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Polarity Reversal of the Polar Magnetic Fields in Solar Cycle 25

Using Hinode satellite vector magnetic field data, this study determines that the polar magnetic field polarity reversals for Solar Cycle 25 occurred in October 2024 (southern) and November 2024 (northern), revealing a latitudinal trend of earlier reversals at lower polar latitudes consistent with Solar Cycle 24.

Original authors: Yin Li, Shuhong Yang, Yuzong Zhang, Qiao Song, Guiping Zhou, Yuanyong Deng, Jingxiu Wang

Published 2026-06-16

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CC BY 4.0

Original authors: Yin Li, Shuhong Yang, Yuzong Zhang, Qiao Song, Guiping Zhou, Yuanyong Deng, Jingxiu Wang

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

Imagine the Sun as a giant, glowing lighthouse that doesn't just spin; it has a massive, invisible magnetic engine inside it. Every 11 years, this engine goes through a complete "reset." One of the most dramatic signs of this reset is that the Sun's magnetic North and South poles swap places, just like a compass needle suddenly flipping 180 degrees.

This paper is a detailed report card on exactly when and how this flip happened during the Sun's current 11-year cycle (known as Solar Cycle 25). Here is the story, broken down simply:

The Mission: Watching the Poles Flip

Scientists have been watching the Sun's poles using a high-tech telescope on a satellite called Hinode. Think of Hinode as a very patient photographer taking high-resolution "top-down" snapshots of the Sun's North and South poles every year, specifically in March (for the South) and September (for the North), when the view is clearest.

The team took these snapshots from 2022 to 2025 and stitched them together to create a movie of the magnetic fields changing.

The Plot: A Slow Dance of Magnetic Fields

For a long time, the Sun's North Pole was dominated by "positive" magnetic fields, and the South Pole was dominated by "negative" ones. But as the Sun reached its peak activity (solar maximum) around 2024, new magnetic fields started creeping in from lower latitudes, pushing the old fields out.

The North Pole: The old positive fields slowly faded away, and negative fields took over. By late 2024, the North Pole had officially flipped.
The South Pole: The opposite happened here. The negative fields faded, and positive fields took charge. This flip happened slightly earlier, around late 2024 as well.

The Timing: Who Flipped First?

One of the big questions in solar physics is: When does the flip happen relative to the Sun's "birthday party" (solar maximum)?

The South Pole: It flipped almost exactly when the South side of the Sun had its most sunspots (around October 2024). It was right on time.
The North Pole: It was a bit more relaxed. It flipped about 19 months after the North side had its peak sunspot activity (around November 2024).

The researchers also noticed a pattern: the flip didn't happen all at once like a light switch. Instead, it was like a wave rolling in. The magnetic fields started changing at the "edges" of the poles (lower latitudes, around 70°) and slowly moved inward toward the very top of the pole (90°). It took a few months for the whole polar region to fully turn over.

Why This Matters

Think of the Sun's magnetic field at the poles as the "seed" for the next 11-year cycle. Just as a farmer needs to know when the soil is ready to plant the next crop, solar physicists need to know exactly when the magnetic poles flip to understand how the Sun's internal engine works.

This paper provides a very precise "timestamp" for when the poles flipped in the current cycle. By comparing this real-world data with computer models, scientists can test if their theories about how the Sun's magnetic engine works are correct.

The Bottom Line

Using high-quality data from the Hinode satellite, the authors confirmed that the Sun's magnetic poles flipped in late 2024. The South Pole flipped first, right alongside its peak activity, while the North Pole took its time, flipping about a year and a half after its peak. The flip happened gradually, starting from the lower edges of the poles and moving inward, confirming that the Sun's magnetic engine is running on a predictable, though slightly lopsided, schedule.

Technical Summary: Polarity Reversal of the Polar Magnetic Fields in Solar Cycle 25

Problem Statement
The polarity reversal of solar polar magnetic fields is a fundamental signature of solar cycle evolution, serving as a critical constraint for dynamo theory validation and a precursor for predicting the amplitude of subsequent solar cycles. While previous studies have analyzed reversals in Solar Cycles (SC) 21 through 24, observational data for SC 25 remains scarce as the cycle has only recently passed its maximum. Existing predictions for SC 25 reversal times vary, and detailed investigations of the polar magnetic field evolution during this specific cycle are lacking. This study aims to precisely determine the timing and latitudinal progression of the polar polarity reversal in SC 25 using high-resolution vector magnetic field data.

Methodology
The authors utilized vector magnetic field data from the Spectropolarimeter (SP) on board the Hinode satellite, acquired during March and September of each year from 2022 to 2025. These specific observation windows were selected to maximize visibility of the southern and northern polar regions, respectively.

The data processing pipeline involved several rigorous steps:

Coordinate Calibration: A geometric transformation was applied to align Hinode SP magnetograms with HMI (Solar Dynamics Observatory) magnetograms to correct coordinate mismatches and solar limb positions.
Disambiguation: The inherent 180° azimuth ambiguity in the vector magnetograms was resolved using a method based on the physical assumption that magnetic fields are predominantly vertical or horizontal, following the approach of Ito et al. (2010).
Spurious Polarity Correction: To address projection effects that create spurious polarities near the solar limb, the authors identified strong magnetic patches (intrinsic total field > 200 G) and applied a thresholding process. Regions where the spurious polarity area exceeded 150% of the true polarity area were flagged, and their radial magnetic flux density ( $B_r$ ) values were reversed. Only data within ±90° longitude were retained for final analysis to ensure reliability.
Composite Map Construction: The processed magnetograms were geometrically transformed into a "top-down" polar view. Using a solar differential rotation formula with an 80° benchmark, the images were rotated and stitched to create annual composite maps for both hemispheres.
Quantitative Analysis: The authors calculated the average radial magnetic flux density and total radial magnetic flux for the polar caps (70°–90° latitude) and specific 5° latitudinal bands. These metrics were compared against 13-month smoothed hemispheric sunspot numbers (SSN) from SILSO. Additionally, a latitude-time plot was constructed using HMI synoptic maps to validate the reversal timing.

Key Results

Reversal Timing: The study estimates that the northern polar cap (above 70°N) completed its polarity reversal in November 2024, while the southern polar cap (above 70°S) reversed in October 2024.
Relation to Sunspot Maximum: The northern reversal lagged the northern hemispheric sunspot number maximum (April 2023) by approximately 19 months. In contrast, the southern reversal occurred almost simultaneously with the southern hemispheric sunspot maximum (October 2024).
Latitudinal Progression: The reversal process exhibited a sequential progression from lower to higher latitudes in both hemispheres, consistent with flux transport models.
- Northern Hemisphere: Reversal began in the 70°–75° band around October 2024 and concluded in the 80°–85° band around February 2025.
- Southern Hemisphere: Reversal began in the 70°–75° band around August 2024 and concluded in the 85°–90° band around December 2024.
Flux Evolution: From 2022 to 2025, the northern polar cap showed a decrease in radial magnetic flux at a rate of approximately $3.66 \times 10^{21}$ Mx yr $^{-1}$ , while the southern cap increased at a rate of $2.87 \times 10^{21}$ Mx yr $^{-1}$ .
Comparison with SC 24: Unlike SC 24, where the southern reversal lagged the northern by about 1.5 years, SC 25 shows the southern reversal preceding the northern by only about one month.

Significance and Claims
The paper claims that these results provide essential observational references for modeling polar polarity reversals in solar cycles. By utilizing high-resolution Hinode data to construct detailed polar composite maps, the study offers a precise timeline of the SC 25 reversal, which is crucial for:

Validating Dynamo Models: The precise timing and latitudinal sequence of the reversal serve as constraints for surface flux transport (SFT) and solar dynamo models.
Predictive Capability: The findings offer empirical data to refine predictions of the next solar cycle's amplitude, as polar field strength at minimum is a known precursor.
Understanding Cycle Asymmetry: The results highlight the asymmetry between hemispheres in SC 25, noting that the southern reversal coincided with the sunspot maximum while the northern lagged significantly, a pattern that differs from the behavior observed in SC 24.

The authors conclude that the observed sequential reversal from lower to higher latitudes supports the flux transport model, where trailing-polarity flux from active regions is transported poleward by meridional flow, driving the field reversal.

The Mission: Watching the Poles Flip

The Plot: A Slow Dance of Magnetic Fields

The Timing: Who Flipped First?

Why This Matters

The Bottom Line

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