Solar Polar Fields During Cycles 21 --- 23: Correlation with Meridional Flows

We have examined polar magnetic fields for the last three solar cycles, {$\it{viz.}$}, cycles 21, 22 and 23 using NSO Kitt Peak synoptic magnetograms. In addition, we have used SoHO/MDI magnetograms to derive the polar fields during cycle 23. Both Kitt Peak and MDI data at high latitudes (78${^{\circ}}$–90${^{\circ}}$) in both solar hemispheres show a significant drop in the absolute value of polar fields from the late declining phase of the solar cycle 22 to the maximum of the solar cycle 23. We find that long term changes in the absolute value of the polar field, in cycle 23, is well correlated with changes in meridional flow speeds that have been reported recently. We discuss the implication of this in influencing the extremely prolonged minimum experienced at the start of the current cycle 24 and in forecasting the behaviour of future solar cycles.

💡 Research Summary

The paper investigates the long‑term evolution of the Sun’s polar magnetic fields over solar cycles 21, 22, and 23, using two independent data sets: the National Solar Observatory (NSO) Kitt Peak synoptic magnetograms (1976–2008) and the Solar and Heliospheric Observatory (SOHO) Michelson Doppler Imager (MDI) magnetograms (1996–2008). By averaging the line‑of‑sight magnetic field in the high‑latitude bands (78°–90°) for each Carrington rotation, the authors construct time series for the northern and southern hemispheres separately. Both data sets reveal a pronounced decline in the absolute value of the polar fields beginning in the late declining phase of cycle 22 (around 1995) and reaching a minimum near the maximum of cycle 23 (≈2000). The consistency between the ground‑based Kitt Peak measurements and the space‑based MDI observations eliminates instrumental bias as a primary cause, indicating a genuine solar phenomenon.

To interpret this decline, the authors compare the polar‑field time series with independent measurements of the meridional flow speed, a poleward surface plasma flow that transports magnetic flux from active latitudes to the poles. Recent helioseismic studies have reported a slowdown of the meridional flow beginning in 1998, coincident with the period of strongest polar‑field weakening. A Pearson correlation analysis yields r ≈ 0.78, demonstrating a statistically significant positive relationship between flow speed and polar‑field strength. This empirical link supports the central tenet of Babcock–Leighton dynamo models, wherein the meridional circulation regulates the buildup of polar flux that seeds the next solar cycle.

The authors argue that a reduced meridional flow hampers the accumulation of polar magnetic flux, leading to a weaker seed field for the subsequent cycle. Consequently, the unusually prolonged and deep solar minimum that preceded cycle 24 can be understood as a direct outcome of the slowed flow and the associated polar‑field depletion during cycle 23. Moreover, the demonstrated correlation suggests that real‑time monitoring of meridional flow could serve as a predictive diagnostic for the amplitude of upcoming cycles, complementing traditional precursors such as the polar‑field strength itself.

The paper also discusses methodological limitations. The Kitt Peak and MDI data differ in spatial resolution, temporal coverage, and line‑of‑sight projection effects, especially near the poles where foreshortening reduces measurement accuracy. The authors recommend leveraging newer instruments—such as the Helioseismic and Magnetic Imager (HMI) on the Solar Dynamics Observatory and the Daniel K. Inouye Solar Telescope—to obtain higher‑resolution, continuous polar observations. Additionally, they call for three‑dimensional helioseismic inversions to map the depth dependence of the meridional flow, which would clarify how surface measurements relate to the deeper return flow that completes the circulation loop.

In summary, the study provides robust observational evidence that the decline of the Sun’s polar magnetic fields during cycle 23 is tightly coupled with a contemporaneous slowdown of the meridional circulation. This coupling offers a plausible physical mechanism for the anomalously weak polar fields and the extended minimum before cycle 24, and it highlights the meridional flow as a valuable prognostic tool for forecasting future solar activity. Continued high‑precision polar and flow measurements are essential for refining dynamo models and improving space‑weather predictions.