Bibhuti Kumar Jha & Lisa Upton
Southwest Research Institute, Boulder, CO 80304

The Sun, our life-giving star, is a significant driver of space weather and exhibits an intrinsic magnetic cycle roughly every 11 years, reflected in the waxing and waning of sunspots. This cycle is believed to be propelled by a solar dynamo process, which, in simple terms, converts and amplifies the poloidal magnetic field into a toroidal field. These toroidal fields ascend from the photosphere, giving rise to the observed bipolar magnetic regions (BMRs) or sunspots. BMRs interact with the surrounding plasma, with residual fields carried to the poles, leading to the reversal in the polarity and the accumulation of magnetic field with the new polarity. These new fields then serve as the seed for the subsequent cycle. Understanding this cycle, especially the timing of polar field reversals, is crucial for predicting the timing of solar maxima and potential impacts of space weather on Earth’s infrastructure and technological systems.

Recent advancements in Surface Flux Transport (SFT) models, notably the Advective Flux Transport (AFT) model^[1,2], have yielded impressive results in forecasting the evolution of the Sun’s global magnetic field^[1]. AFT assimilates the observed line-of-sight magnetic field data from HMI to model the evolution of global magnetic field, which we call the “Baseline Mode”. However, AFT is not limited to only run in Baseline Mode, it has also demonstrated its capability of using alternative source date to simulate future global field evolution for the purpose of prediction.

This study utilizes AFT’s predictive capabilities to forecast the timing of polar field reversals during Solar Cycle 25. However, a key challenge arises: the inherently stochastic nature of AR emergence precludes perfect predictions. To address this, the study employs a state-of-the-art Synthetic Active Region Generator (SARG) based on established statistical properties of solar activity cycles and ARs^[3]. While SARG shows promise in generating realistic ARs, the chaotic nature of AR emergence necessitates a probabilistic approach. Therefore, the study incorporates 30 distinct SARG realizations within the AFT model, generating an ensemble of forecasts and associated uncertainties. In Figure 1, we show the magnetic butterfly diagram generated based on one of the selected realizations of SARG. Notably, it is almost impossible for human eyes to distinguish between the two segments of the butterfly diagram: the Baseline Mode, where HMI data is assimilated (left of the white line), and the segment based on SARG (right of the white line). This demonstrate the capability of SARG to generate the synthetic active regions that are consistent with observations.

Figure 1| The magnetic butterfly diagram, constructed using AFT baseline map until 2023 August 31 (marked using white dashed vertical line) and after that using one of the realizations of synthetic ARs in AFT’s predictive mode.

This ensemble analysis suggests it is most likely that the northern hemisphere’s polar field reversal will occur around August 2024 (see Figure 2), with a 50% confidence spanning over June to November 2024. Similarly, the southern hemisphere’s reversal is anticipated around February 2025, with a range (50% confidence) extending from November 2024 to July 2025. Here, we emphasize that the timing of reversal is calculated based on the average flux above 60° latitude. We refer the interested reader to Ref. [3] to learn more about the role of various latitude limits on the timing of polarity reversal. Furthermore, we would like to draw the reader’s attention to the flatness of the polar field curve in the Northern hemisphere (see Figure 1), very close to zero, a small fluctuation in this value can change the sign of the polar field and momentarily show the reversal of filed. However, we believe that the overall behavior of polar field will remain the same for a few months before it starts rising from zero.

Figure 2| The evolution of Sun’s polar field with time. The shaded region indicates when AFT is used for prediction, while the other regions show baseline mode, using direct HMI Magnetogram data in the model.

Based on these findings, the study also suggests that Solar Cycle 25 could peak in the latter half of 2024, which aligns with predictions based on other proxies of solar cycles^[4]. While precise predictions remain elusive due to inherent solar variability, this ensemble approach offers valuable insights into the potential timing and uncertainties associated with the upcoming polar field reversals.

References

[1] Upton, L., & Hathaway, D. H. 2014, ApJ, 780, 5
[2] Upton, L., & Hathaway, D. H. 2014, ApJ, 792, 142
[3] Jha, B. K., & Upton, L. 2024, ApJL, 962, L15
[4] Upton, L., & Hathaway, D. H. 2023, JGR (Space Phys), 128, e2023JA031681