The Impending Reversal of the Sun’s Polar-Cap Fields

The Sun’s polar-caps reverse polarity around the time of maximum in each solar cycle. Typically, one hemisphere changes sign before the other. For example, in Cycle 24 the north pole reached a zero point in late 2012 and the south reversed about a year later. The polar reversal is driven by the arrival of ‘new’ polarity from following-polarity flux concentrations that first emerge in the activity belts and travel poleward. The reversals are not always smooth, but Cycle 25 seems unusually unsteady during the decay phase that is currently underway. High-latitude observations from the Helioseismic and Magnetic Imager (HMI) show that after a long gradual decay that started in 2016, the south polar cap plunged quickly in mid-2022 and then plateaued in early 2023 at a level about one quarter of its peak value. The northern cap gradually strengthened after 2014, not reaching a peak until 2019. In late 2022 the arrival of negative-polarity flux quickly eroded the field strength in the north. Both poles, particularly the south, have been relatively stable since early 2023, and the two are presently relatively equal in strength. However, new-polarity surges are rapidly approaching from mid-latitudes in both hemispheres, perhaps arriving sooner in the south. Characteristics of the decay phases of the five polar field reversals since 1979 observed by the Wilcox Solar Observatory will be compared.