🔭 astrophysics

Solar filament migration across activity cycles: The emergence of a late-cycle high-latitude population

This study analyzes over a century of solar filament data to reveal that a distinct high-latitude filament population emerges late in each activity cycle, serving as a precursor that traces the transition to the next cycle and appears approximately 1.1 to 2.2 years before the onset of the following cycle's sunspot activity.

Original authors: Jouni Takalo

Published 2026-09-15

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CC BY 4.0

Original authors: Jouni Takalo

Original paper licensed under CC BY 4.0 (https://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 Sun is not a static ball of fire; it is a churning, magnetic engine that goes through regular periods of intense activity followed by quieter times. This rhythm, known as the solar cycle, lasts roughly eleven years. During the peak of this cycle, the Sun's surface becomes crowded with dark spots called sunspots, which are regions of intense magnetic fields. These spots do not appear randomly; they follow a predictable pattern. They first emerge at mid-latitudes, roughly thirty degrees north and south of the equator, and then slowly drift toward the equator as the cycle progresses. This migration creates a shape on a graph that resembles the wings of a butterfly, a pattern astronomers have recognized for over a century.

However, the Sun's magnetic story is more complex than just sunspots. Suspended in the Sun's upper atmosphere are vast, cool clouds of gas called filaments. When seen against the bright disk of the Sun, these look like dark, winding threads. Like sunspots, they are held in place by magnetic fields, but they can form across a much wider range of latitudes, reaching all the way to the poles. Because they trace the large-scale magnetic structure of the Sun, filaments offer a different, broader view of the solar cycle than sunspots alone. Scientists have long known that while low-latitude filaments drift toward the equator, high-latitude ones can drift toward the poles. But a lingering question remained: what happens to the overall distribution of these filaments as one solar cycle ends and the next begins? Does the entire population of filaments suddenly reverse direction, or is something else happening?

A recent study by Jouni Takalo investigates this question by looking at a century of observations, covering solar cycles fifteen through twenty-four, from 1909 to 2022. The researcher used a massive, unified record of filament areas compiled from fifteen different historical and modern archives. Instead of tracking individual filaments, the study measured the total amount of filament material present at every degree of latitude over time. To make the cycles comparable, the data was adjusted so that each eleven-year period was stretched or compressed to fit a standard timeline, allowing patterns that repeat every cycle to stand out clearly against the background noise of cycles that are slightly longer or shorter.

The analysis confirmed the expected behavior of the main group of filaments: the low and middle-latitude filaments steadily migrate toward the equator as the cycle matures. But as the cycle approached its end, the average latitude of all filaments on the Sun began to rise again. At first glance, this might suggest that the main group of filaments had suddenly turned around and started heading back toward the poles. The study, however, explicitly rules out this idea. By separating the data into low-latitude and high-latitude groups, the researcher found that the low-latitude filaments never reversed their course; they continued their steady drift toward the equator until the cycle ended.

The rise in the average latitude was caused by the arrival of a completely different group: a population of filaments appearing at high latitudes, near the poles. This high-latitude population grows significantly during the final stages of a solar cycle, just as the low-latitude activity is winding down. Because these new, high-latitude filaments are present at the same time as the old, low-latitude ones, they pull the average position of all filaments upward, creating the illusion of a reversal. The study shows that this is not a change in direction for the main group, but rather a mixing of two distinct populations: the fading remnants of the old cycle and the emerging structures of the new one.

To understand the timing of this transition, the study compared the appearance of these high-latitude filaments with the first signs of the next sunspot cycle. The researchers looked for the very first sunspots of a new cycle and compared their arrival time to the rise of the high-latitude filaments. The results showed a clear sequence. In eight out of nine cycle transitions, the high-latitude filaments appeared before the first sunspots of the new cycle. When the researchers looked for a more established, persistent pattern of sunspots rather than just the first isolated group, the delay was even more pronounced. The high-latitude filaments consistently appeared about two years before the new sunspot activity became firmly established.

This timing suggests that the high-latitude filaments are a sign of the Sun's magnetic field beginning to reorganize for the next cycle, long before the new cycle's sunspots become visible. The filaments act as an early indicator, tracing the magnetic changes that occur during the overlap between two solar cycles. While the study does not prove that these high-latitude filaments belong magnetically to the new cycle, their systematic appearance before the new sunspots indicates they are part of the transition process. The findings reinforce the idea that solar cycles are not isolated, eleven-year blocks of time, but rather overlapping phases where the magnetic structures of the past and future coexist on the solar surface. By watching these high-latitude filaments, scientists can see the Sun preparing for its next phase of activity years in advance.

Technical Summary: Solar Filament Migration Across Activity Cycles

Problem Statement
Solar filaments, which trace the large-scale magnetic structure of the Sun, exhibit a complex latitude–time evolution that differs from the classic sunspot butterfly diagram. While low- and intermediate-latitude filaments migrate equatorward, historical observations have identified a distinct poleward-moving branch at high latitudes. A specific phenomenon observed in long-term data is an increase in the mean absolute latitude of filaments during the declining phase of a solar cycle. It remains unclear whether this increase signifies a reversal of the main equatorward migration or if it results from the emergence of a separate high-latitude population that overlaps with the declining low-latitude activity. Furthermore, the relationship between this late-cycle high-latitude filament population and the onset of the following solar activity cycle requires clarification.

Methodology
The study analyzes the latitude–time evolution of solar filament area over Solar Cycles 15–24 (1909–2022) using a composite Hα filament-area record constructed by Chatzistergos et al. (2023b). The dataset comprises daily projected filament areas across 180 heliographic latitude bins derived from 15 historical and modern archives.

Key methodological steps include:

Phase Normalization: To account for variations in cycle duration, individual cycles were transformed into a common normalized phase coordinate ( $\phi$ ), where $\phi=0$ and $\phi=1$ correspond to consecutive sunspot minima. This allows for the direct averaging of recurrent structures across cycles of different lengths.
Statistical Separation: The filament area was analyzed as a total population and separated into low-latitude ( $|\lambda| < 40^\circ$ ) and high-latitude ( $|\lambda| \ge 40^\circ$ or $|\lambda| \ge 50^\circ$ ) components to distinguish migration patterns.
Onset Detection: The onset of late-cycle high-latitude activity was defined statistically as the first of two consecutive phase bins where the high-latitude filament fraction exceeded a threshold set at 20% above the minimum value within the late-cycle interval ( $\phi \ge 0.65$ ).
Comparative Timing: The filament onset times were compared against independently identified sunspot butterfly wings of the following cycles (SC16–SC24) using the catalogue of Leussu et al. (2017b). Two sunspot onset criteria were used: the "first-wing onset" (earliest assigned group) and the "persistent-wing onset" (establishment of sustained activity in consecutive 3-month bins).
Statistical Robustness: Significance was evaluated using exact sign-flip randomization tests and non-parametric cycle bootstrapping (50,000 realizations) to ensure the temporal ordering was not driven by individual cycles or arbitrary definitions of persistence.

Key Results

Nature of Late-Cycle Latitude Increase: The analysis confirms that the increase in mean absolute filament latitude near the end of a solar cycle is not caused by a reversal of the equatorward migration of the main low- and intermediate-latitude population. When filaments above $40^\circ$ are excluded, the mean latitude continues to decrease toward the cycle minimum. Instead, the increase is driven by the growing contribution of a distinct high-latitude population that emerges while low-latitude activity from the current cycle is still present.
Cycle-to-Cycle Stability: This pattern of overlapping populations—a declining low-latitude branch and an emerging high-latitude branch—is a persistent feature across all ten analyzed cycles (SC15–SC24).
Temporal Ordering with Following Cycles: A systematic temporal sequence was established between the high-latitude filament population and the following sunspot cycle:
- The high-latitude filament onset precedes the first identified sunspot groups of the following butterfly wing in 8 out of 9 cycle transitions, with a mean lead time of approximately 1.1 years.
- The high-latitude filament onset precedes the establishment of persistent sunspot-wing activity in all 9 cycle transitions, with a mean lead time of approximately 2.2 years.
- This ordering is robust to variations in the definition of "persistent" activity (tested with bin sizes of 2–4 months and 2–3 consecutive bins).

Significance and Claims
The paper claims that the late-cycle high-latitude filament population serves as a statistical tracer of the transition between successive solar activity cycles. The systematic appearance of this population before the persistent emergence of the following sunspot butterfly wing suggests that large-scale magnetic reorganization occurs during the overlap of cycles, preceding the full establishment of new sunspot activity.

The author emphasizes that while the timing suggests a connection to the following cycle's magnetic evolution, the study does not definitively prove that these filaments belong magnetically to the following cycle. The results support the concept of an "extended solar cycle" where magnetic activity signatures of consecutive cycles overlap in time and latitude. The findings provide an additional observational perspective on solar cycle evolution, highlighting that filament distributions offer a complementary view to sunspots, particularly regarding the early stages of magnetic reorganization that precede sustained sunspot emergence.

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