Proceedings PaperMeasuring storage and release of magnetic free-energy
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The discovery of the large scale patterns of the solar magnetic field has roots in the Babcock model of the sunspot cycle. This explanation of developing toroidal field from the differential rotation, is equivalent to the development of a global poloidal electric current distribution such as shown in Figure 1. We sketched the currents' path as mostly closing through the photosphere but the actual path could well go through the corona, and can also change during the solar cycle. The picture of Fig. 1 is consistent with the induction of poloidal EMF as result of the poloidal field and toroidal plasma velocity. There is still pending the question of how the 22 years magnetic cycle works. Several ideas have been proposed, but so far none is very well developed and tested. Schematically, the surface field reversal is believed to result from the disappearance of the previous cycle flux, and replacement by emerging opposite directed flux. The closed (by Maxwell's laws) magnetic field and associated toroidal electric current must either be expelled from the Sun or decay and dissipate in place. Field reconnection may play a role for eliminating the old field, but does not destroy the field. Eliminating the field implies destroying the associated electric currents. The scheme of Figure 2 shows the implications of these ideas. Both Figs. I and 2 deal with cylindrical symmetric schemes with base latitudinal mode. After observing many solar cycles, it is clear now that there is no such simple symmetry in the solar fields, thus, although these schemes may be partially valid there is a far richer spectrum of possibilities. These possibilities arise from added degrees of freedom due to field structures of sizes smaller than the whole Sun. A few of such possible temporal evolution schemes are shown in the Figure 3 for some low latitudinal and longitudinal modes. Since higher angular mode cases are also distorted by global differential rotation, a number of features of the symmetric models also show up. However, the closed field loop bundles can not only become distorted, but also move and rotate in many ways.