Prediction of the Solar Polar Fields in 2026: An Unusually Weak Level Across the Last Five Solar Cycles

TL;DR

This study uses a surface flux-transport model with a meridional-flow profile optimized against HMI data to predict solar polar fields one year ahead, demonstrating that short-term forecasts (1–2 years) can be accurate without forecasting future AR emergence. Validation against 2020–2025 data and hindcasting of 2023–2024 reversals show that the Flow 1 profile reproduces polar-field evolution better than a traditional Flow 2, enabling a predicted 2025–2026 evolution where the northern field declines sharply while the southern field remains weak. The results suggest cycle-25 polar fields are unusually weak, raising concerns about the cycle-26 minimum and amplitude, with dominant uncertainties from transport-parameter choices and missing far-side ARs. The work highlights the value of optimized meridional flow in SFT models and foreshadows improved polar-field predictions as new observations (e.g., from Solar Orbiter) become available.

Abstract

Solar polar fields are essential for the solar cycle and the heliospheric magnetic field. Cycle 25 is now entering its declining phase, the critical period during which most of the cycle's polar fields are established. Therefore, reliable polar-field prediction is now especially important. Polar-field evolution is governed by the poleward transport of already-emerged active-region (AR) flux over a timescale of a few years. Thus, surface flux-transport models can reliably provide one-year predictions without requiring information about future AR emergence. Our prediction method is validated using simulations of the surface magnetic field from 2020-2025 and hindcasts of the 2023-2024 polar fields, employing a newly constrained profile of the meridional flow. Using the most recent HMI synoptic magnetogram as the initial condition, we predict the polar-field evolution from October 2025 to October 2026. The southern polar field is predicted to strengthen gradually, while the northern field is expected to decline sharply until March 2026 due to some ARs with abnormal polarity. By that time, the northern polar field becomes exceptionally weak, and the southern field remains relatively weak, raising concerns about the polar-field strength at the cycle 25/26 minimum and the amplitude of cycle 26.

Figures (5)

• Figure 1: Comparison of the two meridional flows. Flow 1 is the profile obtained by optimizing the simulated polar fields within $60^{\circ}$--$75^{\circ}$ latitude against HMI observations. Flow 2 follows the form given by van_Ballegooijen1998.
• Figure 2: Comparison of the surface magnetic field from HMI synoptic magnetograms and the SFT simulation during 2020--2025. Panels (a) and (b) show the magnetic butterfly diagrams from the HMI observations and the SFT simulation with Flow 1, respectively. Panel (c) presents the axial dipole strength, and Panel (d) shows the polar fields, averaged over $60^\circ$-$75^\circ$ latitude. The dash-dotted lines in Panels (c) and (d) indicate the zero level. Panels (c) and (d) each show two SFT simulations using Flow 1 and Flow 2, whose profiles are given in Equations \ref{['eq:MF_new']} and \ref{['eq:MF_VB']}.
• Figure 3: Comparison of magnetic butterfly diagrams and polar fields between HMI observations and SFT predictions for 2023--2024. (a) HMI magnetic butterfly diagram. (b) Magnetic butterfly diagram combining HMI observations (before 2023) and SFT predictions (from 2023 onward). (c) Comparison of polar fields averaged over $60^{\circ}-75^{\circ}$. (d) Comparison of polar fields averaged over $60^{\circ}-90^{\circ}$. The vertical dash-dotted line marks the start of the prediction, and the horizontal dash-dotted line marks the zero level. In (c) and (d), solid and dashed lines show the northern and southern polar fields. Thick black lines show the nine-CR moving-averaged fields, thin gray lines the original fields, and red lines the predicted fields, with red shading indicating the prediction uncertainties.
• Figure 4: Prediction of polar fields averaged over $60^{\circ}-75^{\circ}$ (a), polar fields averaged over $60^{\circ}-90^{\circ}$ (b), and the magnetic butterfly diagram (c) for CRs 2303--2317 (2025.10--2026.10). Line styles and colors follow the same convention as in Figure \ref{['fig:test_pred']}.
• Figure 5: Comparison of the predicted unusually weak polar fields across the five cycles. WSO polar fields (black) are averaged over latitudes $55^\circ$--$90^\circ$, while HMI fields (blue) and SFT predictions (red) are averaged over latitudes $60^\circ$–-$75^\circ$. WSO line-of-sight magnetic fields are converted to radial fields by dividing by a projection factor of 0.2482 and then calibrated to HMI fields by multiplying by 1.37. The vertical dash-dotted line indicates the start of the prediction.