In 1990, Peter Wilson and his colleagues began a long series of papers on the reversal of the Sun’s polar magnetic fields (Wilson et al., 1990; Wilson and McIntosh, 1991; Wilson, 1992; Murray and Wilson, 1992; Wilson and Giovannis, 1994). At first, they argued that the flux-transport model could not work because the surface fields ought to be linked to deep subsurface toroids, which would prevent them from moving freely to the poles. Next, they began their own simulations and concluded that the large-scale fields are not entirely the result of flux from active regions, but require additional contributions from sources in the network. They continued on to papers III (large-scale fields and the first major active regions of cycle 22), IV (polar fields near sunspot maximum), and V (reversal of the polar fields in cycle 22), always concluding that the flux in active regions was not enough to reproduce the large-scale field.
But as they considered the polar field reversals in sunspot cycles 22 (Snodgrass et al., 2000; Kress and Wilson, 2000) and 23 (Durrant and Wilson, 2003), the emphasis of the Australian group seems to have changed. In their most recent paper, Wilson and his colleagues use supergranular diffusion at a rate of 600 km2 s–1 to obtain an improved estimate of the poorly observed polar field reversal time. In addition, McCloughan and Durrant (2002) and Durrant and McCloughan (2004) devised a method for studying the evolution of synoptic maps of the photospheric field and used it to compare their simulations with observations of the polar fields. McCloughan’s 99-page thesis can be found online (McCloughan, 2002).
This change of emphasis seems to be bringing the various schools closer together. Instead of arguing whether the flux-transport model is valid, we were exploiting it to understand properties of the Sun’s magnetic fields. Two new ideas immediately come to mind. First, because the flux does drift to the poles, it must not be permanently attached to subsurface toroids, but must reconnect freely as it seems to do in the corona (Sheeley Jr and Wang, 2002; Sheeley Jr et al., 2004). If the reconnecting fields are so interesting above the surface, what must they be like below the surface? Second, differences in the polar field reversals from one sunspot cycle to the next may reflect differences in the rates of meridional flow. Flow speed variations were suggested by the episodic poleward surges (Labonte and Howard, 1982) which have now been observed in every sunspot cycle since 1964, and by the short-term fluctuations in the numbers of polar faculae since 1905 (Sheeley Jr, 1991). As we shall see in the next section, secular changes in the flow rate of order ± 6 m s–1 are sufficient to preserve the polar field reversal from cycle to cycle (Wang et al., 2002a).
By contrast, the role of small-scale background eruptions is poorly understood. On the one hand, we have been able to reproduce the evolution of the large-scale field without including ephemeral regions in the model. In fact, Sheeley Jr et al. (1985) found that 85% of the sources provided about 50% of the total erupted flux and doublet moment during sunspot cycle 21, but that their exclusion had little effect on either the strength or polarity pattern of the mean field. Similarly, Wang and Sheeley Jr (1991) found that ephemeral regions had no effect on the evolution of the Sun’s axial dipole moment, nor did they give rise to an effective diffusion of flux. Also, at the November 1 – 5, 1993 Soesterberg Workshop honoring Kees Zwaan, Karen Harvey (1994) presented observations demonstrating that the flux in large-scale unipolar magnetic regions originates in active regions and activity ‘nests’ of the kind that were studied by Gaizauskas et al. (1983). She found that during sunspot cycle 21 the dipole contribution of ephemeral regions was only one-sixth that of active regions and of opposite sign.
On the other hand, Stenflo (1992) and Snodgrass and Wilson (1993) suggested that large-scale regions may sometimes (in Karen’s words) ‘form in situ from a clustering and preferential alignment of the magnetic poles of many small-scale emerging bipolar regions’. More recently, Solanki et al. (2002) argued that ephemeral regions may have contributed to the Sun’s open flux in the past, especially during quiet times like the Maunder Minimum when large active regions were rare. Of course, a sufficiently large number of ephemeral regions will contribute to the large-scale field if the orientations of their doublet moments are not random (as Karen found in cycle 21). However, at present we do not know if these weak background eruptions are the tail of the active-region size distribution with orientations that have been partially randomized during their transit through the convection zone, or an independent component of randomized ‘magnetic foam’. Perhaps future studies of high cadence observations like those of Hagenaar (2001) and high resolution simulations like those of Schrijver (2001) will help to answer this question.
© Max Planck Society and the author(s)