at the expense of dynamic range). As a consequence, sunspots and plages are not as clearly delineated as on slide #5, but show up globally as large regions of a given magnetic polarity. The white lines trace the paths of magnetic neutral lines. The equator runs more or less horizontally across each magnetogram. As noted previously (cf. slide #5), most regions of strong magnetic fields are grouped in pairs of opposite polarities; furthermore, at any given time the ordering of positive/negative regions with respect to the E---W direction (the direction of rotation, from left to right on these images) is the same in a given hemisphere, but is reversed from northern to southern hemispheres. This was first determined observationally in the first decade of the 20 century by G.H. Hale, and is known as Hale's Polarity Law. Shortly after this discovery, ongoing studies of the magnetic polarities of sunspot pairs by Hale and collaborators revealed yet another intriguing pattern: from one sunspot cycle to the next, the magnetic polarities of sunspot pairs undergo a reversal in each hemisphere. This polarity reversal pattern is apparent on this slide.

Hale's Polarity Law is evidence for large-scale order underlying what would otherwise seem to be a purely stochastic phenomena. The source of these regularities is rooted in the solar dynamo, the mechanism leading to the continuous regeneration of the solar magnetic field. This involves the interaction between large-scale flows deep in the solar interior and the existing solar magnetic field, and requires the presence of the rotationally-induced Coriolis force to break the global symmetry that would otherwise characterize those fluid motions. As a discussion of the physical nature and mode of operation of the solar dynamo would entail an overly lengthy digression, we refer the interested reader to some of the textbooks listed at the end of this text. Nevertheless, it should now be apparent that in terms of the global configuration of the large-scale solar magnetic field, it takes two sunspot cycles for the same pattern of magnetic polarities to reappear. From a physical standpoint, the true length of the solar cycle is not 11 years, but rather 22 years. Yet astronomers are creatures of tradition, and solar astronomers are no exception; nearly a century after Hale's discovery of the sunspot polarity law, it remains customary to speak of the ``11 year solar cycle''.