On Polar Magnetic Field Reversal in Solar Cycles 21, 22, 23, and 24

Abstract

The Sun’s polar magnetic fields change their polarity near the maximum of sunspot activity. We analyzed the polarity reversal epochs in Solar Cycles 21 to 24. There was a triple reversal in the N-hemisphere in Solar Cycle 24 and single reversals in the rest of the cases. We used data of the Wilcox Solar Observatory (WSO) to determine the polarity reversals at high latitudes and then compared these timings with the epochs when the reversals were completed in the N- and S-hemispheres. The reversal times were compared with hemispherical sunspot activity and with the Heliospheric Current Sheet (HCS) tilts, too. It was found that reversals occurred at the epoch of the sunspot activity maximum in Solar Cycles 21 and 23, and after the corresponding maxima in Solar Cycles 22 and 24, and one–two years after maximal HCS tilts calculated in WSO. Reversals in Solar Cycles 21, 22, 23, and 24 were completed first in the N-hemisphere and then in the S-hemisphere with a delay of 0.6, 1.1, 0.7, and 0.9 years, respectively. The polarity inversion in the near-polar latitude range ${\pm}\,(55\,\mbox{--}\,90)^{\circ}$ occurred from 0.5 to 2.0 years earlier than the times when the reversals were completed in the corresponding hemisphere. Using the maximal smoothed WSO polar field as precursor we estimated that the amplitude of Solar Cycle 25 will reach $116\pm 12$ in values of smoothed monthly sunspot numbers which will be comparable with the current cycle amplitude of 116.4.