Polar faculae near the Sun's south pole
posted: September 9, 2015

Solar polar faculae (PF) appear in white light as bright points near the northern and southern solar poles, usually at heliographic latitudes greater than about 60 degrees. In contrast to their equatorial counterparts (the main zone faculae), they are pointlike, less bright, and randomly distributed. Faculae are local concentrations of magnetic field, but not that strong that they can form sunspots. The SDO/HMI images underneath show main zone faculae fields (in the gray dashed circles) on 26 July 2015, as well as -for the same day- a highly magnified view of the south pole with some polar faculae indicated by arrows (bright dots).

An important difference between the PF and main zone faculae is that the number of PF evolves oppositely to the "normal" solar cycle, i.e. PF are most numerous during solar cycle minima, and (near) zero during solar cycle maxima. The latter corresponds to the period of polar magnetic field reversal, offering an explanation for the absence of the PF during these years. Interestingly, over the last few months, observers of the Belgian solar section (see their newsletters) noticed a significant increase in the number of PF near the Sun's south pole. This can be seen in graph underneath, showing an observer's smoothed number of polar faculae for the northern (blue) and southern solar pole (red). The sudden increase in southern polar faculae at the end of the graph, is a bit unexpected because the next solar minimum is still many years away.

What seems to be happening is that a surge of negative magnetic field flux reached the south pole late in 2014, cancelling and effectively overturning the previously existing southern polar magnetic field of opposite polarity. This can be seen in the annotated magnetic diagram underneath from NASA/Ames Research Center. Yellow is positive polarity and blue is negative. Increased PF activity associated to the arrival of such surges has been documented in the past by professional solar observers such as N. R. Sheeley (e.g. in 1974). No such *strong* surge has been observed near the north pole so far this cycle, despite the polar field reversal having taken place there more than a year before the southern one. Completely conform to these findings, no obvious increase in PF has been observed near the north pole during the last two years.

Magnetic field measurements by the Wilcox Solar Observatory independently confirm the magnetic ongoings near the Sun's poles, with neutral (close to zero) values near the north pole (in "blue"), and a sharp upturn in magnetic field strength near the south pole (in "red"; see chart underneath). Just as for the PF, the undulations (regular up-and-downs) are caused by the fact that Earth has a better view on the Sun's south pole during spring months, and on the north pole during autumn because of the tilt of the solar rotation axis. The values of the polar field strength can be used for the prediction of the strength of the upcoming solar cycle, but for solar cycle 25, an accurate prediction is still a few years away (awaiting stable fields at maximum strength). For SC24 predictions, one can refer to e.g. Svalgaard (2005) and Schatten (2005).