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Arnab Rai Choudhuri - One of the best experts on this subject based on the ideXlab platform.
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The Meridional Circulation of the Sun: Observations, theory and connections with the solar dynamo
Science China Physics Mechanics & Astronomy, 2020Co-Authors: Arnab Rai ChoudhuriAbstract: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 over turbulence to be axisymmetric, we study the large-scale flows in solar-like stars on the basis of a 2D mean field theory. Turbulent stresses in a rotating star can transport angular momentum, setting up a differential rotation. The Meridional Circulation arises from a slight imbalance between two terms which try to drive it in opposite directions: a thermal wind term (arising out of the higher efficiency of convective heat transport in the polar regions) and a centrifugal term (arising out of the differential rotation). To make these terms comparable, the poles of the Sun should be slightly hotter than the equator. We discuss the important role played by the Meridional Circulation in the flux transport dynamo model. The poloidal field generated by the Babcock-Leighton process at the surface is advected poleward, whereas the toroidal field produced at the bottom of the convection zone is advected equatorward. The fluctuations in the Meridional Circulation (with coherence time of about 30–40 yr) help in explaining many aspects of the irregularities in the solar cycle. Finally, we discuss how the Lorentz force of the dynamo-generated magnetic field can cause periodic variations in the large-scale flows with the solar cycle.
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Explaining the variation of the Meridional Circulation with the solar cycle
Proceedings of the International Astronomical Union, 2018Co-Authors: Gopal Hazra, Arnab Rai ChoudhuriAbstract: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 $\phi$ 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.
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A theoretical model of the variation of the Meridional Circulation with the solar cycle
Monthly Notices of the Royal Astronomical Society, 2017Co-Authors: Gopal Hazra, Arnab Rai ChoudhuriAbstract:Observations of the Meridional Circulation of the Sun, which plays a key role in the operation of the solar dynamo, indicate that its speed varies with the solar cycle, becoming faster during the solar minima and slower during the solar maxima. To explain this variation of the Meridional Circulation with the solar cycle, we construct a theoretical model by coupling the equation of the Meridional Circulation (the $\phi$ component of the vorticity equation within the solar convection zone) with the equations of the flux transport dynamo model. We consider the back reaction due to the Lorentz force of the dynamo-generated magnetic fields and study the perturbations produced in the Meridional Circulation due to it. This enables us to model the variations of the Meridional Circulation without developing a full theory of the Meridional Circulation itself. We obtain results which reproduce the observational data of solar cycle variations of the Meridional Circulation reasonably well. We get the best results on assuming the turbulent viscosity acting on the velocity field to be comparable to the magnetic diffusivity (i.e. on assuming the magnetic Prandtl number to be close to unity). We have to assume an appropriate bottom boundary condition to ensure that the Lorentz force cannot drive a flow in the subadiabatic layers below the bottom of the tachocline. Our results are sensitive to this bottom boundary condition. We also suggest a hypothesis how the observed inward flow towards the active regions may be produced.
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Is a deep one-cell Meridional Circulation essential for the flux transport Solar Dynamo?
The Astrophysical Journal, 2014Co-Authors: Gopal Hazra, Bidya Binay Karak, Arnab Rai ChoudhuriAbstract:The solar activity cycle is successfully modeled by the flux transport dynamo, in which the Meridional Circulation of the Sun plays an important role. Most of the kinematic dynamo simulations assume a one-cell structure of the Meridional Circulation within the convection zone, with the equatorward return flow at its bottom. In view of the recent claims that the return flow occurs at a much shallower depth, we explore whether a Meridional Circulation with such a shallow return flow can still retain the attractive features of the flux transport dynamo (such as a proper butterfly diagram, the proper phase relation between the toroidal and poloidal fields). We consider additional cells of the Meridional Circulation below the shallow return flow---both the case of multiple cells radially stacked above one another and the case of more complicated cell patterns. As long as there is an equatorward flow in low latitudes at the bottom of the convection zone, we find that the solar behavior is approximately reproduced. However, if there is either no flow or a poleward flow at the bottom of the convection zone, then we cannot reproduce solar behavior. On making the turbulent diffusivity low, we still find periodic behavior, although the period of the cycle becomes unrealistically large. Also, with a low diffusivity, we do not get the observed correlation between the polar field at the sunspot minimum and the strength of the next cycle, which is reproduced when diffusivity is high. On introducing radially downward pumping, we get a more reasonable period and more solar-like behavior even with low diffusivity.
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Quenching of Meridional Circulation in Flux Transport Dynamo Models
Solar Physics, 2012Co-Authors: Bidya Binay Karak, Arnab Rai ChoudhuriAbstract:Guided by the recent observational result that the Meridional Circulation of the Sun becomes weaker at the time of the sunspot maximum, we have included a parametric quenching of the Meridional Circulation in solar dynamo models such that the Meridional Circulation becomes weaker when the magnetic field at the base of the convection zone is stronger. We find that a flux transport solar dynamo tends to become unstable on including this quenching of Meridional Circulation if the diffusivity in the convection zone is less than about 2 * 10^{11} cm^2/s. The quenching of alpha, however, has a stabilizing effect and it is possible to stabilize a dynamo with low diffusivity with sufficiently strong alpha-quenching. For dynamo models with high diffusivity, the quenching of Meridional Circulation does not produce a large effect and the dynamo remains stable. We present a solar-like solution from a dynamo model with diffusivity 2.8 * 10^{12} cm^2/s in which the quenching of Meridional Circulation makes the Meridional Circulation vary periodically with solar cycle as observed and does not have any other significant effect on the dynamo.
Frédéric Masset - One of the best experts on this subject based on the ideXlab platform.
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Meridional Circulation in turbulent protoplanetary disks
Astronomy & Astrophysics, 2011Co-Authors: Sebastien Fromang, Wladimir Lyra, Frédéric MassetAbstract:Context. Based on the viscous disk theory, a number of recent studies have suggested there is large-scale Meridional Circulation in protoplanetary disks. Such a flow could account for the presence of crystalline silicates, including calcium- and aluminum-rich inclusions (CAIs), at large distances from the sun. Aims. This paper aims at examining whether such large-scale flows exist in turbulent protoplanetary disks. Methods. High-resolution global hydrodynamical and magnetohydrodynamical (MHD) numerical simulations of turbulent protoplanetary disks were used to infer the properties of the flow in such disks. Results. By performing hydrodynamic simulations using explicit viscosity, we demonstrate that our numerical setup does not suffer from any numerical artifact. The aforementioned Meridional Circulation is easily recovered in viscous and laminar disks and is quickly established. In MHD simulations, the magnetorotational instability drives turbulence in the disks. Averaging out the turbulent fluctuations on a long timescale, the results fail to show any large-scale Meridional Circulation. A detailed analysis of the simulations show that this lack of Meridional Circulation is due to the turbulent stress tensor having a vertical profile different from the viscous stress tensor. A simple model is provided that successfully accounts for the structure of the flow in the bulk of the disk. In addition to those results, possible deviations from standard vertically averaged α disk models are suggested by the simulations and should be the focus of future work. Conclusions. Global MHD numerical simulations of fully ionized and turbulent protoplanetary disks are not consistent with the existence of a large-scale Meridional flow. As a consequence, the presence of crystalline silicates at large distance of the central star cannot be accounted for by that process as suggested by recent models based on viscous disk theory.
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Meridional Circulation in turbulent protoplanetary disks
arXiv: Earth and Planetary Astrophysics, 2011Co-Authors: Sebastien Fromang, Wladimir Lyra, Frédéric MassetAbstract:Based on the viscous disk theory, a number of recent studies have suggested there is large scale Meridional Circulation in protoplanetary disks. Such a flow could account for the presence of crystalline silicates, including calcium- and aluminum-rich inclusions (CAIs), at large distances from the sun. This paper aims at examining whether such large-scale flows exist in turbulent protoplanetary disks. High-resolution global hydrodynamical and magnetohydrodynamical (MHD) numerical simulations of turbulent protoplanetary disks were used to infer the properties of the flow in such disks. By performing hydrodynamic simulations using explicit viscosity, we demonstrate that our numerical setup does not suffer from any numerical artifact. The aforementioned Meridional Circulation is easily recovered in viscous and laminar disks and is quickly established. In MHD simulations, the magnetorotational instability drives turbulence in the disks. Averaging out the turbulent fluctuations on a long timescale, the results fail to show any large-scale Meridional Circulation. A detailed analysis of the simulations show that this lack of Meridional Circulation is due to the turbulent stress tensor having a vertical profile different from the viscous stress tensor. A simple model is provided that successfully accounts for the structure of the flow in the bulk of the disk. In addition to those results, possible deviations from standard vertically averaged alpha disk models are suggested by the simulations and should be the focus of future work. Global MHD numerical simulations of fully ionized and turbulent protoplanetary disks are not consistent with the existence of a large-scale Meridional flow. As a consequence, the presence of crystalline silicates at large distance for the central star cannot be accounted for by that process as suggested by recent models based on viscous disk theory.
Bidya Binay Karak - One of the best experts on this subject based on the ideXlab platform.
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Is a deep one-cell Meridional Circulation essential for the flux transport Solar Dynamo?
The Astrophysical Journal, 2014Co-Authors: Gopal Hazra, Bidya Binay Karak, Arnab Rai ChoudhuriAbstract:The solar activity cycle is successfully modeled by the flux transport dynamo, in which the Meridional Circulation of the Sun plays an important role. Most of the kinematic dynamo simulations assume a one-cell structure of the Meridional Circulation within the convection zone, with the equatorward return flow at its bottom. In view of the recent claims that the return flow occurs at a much shallower depth, we explore whether a Meridional Circulation with such a shallow return flow can still retain the attractive features of the flux transport dynamo (such as a proper butterfly diagram, the proper phase relation between the toroidal and poloidal fields). We consider additional cells of the Meridional Circulation below the shallow return flow---both the case of multiple cells radially stacked above one another and the case of more complicated cell patterns. As long as there is an equatorward flow in low latitudes at the bottom of the convection zone, we find that the solar behavior is approximately reproduced. However, if there is either no flow or a poleward flow at the bottom of the convection zone, then we cannot reproduce solar behavior. On making the turbulent diffusivity low, we still find periodic behavior, although the period of the cycle becomes unrealistically large. Also, with a low diffusivity, we do not get the observed correlation between the polar field at the sunspot minimum and the strength of the next cycle, which is reproduced when diffusivity is high. On introducing radially downward pumping, we get a more reasonable period and more solar-like behavior even with low diffusivity.
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Quenching of Meridional Circulation in Flux Transport Dynamo Models
Solar Physics, 2012Co-Authors: Bidya Binay Karak, Arnab Rai ChoudhuriAbstract:Guided by the recent observational result that the Meridional Circulation of the Sun becomes weaker at the time of the sunspot maximum, we have included a parametric quenching of the Meridional Circulation in solar dynamo models such that the Meridional Circulation becomes weaker when the magnetic field at the base of the convection zone is stronger. We find that a flux transport solar dynamo tends to become unstable on including this quenching of Meridional Circulation if the diffusivity in the convection zone is less than about 2 * 10^{11} cm^2/s. The quenching of alpha, however, has a stabilizing effect and it is possible to stabilize a dynamo with low diffusivity with sufficiently strong alpha-quenching. For dynamo models with high diffusivity, the quenching of Meridional Circulation does not produce a large effect and the dynamo remains stable. We present a solar-like solution from a dynamo model with diffusivity 2.8 * 10^{12} cm^2/s in which the quenching of Meridional Circulation makes the Meridional Circulation vary periodically with solar cycle as observed and does not have any other significant effect on the dynamo.
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Is Meridional Circulation important in modelling irregularities of the solar cycle
Proceedings of the International Astronomical Union, 2011Co-Authors: Bidya Binay Karak, Arnab Rai ChoudhuriAbstract:We explore the importance of Meridional Circulation variations in modelling the irregularities of the solar cycle by using the flux transport dynamo model. We show that a fluctuating Meridional Circulation can reproduce some features of the solar cycle like the Waldmeier effect and the grand minimum. However, we get all these results only if the value of the turbulent diffusivity in the convection zone is reasonably high.
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importance of Meridional Circulation in flux transport dynamo the possibility of a maunder like grand minimum
The Astrophysical Journal, 2010Co-Authors: Bidya Binay KarakAbstract:Meridional Circulation is an important ingredient in flux transport dynamo models. We have studied its importance on the period, the amplitude of the solar cycle, and also in producing Maunder-like grand minima in these models. First, we model the periods of the last 23 sunspot cycles by varying the Meridional Circulation speed. If the dynamo is in a diffusion-dominated regime, then we find that most of the cycle amplitudes also get modeled up to some extent when we model the periods. Next, we propose that at the beginning of the Maunder minimum the amplitude of Meridional Circulation dropped to a low value and then after a few years it increased again. Several independent studies also favor this assumption. With this assumption, a diffusion-dominated dynamo is able to reproduce many important features of the Maunder minimum remarkably well. If the dynamo is in a diffusion-dominated regime, then a slower Meridional Circulation means that the poloidal field gets more time to diffuse during its transport through the convection zone, making the dynamo weaker. This consequence helps to model both the cycle amplitudes and the Maunder-like minima. We, however, fail to reproduce these results if the dynamo is in an advection-dominated regime.
Georges Michaud - One of the best experts on this subject based on the ideXlab platform.
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Abundances anomalies and Meridional Circulation in horizontal branch stars
Astronomy & Astrophysics, 2009Co-Authors: D. Quievy, Georges Michaud, Paul Charbonneau, Jacques RicherAbstract:Context. Photospheric chemical abundances on the horizontal branch (HB) show some striking variations with effective temperature (Teff). The most straightforward explanation is that these anomalies develop through diffusion processes, in particular gravitational settling and radiative levitation. However, the abrupt disappearance of strong abundance anomalies as one moves below about 11 000 K on the HB suggests that another factor plays an important role. Aims. We test an extension to the HB of the diffusion model for main-sequence HgMn stars, where strong anomalies can only develop in the slower rotators. In these rotators the gravitational settling of helium leads to the disappearance of its superficial convection zone, so that chemical separation by radiative levitation can occur all the way to the photosphere. Methods. More specifically, we calculate the critical rotational velocity at which He settling is prevented by rotationally-induced Meridional Circulation, in a suite of stellar models spanning the zero-age HB. Helium settling serves as the measure of the atomic diffusion of all species. Results. Our abundance evolution calculations show that, for models with Teff less than about 11 500 K, corresponding to stars typically observed with the same metal composition as giants, Meridional Circulation is efficient enough to suppress He settling for rotational velocities, in good agreement with observed values. Once the Meridional Circulation profile of a star rotating as a near rigid body has been adopted, no adjustable parameter is involved. Conclusions. The Teff dependence of abundance anomalies observed on the HB can be explained by atomic diffusion transport if one introduces the competition of Meridional Circulation with the observed Teff dependence of rotation velocity of HB stars.
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A finite volume code for Meridional Circulation in stars
Journal of Computational Physics, 2002Co-Authors: Suzanne Talon, Alain P. Vincent, Georges Michaud, J. RicherAbstract:To understand the driving of both Meridional Circulation and differential rotation in radiative envelopes of stars, one has to solve for 3D mass, momentum, and energy conservation equations for a compressible gas in a central gravity field. In this study, we propose a novel finite volume technique that uses Cartesian geometry thus reducing greatly the complexity of spherical operators. The boundary conditions are efficiently imposed at the surface of the star using the fictitious points technique. We use the anelastic approximation and the Poisson equation for pressure is solved by the Jacobi method which preserves natural symmetries. We present analytical test cases of the fictitious domain technique, and show our results of asymptotic Circulation in a model with little stratification and a large viscosity.
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On Meridional Circulation in Stars
Symposium - International Astronomical Union, 2000Co-Authors: Suzanne Talon, Georges Michaud, Alain VincentAbstract:Even though the existence of Meridional currents in stars has been known for quite a long time (Eddington 1925, Vogt 1925), its exact structure as well as its influence on stellar evolution is still unclear. Some authors concentrated on finding the exact shape of Meridional Circulation in a rotating star, while others tried to model its effect on the chemical distribution in the interior. In all studies performed so far however, Meridional Circulation is considered in an asymptotic regime in which the advection of entropy by the Meridional currents is supposed to balance exactly the source term of the non-zero radiative flux divergence. Other terms could however be added to that asymptotic regime which could turn out to dominate the transport of chemicals. We wish to present here preliminary results of 3D numerical simulation attempted to tackle this problem.
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Meridional Circulation and diffusion in A and early F stars
The Astrophysical Journal, 1991Co-Authors: Paul Charbonneau, Georges MichaudAbstract:Time-dependent two-dimensional calculations of diffusion in the presence of Meridional Circulation are presented for stellar models pertaining to FmAm stars. It is shown that, once the helium superficial convection zone (HSCZ) has disappeared, the Meridional Circulation has little influence on chemical separation. In stars rotating too rapidly to become FmAm stars, chemical separation remains possible under the HSCZ. Meridional Circulation does not completely wipe out chemical separation at these velocities, and cannot by itself lead to the abundance patterns characteristic of Lambda Booti stars. Upper limits to turbulence are set. In the presence of Meridional Circulation, helium settling in stars rotating at the observed cutoff for FmAm stars remains possible for values of vertical turbulent diffusion coefficients of order 1000 sq cm/s under the helium convection zone. This sets extremely tight constraints on turbulence in stars with equatorial rotational velocities of 100 km/s or less. 52 refs.
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Main sequence abundances, mass loss and Meridional Circulation
Atmospheric Diagnostics of Stellar Evolution: Chemical Peculiarity Mass Loss and Explosion, 1Co-Authors: Georges MichaudAbstract:Constraints that abundance anomalies observed on main sequence stars put on turbulence, Meridional Circulation and mass loss are reviewed. The emphasis is on recent observations of Li abundances.
Yuhji Kuroda - One of the best experts on this subject based on the ideXlab platform.
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On the influence of the Meridional Circulation and surface pressure change on the Arctic Oscillation
Journal of Geophysical Research, 2005Co-Authors: Yuhji KurodaAbstract:[1] Eddy-forced Meridional Circulation and the corresponding surface pressure change associated with the month-to-month variability of the Arctic Oscillation (AO) are examined in the framework of Eulerian mean dynamics and are compared with the AO-like variability associated with stratospheric vacillation known as the Polar-night Jet Oscillation (PJO). Surface signals associated with both the AO and the AO-like variability associated with the PJO are produced through the eddy-forced Meridional Circulation. In the case of the AO, however, a surface pressure change is found to be produced by Meridional Circulation driven mainly by the mechanical forcing of zonal wave number 2 or 3 and high-frequency transient eddies in the troposphere. This largely contrasts with the AO-like variability associated with the PJO, which is mainly produced by the eddy forcings of zonal wave number 1 in the troposphere and stratosphere. A close relationship among the eddy forcing, Meridional velocity, surface pressure change, and AO index was found to exist not only for the month-to-month variability but also on a decadal timescale. The separation of the AO index into tropospheric and stratospheric components in the decadal timescale revealed that the recent increasing trend mainly comes from the stratosphere whereas the decadal variation comes from the troposphere.
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Meridional Circulation and the surface pressure change associated with the Southern Annular Mode: Comparison with the Arctic Oscillation
Journal of Geophysical Research, 2005Co-Authors: Yuhji KurodaAbstract:[1] Eddy forced Meridional Circulation and corresponding surface pressure change associated with the month-to-month variability of the Southern Annular Mode (SAM) are examined in the framework of the Eulerian mean dynamics, and they are compared with those of the Arctic Oscillation (AO). Effect of wave forcings on the surface pressure was diagnosed by means of a zonal mean quasi-geostrophic model on the sphere. It is found that the surface pressure change associated with the SAM is mainly produced by the Meridional Circulation driven by mechanical and thermal eddy forcings of zonal wave number 1 and mechanical forcing by high-frequency transient eddies. This contrasts with the surface pressure change associated with the AO, which is produced by the mechanical forcing by zonal wave number 2 or 3 and high-frequency transient eddies. Close relationship between the Meridional Circulation and surface pressure was found not only for the month-to-month variability but also for the decadal variability as also observed for the AO.
