arXiv subjects

Yukun Luo

Publications and source records attributed to Yukun Luo.

14 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↗

arXiv2026

LP-NavOA: Integrated Local Navigation and Obstacle Avoidance for Humanoid Robots under Limited Perception

Humanoid local navigation in cluttered environments must jointly resolve obstacle avoidance, sparse-goal recovery, and stable whole-body locomotion under short-range and partially observable sensing. Explicit planner-control decompositions introduce latency and can mismatch agile humanoid command-tracking limits, while purely reactive controllers may lose the goal after obstacle occlusion. We present LP-NavOA, a limited-perception navigation and obstacle-avoidance framework for humanoid robots. A raycast-conditioned perception-action proximal policy optimization (PPO) locomotion backbone is first trained with a robot-centered circular heading-speed command and a shared command-side safety filter. With this backbone frozen, A-star and waypoint teachers generate rollouts for distilling a recurrent local planner that overwrites only the heading command at deployment, leaving the whole-body policy intact. At runtime, LP-NavOA uses proprioception, short-range local range sensing, and a body-frame goal direction, requiring no global map, waypoint stream, or external planner. In MuJoCo open-wall and indoor layouts, the distilled planner produces obstacle bypassing and post-avoidance goal recovery, raising teacher-calibrated on-time arrival from 38--40\% to 85--97\% and reducing brush/contact-heavy progress relative to a backbone-only controller. Ablations show that dynamic route shaping, teacher-active data collection, and the circular command interface are important for navigation efficiency and for training the 3.0\,m/s backbone. A Unitree G1 deployment analysis demonstrates hardware executability without continuous joystick steering.

cs.RO↗

arXiv2026

Understanding mechanisms underlying solar cycle predictability with a general framework

The large-scale magnetic field observed at the solar surface is produced by the interior dynamo process. Whether this surface field also provides the dominant seed for the subsequent dynamo cycle, however, remains controversial, with important consequences for the predictive skill of solar dynamo models.We investigate the physical conditions under which this predictive skill of the surface field arises in dynamo models within a general framework.By applying Stokes' theorem to the magnetic induction equation, we establish a direct physical link between the surface magnetic field and the subsequent dynamo process. The dominance of the surface induction integral in the net toroidal flux generation rate provides a quantitative criterion for assessing dynamo predictability, which we apply to five representative dynamo models.This general framework shows that the surface magnetic field acquires predictive power when the surface poloidal field is efficiently coupled back into the dynamo loop through flux-transport processes (e.g., meridional circulation), a condition that can be satisfied in both Babcock-Leighton (BL)-type and $\alpha-\Omega$ mean-field dynamo models. The framework further identifies a new condition under which the surface magnetic field acquires predictive power: namely, that it represents the radial component of the interior poloidal field, as in the original BL-type dynamo scenario. In addition, the non-zero net toroidal flux across different dynamo models supports its use as a proxy linking the interior toroidal field to surface flux emergence.

astro-ph.SR↗

arXiv2026

Effects of Various Bipolar Approximations of Active Regions on Solar Surface Magnetic Field Simulations

The evolution of solar surface magnetic fields is essential for understanding solar activity and the underlying dynamic process. The surface flux transport (SFT) model is a widely used and effective tool for simulating this evolution. Active regions are incorporated as magnetic flux sources of the SFT model, but their configurations are usually simplified as symmetric or asymmetric bipolar magnetic regions (BMRs). Here, we aim to quantitatively and systematically assess how such flux source approximations affect SFT results and explore improved approximation methods using our recently developed SFT code. By comparing simulations that incorporate realistic active region configurations from solar cycle 23 through the ongoing cycle 25, we show that approximating active regions as symmetric BMRs leads to a systematic overestimation of the axial dipole strength at solar minimum. This result is independently confirmed using an algebraic quantification that evaluates the axial dipole contribution of individual active regions. The overestimation can be partially reduced by monotonically decreasing the size of the approximated BMRs, but it cannot be fully eliminated. When active regions are instead represented by morphologically asymmetric BMRs, the simulated axial dipole strength exhibits a strong and nearly linear negative dependence on the size ratio between the following and leading polarities. Based on these results, we propose a combination of BMR size and polarity size ratio that yields an axial dipole evolution comparable to that obtained with fully incorporated realistic active region configurations. This study provides a new quantitative constraint for improving future simulations with approximated BMRs.

astro-ph.SR↗

arXiv2025

Prediction of the Solar Polar Fields in 2026: An Unusually Weak Level Across the Last Five Solar Cycles

Solar polar fields are essential for the solar cycle and the heliospheric magnetic field. Cycle 25 is now entering its declining phase, the critical period during which most of the cycle's polar fields are established. Therefore, reliable polar-field prediction is now especially important. Polar-field evolution is governed by the poleward transport of already-emerged active-region (AR) flux over a timescale of a few years. Thus, surface flux-transport models can reliably provide one-year predictions without requiring information about future AR emergence. Our prediction method is validated using simulations of the surface magnetic field from 2020-2025 and hindcasts of the 2023-2024 polar fields, employing a newly constrained profile of the meridional flow. Using the most recent HMI synoptic magnetogram as the initial condition, we predict the polar-field evolution from October 2025 to October 2026. The southern polar field is predicted to strengthen gradually, while the northern field is expected to decline sharply until March 2026 due to some ARs with abnormal polarity. By that time, the northern polar field becomes exceptionally weak, and the southern field remains relatively weak, raising concerns about the polar-field strength at the cycle 25/26 minimum and the amplitude of cycle 26.

astro-ph.SR↗

arXiv2025

Constraining the outer boundary condition for the Babcock-Leighton dynamo models

The evolution of the Sun's large-scale surface magnetic field is well captured by surface flux transport models, which can therefore provide a natural constraint on the outer boundary condition (BC) of Babcock-Leighton (BL) dynamo models. For the first time, we propose a zero radial diffusion BC for BL dynamo models, enabling their surface field evolution to align consistently with surface flux transport simulations. We derive a zero radial diffusion BC from the Magnetohydrodynamic induction equation and evaluate its effects in comparison with two alternatives: (i) a radial outer BC, and (ii) a radial outer BC combined with strong near-surface radial pumping. The comparison is carried out both for the evolution of a single bipolar magnetic region and within a full BL dynamo model. The zero radial diffusion outer BC effectively suppresses radial diffusion across the surface, ensuring consistency between the evolution of the bipolar magnetic region in the BL dynamo and the surface flux transport model. With this outer BC, the full BL dynamo model successfully reproduces the fundamental properties of the solar cycle. In addition, the model naturally produces a surface magnetic field that is not purely radial, in closer agreement with solar observations. The physically motivated zero radial diffusion boundary condition paves the way for deeper insight into the solar and stellar cycles.

astro-ph.SR↗

arXiv2025

Simulation of Solar Surface Flux Transport Constrained by Magnetic Power Spectra. I. Flux Transport Parameter

The multi-scale structure of the solar surface magnetic field is essential for understanding both the Sun's internal dynamo processes and its external magnetic activity. The surface flux transport (SFT) model has been successful in describing the large-scale evolution of the surface field, but its ability to capture observed multi-scale features, quantified by magnetic power spectra, remains uncertain. Here, we evaluate the SFT model by comparing observed and simulated power spectra across a broad range of spatial scales and by analyzing the effects of key transport parameters. We find that the simulations reproduce the observed spectra well at spherical harmonic degrees $l\lesssim60$, but diverge progressively at smaller spatial scales $l\gtrsim60$. This divergence likely arises from the diffusion approximation used to model the random walk of supergranulation. Power at $20 \lesssim l \lesssim 60$ is primarily determined by the magnetic flux sources, while at $l \lesssim 20$, the spectra are more sensitive to transport parameters. The meridional flow profile, including its equatorial gradient, peak latitude, and polar distribution, along with turbulent diffusivity, has distinct impacts on the low-degree modes ($l \leq 5$). In particular, a comparison of the $l$=3 and $l$=5 multipoles strengths suggests that the poleward flow above $\sim\pm60^\circ$ latitudes is very weak. This study presents the first quantitative validation of SFT models using magnetic power spectra and provides new constraints on surface flux transport physics.

astro-ph.SR↗

arXiv2025

Solar Surface Magnetic Field Simulation from 2010 to 2024 and Anomalous Southern Poleward Flux Transport in Cycle 24

The solar surface magnetic field is fundamental for modeling the coronal magnetic field, studying the solar dynamo, and predicting solar cycle strength. We perform a continuous simulation of the surface magnetic field from 2010 to 2024, covering solar cycle 24 and the ongoing cycle 25, using the surface flux transport model with assimilated observed active regions (ARs) as the source. The simulation reproduces the evolution of the axial dipole strength, polar field reversal timing, and magnetic butterfly diagram in good agreement with SDO/HMI observations. Notably, these results are achieved without incorporating radial diffusion or cyclic variations in meridional flow speed, suggesting their limited impact. Poleward surges of the following polarity typically dominate throughout the cycle, but in the southern hemisphere during cycle 24, they are limited to a short period from 2011 to 2016. This anomalous pattern arises from intermittent AR emergence, with about 46% of total unsigned flux contributed by ARs emerging during Carrington Rotations 2141-2160 (September 2013 - February 2015). These ARs show a strong active longitude at Carrington longitudes 200-260 degree and a weaker one at 80-100 degree. After 2016, poleward migrations of leading-polarity flux become dominant, despite most ARs following Joy's and Hale's laws. This reversal is likely due to prolonged intervals between AR emergences, which allow leading-polarity flux to distribute across a broad latitude range before cancellation by subsequent ARs. These findings highlight the importance of the temporal interval of AR emergence in driving the flux transport pattern.

astro-ph.SR↗

arXiv2024

Long-term variation of the solar polar magnetic fields at different latitudes

The polar magnetic fields of the Sun play an important role in governing solar activity and powering fast solar wind. However, because our view of the Sun is limited in the ecliptic plane, the polar regions remain largely uncharted. Using the high spatial resolution and polarimetric precision vector magnetograms observed by Hinode from 2012 to 2021, we investigate the long-term variation of the magnetic fields in polar caps at different latitudes. The Hinode magnetic measurements show that the polarity reversal processes in the north and south polar caps are non-simultaneous. The variation of the averaged radial magnetic flux density reveals that, in each polar cap, the polarity reversal is completed successively from the 70 degree latitude to the pole, reflecting a poleward magnetic flux migration therein. These results clarify the polar magnetic polarity reversal process at different latitudes.

astro-ph.SR↗

arXiv2024

Meridional flow in the solar polar caps revealed by magnetic field observation and simulation

As a large-scale motion on the Sun, the meridional flow plays an important role in determining magnetic structure and strength and solar cycle. However, the meridional flow near the solar poles is still unclear. The Hinode observations show that the magnetic flux density in polar caps decreases from the lower latitudes to the poles. Using a surface flux transport model, we simulate the global radial magnetic field to explore the physical process leading to the observed polar magnetic distribution pattern. For the first time, the high-resolution observations of the polar magnetic fields observed by Hinode are used to directly constrain the simulation. Our simulation reproduces the observed properties of the polar magnetic fields, suggesting the existence of a counter-cell meridional flow in the solar polar caps with a maximum amplitude of about 3 m s$^{-1}$.

astro-ph.SR↗

arXiv2024

The Sun's Magnetic Power Spectra over Two Solar Cycles. \uppercase\expandafter{\romannumeral2}. Cycle Dependence of Active Region, Magnetic Network, and Their Relation

The multi-scaled solar magnetic field consists of two major components: active regions (ARs) and magnetic network. Unraveling the cycle-dependent properties and interrelations of these components is crucial for understanding the evolution of the solar magnetic field. In this study, we investigate these components using magnetic power spectra derived from high-resolution and continuous synoptic magnetograms since cycle 23 onwards. Our results show that the size of the magnetic network ranges from 26 Mm to 41 Mm without dependence on the solar cycle. The power of the network field ($P_{NW}$) accounts for approximately 20\% of the total power during any phase of solar cycles. In contrast to the AR power ($P_{AR}$), $P_{NW}$ displays a weaker cycle dependence, as described by the relationship $P_{NW}$ $\approx$ 0.6* $P_{AR}$ + 40. The power-law index between AR sizes and magnetic network sizes presents a strong anti-correlation with the activity level. Additionally, our study indicates that in the absence of sunspots on the solar disc, the magnetic power spectra remain time-independent, consistently exhibiting similarity in both shape and power. This study introduces a new method to investigate the properties of the magnetic network and provides magnetic power spectra for high-resolution simulations of the solar magnetic field at the surface at various phases of solar cycles.

astro-ph.SR↗

arXiv2024

Toward a live homogeneous database of solar active regions based on SOHO/MDI and SDO/HMI synoptic magnetograms.II.parameters for solar cycle variability

Solar active regions (ARs) determine solar polar fields and cause solar cycle variability within the framework of the Babcock-Leighton (BL) dynamo. The contribution of an AR to the polar field is measured by its dipole field, which results from flux emergence and subsequent flux transport over the solar surface. The dipole fields contributed by an AR before and after the flux transport are referred to as the initial and final dipole fields, respectively. For a better understanding and prediction of solar cycles, in this paper, we provide a database including AR's initial and final dipole fields and the corresponding results of their bipolar magnetic region (BMR) approximation from 1996 onwards. We also identify the repeated ARs and provide the optimized transport parameters. Based on our database, we find that although the commonly used BMR approximation performs well for the initial dipole field, it exhibits a significant deviation for the final dipole field. To accurately assess an AR's contribution to the polar field, the final dipole field with its real configuration should be applied. Despite the notable contributions of a few rogue ARs, approximately the top 500 ARs ordered by their final dipole fields are necessary to derive the polar field at the cycle minimum. While flux transport may increase or decrease the dipole field for an individual AR, its collective impact over all ARs in a cycle is a reduction in their total dipole field.

astro-ph.SR↗

arXiv2023

The Sun's Magnetic Power Spectra Over Two Solar Cycles. I. Calibration Between SDO/HMI And SOHO/MDI Magnetograms

The Sun's magnetic field is strongly structured over a broad range of scales. The magnetic spatial power spectral analysis provides a powerful tool to understand the various scales of magnetic fields and their interaction with plasma motion. We aim to investigate the power spectra using spherical harmonic decomposition of high-resolution SOHO/MDI and SDO/HMI synoptic magnetograms covering three consecutive solar cycle minima in a series of papers. As the first of the series, we calibrate and analyze the power spectra based on co-temporal SDO/HMI and SOHO/MDI data in this paper. For the first time, we find that the calibration factor $r$ between SOHO/MDI and SDO/HMI varies with the spatial scale $l$ of the magnetic field, where $l$ is the degree of a spherical harmonics. The calibration factor satisfies $r(l)=\sqrt{-0.021 l^{0.64}+2} \quad(5<\mathrm{l}\leq539)$. With the calibration function, most contemporaneous SOHO/MDI and SDO/HMI magnetograms show consistent power spectra from about 8 Mm to the global scales over about 3 orders of magnitudes. Moreover, magnetic power spectra from SOHO/MDI and SDO/HMI maps show peaks/knees at $l\approx120$ corresponding to the typical supergranular scale (about 35 Mm) constrained from direct velocimetric measurements. This study paves the way for investigating the solar-cycle dependence of supergranulation and magnetic power spectra in subsequent studies.

astro-ph.SR↗

arXiv2023

Towards a live homogeneous database of solar active regions based on SOHO/MDI and SDO/HMI synoptic magnetograms. I. Automatic detection and calibration

Recent studies indicate that a small number of rogue solar active regions (ARs) may have a significant impact on the end-of-cycle polar field and the long-term behavior of solar activity. The impact of individual ARs can be qualified based on their magnetic field distribution. This motivates us to build a live homogeneous AR database in a series of papers. As the first of the series, we develop a method to automatically detect ARs from 1996 onwards based on SOHO/MDI and SDO/HMI synoptic magnetograms. The method shows its advantages in excluding decayed ARs and unipolar regions and being compatible with any available synoptic magnetograms. The identified AR flux and area are calibrated based on the co-temporal SDO/HMI and SOHO/MDI data. The homogeneity and reliability of the database are further verified by comparing it with other relevant databases. We find that ARs with weaker flux have a weaker cycle dependence. Stronger ARs show the weaker cycle 24 compared with cycle 23. Several basic parameters, namely, location, area, and flux of negative and positive polarities of identified ARs are provided in the paper. This paves the way for AR's new parameters quantifying the impact on the long-term behavior of solar activity to be presented in the subsequent paper of the series. The constantly updated database covering more than two full solar cycles will be beneficial for the understanding and prediction of the solar cycle. The database and the detection codes are accessible online.

astro-ph.SR↗