arXiv subjects

Zi-Fan Wang

Publications and source records attributed to Zi-Fan Wang.

7 recordsLinked to original sources

arXiv2026

The recent anomalously weak polar field does not imply a weak field at solar cycle 25 minimum

The ongoing solar cycle 25 has progressed past its peak of sunspot numbers, being stronger than the previous cycle 24. However, the present polar field is rather weak compared to previous cycles at the same evolution phase, particularly in the northern hemisphere, where it has been decreasing since mid-2025 till present. A prominent poleward surge is observed to cause the decrease of the polar field. This raises concerns to the polar field at cycle 25 minimum, which is the precursor to the strength of the next cycle 26. To predict whether the polar field at cycle 25 minimum will be weak as expected, we use observation-based statistical properties to predict the active region emergence during the latter half of cycle 25, and use a surface flux transport model to simulate the evolution of the large-scale magnetic field. We predict the polar field at cycle 25 minimum to be $-5.62\pm1.61$G in the north and $5.51\pm1.48$G in the south, both stronger than those at cycle 24 minimum. This is because the poleward surge causing the temporal decrease of the polar field originates from a group of active regions that produces net increase to the polar field, instead of active regions with non-Joy's tilt. Our results suggest the weak polar field at present is unlikely to cause a weak minium and an exceptionally weak cycle 26, clarifying that the short term evolution should not be simply correlated to the long term properties of the solar large-scale field.

astro-ph.SR↗

arXiv2025

Observation-Based Iterative Map for Solar Cycles. II. The Gnevyshev-Ohl Rule and its Generation Mechanism

The Gnevyshev-Ohl (G-O) rule, or even-odd effect, is an important observational phenomenon in solar cycles, originally suggesting that even-numbered cycles are typically followed by stronger odd-numbered ones. However, subsequent studies have reported varied forms and often conflicting manifestations of this rule, along with diverse interpretations of its physical origin. Using an observation-based iterative map, we investigate these different forms of the G-O rule and propose a more general underlying rule: statistically, a given solar cycle is more likely to be followed by a stronger one, regardless of even-odd numbering. This transition asymmetry arises from the map's inherent asymmetry relative to the diagonal. Over timescales comparable to historical observations, both the G-O rule and its reversal can arise randomly, without a consistent preference. The short-term behavior of the rule is sensitive to the initial cycle, the selected time interval, and the parameters of the recursion function. These findings reconcile previously conflicting reports and point to a general generation mechanism: G-O-like behavior arises naturally from nonlinear stochastic dynamics. While different recursion parameters may lead to varying short-term patterns and statistical strengths, the emergence of G-O-like features appears to be a generic property of such systems.

astro-ph.SR↗

arXiv2025

Observation-Based Iterative Map for Solar Cycles. I. Nature of Solar Cycle Variability

Inter-cycle variations in the series of 11-year solar activity cycles have a significant impact on both the space environment and climate. Whether solar cycle variability is dominated by deterministic chaos or stochastic perturbations remains an open question. Distinguishing between the two mechanisms is crucial for predicting solar cycles. Here we reduce the solar dynamo process responsible for the solar cycle to a one-dimensional iterative map, incorporating recent advance in the observed nonlinearity and stochasticity of the cycle. We demonstrate that deterministic chaos is absent in the nonlinear system, regardless of model parameters, if the generation of the poloidal field follows an increase-then-saturate pattern as the cycle strength increases. The synthesized solar cycles generated by the iterative map exhibit a probability density function (PDF) similar to that of observed normal cycles, supporting the dominant role of stochasticity in solar cycle variability. The parameters governing nonlinearity and stochasticity profoundly influence the PDF. The iterative map provides a quick and effective tool for predicting the range, including uncertainty of the subsequent cycle strength when an ongoing cycle amplitude is known. Due to stochasticity, a solar cycle loses almost all its original information within 1 or 2 cycles. Although the simplicity of the iterative map, the behaviors it exhibits are generic for the nonlinear system. Our results provide guidelines for analyzing solar dynamo models in terms of chaos and stochasticity, highlight the limitation in predicting solar cycle, and motivate further refinement of observational constraints on nonlinear and stochastic processes.

astro-ph.SR↗

arXiv2022

The nonuniformity of poleward flux transport on the solar surface: a statistical method applied to solar cycles 21-24

The poleward migration of the active regions' magnetic flux on the solar surface plays an important role in the development of the large-scale field development, especially the polar field reversal, which is a key process in the Babcock-Leighton-type solar dynamos. The poleward flux transport is nonuniform, centered around poleward surges as suggested by previous observations. The strong, long-lasting surges are related to activity complexes, and often result in violent polar field reversal. However, the nonuniformity of poleward flux transport has not been evaluated quantitatively. We propose a statistical method to analyze the poleward flux transport during solar cycles 21-24 by considering the frequency distributions of the magnetic field at latitudes of poleward surges occurrence during solar cycles. The nonuniformity is quantified as the kurtosis statistics representing the tailedness of the distributions. We test the method on results of surface flux transport simulations, and apply to WSO, NSO, MWO, and HMI data. We confirm that the poleward surges are of significance during solar cycles 21-24 in general. The kurtosis within a solar cycle is affected by different latitudes of the magnetic field and different data sources. The southern hemisphere of cycle 24 exhibits the largest kurtosis, agreeing the super surge concept from previous work. The significant nonuniformity of poleward flux transport originates from the nonrandomness of active regions, which favors the activity complexes origin of poleward surges.

astro-ph.SR↗

arXiv2021

Sunspot tilt angles revisited: Dependence on the solar cycle strength

The tilt angle of sunspot groups is crucial in the BL type dynamo. Some studies have shown that the tilt coefficient is anti-correlated with the cycle strength. If the anti-correlation exists, it will be shown to act as an effective nonlinearity of the BL-type dynamo to modulate the solar cycle. However, some studies have shown that the anti-correlation has no statistical significance. We aim to investigate the causes behind the controversial results of tilt angle studies and to establish whether the tilt coefficient is indeed anti-correlated with the cycle strength. We first analyzed the tilt angles from DPD. Based on the methods applied in previous studies, we took two criteria to select the data, along with the linear and square-root functions to describe Joy's law, and three methods to derive the tilt coefficients for cycles 21-24. This allowed us to evaluate different methods based on comparisons of the differences among the tilt coefficients and the tilt coefficient uncertainties. Then we utilized Monte Carlo experiments to verify the results. Finally, we extended these methods to analyze the separate hemispheric DPD data and the tilt angle data from Kodaikanal and Mount Wilson. The tilt angles exhibit an extremely wide scatter due to both the intrinsic mechanism for its generation and measurement errors, for instance, the unipolar regions included in data sets. Different methods to deal with the uncertainties are mainly responsible for the controversial character of the previous results. The linear fit to the tilt-latitude relation of sunspot groups with $Δs>2.5$ of a cycle carried out without binning the data can minimize the effect of the tilt scatter on the uncertainty of the tilt coefficient. Based on this method the tilt angle coefficient is anti-correlated with the cycle strength with strong statistical significance.

astro-ph.SR↗

arXiv2021

Algebraic quantification of an active region's contribution to the solar cycle

The solar dipole moment at cycle minimum is considered to be the most successful precursor for the amplitude of the subsequent cycle. Numerical simulations of the surface flux transport (SFT) model are widely used to effectively predict the dipole moment at cycle minimum. Recently an algebraic method has been proposed to quickly predict the contribution of an active region (AR) to the axial dipole moment at cycle minimum instead of SFT simulations. However, the method assumes a bipolar magnetic region (BMR) configuration of ARs. Actually most ARs are asymmetric in configuration of opposite polarities, or have more complex configurations. Such ARs evolve significantly differently from that of BMR approximations. We propose a generalized algebraic method to describe the axial dipole contribution of an AR with an arbitrary configuration, and evaluate its effectiveness compared to the BMR-based method. We employ mathematical deductions to obtain the generalized method. We compare the results of the generalized method with SFT simulations of observed ARs, artificially created BMRs, and ARs with more complex configurations. We also compare the results with that from the BMR-based method. The generalized method is equivalent to the SFT model, and precisely predicts the ARs' contributions to the dipole moment. The method has a much higher computational efficiency than SFT simulations. Although the BMR-based method has similar computational efficiency as the generalized method, it is only accurate for symmetric bipolar ARs. The BMR-based method systematically overestimates the dipole contributions of asymmetric bipolar ARs, and randomly miscalculate the contributions of more complex ARs. The generalized method provides a quick and precise quantification of an AR's contribution to the solar cycle evolution, which paves the way for the application into the physics-based solar cycle prediction.

astro-ph.SR↗

arXiv2020

Activity Complexes and A Prominent Poleward Surge During Solar Cycle 24

Long-lasting activity complexes (ACs), characterised as a series of closely located, continuously emerging solar active regions (ARs), are considered generating prominent poleward surges from observations. The surges lead to significant variations of the polar field, which are important for the modulation of solar cycles. We aim to study a prominent poleward surge during solar cycle 24 on the southern hemisphere, and analyse its originating ACs and the effect on the polar field evolution. We automatically identify and characterize ARs based on synoptic magnetograms from the Solar Dynamic Observatory. We assimilate these ARs with realistic magnetic configuration into a surface flux transport model, and simulate the creation and migration of the surge. Our simulations well reproduce the characteristics of the surge and show that the prominent surge is mainly caused by the ARs belonging to two ACs during Carrington Rotations 2145-2159 (December 2013-January 2015). The surge has a strong influence on the polar field evolution of the southern hemisphere during the latter half of cycle 24. Without the about one-year-long flux emergence in the form of ACs, the polar field around the cycle minimum would have remained at a low level and even reversed to the polarity at cycle 23 minimum. Our study also shows that the long-lived unipolar regions due to the decay of the earlier emerging ARs cause an intrinsic difficulty of automatically identifying and precisely quantifying later emerging ARs in ACs.

astro-ph.SR↗