(last text update: 2025.08.17)
The development of the solar polar field strength throughout a solar sunspot cycle can be used to predict the magnitude of the next cycle and the peak of the current cycle. Polar field reversals typically occur within a year of sunspot maximum. It is not uncommon for the northern and southern polar fields to have significant differences in field strength and develop asynchronously over time. The Wilcox Solar Observatory has been collecting solar polar field data since 1975. Non-filtered data can be viewed in this plot. Below you will find a plot where only the filtered (by a 20 nHz lowpass filter) field strength data is displayed. Vertical lines representing field reversals and sunspot cycle min/max have been added to increase the potential usefulness of the plot.
For solar cycle 25 the northern polar field changed polarity the first time in May 2023 and stayed near neutral until February 2024. Since then the northern polar field has strengthened faster than the southern polar field. Both the southern polar field and the total field reversed in September 2023.
The SC25 365 day smoothed sunspot numbers (ISN, NOAA, STAR 1K and STAR 2K) all peaked on October 11-12, 2024.
For a comparison of WSO and SDO/HMI data take a look at this presentation: http://www.leif.org/research/Comparing-HMI-WSO-Polar-Fields.pdf
Wilcox Solar Observatory data used in this study was obtained via the web site http://wso.stanford.edu courtesy of J.T. Hoeksema.