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

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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

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

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

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

Evolution of a confined gravelly river to a braided-meandering river in the Late Cretaceous Khasi group,southern Shillong plateau,NE India

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Journal of Palaeogeography 2024年第4期13卷 754-774页

作者： Adrita choudhuri Prem Raj Uddandam Debarati Nag Birbal Sahni Institute of Palaeosciences Lucknow226007India Savitribhai Phule Pune University PuneIndi

This paper intends to unravel the depositional condition of the Basal Conglomerate/Jadukata Formation and the Lower Mahadek Formation in southern Shillong Plateau,NE India deposited during the Late Cretaceous *** studied succession is entirely siliciclastics in composition and was built up on the Precambrian basement ***-of-art facies analysis and palaeocurrent evidence helped us to reconstruct the palaeogeography and *** facies tracts reveal that frequent hyperconcentrated mass flow deposits dominated the proximal part of the river ***,the river evolved to a sandy braided and finally mud-dominated,meandering *** river channel changed its pattern through onlapping and progressive elimination of the accommodation space created by the tectonic subsidence/*** shifts in palaeocurrent direction at different stages of the river evolution confirm the role of tectonics on the river channel *** detrital modes of the sandstones reflecting lithic wacke composition and the recycled orogeny corroborate the *** lateral and vertical facies transition suggests a pull-apart type basin due to ongoing *** analyses also show signatures of the overlapping tectonic conditions supporting the pull-apart basin model.

关键词： Tectonics Hyperconcentrated flows Braided-meandering system Pull-apart rift basin Late Cretaceous south Shillong Plateau

Computational Studies of Photocatalysis with Metal-Organic Frameworks

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Energy & Environmental Materials 2019年第4期2卷 251-263页

作者： Xin-Ping Wu Indrani choudhuri Donald G.Truhlar Department of Chemistry Chemical Theory Centerand Minnesota Supercomputing InstituteUniversity of MinnesotaMinneapolis MN 55455-0431USA

Metal–organic frameworks (MOFs) as photocatalysts and photocatalyst supports combine several advantages of homogeneous and heterogeneous catalyses, including stability, post-reaction separation, catalyst reusability,and tunability, and they have been intensively studied for photocatalytic applications. There are several reviews that focus mainly or even entirely on experimental work. The present review is intended to complement those reviews by focusing on computational work that can provide a further understanding of the photocatalytic properties of MOF photocatalysts. We first present a summary of computational methods, including density functional theory, combined quantum mechanical and molecular mechanical methods, and force fields for MOFs. Then, computational investigations on MOF-based photocatalysis are briefly discussed. The discussions focus on the electronic structure, photoexcitation, charge mobility, and photoredox catalysis of MOFs, especially the widely studied Ui O-66-based MOFs.

关键词： computation metal doping metal-organic framework photocatalysis QM/MM

Microbial mat-related structures shared by both siliciclastic and carbonate formations

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Journal of Palaeogeography 2016年第3期5卷 278-291页

作者： Subir Sarkar Adrita choudhuri Sunipa Mandal Patrick G. Eriksson Department of Geological Sciences Jadavpur University Kolkata 700032 India Department of Geology University of Pretoria Pretoria 0002 South Africa

Microbiota has always been the dominant life form, records of which are preserved in delicate forms within siticiclastic rocks. More pronounced record in the form of stromatotites possibly obscured the fact that many of the same delicate structures may be recognizable within carbonate rocks too. The Neo- proterozoic Bhander Limestone in central India bears many such structures that are quintessentially similar to microbial mat-related structures reported from the Paleoproterozoic Chorhat Sandstone preserved within the same, Vindhyan Basin. Extensive microscopic, ultramicroscopic, and geochemical studies address the appre- hension that such bedding plane structures in carbonate rocks could be merely weathering products. Trapping, binding and stabilitization of sediment by microbial mats are atl evident. Preferred pyritization along the inferred, predefined microbial mats confirmed on the basis of EPMA （Electron Probe Microanalysis） results, and the enhanced carbon content along these mats layers and within suspected mat chips associated with them, are revealing. Raman spectroscopy, indeed, evinces enhanced kerogen content within both mats and mat chips. Interestingly, these microbial mat layers are recognized selectively within the lower of the two tiers of the Bhander Limestone. The lagoonal carbonate of the lower tier of the Bhander Limestone is muddy and contains a substantial proportion of silt-sized quartz grains that possibly impeded stromatolite growth. Stromatolites abound in the wave agitated upper tier of the Bhander Limestone which is dominated by oosparite. This paper provides evidence that the delicate microbial mat-related structures reported so far only from siticictastic rocks can *** be recognized within carbonate formations, and hopes to stimulate the search for additional such features, more preferably within carbonates originated in shallow and quiet water.

关键词： Microbiat mat-retated structures Bhander timestone Chorhat sandstone Kerogen Preferredpyritization Impeded stromatotite ~rowth

Lakshanhatti stromatolite, India： Biogenic or abiogenic？

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Journal of Palaeogeography 2016年第3期5卷 292-310页

作者： Adrita choudhuri Subir Sarkar Wladyslaw Altermann Soumik Mukhopadhyay Pradip K.Bose Department of Geological Sciences Jadavpur University Kolkata 700032 India Department of Geology University of Pretoria Pretoria 0002 South Africa

Abstract The extraordinary lateral continuity of isopachous stromatotite laminae in the -87 m-thick Mesoproterozoic Lakshanhatti Dolomite （India） evinces chemical precipitation. Fan-shaped crystals grown on tamina surfaces further corroborate this contention; growth of fan-shaped crystats under the overhanging stromatolite column-margin indicates direct carbonate precipitation from ambient waters. The fan-shaped crystats are stacked up, separated onty by thin dark micritic taminae. In a retativety upper stratigraphic in- terval of the formation, tighter laminae characterized by a dotted texture and traversed by numerous winding tubutar voids change graduatty upwards into dark micritic taminae. Some sporadicatty distributed lenticutar intraciastic beds also have the similarly dark micritic coatings. Ctear carbonate cement crusts atso occur between laminae and between successive dark micritic coats around intraciasts. DuU cathodotuminescence （CL） characterizes this cement as weU as the cement lining within earty dia^enetic voids, in contrast, the taminae with dotted textures show dirty orange luminescence, white the dark micritic taminae and the dark micritic grain-coats display dear bright orange luminescence. Pyrite and its pseudomorphs are preferabty concentrated along the dark micritic laminae. Carbon content in these dark micritic components, whether laminae or coats, is much higher than in the lighter components, exceeding what can be accounted for their CaMg（COs）2 composition. A large part of this carbon is kerogen, ptausibly biogenic. The dark components are, therefore, reasonably, though not unequivocally, assumed to be microbial mats. Degradation of the mats might have given rise to the tight laminae with dotted textures. The fuzzy-margin tubes within the tight laminae probabty manifest the escape of gases generated during organic matter decomposition. Si-At-rich terrigenous fines thinty draping the dark carbonaceous taminae was possibty the resutt of baft

关键词： StromatoHte Lamina continuity Mesoproterozoic Lakshanhatti Dolomite Member Inorganicprecipitation Kerogen enrichment Filamentous microbiota

Life style modification improves insulin resistance and liver histology in patients with non-alcoholic fatty liver disease

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World Journal of Hepatology 2012年第7期4卷 209-217页

作者： Ganesh Bhat Chalamalasetty Sreenivasa Baba Amaresh Pandey Neeraj Kumari Gourdas choudhuri Department of Gastroenterology Kasturba Medical College Manipal University Manipal 576104 India Department of Gastroenterology Sanjay Gandhi Postgraduate Institute of Medical Sciences Lucknow 226014 India Department of Pathology Sanjay Gandhi Postgraduate Institute of Medical Sciences Lucknow 226014 India

AIM: To study the effect of regular aerobic exercise on insulin resistance, serum aminotransferase and liver histology in nonalcoholic fatty liver disease (NAFLD) patients. METHODS: Sixty (mean age 40.0 ± 8.5 years, 75% male) NAFLD patients were included in the study. After baseline anthropometric measurement i.e., body mass index (BMI), waist circumference (WC); all patients were advised regular aerobic exercise for 30 min/d, for at least 5 d/wk and trained to achieve around 70% of his maximal heart rate. In addition, moderately energy restricted diet was advised to patients with high BMI (> 25 kg/m 2 ). Monthly follow up was done by measuring BMI, WC, aspartate aminotransferase, and alanine aminotransferase (ALT). Insulin resistance was calculated using homeostasis model assessment (HOMA) of insulin resistance (HOMA-IR) model, at baseline and after 6mo. Insulin resistance was arbitrarily considered altered when it was ≥ 2. Liver biopsy was done in a section of patients at baseline and after 6 mo. RESULTS: Seventy percent (42/60) patients were overweight or obese; 95% (57/60) had central obesity (WC > 90 cm in men, > 80 cm in women). In the 45 exercise compliant patients insulin resistance decreased from 6.4 ± 6.1 to 1.3 ± 1.0, BMI from 26.7 ± 3.3 kg/m 2 to 25.0 ± 3.3 kg/m 2 , WC from 95.7 ± 8.9 cm to 90.8 ± 7.3 cm and ALT from 84.8 ± 43.5 U/L to 41.3 ± 18.2 U/L (P < 0.01). In 15 exercise noncompliant patient's insulin resistance, BMI, WC and ALT did not show significant change at 6 mo follow up. Six of 8 patients in compliant group on repeat liver biopsy showed significant change in steatosis and necroinflammation. Nonalcoholic steatohepatitis scores improved form 5.3 ± 1.5 to 3.35 ± 1.5. The decline in insulin resistance correlated with decline in ALT (P = 0.01, r s = 0.90) and liver histology (P = 0.03, r s = 0.73). CONCLUSION: Life style modification improves insulin resistance resulting in improvement in ALT and liver histology in NAFLD patients.

关键词： Lifestyle changes Insulin resistance Metabolic syndrome Nonalcoholic steatohepatitis Liver histology