6.1 What is the primary poloidal field regeneration mechanism?

Given the amount of effort having gone into building detailed dynamo models of the solar cycle, it is quite sobering to reflect upon the fact that the physical mechanism responsible for the regeneration of the poloidal component of the solar magnetic field has not yet been identified with confidence. As discussed at some length in Section 4, current models relying on distinct mechanisms all have their strengths and weaknesses, in terms of physical underpinning as well as comparison with observations.

Modelling of the evolution of the Sun’s surface magnetic flux has abundantly confirmed that the Babcock-Leighton mechanism is operating on the Sun, in the sense that magnetic flux liberated by the decay of tilted bipolar active regions does accumulate in the polar regions, where it triggers polarity reversal of the poloidal component (see Wang and Sheeley Jr, 1991 ; Schrijver et al., 2002 ; Wang et al., 2002 ; Baumann et al., 2004 , and references therein). The key question is whether this is an active component of the dynamo cycle, or a mere side-effect of active region decay. Likewise, the buoyant instability of magnetic flux tubes (Section 4.7) is, in some sense, unavoidable; here again the question is whether or not the associated azimuthal mean electromotive force contributes significantly to dynamo action in the Sun.

Petrovay (2000 ) makes an interesting proposal, namely that one would be better off to categorize dynamo models according to the following two criteria: (i) whether or not the source regions for the poloidal and toroidal components are spatially coincident, and (ii) whether the sunspot butterfly diagram is to be understood in terms of the Parker-Yoshimura sign rule, or to advection by meridional circulation. All dynamo models described above fall into one of four possible categories (see Table 1 in Petrovay, 2000 , and accompanying discussion). The challenge is then to devise observational criteria allowing to meaningfully distinguish between these four possible classes.

A noteworthy contribution along these lines is the recent work of Hathaway et al. (2003 ), who have shown that the duration of individual sunspot cycles is inversely correlated with the slope of the dynamo wave in the butterfly diagram. The latter, in models relying on advection by meridional circulation, is itself expected to be set by the equatorward meridional flow speed at the core-envelope interface, which can be related to the observable surface meridional flow speed via the mass conservation constraint; although uncertainties remain at that level, the analysis of Hathaway et al. (2003 ) supports the idea that the cycle period is indeed set by the meridional flow speed (but do see Schmitt and Schüssler, 2004 , for an opposing viewpoint). This kind of work must be pursued.