Meridional flow is crucial in generating the solar poloidal magnetic field by facilitating the poleward transport of the field from the decayed Bipolar Magnetic Regions (BMRs). As the meridional circulation changes with the stellar rotation rate, the properties of stellar magnetic cycles are expected to be influenced by this flow. In this study, we explore the role of meridional flow in generating magnetic fields in Sun and sun-like stars using STABLE, Surface flux Transport And Babcock-Leighton, dynamo model. We find that a moderate meridional flow increases the polar field by efficiently driving the trailing polarity flux toward the pole, while a strong flow tends to transport both polarities of BMRs poleward, potentially reducing the polar field. Our findings are in perfect agreement with what one can expect from the surface flux transport model. Similarly, the toroidal field initially increases with moderate flow speeds and then decreases after a certain value. This trend is due to the competitive effects of shearing and diffusion. Furthermore, our study highlights the impact of meridional flow on the cycle strength and duration in stellar cycles. By including the meridional flow from a mean-field hydrodynamics model in STABLE, we show that the magnetic field strength initially increases with the stellar rotation rate and then declines in rapidly rotating stars, offering an explanation of the observed variation of stellar magnetic field with rotation rate.
The meridional circulation of the Sun is observationally found to vary with the solar cycle, becoming slower during the solar maxima. We explain this by constructing a theoretical model in which the equation of the meridional circulation (the $ϕ$ component of the vorticity equation) is coupled with the equations of the flux transport dynamo model. We find that the Lorentz force of the dynamo-generated magnetic fields can slow down the \MC\ during the solar maxima in broad conformity with the observations.
Ensemble Kalman filter data assimilation in a Babcock-Leighton solar dynamo model: an observation system simulation experiment for reconstructing meridional flow-speed
Can meridional flow variations explain the observed rising/declining phase asymmetry in the solar cycle?
Département d’Astrophysique/AIM CEA/IRFU, CNRS/INSU, Univ. Paris-Saclay & Univ. de Paris Cité（天体物理系/AIM cea/IRFU，CNRS/INSU，巴黎萨克雷大学与巴黎西岱大学）；Toulouse Univ., Institut de Recherche en Astrophysique et Planétologie, CNRS, CNES（图卢兹大学，天体物理学与行星学研究所以及CNRS、CNES）
Sunspot position data obtained from Kanzelhöhe Observatory for Solar and Environmental Research (KSO) sunspot drawings and white light images in the period 1964 to 2016 were used to calculate the rotational and meridional velocities of the solar plasma. Velocities were calculated from daily shifts of sunspot groups and an iterative process of calculation of the differential rotation profiles was used to discard outliers. We found a differential rotation profile and meridional motions in agreement with previous studies using sunspots as tracers and conclude that the quality of the KSO data is a…
The results of determinations of the azimuthal and meridional velocities by time-distance helioseismology from Helioseismic and Magnetic Imager (HMI) onboard Solar Dynamics Observatory (SDO) from May 2010 to September 2020 at latitudes from -60° to +60° and depths to about 19 Mm below the photosphere are used to analyze spatiotemporal variations of the solar differential rotation and meridional circulation. The pattern of torsional oscillations, or latitudinal belts of alternating `fast' and `slow' zonal flows migrating from high latitudes towards the equator, is found to extend in the time--l…
Solar Meridional Circulation from Doppler Shifts of the Fe I line at lambda 5250 Angstroms as Measured by the 150-foot Solar Tower Telescope at the Mt. Wilson Observatory
The Sun's axisymmetric flows, differential rotation and meridional flow, govern the dynamics of the solar magnetic cycle and variety of methods are used to measure these flows, each with its own strengths and weaknesses. Flow measurements based on cross-correlating images of the surface magnetic field have been made since the 1970s which require advanced numerical techniques that are capable of detecting movements of less than the pixel size in images of the Sun. We have identified several systematic errors in addition to the center-to-limb effect that influence previous measurements of these …
Employing the standard solar interior model as input we construct a dynamically-consistent nonlinear dynamo model that takes into account the detailed description of the Λ- effect, turbulent pumping, magnetic helicity balance, and magnetic feedback on the differential rotation and meridional circulation. The background mean-field hydrodynamic model of the solar convection zone accounts the solar-like angular velocity profile and the double-cell meridional circulation. We investigate an impact of the nonlinear magnetic field generation effects on the long-term variability and properties of the …
Recent advances in helioseismology, numerical simulations and mean-field theory of solar differential rotation have shown that the meridional circulation pattern may consist of two or more cells in each hemisphere of the convection zone. According to the mean-field theory the double-cell circulation pattern can result from the sign inversion of a nondiffusive part of the radial angular momentum transport (the so-called $Λ$-effect) in the lower part of the solar convection zone. Here, we show that this phenomenon {can result} from the radial inhomogeneity of the Coriolis number, which depends o…
We measure differential rotation and meridional flow in the Sun's surface shear layer by tracking the motions of the magnetic network seen in magnetograms from SOHO/MDI and SDO/HMI over solar cycles 23, 24, and the start of 25 (1996-2022). We examine the axisymmetric flows derived from 15-24 daily measurements averaged over individual 27-day Carrington rotations. Variations in the differential rotation include the equatorial torsional oscillation - cyclonic flows centered on the active latitudes with slower flows on the poleward sides of the active latitudes and faster flows equatorward. The f…
Accurate inference of solar meridional flow is of crucial importance for the understanding of solar dynamo process. Wave travel times, as measured on the surface, will change if the waves encounter perturbations e.g. in the sound speed or flows, as they propagate through the solar interior. Using functions called sensitivity kernels, we may image the underlying anomalies that cause measured shifts in travel times. The inference of large-scale structures e.g meridional circulation requires computing sensitivity kernels in spherical geometry. Mandal et al. (2017) have computed such spherical ker…
Stellar variability due to magnetic activity and flows at different spatial scales strongly impacts radial velocities. This variability is seen as oscillations, granulation, supergranulation, and meridional flows. The effect of this latter process is poorly known but could affect exoplanet detectability. We aim to quantify its amplitude when integrated over the disc and its temporal variability, first for the Sun, seen with different inclinations, and then for other solar-type stars. We used long time series of solar latitudinal meridional circulation to reconstruct its integrated contribution…
In this thesis, various studies are performed leading to better understanding of the 11-year solar cycle and its theoretical modeling with the flux transport dynamo model. Although this is primarily a theoretical thesis, there is a part dealing with the analysis of observational data. The various proxies of solar activity from various observatory including the sunspot area records of Kodaikanal Observatory have been analyzed to study the irregular aspects of solar cycles and an analysis has been carried out on the correlation between the decay rate and the next cycle amplitude. Theoretical ana…
The meridional circulation of the Sun, which is observed to be poleward at the surface, should have a return flow at some depth. Since large-scale flows like the differential rotation and the meridional circulation are driven by turbulent stresses in the convection zone, these flows are expected to remain confined within this zone. Current observational (based on helioseismology) and theoretical (based on dynamo theory) evidences point towards an equatorward return flow of the meridional circulation at the bottom of the convection zone. Assuming the mean values of various quantities averaged o…
我们探讨了一个新发展的物理假说，描述了一个弱耦合的轨道和旋转运动的扩展机构的太阳发电机的后果。耦合由- c（（dL/dt）x omega sub alpha）x r给出，其中dL/dt表示重心轨道角动量的变化率，omega sub alpha是旋转的角速度，r是标识随太阳旋转的坐标系中的特定位置的位置矢量，并且c是耦合效率系数。这种形式的耦合不依赖于潮汐。耦合表达式定义了一个非轴对称的全球尺度的加速度场，在空间和时间上都不同。加速度的经向分量通常在赤道和中纬度地区占主导地位，而纬向加速度在高纬度地区变得越来越重要。假定的动力学强迫函数的波形与太阳黑子周期23的实测太阳纬向流速度的时间序列的比较产生的相关性显着在99.9%的水平。我们介绍了预测的大规模流动（由于轨道自旋耦合）和发电机机制（S）的22年的磁活动周期之间的破坏性相互作用的可能性;在最近的周期中的观测较高的纬向流速度在减少太阳黑子活动的插曲可能被解释为磁周期的相位和强迫函数的相位之间的差异的结果。提供了计算太阳内纬向和纬向轨道-自旋耦合加速度的算法，作为纬度、经度、深度和时间的函数。
Quenching of Meridional Circulation in Flux Transport Dynamo Models
在最近的观测结果（太阳黑子极大值时太阳的经向环流变得较弱）的指导下，我们在太阳发电机模型中纳入了经向环流的参数猝灭，使得当对流区底部的磁场较强时，经向环流变得较弱。我们发现，如果对流区的扩散率小于2 * 10^{11} cm ^2/s，则通量传输太阳发电机在包含对流环流的淬灭时趋于不稳定。然而，α的淬灭具有稳定效果，并且可以用足够强的α淬灭来稳定具有低扩散率的发电机。对于具有高扩散系数的发电机模型，对流循环的猝灭不会产生大的影响，发电机保持稳定。我们从扩散率为2.8 * 10^{12} cm ^2/s的发电机模型中给出了一个类太阳解，其中纬向环流的猝灭使纬向环流随观测到的太阳活动周期而周期性变化，并且对发电机没有任何其他显著的影响。
Recent observations suggest the existence of two characteristic cycle times for early-type M stars dependent on the rotation period. They are of order one year for the fast rotators ($P_{\rm rot}<1$ day) and of order 4 years for the slower rotators. Additionally, the equator-to-pole differences of the rotation rates with $δΩ$ up to 0.03 rad d$^{-1}$ are known from Kepler data for the fast-rotating stars. These values are well-reproduced by the theory of large-scale flows in rotating convection zones on the basis of the $Λ$ effect. The resulting amplitudes $u^{\rm m}$ of the bottom value of the…
Active-region Modulation of Subsurface Meridional Flows and Magnetic Flux Transport on the Sun
Indian Institute of Astrophysics（印度天体物理研究所）；Pondicherry University（本地治里大学）；W. W. Hansen Experimental Physics Laboratory, Stanford University（斯坦福大学W.W.汉森实验物理实验室）；National Solar Observatory（国家太阳天文台）
We studied the solar surface flows (differential rotation and meridional circulation) using a magnetic element feature tracking technique by which the surface velocity is obtained using magnetic field data. We used the line-of-sight magnetograms obtained by the Helioseismic and Magnetic Imager aboard the Solar Dynamics Observatory from 01 May 2010 to 16 August 2017 (Carrington rotations 2096 to 2193) and tracked the magnetic element features every hour. Using our method, we estimated the differential rotation velocity profile. We found rotation velocities of $\sim$ 30 and -170 m s$^{-1}$ at la…
Context. It is possible to detect and track coronal bright points (CBPs) in SDO/AIA images. Combination of high resolution and high cadence provides a wealth of data that can be used to determine velocity flows on the solar surface with very high accuracy. Aims. We derived a very accurate solar rotation profile and investigated meridional flows, torsional oscillations and horizontal Reynolds stress based on $\approx$6 months of SDO/AIA data. Methods. We used a segmentation algorithm to detect CBPs in SDO/AIA images. We also used invariance of the solar rotation profile with central meridian di…
