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The meridional circulation of the Sun: Observations, theory and connections with the solar dynamo

Science China(Physics,Mechanics & Astronomy) 2021年第3期64卷 46-72页

作者： arnab rai choudhuri Department of Physics Indian Institute of ScienceBangalore560012India

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.

关键词： astrophysical plasma solar physics Sun:solar magnetism Sun:helioseismology fluid flow:rotational

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The hemispheric asymmetry of solar activity during the last century and the solar dynamo

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Chinese Journal of Astronomy and Astrophysics 2009年第1期9卷 115-126页

作者： Ashish Goel arnab rai choudhuri Department of Physics Indian Institute of Technology Department of Physics Indian Institute of Science

We believe the Babcock-Leighton process of poloidal field generation to be the main source of irregularity in the solar cycle. The random nature of this process may make the poloidal field in one hemisphere stronger than that in the other hemisphere at the end of a cycle. We expect this to induce an asymmetry in the next sunspot cycle. We look for evidence of this in the observational data and then model it theoretically with our dynamo code. Since actual polar field measurements exist only from the 1970s, we use the polar faculae number data recorded by Sheeley （1991, 2008） as a proxy of the polar field and estimate the hemispheric asymmetry of the polar field in different solar minima during the major part of the twentieth century. This asymmetry is found to have a reasonable correlation with the asymmetry of the next cycle. We then run our dynamo code by feeding information about this asymmetry at the successive minima and compare the results with observational data. We find that the theoretically computed asymmetries of different cycles compare favorably with the observational data, with the correlation coefficient being 0.73. Due to the coupling between the two hemispheres, any hemispheric asymmetry tends to get attenuated with time. The hemispheric asymmetry of a cycle either from observational data or from theoretical calculations statistically tends to be less than the asymmetry in the polar field （as inferred from the faculae data） in the preceding minimum. This reduction factor turns out to be 0.43 and 0.51 respectively in observational data and theoretical simulations.

关键词： Sun activity -- Sun magnetic fields -- sunspots

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A possible explanation of the Maunder minimum from a flux transport dynamo model

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Research in Astronomy and Astrophysics 2009年第9期9卷 953-958页

作者： arnab rai choudhuri Bidya Binay Karak Department of Physics Indian Institute of ScienceBangalore-560012India

We propose that the poloidal field at the end of the last sunspot cycle before the Maunder minimum fell to a very low value due to fluctuations in the Babcock-Leighton process. With this assumption, a flux transport dynamo model is able to explain various aspects of the historical records of the Maunder minimum remarkably well by suitably choosing the parameters of the model to give the correct growth time.

关键词： Sun: activity -- Sun: magnetic fields -- sunspots

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Studies of grand minima in sunspot cycles by using a flux transport solar dynamo model

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Research in Astronomy and Astrophysics 2013年第11期13卷 1339-1357页

作者： Bidya Binay Karak arnab rai choudhuri Department of Physics Indian Institute of Science

We propose that grand minima in solar activity are caused by simultane- ous fluctuations in the meridional circulation and the Babcock-Leighton mechanism for the poloidal field generation in the flux transport dynamo model. We present the following results：（a） fluctuations in the meridional circulation are more effective in producing grand minima; （b） both sudden and gradual initiations of grand minima are possible; （c） distributions of durations and waiting times between grand minima seem to be exponential; （d） the coherence time of the meridional circulation has an effect on the number and the average duration of grand minima, with a coherence time of about 30 yr being consistent with observational data. We also study the occurrence of grand maxima and find that the distributions of durations and waiting times between grand maxima are also exponential, like the grand minima. Finally we address the question of whether the Babcock-Leighton mechanism can be operative during grand minima when there are no sunspots. We show that an a-effect restricted to the upper portions of the convection zone can pull the dynamo out of the grand minima and can match various observational requirements if the amplitude of this a-effect is suitably fine-tuned.

关键词： magnetic fields -- (magnetohydrodynamics:) MHD -- Sun: magneticfields -- (Sun:) sunspots

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印度的天文学与天体物理学

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中国国家天文 2012年第10期6卷 50-52页

作者： arnab rai choudhuri 吴蕴豪(译)

在古印度，天文学被认为是吠陀经（印度教经典——译者注）教育中不可或缺的一部分。Aryabhata（公元476年-？）、Brahmagupta（公元598年-？）和Bhaskaracarya（公元1114年-？）都是重要的天文学家，他们的天文学论著经受住了时间的考... 详细信息

在古印度，天文学被认为是吠陀经（印度教经典——译者注）教育中不可或缺的一部分。Aryabhata（公元476年-？）、Brahmagupta（公元598年-？）和Bhaskaracarya（公元1114年-？）都是重要的天文学家，他们的天文学论著经受住了时间的考验而流传至今。

关键词：天文学家古印度天体物理学印度教

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Starspots, stellar cycles and stellar flares： Lessons from solar dynamo models

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Science China(Physics,Mechanics & Astronomy) 2017年第1期60卷 1-16页

作者： Amab rai choudhuri Department of Physics Indian Institute of Science Bangalore 560012 India

In this review, we discuss whether the present solar dynamo models can be extrapolated to explain various aspects of stellar activity. We begin with a summary of the following kinds of data for solar-like stars:(i) data pertaining to stellar cycles from Ca H/K emission over many years;(ii) X-ray data indicating hot coronal activity;(iii) starspot data(especially about giant polar spots); and(iv) data pertaining to stellar superflares. Then we describe the current status of solar dynamo modelling—giving an introduction to the flux transport dynamo model, the currently favoured model for the solar cycle. While an extrapolation of this model to solar-like stars can explain some aspects of observational data, some other aspects of the data still remain to be theoretically explained. It is not clear right now whether we need a different kind of dynamo mechanism for stars having giant starspots or producing very strong superflares.

关键词： sunspots solar dynamo solar cycle stellar dynamo, stellar activity

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