🔭 astrophysics

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

Using a surface flux-transport model constrained by recent meridional flow profiles and HMI magnetogram data, the study predicts that solar cycle 25 will conclude with unusually weak polar fields in 2026, particularly in the north, raising concerns about the amplitude of the upcoming cycle 26.

Original authors: Ruihui Wang, Jie Jiang, Yukun Luo

Published 2026-03-31

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Ruihui Wang, Jie Jiang, Yukun Luo

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Solar "Compass" is Getting Weak

Imagine the Sun as a giant, spinning magnet. Like Earth, it has a North and South magnetic pole. These poles are crucial because they act like a compass for the entire solar system, guiding the solar wind and protecting us from cosmic rays.

Every 11 years, the Sun goes through a "heartbeat" called a solar cycle. At the peak of this cycle, the Sun's magnetic poles flip (North becomes South, and vice versa). By the time the cycle is over and the Sun is "calming down" (entering its declining phase), these poles need to rebuild themselves to be strong and ready for the next cycle.

The Big News: A team of scientists from Beihang University has run a simulation to predict what these poles will look like in late 2025 and 2026. Their forecast? The Sun's North Pole is about to get unusually weak.

How They Predicted the Future (Without a Crystal Ball)

Predicting the Sun is hard because new magnetic storms (called Active Regions) pop up randomly, like unpredictable weather. Usually, you have to guess where these storms will appear to know how they will affect the poles.

However, the scientists used a clever trick. They realized that the magnetic "stuff" from storms takes about 1–2 years to travel from the Sun's equator all the way to the poles. It's like a slow-moving conveyor belt.

The Analogy: Imagine you are standing at the end of a conveyor belt (the pole). You don't need to know what packages (storms) will be put on the belt tomorrow to know what will arrive next year. You just need to see what is already on the belt right now.
The Method: They took a snapshot of the Sun's current magnetic field (using NASA's SDO satellite) and ran a computer model that simulates how that magnetic "dust" drifts toward the poles over the next year. They didn't need to guess about future storms; they just tracked the ones already in motion.

The "Traffic Jam" at the North Pole

The model predicts a specific drama unfolding at the Sun's North Pole:

The Problem: A "surge" of magnetic energy with the wrong polarity (think of it as a traffic jam of cars driving the wrong way) is currently heading toward the North Pole.
The Result: This wrong-way traffic will collide with the existing North Pole field and cancel it out.
The Prediction: By March 2026, the North Pole will be exceptionally weak—almost flat. The South Pole will also be weak, but it will slowly start to recover.

This is unusual. In the last five solar cycles, the poles were much stronger at this stage of the game.

Why Should We Care?

You might ask, "So the Sun's magnet is weak. Does that matter?"

Yes, it matters a lot.
Think of the solar cycle like a battery. The strength of the magnetic poles at the end of a cycle is the best indicator of how "charged up" the Sun will be for the next cycle.

Strong poles = A powerful, energetic next cycle (lots of solar flares and auroras).
Weak poles = A quiet, sleepy next cycle.

Because the poles are so weak right now, scientists are worried that Solar Cycle 26 (which starts around 2026/2027) might be a very weak one. This could mean fewer spectacular auroras and a quieter Sun for the next decade.

The "Butterfly" and the "Flow"

To make this prediction accurate, the scientists had to fix a flaw in their "conveyor belt" model.

The Old Model: It assumed the magnetic dust moved at a standard speed, like a car on a highway with a set speed limit.
The New Model: They realized the "wind" (called meridional flow) that pushes the dust is actually faster near the equator and slower near the poles, and they tweaked their model to match real observations.
The Result: This new "traffic map" allowed them to accurately predict the past (2023–2024) and gives them high confidence in their 2026 forecast.

The Bottom Line

The Sun is currently in a "declining phase," but it's behaving strangely. A magnetic "surge" is about to wipe out the North Pole's strength, making it the weakest it has been in 20 years.

While this might sound alarming, it's just the Sun doing its 11-year dance. However, this weak start suggests the next solar cycle might be a "chill" one. The scientists will keep watching, and with new satellites like the Solar Orbiter getting better views of the poles, we'll know for sure if the Sun is taking a nap or just stretching before its next big workout.

1. Problem Statement

Solar polar fields are critical drivers of the solar cycle and the heliospheric magnetic field. Their strength at solar minimum serves as a reliable precursor for the amplitude of the subsequent solar cycle. As Solar Cycle 25 (SC25) enters its declining phase, accurately predicting the evolution of polar fields is essential.

However, traditional Surface Flux Transport (SFT) models face two major limitations in polar field prediction:

Source Uncertainty: Predicting future Active Region (AR) emergence involves significant stochasticity, making long-term forecasts unreliable.
Parameter Constraints: Flux-transport parameters, particularly the meridional flow profile, are poorly constrained by observations. Most models are tuned to reproduce the axial dipole strength or magnetic butterfly diagrams but often fail to accurately reproduce the temporal evolution of the polar fields themselves.

The authors aim to provide a reliable one-year prediction of polar fields (October 2025–October 2026) by overcoming these limitations, specifically by optimizing the meridional flow profile against observations.

2. Methodology

The study employs a Surface Flux Transport (SFT) model governed by the standard advection-diffusion equation for the radial magnetic field. The key methodological innovations include:

Optimized Meridional Flow (Flow 1): Instead of using standard theoretical profiles, the authors derived a new meridional flow profile by optimizing simulated mean fields against Helioseismic and Magnetic Imager (HMI) observations within the 60°–75° latitude band.
- Profile Characteristics: Flow 1 is slower between 20°–70° latitude compared to the standard van Ballegooijen profile (Flow 2). Its peak speed is lower (13 m/s vs. 15 m/s) and shifted equatorward (29°), with a steeper equatorial latitudinal gradient consistent with helioseismology measurements.
Initial Conditions & Source Handling:
- Validation (2020–2025): The model was validated by simulating the surface magnetic field from Carrington Rotation (CR) 2225 to CR 2303 using the ARISE active region database as the source term.
- Prediction (2025–2026): For the actual forecast, the source term $S(\theta, \phi, t)$ was set to zero. The model relies solely on the transport of already-emerged flux from the initial condition (HMI synoptic magnetogram of CR 2303). This approach avoids the uncertainty of predicting future AR emergence, as AR flux typically takes 1–2 years to reach the poles.
Uncertainty Quantification: Prediction errors were estimated by combining HMI measurement limitations ( $\sigma_{HMI} \approx 0.059$ G) and short-term observational fluctuations ( $\sigma_{smt} \approx 0.175$ G for 60°–75°), resulting in a total uncertainty of $\approx 0.185$ G.

3. Key Contributions

Novel Flow Profile: The derivation of "Flow 1," a meridional flow profile specifically constrained to reproduce observed polar field evolution, rather than just the axial dipole.
Source-Free Short-Term Prediction: Demonstrating that reliable 1–2 year polar field predictions can be made without generating synthetic future ARs, provided the initial magnetic field is accurate and the transport parameters are optimized.
Validation of Hindcasts: Successfully hindcasting the complex polar field surges of 2023–2024, a period characterized by significant magnetic surges, proving the model's robustness.

4. Key Results

Model Validation:
- The optimized Flow 1 successfully reproduced the magnetic butterfly diagram, axial dipole strength, and the temporal evolution of polar fields (60°–75° and 60°–90°) from 2020 to 2025.
- In contrast, the standard Flow 2 failed to match observed polar fields, particularly after the 2023 polarity reversal, despite reproducing the axial dipole.
- Hindcasts for 2023–2024 showed good agreement with HMI observations, with observed values falling within the predicted uncertainty bounds.
2025–2026 Prediction:
- Northern Hemisphere: The model predicts a sharp decline in the northern polar field until March 2026, reaching an exceptionally weak level of $-0.72 \pm 0.21$ G (averaged over 60°–75°). This decline is driven by a strong positive magnetic surge (N1) originating from ARs with abnormal polarity that emerged between June 2024 and March 2025. A subsequent negative surge (N2) will begin to recover the field after March 2026.
- Southern Hemisphere: The southern polar field is predicted to remain relatively stable initially and then gradually strengthen due to a positive surge (S2) from ARs emerging in mid-2025.
Historical Comparison:
- The predicted northern polar field strength at 6 years into SC25 is the weakest among the last five solar cycles (SC21–SC25). It is significantly weaker than SC21–SC23 and even weaker than the southern field of SC24.
- However, the ratio of the field strength at 6 years to the field strength at solar minimum ( $B_{t=6y}/B_{t=min}$ ) varies widely across cycles (13.9% to 151.6%). Therefore, while the current level is weak, it does not definitively predict a weak SC26, as significant field buildup could still occur during the declining phase.

5. Significance

Scientific Implication: The paper highlights that the polar field strength at the SC25/26 minimum may be unusually weak, particularly in the northern hemisphere. This raises concerns regarding the potential amplitude of Solar Cycle 26.
Methodological Advancement: The study establishes a robust framework for short-term solar cycle prediction that bypasses the stochastic uncertainty of AR emergence. By focusing on the transport of existing flux and optimizing transport parameters against observations, it offers a more reliable tool for space weather forecasting.
Future Outlook: The authors note that upcoming observations from the Solar Orbiter, which can view the poles from outside the ecliptic plane, will further improve measurement quality and refine future SFT models and predictions.