Solar Polar Field Reversals as the Result of the Global Magnetic field Meridional Flows

TL;DR

The paper investigates solar polar field reversals in cycles 21–25 using Wilcox Solar Observatory data to show that reversals are driven by global GMF meridional flows, not local active-region fields. It derives latitudinal-flow formulas for positive- and negative-polarity GMF fluxes, enabling predictions of reversal timing tied to cycle maxima and allowing retrospective checks against past cycles. Through source-surface maps and spherical-harmonic analyses, the study reveals how sectorial and zonal harmonic components evolve during reversals and how the HCS neutral line and polar-angle oscillations respond to GMF restructuring. The results advance cycle prediction and imply a tachocline-linked mechanism for GMF transport, offering practical formulas to forecast reversal times and, by extension, solar cycle maxima.

Abstract

Based on data obtained from Wilcox Solar Observatiry the solar polar magnetic fields reversals in cycles 21\,--\,25 were considered. The results indicate that the polarity reversal occurs at the maximum of sunspot activity of each cycle, but the beginning, end, and duration of the reversals did not demonstrate any association with the Wolf numbers, which are characteristics of local magnetic fields. Moreover, during the periods of polarity reversal, the correlation between global magnetic field (GMF) parameters and Wolf numbers decreased and even moved into anti-correlation mode. The polar field reversals are determined by the GMF flows of positive- and negative-polarity magnetic fields, which cyclically migrate from one pole to the opposite pole. The new polarity magnetic fields are delivered to the poles by a certain flow, and then carried away by the same flow to the opposite pole. The increase in the polar magnetic field strength to its maximal values at the solar activity minimum and following decrees to the next cycle maximum coincides with the latitudinal changes in corresponding magnetic field flow. Formulas for calculating the meridional circulation of positive- and negative-polarity magnetic field flows were proposed. They allow predict the time of polarity reversals, and since polarity reversals occur at the maxima of cycles, then also the time of maxima of both the future and past cycles.

Figures (12)

• Figure 1: Illustration of the process of transition of the GMF structure from zonal at the cycle minimum to sectorial at the maximum and back to zonal at the minimum of the next cycle with the redistribution of positive- and negative-polarity magnetic field domination in the northern and southern hemispheres in cycles 21 -- 25. WSO calculated source surface synoptic maps (R=2.5 R$\odot$) are shown. Each map is marked with a CR number.
• Figure 2: Polar magnetic field strength evolution in each cycle separately for the North (b1 --ḟ1) and South (b2 --ḟ2) poles and their sum (a1, a2). (b3 -- f3) combined North (solid line) and South (dashed line) polar magnetic fields and the sum of their moduli (thick lines) for each cycle and for all cycles (a3).
• Figure 3: (a) variations in the polar magnetic fields at the North (blue) and South (red) poles in cycles 21 -- 25. The sum of their moduli is shown in black; (b) variations of the positive- and negative-polarity magnetic field strength in the latitudinal ranges: 45$^\circ$ -- 55$^\circ$ are shown in green, 55$^\circ$ -- 65$^\circ$ -- in blue, and 65$^\circ$ -- 70$^\circ$ -- in red in the North hemisphere and (c) that in the South hemisphere; (d) mean magnetic field strengths for each latitudinal interval (N1 -- N3) in the North hemisphere and (e) that in the South hemisphere (S1 -- S3); (f) imbalances between the mean magnetic field strength in the North and South hemispheres for each latitudinal interval. Thin lines indicate the magnetic fields averaged over each CR, and thick lines that over seven CRs. The maxima and minima of Cycles 21 -- 25 are marked at the top.
• Figure 4: Cyclic variations of large-scale positive- (red, (a)) and negative-polarity (green, (b)) magnetic fields calculated on the source surface; (c) the trajectories of the flow centers (red and green respectively). Variations in the positive polarity (dark blue) and negative polarity (light blue) polar magnetic field strength at the North (N) and South (S) poles. On the right 'y'-axes the strength of the polar magnetic field is given in absolute values; (d) speed of displacement by latitude of positive- and negative-polarity magnetic field flows. The maxima and minima of Cycles 21 -- 25 are marked at the top.
• Figure 5: Time-latitude distribution of the flow centers of longitude-averaged positive-polarity (a) and negative-polarity (b) magnetic field flows; (c) forecast of the polarity reversal time in Cycles 26 and 27; (d) estimated time of polarity reversals in Cycles 11 -- 20.