Polar Filaments Capture High Latitude Solar Poloidal Field Interactions and can Foretell the Future Sunspot Cycle Amplitude before Polar Field Precursors

TL;DR

This study tackles the challenge of forecasting solar-cycle amplitude by leveraging polar filaments as observable proxies of the BL poloidal-field generation and polar flux evolution. By combining Meudon synoptic maps (cycles 16–21) with a data-driven surface flux transport model (SPhoTraM), the authors quantify how high-latitude filaments relate to polar-field buildup and the cycle’s strength, introducing remnant metrics $\Delta\langle A_N\rangle$ and $\Delta\langle L_N\rangle$. They find strong positive correlations between these remnant filament quantities and the end-of-cycle polar flux, and crucially, significant correlations with the maximum sunspot area of cycle $N+1$ that persist up to $\Delta t \approx 5$ years before cycle end, implying a forecast window of about $10$ years before the cycle peak. This work suggests that polar filaments capture the physics of interactions between the previous and current cycle poloidal fields, offering a potential early precursor to solar-cycle strength while highlighting the need for cross-dataset validation and clarification of timing predictions.

Abstract

Polar fields at the minimum of a sunspot cycle -- which are a manifestation of the radial component of the Sun's poloidal field -- are deemed to be the best indicator of the strength of the toroidal component, and hence the amplitude of the future sunspot cycle. However, the Sun's polar magnetic fields are difficult to constrain with ground-based or space-based observations from near the plane-of-ecliptic. In this context, polar filaments -- dark, elongated structures that overlie polarity inversion lines -- are known to offer critical insights into solar polar field dynamics. Through investigations of the long-term evolution of polar filament areas and length acquired from the Meudon Observatory and complimentary solar surface flux transport simulations, here we establish the common physical foundation connecting the Babcock-Leighton solar dynamo mechanism of solar polar field reversal and build-up with the origin and evolution of polar filaments. We discover a new relationship connecting the residual filament area of adjacent solar cycles with the amplitude of the next sunspot cycle -- which can serve as a new tool for solar cycle forecasts -- advancing the forecast window to earlier than polar field based precursors. We conclude that polar filament properties encapsulate the physics of interaction of the poloidal magnetic field of the previous and current sunspot cycles, the resultant of which is the net poloidal magnetic field at the end of the current cycle, thus encoding as a precursor the strength of the upcoming solar cycle.

Figures (5)

• Figure 1: (Left Panel) A representative Meudon hand-drawn synoptic chart after calibration corresponding to the Carrington rotation 1123. (Right Panel) A binary segmented image showcasing the extracted filaments, while the polar filaments ($|\theta|\geq50^{\circ}$) are separately identified with a red contour from the same.
• Figure 2: Latitudinal distribution and temporal evolution of polar filament (a) area and (b) length, marked by hemisphere for solar cycles 16–21. (Middle Panels) The latitudinal distribution of all filaments extracted is presented, revealing a characteristic butterfly-like pattern, with the polar filaments ($|\theta|\geq 50^{\circ}$) highlighted in red and blue for the North and South hemispheres, respectively. (Top Panels) The temporal evolution of (a) area and (b) length of the polar filaments in the northern hemisphere (shown in red continuous lines), compared with the sunspot area (shown in dashed black lines) in the same. The comparatively lighter red and black curves in the background show carrington rotation aggregated and monthly averaged polar filament parameters and sunspot area, respectively. Dimgrey dotted lines indicate the end epoch of each cycle. (Bottom Panels) The same in the southern hemisphere. All time series have been smoothed using a 13-month running average to enhance clarity. A coloured version of this diagram is available for the online version of this article.
• Figure 3: Top panel: The red curve shows the time series of polar filament area in the northern hemisphere from solar cycles 16 to 21, while the black curve represents the northern hemispheric sunspot area obtained from Mandal2020. The lighter red and black curves in the background indicate the show carrington rotation aggregated and monthly averaged polar filament area and sunspot area, respectively, while the darker red and black curves show their corresponding 13-month running averages. The magenta curve denotes the northern hemispheric polar flux generated from data-driven optimized SFT model. Green dashed lines indicate the epochs of polar field reversal. Middle panel: This panel shows the spatio-temporal distribution of the radial magnetic field ($\mathrm{B_r}$) from solar cycles 16 to 21, generated using a data-driven optimized Surface Flux Transport (SFT) model (Pal and Nandy, in preparation). Red and black colors represent regions of positive and negative magnetic polarity, respectively. Bottom panel: The red curve shows the time series of polar filament area in the southern hemisphere from solar cycles 16 to 21, while the black curve represents the southern hemispheric sunspot area obtained from Mandal2020. The lighter red and black curves in the background indicate show carrington rotation aggregated and monthly averaged polar filament parameters and sunspot area, respectively, while the darker red and black curves represent their corresponding 13-month running averages. The blue curve denotes the southern hemispheric polar flux generated from data-driven optimized. Green dashed lines indicate the epochs of polar field reversal.
• Figure 4: The left panel shows the correlation between the remnant average polar filament area, $\mathrm{ \Delta \langle A_N\rangle}$, and the polar flux at the end of the $\mathrm{N^{th}}$ cycle. The right panel displays the corresponding correlation between $\mathrm{ \Delta \langle L_N\rangle}$ and the polar flux strength of the end of sunspot cycle N. Red and blue circles represent data from the northern and southern hemispheres, respectively. The cycle number labeled at each circle refers to cycle $\mathrm{N}$. The black dashed line in both panels indicates the best linear fit to the data.
• Figure 5: Correlation of polar filament remnant over solar cycle (SC) $N$ and $N-1$ with maximum sunspot area (SSA) of cycle $N+1$. The top-left panel shows the correlation between the remnant average polar filament area, $\mathrm{ \Delta \langle A_N\rangle}$, and the maximum sunspot area of the $\mathrm{{N+1}^{th}}$ cycle. The top-right panel displays the corresponding correlation between $\mathrm{ \Delta \langle L_N\rangle}$ and the peak sunspot area of the next cycle. Red and blue circles represent data from the northern and southern hemispheres, respectively. The cycle number labeled at each circle refers to cycle $\mathrm{N}$. The black dashed line in both panels indicates the best linear fit to the data. Bottom panels show the result from sliding the correlation coefficient in time $\Delta$t before the end of cycle N calculated for the $\mathrm{ \Delta \langle A_N\rangle}$ (left) and $\mathrm{ \Delta \langle L_N\rangle}$ (right), showing statistically significant (p-value$<$0.05) correlation with $SSA_{max}^{N+1}$ for $\Delta t \leq 5$ years measured relative to the solar minimum of cycle $N$.