M. Švanda1,2, A. S. Brun3, Th. Roudier4, & L. Jouve4,3
1 Astronomical Institute, Academy of Sciences of the Czech Republic, Ondřejov, Czech Republic
2 Astronomical Institute, Charles University in Prague, Czech Republic
3 CEA Saclay, France
4 IRAP, Université de Toulouse, France
Figure 1 | Top: Magnetic butterfly diagram from MDI and HMI synoptic maps shows the prevailing polarity of the magnetic field in polar caps, reversals of the global magnetic field and the flux transported to the polar regions from the relics of the active regions. Bottom: Magnetic butterfly diagram of the pores outside active regions. At the top and the bottom the polar cap from magnetic butterfly from above is inserted. The intensity of the average magnetic field in the inserts is boosted by the factor of 5 to make them visible. This figure clearly shows that the pores follow the cycle. The inset in the lower panel demonstrates the mixed polarity in magnification.
We found that the pores outside SARs do follow the solar cycle (Figure 1). They seem to broaden the activity belt. At first glance, their polarity seems to be mixed and with no obvious bias — such bias appears only after averaging in time. There is a net magnetic flux of ~1021 Mx per hemisphere in those pores alone, which (if transported towards poles) is almost sufficient to cause a reversal of polar caps in a weak cycle. The hemispheric polarity bias was the right one to contribute to the reversal.
Figure 2 | Binned index of match of the flux polarity in pores and their closest SAR as a function of distance from the closest SAR and time. One can see that the pores in the intermediate distances 40–100 Mm from closest SAR depict systematically opposite polarity to that SAR.
We further investigated the origin of these pores. We found that those with polarity bias are located at distances 40 to 100 (possibly up to 140) Mm from the flux-weighted center of the closest SARs (Figure 2). We further found that a vast majority of these pores form in weaker bipolar active regions located to the west of the closest SARs (hence closer to their trailing parts). The locations of the pores do not necessarily coincide with previously existing, strong SARs (within 4 rotations).We speculate that these pores originate from “failed” emergence of sub-surface flux ropes, which either did not reach the surface or only partly reached the surface. They were then dispersed to the neighborhood by large-scale convection, e.g., giant cells. This would explain their typical distance (40 to 100 Mm) from the closest SARs.We therefore speculate that during the grand-minima, a weak source term akin to a BL mechanism was possibly operating, as there were organized magnetic fields present on the Sun.
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