explanation      blue bibcodes open ADS page with paths to full text
Author name code: choudhuri
ADS astronomy entries on 2022-09-14
author:"Choudhuri, Arnab Rai" 

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Title: A theoretical model of the near-surface shear layer of the Sun
Authors: Jha, Bibhuti Kumar; Choudhuri, Arnab Rai
2021MNRAS.506.2189J    Altcode: 2021arXiv210514266J; 2021MNRAS.tmp.1537J; 2021MNRAS.tmp.1557J
  The Sun has a near-surface shear layer (NSSL), within which the angular
  velocity decreases rapidly with radius. We provide an explanation
  of this layer based on the thermal wind balance equation. Since
  convective motions are not affected by solar rotation in the top
  layer of the convection zone, we argue that the temperature falls at
  the same rate at all latitudes in this layer. This makes the thermal
  wind term very large in this layer and the centrifugal term has also
  to become very large to balance it, giving rise to the NSSL. From
  the values of differential rotation Ω(r &lt; r<SUB>c</SUB>, θ)
  at radii less than a radius r<SUB>c</SUB>, we can calculate the
  temperature difference ΔT(r, θ) with respect to the standard solar
  model at different points of the convection zone by making use of
  the thermal wind balance equation. Then, we again use this equation
  in the top layer to calculate Ω(r &gt; r<SUB>c</SUB>, θ) there
  from ΔT(r, θ). We carry on this exercise using both an analytical
  expression of the differential rotation and the actual data provided
  by helioseismology. We find that our theoretical results of the NSSL
  match the observational data reasonably well for r<SUB>c</SUB> ≍
  0.96R<SUB>⊙</SUB>, giving an estimate of the radius till which the
  convective motions are affected by the solar rotation.

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Title: Professor Govind Swarup's contribution to Indian science:
    the recollections of a non-radio astronomer
Authors: Choudhuri, Arnab Rai
2021JAHH...24....3C    Altcode:
  Professor Govind Swarup FRS, the doyen of Indian astrophysics, passed
  away on 7 August 2020 at 9 pm. In this short tribute to 'The Father
  of Indian Radio Astronomy', I want to write about him from a personal
  perspective, primarily in a non-radio astronomical context.

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Title: The meridional circulation of the Sun: Observations, theory
    and connections with the solar dynamo
Authors: Choudhuri, Arnab Rai
2021SCPMA..6439601C    Altcode: 2020arXiv200809347C
  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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Title: A Theoretical Estimate of the Pole-Equator Temperature
    Difference and a Possible Origin of the Near-Surface Shear Layer
Authors: Choudhuri, Arnab Rai
2021SoPh..296...37C    Altcode: 2020arXiv200802983C
  Convective motions in the deep layers of the solar convection zone
  are affected by rotation, making the convective heat transport
  latitude-dependent, but this is not the case in the top layers near
  the surface. We use the thermal wind balance condition in the deeper
  layers to estimate the pole-equator temperature difference. Surface
  observations of this temperature difference can be used for estimating
  the depth of the near-surface layer within which convection is not
  affected by rotation. If we require that the thermal wind balance
  holds in this layer also, then we have to conclude that this must be
  a layer of strong differential rotation and its characteristics which
  we derive are in broad agreement with the observational data of the
  near-surface shear layer.

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Title: M.K. Das Gupta, the first Indian radio astronomer, and his
    connection with the 2020 Physics Nobel Prize
Authors: Choudhuri, Arnab Rai; Chatterjee, Ritaban
2020arXiv201201001C    Altcode:
  Half of the 2020 Nobel Prize is awarded for discovering a super-massive
  black hole at the centre of our Galaxy. One of the first indications of
  the existence of a black hole at the centre of a galaxy was found by
  Jennison and Das Gupta in 1953 while carrying on a radio observation
  of the source Cygnus A. Mrinal Das Gupta, who was doing his PhD at
  Manchester University at the time of this discovery, spent the major
  part of his professional career at Calcutta University. We give an
  outline of Das Gupta's life and explain the scientific significance
  of his discovery.

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Title: Bio-inspired guidance method for a soft landing on a Near-Earth
    Asteroid
Authors: Valenzuela Najera, R.; Everett, L.; Ortega, A. G.; Choudhuri,
   A.; Flores-Abad, A.
2020AdSpR..66.2402V    Altcode:
  Achieving a soft landing over the surface of small celestial bodies
  is an essential maneuver in space to advance the status of space
  exploration, sample collecting and in-situ resource utilization, among
  other on-orbit tasks. Landing on these bodies is challenging due to
  the reduced-gravity and airless environment. The correct planning
  of execution of the trajectory to land on the surface of the body
  is of cumbersome importance to prevent the vehicle from bouncing up
  and eventually reach escape velocity. In this paper, a bio-inspired
  trajectory planning method to land on the surface of a Near-Earth
  Asteroid (NEA) with zero relative velocity is proposed. The method
  is based on Tau theory, which has been demonstrated to explain the
  way that humans and some other animals' approach to different target
  spots to perform tasks such as perching, landing, and grasping. We
  have selected the NEA Apophis asteroid as our case study due to its
  accessibility, and small rotational velocity and orbit condition
  code. Two landing scenarios are studied; one considers the case where
  the satellite is hovering at a low altitude; the other corresponds
  to a landing maneuver right after a deorbiting or breaking phase,
  which may cause residual initial velocity in the vehicle prior to
  the landing maneuver. Once the descending trajectory is obtained, a
  closed-loop controller is in charge of achieving trajectory tracking
  and calculating the continuous and on/off thrust control signals. The
  simulation results show that the introduced approach can achieve zero
  final relative velocity in both cases for different initial condition,
  which is a requirement for a soft landing. Besides, different kinematic
  behaviors can be obtained by modifying the single variable named the
  Tau constant. The particular advantages of the method with respect to
  a commonly used approach are devised and analyzed as well.

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Title: A New Formula for Predicting Solar Cycles
Authors: Hazra, Gopal; Choudhuri, Arnab Rai
2019ApJ...880..113H    Altcode: 2018arXiv181101363H
  A new formula for predicting solar cycles based on the current
  theoretical understanding of the solar cycle from the flux transport
  dynamo is presented. Two important processes—fluctuations
  in the Babcock-Leighton (BL) mechanism and variations in the
  meridional circulation (MC), which are believed to be responsible for
  irregularities of the solar cycle—are constrained using observational
  data. We take the polar field near minima of the cycle as a measure of
  the randomness in the BL process, and the decay rate near the minima as
  a consequence of the change in MC. We couple these two observationally
  derived quantities into a single formula to predict the amplitude of
  the future solar cycle. Our new formula suggests that cycle 25 would
  be a moderate cycle. Whether this formula for predicting the future
  solar cycle can be justified theoretically is also discussed using
  simulations from the flux transport dynamo model.

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Title: How the Saha Ionization Equation Was Discovered
Authors: Choudhuri, Arnab Rai
2018arXiv181010898C    Altcode:
  Although the Saha ionization equation is a standard topic in advanced
  statistical physics and most professional physicists would have some
  knowledge of it, the exact nature of Meghnad Saha's contributions
  in this subject is not widely known. Based on an analysis of Saha's
  original papers and other relevant papers by contemporary scientists,
  as well as other source materials such as letters exchanged between
  scientists, I discuss how Saha used the theory of thermal ionization
  to solve some of the most important astrophysics problems of that era.

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Title: Flux transport dynamo: From modelling irregularities to
    making predictions
Authors: Choudhuri, Arnab Rai
2018JASTP.176....5C    Altcode: 2018arXiv180808550C
  The flux transport dynamo, in which the poloidal magnetic field
  is generated by the Babcock-Leighton mechanism and the meridional
  circulation plays a crucial role, has emerged as an attractive model
  for the solar cycle. Based on theoretical calculations done with this
  model, we argue that the fluctuations in the Babcock-Leighton mechanism
  and the fluctuations in the meridional circulation are the most likely
  causes of the irregularities of the solar cycle. With our increased
  theoretical understanding of how these irregularities arise, it can
  be possible to predict a future solar cycle by feeding the appropriate
  observational data in a theoretical dynamo model.

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Title: Predicting a Solar Cycle Before its Onset Using a Flux
    Transport Dynamo Model
Authors: Choudhuri, Arnab Rai
2018IAUS..335..177C    Altcode: 2018arXiv180808543C
  We begin with a review of the predictions for cycle 24 before its
  onset. After summarizing the basics of the flux transport dynamo
  model, we discuss how this model had been used to make a successful
  prediction of cycle 24, on the assumption that the irregularities of
  the solar cycle arise due to the fluctuations in the Babcock-Leighton
  mechanism. We point out that fluctuations in the meridional circulation
  can be another cause of irregularities in the cycle.

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Title: The Sun as a Laboratory for Plasma Physics
Authors: Choudhuri, Arnab Rai
2018arXiv180810186C    Altcode:
  Several phenomena connected with the magnetic field of the Sun (the cool
  sunspots, the hot corona, solar flares, the solar wind) are collectively
  known as solar activity. This paper discusses how one uses the MHD
  equations to understand how the magnetic field of the Sun is produced
  by the dynamo process and then gives rise to these diverse activities,
  making the Sun the best laboratory for plasma physics in the limit of
  high magnetic Reynolds number (defined at the end of the Introduction).

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Title: My Life and My Journey through Solar Physics
Authors: Choudhuri, Arnab Rai
2018arXiv180909709C    Altcode:
  This is the talk I gave at the workshop "Solar-Stellar Magnetism:
  Past, Present and Future" held in Jaipur on 18 February 2018 on the
  occasion of my 60th year.

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Title: The Sun's polar magnetic field: datasets, proxies and
    theoretical issues
Authors: Choudhuri, Arnab Rai
2018IAUS..340..289C    Altcode: 2018arXiv181005563C
  The polar magnetic field of the Sun is a manifestation of certain
  aspects of the dynamo process and is a good precursor for predicting a
  sunspot cycle before its onset. Although actual synoptic measurements
  of this field exist only from the mid-1970s, it has now been possible
  to determine its evolution from the beginning of the twentieth century
  with the help of various proxies. The recently developed 3D kinematic
  dynamo model can study the build-up of the Sun's polar magnetic field
  more realistically than the earlier surface flux transport model.

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Title: Explaining the variation of the meridional circulation with
    the solar cycle
Authors: Hazra, Gopal; Choudhuri, Arnab Rai
2018IAUS..340..313H    Altcode: 2018arXiv180906600H
  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 meridional circulation during the solar maxima
  in broad conformity with the observations.

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Title: A theoretical model of the variation of the meridional
    circulation with the solar cycle
Authors: Hazra, Gopal; Choudhuri, Arnab Rai
2017MNRAS.472.2728H    Altcode: 2017arXiv170805204H
  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 ϕ
  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 on how the observed inward flow towards the active regions
  may be produced.

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Title: VizieR Online Data Catalog: Polar network index for the solar
    cycle studies (Priyal+, 2014)
Authors: Priyal, M.; Banerjee, D.; Karak, B. B.; Munoz-Jaramillo,
   A.; Ravindra, B.; Choudhuri, A. R.; Singh, J.
2017yCat..17939004P    Altcode:
  The spatial resolution of the Ca K spectroheliograms taken at
  Kodaikanal (hereafter KKL) is about 2 arcsec and the exit slit of the
  spectroheliograph yields a spectral window of 0.5 Å centered at the
  Ca-K line at 3933.67 Å. Ermoli et al. (2009ApJ...698.1000E) pointed
  out that the Kodaikanal archive hosts the longest homogeneous record,
  with fewer variations in spatial resolution. The earlier version of the
  8 bit data at Kodaikanal is sufficient to study those plage area with
  high intensity contrast, but does not provide the required photometric
  accuracy to properly identify the network structures because of the
  small intensity contrast of these features. Therefore, we have designed
  and developed two digitizer units, using a 1 m labsphere with an exit
  port of 350 mm which provides a stable and uniform source of light
  with less than 1% variation from the center to the edge of the light
  source. The CCD camera with 4kx4k format, a pixel size of 15 u square,
  and a 16 bit read out, operating at temperature of -100°C, was used
  to digitize the images. The Ca-K network can be clearly seen because
  of the high spatial resolution of digitization (0.86 arcsec). <P />(4
  data files).

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Title: Starspots, stellar cycles and stellar flares: Lessons from
    solar dynamo models
Authors: Choudhuri, Arnab Rai
2017SCPMA..60a9601C    Altcode: 2016arXiv161202544C; 2017SCPMA..6019601C
  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.

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Title: A Theoretical Study of the Build-up of the Sun’s Polar
    Magnetic Field by using a 3D Kinematic Dynamo Model
Authors: Hazra, Gopal; Choudhuri, Arnab Rai; Miesch, Mark S.
2017ApJ...835...39H    Altcode: 2016arXiv161002726H
  We develop a three-dimensional kinematic self-sustaining model of the
  solar dynamo in which the poloidal field generation is from tilted
  bipolar sunspot pairs placed on the solar surface above regions of
  strong toroidal field by using the SpotMaker algorithm, and then
  the transport of this poloidal field to the tachocline is primarily
  caused by turbulent diffusion. We obtain a dipolar solution within a
  certain range of parameters. We use this model to study the build-up
  of the polar magnetic field and show that some insights obtained from
  surface flux transport models have to be revised. We present results
  obtained by putting a single bipolar sunspot pair in a hemisphere and
  two symmetrical sunspot pairs in two hemispheres. We find that the
  polar fields produced by them disappear due to the upward advection
  of poloidal flux at low latitudes, which emerges as oppositely signed
  radial flux and which is then advected poleward by the meridional
  flow. We also study the effect that a large sunspot pair, violating
  Hale’s polarity law, would have on the polar field. We find that
  there would be some effect—especially if the anti-Hale pair appears
  at high latitudes in the mid-phase of the cycle—though the effect
  is not very dramatic.

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Title: The treatment of magnetic buoyancy in flux transport dynamo
    models
Authors: Choudhuri, Arnab Rai; Hazra, Gopal
2016AdSpR..58.1560C    Altcode: 2015arXiv151103782C
  One important ingredient of flux transport dynamo models is the rise
  of the toroidal magnetic field through the convection zone due to
  magnetic buoyancy to produce bipolar sunspots and then the generation
  of the poloidal magnetic field from these bipolar sunspots due to the
  Babcock-Leighton mechanism. Over the years, two methods of treating
  magnetic buoyancy-a local method and a non-local method-have been
  used widely by different groups in constructing 2D kinematic models of
  the flux transport dynamo. We review both these methods and conclude
  that neither of them is fully satisfactory-presumably because magnetic
  buoyancy is an inherently 3D process. We also point out so far we do
  not have proper understanding of why sunspot emergence is restricted
  to rather low latitudes.

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Title: Presenting the science of the Sun to the general public
Authors: Choudhuri, Arnab Rai
2016cosp...41E1619C    Altcode:
  Although the science behind the Sun is so fascinating, there has not
  been sufficient worldwide effort in presenting this science to the
  general public. My recently published popular science book "Nature's
  Third Cycle: A Story of Sunspots" (Oxford University Press, 2015) is
  probably the first popular science book introducing the phenomenology of
  the solar cycle along with a non-technical account of dynamo theory. I
  shall discuss my perspective of the challenges involved in presenting
  the science of the Sun to the public. The Amazon link of my book is:
  http://www.amazon.co.uk/Natures-Third-Cycle-Story-Sunspots/dp/0199674752/

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Title: Correlation Between Decay Rate and Amplitude of Solar Cycles
    as Revealed from Observations and Dynamo Theory
Authors: Hazra, Gopal; Karak, Bidya Binay; Banerjee, Dipankar;
   Choudhuri, Arnab Rai
2015SoPh..290.1851H    Altcode: 2015SoPh..tmp...80H; 2014arXiv1410.8641H
  Using different proxies of solar activity, we have studied the following
  features of the solar cycle: i) The linear correlation between the
  amplitude of cycle and its decay rate, ii) the linear correlation
  between the amplitude of cycle n and the decay rate of cycle (n −1
  ), and iii) the anti-correlation between the amplitude of cycle n and
  the period of cycle (n −1 ). Features ii) and iii) are very useful
  because they provide precursors for future cycles. We have reproduced
  these features using a flux-transport dynamo model with stochastic
  fluctuations in the Babcock-Leighton α effect and in the meridional
  circulation. Only when we introduce fluctuations in meridional
  circulation, are we able to reproduce different observed features of
  the solar cycle. We discuss the possible reasons for these correlations.

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Title: Book review: Nature's third cycle - a story of sunspots
    (Choudhuri)
Authors: Meadows, P.; Choudhuri, A. R.
2015JBAA..125Q.123M    Altcode:
  No abstract at ADS

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Title: Communicating the science of the 11-year sunspot cycle to
    the general public
Authors: Choudhuri, A. R.
2015HiA....16..638C    Altcode:
  Astrophysics is one branch of science which excites the imagination
  of the general public. Pioneer science popularizers like George Gamow
  and Fred Hoyle wrote on different aspects of astrophysics. However,
  of late, we see a trend which I find disturbing. While it has become
  extremely fashionable to write popular science books on cosmology,
  other areas of astrophysics are grossly neglected.

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Title: A Critical Assessment of the Flux Transport Dynamo
Authors: Choudhuri, Arnab Rai
2015JApA...36....5C    Altcode: 2015JApA..tmp...20C; 2014arXiv1408.3968C
  We first discuss how the flux transport dynamo with reasonably high
  diffusion can explain both the regular and the irregular features of
  the solar cycle quite well. Then, we critically examine the inadequacies
  of the model and the challenge posed by some recent observational data
  about meridional circulation, arriving at a conclusion that this model
  can still work within the bounds of observational data.

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Title: Flux Transport Dynamos: From Kinematics to Dynamics
Authors: Karak, Bidya Binay; Jiang, Jie; Miesch, Mark S.; Charbonneau,
   Paul; Choudhuri, Arnab Rai
2015sac..book..561K    Altcode:
  No abstract at ADS

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Title: Flux Transport Dynamos: From Kinematics to Dynamics
Authors: Karak, Bidya Binay; Jiang, Jie; Miesch, Mark S.; Charbonneau,
   Paul; Choudhuri, Arnab Rai
2014SSRv..186..561K    Altcode: 2014SSRv..tmp...55K
  Over the past several decades, Flux-Transport Dynamo (FTD) models
  have emerged as a popular paradigm for explaining the cyclic nature of
  solar magnetic activity. Their defining characteristic is the key role
  played by the mean meridional circulation in transporting magnetic
  flux and thereby regulating the cycle period. Most FTD models also
  incorporate the so-called Babcock-Leighton (BL) mechanism in which
  the mean poloidal field is produced by the emergence and subsequent
  dispersal of bipolar active regions. This feature is well grounded
  in solar observations and provides a means for assimilating observed
  surface flows and fields into the models in order to forecast future
  solar activity, to identify model biases, and to clarify the underlying
  physical processes. Furthermore, interpreting historical sunspot records
  within the context of FTD models can potentially provide insight into
  why cycle features such as amplitude and duration vary and what causes
  extreme events such as Grand Minima. Though they are generally robust
  in a modeling sense and make good contact with observed cycle features,
  FTD models rely on input physics that is only partially constrained by
  observation and that neglects the subtleties of convective transport,
  convective field generation, and nonlinear feedbacks. Here we review
  the formulation and application of FTD models and assess our current
  understanding of the input physics based largely on complementary 3D
  MHD simulations of solar convection, dynamo action, and flux emergence.

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Title: The irregularities of the sunspot cycle and their theoretical
    modelling
Authors: Choudhuri, Arnab Rai
2014InJPh..88..877C    Altcode: 2013arXiv1312.3408C
  The 11-year sunspot cycle has many irregularities, the most promi- nent
  amongst them being the grand minima when sunspots may not be seen for
  several cycles. After summarizing the relevant observational data about
  the irregularities, we introduce the flux transport dynamo model, the
  currently most successful theoretical model for explaining the 11-year
  sunspot cycle. Then we analyze the respective roles of nonlinearities
  and random fluctuations in creating the irregularities. We also
  discuss how it has recently been realized that the fluctuations in
  meridional circula- tion also can be a source of irregularities. We
  end by pointing out that fluctuations in the poloidal field generation
  and fluctuations in meridional circulation together can explain the
  occurrences of grand minima.

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Title: Polar Network Index as a Magnetic Proxy for the Solar Cycle
    Studies
Authors: Priyal, Muthu; Banerjee, Dipankar; Karak, Bidya Binay;
   Muñoz-Jaramillo, Andrés; Ravindra, B.; Choudhuri, Arnab Rai;
   Singh, Jagdev
2014ApJ...793L...4P    Altcode: 2014arXiv1407.4944P
  The Sun has a polar magnetic field which oscillates with the 11 yr
  sunspot cycle. This polar magnetic field is an important component
  of the dynamo process which operates in the solar convection zone and
  produces the sunspot cycle. We have direct systematic measurements of
  the Sun's polar magnetic field only from about the mid-1970s. There are,
  however, indirect proxies which give us information about this field
  at earlier times. The Ca-K spectroheliograms taken at the Kodaikanal
  Solar Observatory during 1904-2007 have now been digitized with 4k
  × 4k CCD and have higher resolution (~0.86 arcsec) than the other
  available historical data sets. From these Ca-K spectroheliograms,
  we have developed a completely new proxy (polar network index,
  hereafter PNI) for the Sun's polar magnetic field. We calculate PNI
  from the digitized images using an automated algorithm and calibrate
  our measured PNI against the polar field as measured by the Wilcox
  Solar Observatory for the period 1976-1990. This calibration allows
  us to estimate the polar fields for the earlier period up to 1904. The
  dynamo calculations performed with this proxy as input data reproduce
  reasonably well the Sun's magnetic behavior for the past century.

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Title: A Dynamo Model of Magnetic Activity in Solar-like Stars with
    Different Rotational Velocities
Authors: Karak, Bidya Binay; Kitchatinov, Leonid L.; Choudhuri,
   Arnab Rai
2014ApJ...791...59K    Altcode: 2014arXiv1402.1874K
  We attempt to provide a quantitative theoretical explanation for
  the observations that Ca II H/K emission and X-ray emission from
  solar-like stars increase with decreasing Rossby number (i.e.,
  with faster rotation). Assuming that these emissions are caused
  by magnetic cycles similar to the sunspot cycle, we construct flux
  transport dynamo models of 1 M <SUB>⊙</SUB> stars rotating with
  different rotation periods. We first compute the differential rotation
  and the meridional circulation inside these stars from a mean-field
  hydrodynamics model. Then these are substituted in our dynamo code to
  produce periodic solutions. We find that the dimensionless amplitude
  f <SUB>m</SUB> of the toroidal flux through the star increases with
  decreasing rotation period. The observational data can be matched if we
  assume the emissions to go as the power 3-4 of f <SUB>m</SUB>. Assuming
  that the Babcock-Leighton mechanism saturates with increasing rotation,
  we can provide an explanation for the observed saturation of emission
  at low Rossby numbers. The main failure of our model is that it predicts
  an increase of the magnetic cycle period with increasing rotation rate,
  which is the opposite of what is found observationally. Much of our
  calculations are based on the assumption that the magnetic buoyancy
  makes the magnetic flux tubes rise radially from the bottom of the
  convection zone. Taking into account the fact that the Coriolis force
  diverts the magnetic flux tubes to rise parallel to the rotation axis
  in rapidly rotating stars, the results do not change qualitatively.

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Title: Is a Deep One-cell Meridional Circulation Essential for the
    Flux Transport Solar Dynamo?
Authors: Hazra, Gopal; Karak, Bidya Binay; Choudhuri, Arnab Rai
2014ApJ...782...93H    Altcode: 2013arXiv1309.2838H
  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. In addition,
  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.

---------------------------------------------------------
Title: Studies of grand minima in sunspot cycles by using a flux
    transport solar dynamo model
Authors: Karak, Bidya Binay; Choudhuri, Arnab Rai
2013RAA....13.1339K    Altcode: 2013arXiv1306.5438K
  We propose that grand minima in solar activity are caused by
  simultaneous 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 α-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 α-effect is suitably fine-tuned.

---------------------------------------------------------
Title: Flux-transport and mean-field dynamo theories of solar cycles
Authors: Choudhuri, Arnab Rai
2013IAUS..294...37C    Altcode: 2012arXiv1211.0520C
  We point out the difficulties in carrying out direct numerical
  simulation of the solar dynamo problem and argue that kinematic
  mean-field models are our best theoretical tools at present for
  explaining various aspects of the solar cycle in detail. The most
  promising kinematic mean-field model is the flux transport dynamo model,
  in which the toroidal field is produced by differential rotation in
  the tachocline, the poloidal field is produced by the Babcock-Leighton
  mechanism at the solar surface and the meridional circulation plays
  a crucial role. Depending on whether the diffusivity is high or low,
  either the diffusivity or the meridional circulation provides the main
  transport mechanism for the poloidal field to reach the bottom of the
  convection zone from the top. We point out that the high-diffusivity
  flux transport dynamo model is consistent with various aspects of
  observational data. The irregularities of the solar cycle are primarily
  produced by fluctuations in the Babcock-Leighton mechanism and in the
  meridional circulation. We summarize recent work on the fluctuations
  of meridional circulation in the flux transport dynamo, leading to
  explanations of such things as the Waldmeier effect.

---------------------------------------------------------
Title: Modelling grand minima of solar activity using a flux transport
    dynamo model
Authors: Karak, Bidya Binay; Choudhuri, Arnab Rai
2013IAUS..294..433K    Altcode: 2012arXiv1211.0165K
  The occurrence of grand minima like the Maunder minimum is an intriguing
  aspect of the sunspot cycle. We use the flux transport dynamo model
  to explain the grand minima, showing that they arise when either the
  poloidal field or the meridional circulation falls to a sufficiently
  low value due to fluctuations. Assuming these fluctuations to be
  Gaussian and determining the various parameters from the data of
  the last 28 cycles, we carry on a dynamo simulation with both these
  fluctuations. The results are remarkably close to the observational
  data.

---------------------------------------------------------
Title: Can Superflares Occur on Our Sun?
Authors: Shibata, Kazunari; Isobe, Hiroaki; Hillier, Andrew; Choudhuri,
   Arnab Rai; Maehara, Hiroyuki; Ishii, Takako T.; Shibayama, Takuya;
   Notsu, Shota; Notsu, Yuta; Nagao, Takashi; Honda, Satoshi; Nogami,
   Daisaku
2013PASJ...65...49S    Altcode: 2012arXiv1212.1361S
  Recent observations of Sun-like stars, similar to our Sun in their
  surface temperature (5600-6000 K) and slow rotation (rotational period
  &gt; 10 d), using the Kepler satellite by Maehara et al. (2012, Nature,
  485, 478) have revealed the existence of superflares (with energy
  of 10<SUP>33</SUP>-10<SUP>35</SUP> erg). From statistical analyses
  of these superflares, it was found that superflares with energy
  of 10<SUP>34</SUP> erg occur once in 800 yr, and superflares with
  10<SUP>35</SUP> erg occur once in 5000 yr. In this paper, we examine
  whether superflares with energy of 10<SUP>33</SUP>-10<SUP>35</SUP>
  erg could occur on the present Sun through the use of simple
  order-of-magnitude estimates based on current ideas related to
  the mechanisms of the solar dynamo. If magnetic flux is generated
  by differential rotation at the base of the convection zone, as
  assumed in typical dynamo models, it is possible that the present Sun
  would generate a large sunspot with a total magnetic flux of ∼2 ×
  10<SUP>23</SUP> Mx (= G cm<SUP>2</SUP>) within one solar cycle period,
  and lead to superflares with an energy of 10<SUP>34</SUP> erg. To
  store a total magnetic flux of ∼10<SUP>24</SUP> Mx, necessary for
  generating 10<SUP>35</SUP> erg superflares, it would take ∼40 yr. Hot
  Jupiters have often been argued to be a necessary ingredient for the
  generation of superflares, but we found that they do not play any
  essential role in the generation of magnetic flux in the star itself,
  if we consider only the magnetic interaction between the star and the
  hot Jupiter. This seems to be consistent with Maehara et al.'s finding
  of 148 superflare-generating solar-type stars that do not have a hot
  Jupiter-like companion. Altogether, our simple calculations, combined
  with Maehara et al.'s analysis of superflares on Sun-like stars,
  show that there is a possibility that superflares of 10<SUP>34</SUP>
  erg would occur once in 800 yr on our present Sun.

---------------------------------------------------------
Title: Fluctuations in the alpha-effect and grand solar minima
Authors: Olemskoy, S. V.; Choudhuri, A. R.; Kitchatinov, L. L.
2013ARep...57..458O    Altcode: 2013arXiv1305.2660O
  The parameters of a special type of α-effect known in dynamo
  theory as the Babcock-Leighton mechanism are estimated using the
  data of sunspot catalogs. The estimates support the presence of the
  Babcock-Leighton α-effect on the Sun. Fluctuations of the α-effect
  are also estimated. The fluctuation amplitude appreciably exceeds
  themean value, and the characteristic time for the fluctuations is
  comparable to the period of the solar rotation. Fluctuations with
  the parameters found are included in a numericalmodel for the solar
  dynamo. Computations show irregular changes in the amplitudes of
  the magnetic cycles on time scales of centuries and millennia. The
  calculated statistical characteristics of the grand solar minima and
  maxima agree with the data on solar activity over the Holocene.

---------------------------------------------------------
Title: Origin of Grand Minima in Sunspot Cycles
Authors: Choudhuri, Arnab Rai; Karak, Bidya Binay
2012PhRvL.109q1103C    Altcode: 2012arXiv1208.3947C
  One of the most striking aspects of the 11-year sunspot cycle is that
  there have been times in the past when some cycles went missing,
  a most well-known example of this being the Maunder minimum during
  1645-1715. Analyses of cosmogenic isotopes (C14 and Be10) indicated
  that there were about 27 grand minima in the last 11 000 yrs, implying
  that about 2.7% of the solar cycles had conditions appropriate for
  forcing the Sun into grand minima. We address the question of how
  grand minima are produced and specifically calculate the frequency of
  occurrence of grand minima from a theoretical dynamo model. We assume
  that fluctuations in the poloidal field generation mechanism and in the
  meridional circulation produce irregularities of sunspot cycles. Taking
  these fluctuations to be Gaussian and estimating the values of important
  parameters from the data of the last 28 solar cycles, we show from our
  flux transport dynamo model that about 1-4% of the sunspot cycles may
  have conditions suitable for inducing grand minima.

---------------------------------------------------------
Title: Is meridional circulation important in modelling irregularities
    of the solar cycle?
Authors: Karak, Bidya Binay; Choudhuri, Arnab Rai
2012IAUS..286..367K    Altcode: 2011arXiv1111.2144K
  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.

---------------------------------------------------------
Title: Theoretical modeling of grand minima
Authors: Choudhuri, Arnab Rai; Karak, Bidya Binay
2012cosp...39..339C    Altcode: 2012cosp.meet..339C
  We explore whether the flux transport dynamo model of solar cycles
  can explain the occurrence of grand minima. If fluctuations in the
  Babcock-Leighton process make the poloidal field at the end of a cycle
  fall to sufficiently low values, then that can cause a grand minimum
  (Choudhuri and Karak 2009, RAA 9, 953). Additionally, if the meridional
  circulation of the Sun falls to a sufficiently low value, then that
  also can push the dynamo into a grand minimum if the diffusivity is
  sufficiently high (Karak 2010, ApJ 724, 1021). Considering Gaussian
  fluctuations in both poloidal field generation and in meridional
  circulation, we calculate the probability of occurrence of grand minima
  and find our results to be consistent with the observational claim
  that there have been about 27 grand minima in the last 11,000 years
  (Choudhuri and Karak 2012, under preparation).

---------------------------------------------------------
Title: Variable meridional circulation in the dynamo model
Authors: Karak, Bidya Binay; Choudhuri, Arnab Rai
2012cosp...39..879K    Altcode: 2012cosp.meet..879K
  Meridional circulation is an important ingredient in the flux transport
  dynamo models of modeling solar cycles. It connects spatially
  segregated sources of poloidal field generation (near the solar
  surface) and the toroidal field generation (near the bottom of the
  convection zone). Unfortunately the origin and specifically the temporal
  variation of the meridional circulation is very poorly known due to the
  observational limitation and also due to the complex dynamics of the
  solar convection zone. However using the relation based on the flux
  transport dynamo model that the meridional circulation is inversely
  related to the cycle period, we infer that the meridional circulation
  had random variation in past with a coherence time around 30-40 yrs. We
  have shown that this randomly variable meridional circulation helps to
  model irregularities of the solar cycle including some of its features
  like the Waldmeier effect.

---------------------------------------------------------
Title: Dynamo models of grand minima
Authors: Choudhuri, Arnab Rai
2012IAUS..286..350C    Altcode: 2011arXiv1111.1839C
  Since a universally accepted dynamo model of grand minima does not
  exist at the present time, we concentrate on the physical processes
  which may be behind the grand minima. After summarizing the relevant
  observational data, we make the point that, while the usual sources
  of irregularities of solar cycles may be sufficient to cause a grand
  minimum, the solar dynamo has to operate somewhat differently from the
  normal to bring the Sun out of the grand minimum. We then consider three
  possible sources of irregularities in the solar dynamo: (i) nonlinear
  effects; (ii) fluctuations in the poloidal field generation process;
  (iii) fluctuations in the meridional circulation. We conclude that (i)
  is unlikely to be the cause behind grand minima, but a combination
  of (ii) and (iii) may cause them. If fluctuations make the poloidal
  field fall much below the average or make the meridional circulation
  significantly weaker, then the Sun may be pushed into a grand minimum.

---------------------------------------------------------
Title: Quenching of Meridional Circulation in Flux Transport Dynamo
    Models
Authors: Karak, Bidya Binay; Choudhuri, Arnab Rai
2012SoPh..278..137K    Altcode: 2011arXiv1111.1540K
  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<SUP>11</SUP> cm<SUP>2</SUP> s<SUP>−1</SUP>. The
  quenching of α, however, has a stabilizing effect and it is possible
  to stabilize a dynamo with low diffusivity with sufficiently strong
  α-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<SUP>12</SUP> cm<SUP>2</SUP> s<SUP>−1</SUP>
  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.

---------------------------------------------------------
Title: Possible explanations of the Maunder minimum from a flux
    transport dynamo model
Authors: Karak, Bidya Binay; Choudhuri, Arnab Rai
2011IAUS..273..430K    Altcode: 2011arXiv1101.4342K
  We propose that at the beginning of the Maunder minimum the poloidal
  field or amplitude of meridional circulation or both fell abruptly to
  low values. With this proposition, a flux transport dynamo model is
  able to reproduce various important aspects of the historical records
  of the Maunder minimum remarkably well.

---------------------------------------------------------
Title: A theoretical model of torsional oscillations from a flux
    transport dynamo model
Authors: Chatterjee, Piyali; Chakraborty, Sagar; Choudhuri, Arnab Rai
2011IAUS..273..366C    Altcode: 2010arXiv1008.2161C
  Assuming that the torsional oscillation is driven by the Lorentz
  force of the magnetic field associated with the sunspot cycle, we use
  a flux transport dynamo to model it and explain its initiation at a
  high latitude before the beginning of the sunspot cycle.

---------------------------------------------------------
Title: Origin of solar magnetism
Authors: Choudhuri, Arnab Rai
2011IAUS..273...28C    Altcode: 2010arXiv1008.2432C
  The most promising model for explaining the origin of solar magnetism
  is the flux transport dynamo model, in which the toroidal field is
  produced by differential rotation in the tachocline, the poloidal field
  is produced by the Babcock-Leighton mechanism at the solar surface and
  the meridional circulation plays a crucial role. After discussing how
  this model explains the regular periodic features of the solar cycle,
  we come to the questions of what causes irregularities of solar cycles
  and whether we can predict future cycles. Only if the diffusivity
  within the convection zone is sufficiently high, the polar field at
  the sunspot minimum is correlated with strength of the next cycle. This
  is in conformity with the limited available observational data.

---------------------------------------------------------
Title: The origin of the solar magnetic cycle
Authors: Choudhuri, Arnab Rai
2011Prama..77...77C    Altcode: 2011arXiv1103.3385C
  After summarizing the relevant observational data, we discuss how
  a study of flux tube dynamics in the solar convection zone helps us
  to understand the formation of sunspots. Then we introduce the flux
  transport dynamo model and assess its success in modelling both the
  solar cycle and its departures from strictly periodic behaviour.

---------------------------------------------------------
Title: Correlation Between The Rise Rate And The Amplitude Of The
    Solar Magnetic Cycles
Authors: Karak, Bidya Binay; Choudhuri, Arnab Rai
2011arXiv1102.4052K    Altcode:
  We discuss one important aspect of Waldmeier effect which says that
  the stronger cycles rise rapidly than weaker cycles. We studied four
  different data set of solar activity indices, and find strong linear
  correlation between rise rates and amplitudes of solar activity. We
  study this effect theoretically by introducing suitable stochastic
  fluctuations in our regular solar dynamo model.

---------------------------------------------------------
Title: The Waldmeier effect and the flux transport solar dynamo
Authors: Karak, Bidya Binay; Choudhuri, Arnab Rai
2011MNRAS.410.1503K    Altcode: 2010MNRAS.tmp.1490K; 2010arXiv1008.0824K
  We confirm that the evidence for the Waldmeier effect WE1 (the
  anticorrelation between rise times of sunspot cycles and their
  strengths) and the related effect WE2 (the correlation between rise
  rates of cycles and their strengths) is found in different kinds
  of sunspot data. We explore whether these effects can be explained
  theoretically on the basis of the flux transport dynamo models of
  sunspot cycles. Two sources of irregularities of sunspot cycles are
  included in our model: fluctuations in the poloidal field generation
  process and fluctuations in the meridional circulation. We find WE2 to
  be a robust result which is produced in different kinds of theoretical
  models for different sources of irregularities. The Waldmeier effect
  WE1, on the other hand, arises from fluctuations in the meridional
  circulation and is found only in the theoretical models with reasonably
  high turbulent diffusivity which ensures that the diffusion time is
  not more than a few years.

---------------------------------------------------------
Title: Back-reactions of dynamo-generated magnetic fields: torsional
    oscillations and variations in meridional circulation
Authors: Choudhuri, Arnab Rai
2011ASInC...2..131C    Altcode: 2011arXiv1111.2443C
  The periodically varying Lorentz force of the periodic solar magnetic
  field generated by the solar dynamo can induce two kinds of motions:
  torsional oscillations and periodic variations in the mc. Observational
  evidence now exists for both these kinds of motions. We discuss
  our ongoing effort in theoretically studying the variations of the
  mc. Then we present our theoretical model of torsional oscillations,
  which addresses the question why these oscillations start before
  sunspot cycles at latitudes higher than where sunspots are seen.

---------------------------------------------------------
Title: Solar physics in India: developments from the nineteenth
    century to the present era
Authors: Hasan, S. S.; Choudhuri, Arnab Rai; Rajaguru, S. P.
2011ASInC...2..367H    Altcode:
  Modern solar astronomy took root in India with the discovery of helium
  during the total solar eclipse of 1868 and followed by the setting up
  of the Kodaikanal Observatory in 1899. We provide a brief overview of
  the growth of this field, describe the various Indian solar observing
  facilities and summarize the highlights of solar research in India
  during the last few decades.

---------------------------------------------------------
Title: Astrophysics for Physicists
Authors: Choudhuri, Arnab Rai; Smoot, George
2011PhT....64a..57C    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Modeling solar cycles using a variable meridional circulation
    in a flux transport dynamo model
Authors: Karak, Bidya Binay; Choudhuri, Arnab Rai
2011ASInC...3...99K    Altcode:
  The sunspot number - a proxy of solar activity - varies roughly
  periodically with time. However the individual cycle durations and
  amplitudes are found to vary in an irregular way. An important feature
  of the solar cycle is the Maunder minimum during 1645-1715 when there
  were very few sunspots. We explore whether this irregular solar cycle
  can be modeled with the help of a flux transport dynamo model of the
  solar cycle. We model the periods of the last 23 sunspot cycles by
  varying the meridional circulation speed. We find that most of the
  cycle amplitudes also get modeled up to some extent when we model the
  periods. Moreover, under certain situations we are able to reproduce
  Maunder like grand minimum. However, we fail to reproduce these results
  if the value of turbulent diffusivity is reasonably low.

---------------------------------------------------------
Title: Astrophysics for Physicists
Authors: Choudhuri, Arnab Rai
2010asph.book.....C    Altcode:
  1. Introduction; 2. Interaction of radiation with matter; 3. Stellar
  astrophysics I: basic theoretical ideas and observational data;
  4. Stellar astrophysics II: nucleosynthesis and other advanced topics;
  5. End states of stellar collapse; 6. Our galaxy and its interstellar
  matter; 7. Elements of stellar dynamics; 8. Elements of plasma
  astrophysics; 9. Extragalactic astronomy; 10. The spacetime dynamics
  of the Universe; 11. The thermal history of the Universe; 12. Elements
  of tensors and general relativity; 13. Some applications of general
  relativity; 14. Relativistic cosmology; Appendixes; References; Index.

---------------------------------------------------------
Title: UTEP Center for Space Exploration Technology Research:
    Integrated Science and Engineering Approach to Propulsion, In-Situ
    Resource Utilization, and Planetary Science and Aerospace Education
Authors: Choudhuri, A.; Borrok, D. M.; Hurtado, J. M.; Shafirovich, E.
2010LPI....41.2682C    Altcode:
  The purpose of this presentation is to disseminate information on the
  Center for Space Exploration Technology Research (cSETR), established
  at the University of Texas at El Paso (UTEP), effective October 1, 2009.

---------------------------------------------------------
Title: The Waldmeier Effect in Sunspot Cycles
Authors: Karak, B. B.; Choudhuri, A. R.
2010ASSP...19..402K    Altcode: 2010arXiv1008.2931K; 2010mcia.conf..402K
  We discuss two aspects of the Waldmeier Effect, namely (1) the rise
  times of sunspot cycles are anti-correlated to their strengths (WE1) and
  (2) the rates of rise of the cycles are correlated to their strengths
  (WE2). To avoid overlapping effect and the lack of single peaks in some
  cycles, we carefully define rise times to study WE1. From analysis of
  four different data sets, we conclude that WE1 exists in all, although
  the significance levels are poor in some cases due to insufficient
  data. We also find strong evidence for WE2 in all the data sets. We
  study this effect theoretically by introducing suitable stochastic
  fluctuations in our regular solar dynamo model. Some preliminary
  results are presented.

---------------------------------------------------------
Title: Cycle Prediction from Dynamo Theory
Authors: Choudhuri, A. R.
2010ASSP...19..498C    Altcode: 2010mcia.conf..498C
  Many previous efforts in sunspot cycle prediction were based on
  various empirical correlations, most of which have limited statistical
  significance because they were inferred from observations of very
  few cycles. Perhaps the most successful of the empirical methods
  is to use the strength of the polar field in the previous sunspot
  minimum as a precursor for the next cycle. As the polar field at
  the present time is weak, Schatten [2005, Geophys. Res. Lett., 32,
  L21106] and Svalsgaard et al. [2005, Geophys. Res. Lett., 32, L01104]
  have predicted that cycle 24 will be a weak cycle.

---------------------------------------------------------
Title: Why Does the Torsional Oscillation Precede the Sunspot Cycle?
Authors: Chatterjee, P.; Chakraborty, S.; Choudhuri, A. R.
2010ASSP...19..500C    Altcode: 2010mcia.conf..500C
  The Sun's rotation shows a periodic variation with the sunspot cycle,
  called torsional oscillations, the nature of which inside the solar
  convection zone has been determined from helioseismology. Several
  authors developed theoretical models of torsional oscillations by
  assuming that they are driven by the Lorentz force of the Sun's
  cyclically varying magnetic field. If this is true, then one would
  expect the torsional oscillations to follow the sunspot cycles. However,
  the torsional oscillations of a cycle begin a couple of years before
  the sunspots of that cycle appear and at a latitude higher than where
  the first sunspots are subsequently seen. Our aim in this paper is to
  provide an explanation for this seemingly causality defying phenomenon.

---------------------------------------------------------
Title: A possible explanation of the Maunder minimum from a flux
    transport dynamo model
Authors: Choudhuri, Arnab Rai; Karak, Bidya Binay
2009RAA.....9..953C    Altcode: 2009arXiv0907.3106C
  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.

---------------------------------------------------------
Title: Erratum: Why Does the Sun's Torsional Oscillation Begin before
    the Sunspot Cycle? [Phys. Rev. Lett. 102, 041102 (2009)]
Authors: Chakraborty, Sagar; Choudhuri, Arnab Rai; Chatterjee, Piyali
2009PhRvL.103i9902C    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: The hemispheric asymmetry of solar activity during the last
    century and the solar dynamo
Authors: Goel, Ashish; Choudhuri, Arnab Rai
2009RAA.....9..115G    Altcode: 2007arXiv0712.3988G
  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.

---------------------------------------------------------
Title: Why Does the Sun's Torsional Oscillation Begin before the
    Sunspot Cycle?
Authors: Chakraborty, Sagar; Choudhuri, Arnab Rai; Chatterjee, Piyali
2009PhRvL.102d1102C    Altcode: 2009arXiv0907.4842C
  Although the Sun’s torsional oscillation is believed to be driven by
  the Lorentz force associated with the sunspot cycle, this oscillation
  begins 2 3 yr before the sunspot cycle. We provide a theoretical
  explanation of this with the help of a solar dynamo model having a
  meridional circulation penetrating slightly below the bottom of the
  convection zone, because only in such dynamo models does the strong
  toroidal field form a few years before the sunspot cycle and at a
  higher latitude.

---------------------------------------------------------
Title: Prospects for predicting cycle 24
Authors: Choudhuri, Arnab Rai
2008JApA...29...41C    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: A theoretical model for the magnetic helicity of solar
    active regions
Authors: Chatterjee, Piyali; Choudhuri, Arnab Rai; Petrovay, Kristof;
   Nandy, Dibyendu
2008AdSpR..41..893C    Altcode:
  Active regions on the solar surface are known to possess magnetic
  helicity, which is predominantly negative in the northern hemisphere
  and positive in the southern hemisphere. Choudhuri et al. [Choudhuri,
  A.R. On the connection between mean field dynamo theory and flux
  tubes. Solar Phys. 215, 31 55, 2003] proposed that the magnetic helicity
  arises due to the wrapping up of the poloidal field of the convection
  zone around rising flux tubes which form active regions. Choudhuri
  [Choudhuri, A.R., Chatterjee, P., Nandy, D. Helicity of solar active
  regions from a dynamo model. ApJ 615, L57 L60, 2004] used this idea to
  calculate magnetic helicity from their solar dynamo model. Apart from
  getting broad agreements with observational data, they also predict
  that the hemispheric helicity rule may be violated at the beginning
  of a solar cycle. Chatterjee et al. [Chatterjee, P., Choudhuri, A.R.,
  Petrovay, K. Development of twist in an emerging magnetic flux tube
  by poloidal field accretion. A&amp;A 449, 781 789, 2006] study the
  penetration of the wrapped poloidal field into the rising flux tube
  due to turbulent diffusion using a simple 1-d model. They find that
  the extent of penetration of the wrapped field will depend on how
  weak the magnetic field inside the rising flux tube becomes before
  its emergence. They conclude that more detailed observational data
  will throw light on the physical conditions of flux tubes just before
  their emergence to the photosphere.

---------------------------------------------------------
Title: How far are we from a ‘Standard Model’ of the solar dynamo?
Authors: Choudhuri, Arnab Rai
2008AdSpR..41..868C    Altcode:
  Over the last few years, dynamo theorists seem to be converging on a
  basic scenario as to how the solar dynamo operates. The strong toroidal
  component of the magnetic field is produced in the tachocline, from
  where it rises due to magnetic buoyancy to produce active regions
  at the solar surface. The decay of tilted bipolar active regions
  at the surface gives rise to the poloidal component, which is first
  advected poleward by the meridional circulation and then taken below
  the surface to the tachocline where it can be stretched to produce the
  toroidal component. The mathematical formulation of this basic model,
  however, involves the specification of some parameters which are
  still uncertain. We review these remaining uncertainties which have
  resulted in disagreements amongst various research groups and have
  made it impossible to still arrive at something that can be called a
  standard model of the solar dynamo.

---------------------------------------------------------
Title: Solar activity forecast with a dynamo model
Authors: Jiang, Jie; Chatterjee, Piyali; Choudhuri, Arnab Rai
2007MNRAS.381.1527J    Altcode: 2007arXiv0707.2258J; 2007MNRAS.tmp..899J
  Although systematic measurements of the Sun's polar magnetic field exist
  only from mid-1970s, other proxies can be used to infer the polar field
  at earlier times. The observational data indicate a strong correlation
  between the polar field at a sunspot minimum and the strength of the
  next cycle, although the strength of the cycle is not correlated
  well with the polar field produced at its end. This suggests that
  the Babcock-Leighton mechanism of poloidal field generation from
  decaying sunspots involves randomness, whereas the other aspects of
  the dynamo process must be reasonably ordered and deterministic. Only
  if the magnetic diffusivity within the convection zone is assumed to
  be high (of order 10<SUP>12</SUP>cm<SUP>2</SUP>s<SUP>-1</SUP>), we can
  explain the correlation between the polar field at a minimum and the
  next cycle. We give several independent arguments that the diffusivity
  must be of this order. In a dynamo model with diffusivity like this,
  the poloidal field generated at the mid-latitudes is advected toward
  the poles by the meridional circulation and simultaneously diffuses
  towards the tachocline, where the toroidal field for the next cycle
  is produced. To model actual solar cycles with a dynamo model having
  such high diffusivity, we have to feed the observational data of the
  poloidal field at the minimum into the theoretical model. We develop a
  method of doing this in a systematic way. Our model predicts that cycle
  24 will be a very weak cycle. Hemispheric asymmetry of solar activity
  is also calculated with our model and compared with observational data.

---------------------------------------------------------
Title: A New Explanation for the Origin of Trans-equatorial Loops
    based on a Dynamo Model
Authors: Jiang, Jie; Choudhuri, Arnab Rai; Wang, Jingxiu
2007SoPh..245...19J    Altcode:
  Trans-equatorial loops (TLs) are one of the distinct magnetic structures
  in the solar corona and have a close relationship to solar activity. We
  present a systematic study of the origin of TLs linking with the Babcock
  - Leighton dynamo process based on the model of Chatterjee, Nandy,
  and Choudhuri (Astron. Astrophys.427, 1019, 2004). We propose that TLs
  are visible signatures of poloidal field lines across the equator. The
  cycle variation of TL lengths obtained by the connectivities of poloidal
  field lines happens to be roughly in agreement with what one gets by
  considering the positions of sunspots. This explains why this cycle
  variation of TL lengths was found to be in conformity with Spörer's
  law. The active regions always make the poloidal field configuration
  favorable to form TLs, which causes the conformity. The formation of
  TLs is a three-dimensional problem, which will require three-dimensional
  dynamo models for full investigation.

---------------------------------------------------------
Title: An Elementary Introduction to Solar Dynamo Theory
Authors: Choudhuri, Arnab Rai
2007AIPC..919...49C    Altcode:
  The cyclically varying magnetic field of the Sun is believed to be
  produced by the hydromagnetic dynamo process. We first summarize
  the relevant observational data pertaining to sunspots and solar
  cycle. Then we review the basic principles of MHD needed to develop the
  dynamo theory. This is followed by a discussion how bipolar sunspots
  form due to magnetic buoyancy of flux tubes formed at the base of the
  solar convection zone. Following this, we come to the heart of dynamo
  theory. After summarizing the basic ideas of a turbulent dynamo and
  the basic principles of its mean field formulation, we present the
  famous dynamo wave solution, which was supposed to provide a model
  for the solar cycle. Finally we point out how a flux transport dynamo
  can circumvent some of the difficulties associated with the older
  dynamo models.

---------------------------------------------------------
Title: Predicting Solar Cycle 24 With a Solar Dynamo Model
Authors: Choudhuri, Arnab Rai; Chatterjee, Piyali; Jiang, Jie
2007PhRvL..98m1103C    Altcode: 2007astro.ph..1527C
  Whether or not the upcoming cycle 24 of solar activity will be strong
  is being hotly debated. The solar cycle is produced by a complex
  dynamo mechanism. We model the last few solar cycles by “feeding”
  observational data of the Sun’s polar magnetic field into our solar
  dynamo model. Our results fit the observed sunspot numbers of cycles
  21 23 reasonably well and predict that cycle 24 will be about 35%
  weaker than cycle 23.

---------------------------------------------------------
Title: Helical Magnetic Fields in Solar Active Regions: Theory
    vs. Observations
Authors: Petrovay, K.; Chaterjee, P.; Choudhuri, A.
2007astro.ph..2073P    Altcode:
  The mean value of the normalized current helicity in solar active
  regions is on the order of 1e-8 1/m, negative in the northern
  hemisphere, positive in the southern hemisphere. Observations indicate
  that this helicity has a subsurface origin. Possible mechanisms leading
  to a twist of this amplitude in magnetic flux tubes include the solar
  dynamo, convective buffeting of rising flux tubes, and the accretion
  of weak external poloidal flux by a rising toroidal flux tube. After
  briefly reviewing the observational and theoretical constraints on
  the origin of helicity, we present a recently developed detailed model
  for poloidal flux accretion.

---------------------------------------------------------
Title: On Magnetic Coupling Between the Two Hemispheres in Solar
    Dynamo Models
Authors: Chatterjee, Piyali; Choudhuri, Arnab Rai
2006SoPh..239...29C    Altcode: 2006SoPh..tmp...77C
  By introducing an asymmetry between the two hemispheres, we study
  whether the solar dynamo solutions in the two hemispheres remain
  coupled with each other. Our calculations are based on the solar
  dynamo code SURYA, which incorporates the helioseismically-determined
  solar-rotation profile, a Babcock-Leighton α effect concentrated near
  the surface, and a meridional circulation. When the magnetic coupling
  between the hemispheres is enhanced by either increasing the diffusion
  or introducing an α effect distributed throughout the convection zone,
  we find that the solutions in the two hemispheres evolve together with
  a single period even when we make the meridional circulation or the
  α effect different in the two hemispheres. On the other hand, when
  the hemispheric coupling is weaker for other values of parameters,
  an asymmetry between the hemispheres can make solutions in the two
  hemispheres evolve independently with different periods.

---------------------------------------------------------
Title: Forecasting Cycle 24 with a Solar Dynamo Model
Authors: Jiang, Jie; Chatterjee, P.; Choudhuri, A. R.
2006ihy..workE..28J    Altcode:
  A challenge before solar physicists right now is to forecast the
  strength of the next solar cycle (Cycle 24). Several contrary forecasts
  have already been made. Most of the forecasts are based on various
  precursor methods. Only one forecast is based on a dynamo model (Dikpati
  and Gilman 2006). Since we find some aspects of this work questionable,
  it is desirable to have another independent forecast based on a dynamo
  model. We are carrying out an analysis based on our dynamo model,
  using a methodology different from what was used by Dikpati and
  Gilman (2006). We shall present the methodology of our approach and,
  most probably, we shall also have some results by the time of the
  IHY meeting.

---------------------------------------------------------
Title: Some Recent Developments in Solar Dynamo Theory
Authors: Choudhuri, Arnab Rai
2006JApA...27...79C    Altcode:
  We discuss the current status of solar dynamo theory and describe the
  dynamo model developed by our group. The toroidal magnetic field is
  generated in the tachocline by the strong differential rotation and
  rises to the solar surface due to magnetic buoyancy to create active
  regions. The decay of these active regions at the surface gives rise
  to the poloidal magnetic field by the Babcock-Leighton mechanism. This
  poloidal field is advected by the meridional circulation first to
  high latitudes and then down below to the tachocline. Dynamo models
  based on these ideas match different aspects of observational data
  reasonably well.

---------------------------------------------------------
Title: Helical Magnetic Fields in Solar Active Regions: Theory
    vs. Observations
Authors: Petrovay, K.; Chaterjee, P.; Choudhuri, A.
2006PADEU..17....5P    Altcode:
  The mean value of the normalized current helicity
  α_p=čB\cdot(nabla×čB)/B^2 in solar active regions is on the order
  of 10^{-8} m^{-1}, negative in the northern hemisphere, positive in
  the southern hemisphere. Observations indicate that this helicity
  has a subsurface origin. Possible mechanisms leading to a twist
  of this amplitude in magnetic flux tubes include the solar dynamo,
  convective buffeting of rising flux tubes, and the accretion of weak
  external poloidal flux by a rising toroidal flux tube. After briefly
  reviewing the observational and theoretical constraints on the origin
  of helicity, we present a recently developed detailed model for poloidal
  flux accretion.

---------------------------------------------------------
Title: On the Origin of Current Helicity in Active Regions
Authors: Petrovay, K.; Chatterjee, P.; Choudhuri, A.
2006ESASP.617E..67P    Altcode: 2006soho...17E..67P
  No abstract at ADS

---------------------------------------------------------
Title: Development of twist in an emerging magnetic flux tube by
    poloidal field accretion
Authors: Chatterjee, P.; Choudhuri, A. R.; Petrovay, K.
2006A&A...449..781C    Altcode: 2005astro.ph.12472C
  Aims.Following an earlier proposal for the origin of twist in the
  magnetic fields of solar active regions, we model the penetration of
  a wrapped up background poloidal field into a toroidal magnetic flux
  tube rising through the solar convective zone. Methods.The rise of the
  straight, cylindrical flux tube is followed by numerically solving the
  induction equation in a comoving Lagrangian frame, while an external
  poloidal magnetic field is assumed to be radially advected onto the
  tube with a speed corresponding to the rise velocity. Results.One
  prediction of our model is the existence of a ring of reverse current
  helicity on the periphery of active regions. On the other hand, the
  amplitude of the resulting twist depends sensitively on the assumed
  structure (diffuse vs. concentrated/intermittent) of the active
  region magnetic field right before its emergence, and on the assumed
  vertical profile of the poloidal field. Nevertheless, in the model
  with the most plausible choice of assumptions a mean twist comparable
  to the observations results. Conclusions.Our results indicate that the
  contribution of this mechanism to the twist can be quite significant,
  and under favourable circumstances it can potentially account for most
  of the current helicity observed in active regions.

---------------------------------------------------------
Title: How far are we from a 'Standard Model' of the solar dynamo?
Authors: Choudhuri, A. R.
2006cosp...36..595C    Altcode: 2006cosp.meet..595C
  Many aspects of the solar cycle can be explained with kinematic
  solar dynamo models In a kinematic dynamo model it is necessary
  to specify various parameters such as the differential rotation the
  alpha-coefficient the turbulent diffusion and the meridional circulation
  in addition to prescribing some way to treat the magnetic buoyancy
  Within the last few years new developments in helioseismology and
  flux tube simulations have put important constraints on many of these
  parameters Different solar dynamo groups around the world now agree on
  some basics although there still exist major sources of disagreement I
  shall describe the current status of solar dynamo models which take into
  account the various constraints I shall also list the major remaining
  uncertainties and discuss how they may possibly be resolved in future

---------------------------------------------------------
Title: Why do millisecond pulsars have weaker magnetic fields compared
    to ordinary pulsars?
Authors: Choudhuri, A. R.; Konar, S.
2006cosp...36..719C    Altcode: 2006cosp.meet..719C
  Millisecond pulsars with magnetic fields weaker by three to four orders
  compared to those of ordinary pulsars are presumed to be neutron stars
  spun up by binary accretion We expect the magnetic field to get screened
  by the accreted material Our simulation of this screening mechanism
  shows that the field decreases by a purely geometric factor sin^(-7/2)
  theta_(P,i) before freezing to an asymptotic value where theta_(P_i)
  is the initial angular width of the polar cap If theta_(P_i) lies in
  the range 5 - 10 ° then the magnetic field diminution factor turns
  out to be approx 10^3 - 10^4 in conformity with observational data The
  detailed results of the simulation are presented in two publications
  (Choudhuri &amp; Konar 2002, MNRAS 332, 933; Konar &amp; Choudhuri 2004,
  MNRAS 348, 661)

---------------------------------------------------------
Title: A theoretical model for the magnetic helicity of solar
    active regions
Authors: Choudhuri, A. R.; Chatterjee, P.; Petrovay, K.; Nandy, D.
2006cosp...36..714C    Altcode: 2006cosp.meet..714C
  Active regions on the solar surface are known to possess magnetic
  helicity which is predominantly negative in the northern hemisphere
  and positive in the southern hemisphere Choudhuri 2003 Sol Phys 123 217
  proposed that the magnetic helicity arises due to the wrapping up of the
  poloidal field of the convection zone around rising flux tubes which
  form active regions Choudhuri Chatterjee and Nandy 2004 ApJ 615 L57
  used this idea to calculate magnetic helicity from their solar dynamo
  model and found broad agreements with observational data Chatterjee
  Choudhuri and Petrovay 2006 A A in press have studied the penetration
  of the wrapped poloidal field into the rising flux tube and concluded
  that more detailed observational data will throw light on the physical
  conditions of flux tubes just before their emergence to the photosphere

---------------------------------------------------------
Title: Reply to the Comments of Dikpati et al.
Authors: Choudhuri, A. R.; Nandy, D.; Chatterjee, P.
2005A&A...437..703C    Altcode: 2005astro.ph..5232C
  We respond to Dikpati et al.'s criticism of our recent solar dynamo
  model. A different treatment of the magnetic buoyancy is the most
  probable reason for their different results.

---------------------------------------------------------
Title: Full-sphere simulations of a circulation-dominated solar
dynamo: Exploring the parity issue
Authors: Chatterjee, P.; Nandy, D.; Choudhuri, A. R.
2004A&A...427.1019C    Altcode: 2004astro.ph..5027C
  We explore a two-dimensional kinematic solar dynamo model in a full
  sphere, based on the helioseismically determined solar rotation
  profile and with an α effect concentrated near the solar surface,
  which captures the Babcock-Leighton idea that the poloidal field is
  created from the decay of tilted bipolar active regions. The meridional
  circulation, assumed to penetrate slightly below the tachocline, plays
  an important role. Some doubts have recently been raised regarding
  the ability of such a model to reproduce solar-like dipolar parity. We
  specifically address the parity issue and show that the dipolar mode
  is preferred when certain reasonable conditions are satisfied, the
  most important condition being the requirement that the poloidal field
  should diffuse efficiently to get coupled across the equator. Our model
  is shown to reproduce various aspects of observational data, including
  the phase relation between sunspots and the weak, diffuse field.

---------------------------------------------------------
Title: Helicity of Solar Active Regions from a Dynamo Model
Authors: Choudhuri, Arnab Rai; Chatterjee, Piyali; Nandy, Dibyendu
2004ApJ...615L..57C    Altcode:
  We calculate helicities of solar active regions based on the idea that
  poloidal flux lines get wrapped around a toroidal flux tube rising
  through the convection zone, thereby giving rise to the helicity. Rough
  estimates based on this idea compare favorably with the observed
  magnitude of helicity. We use our solar dynamo model based on the
  Babcock-Leighton α-effect to study how helicity varies with latitude
  and time. At the time of solar maximum, our theoretical model gives
  negative helicity in the northern hemisphere and positive helicity in
  the south, in accordance with observed hemispheric trends. However,
  we find that during a short interval at the beginning of a cycle,
  helicities tend to be opposite of the preferred hemispheric trends.

---------------------------------------------------------
Title: Impact Features from Vargeão Dome, Southern Brazil
Authors: Kazzuo-Vieira, C.; Crósta, A. P.; Choudhuri, A.
2004M&PSA..39.5050K    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Diamagnetic screening of the magnetic field in accreting
    neutron stars - II. The effect of polar cap widening
Authors: Konar, Sushan; Choudhuri, Arnab Rai
2004MNRAS.348..661K    Altcode: 2003astro.ph..4490K
  Recently, we have proposed a model for the screening of the magnetic
  field of an accreting neutron star by the accreted material flowing from
  the polar regions towards the equator and sinking there underneath the
  surface. In this earlier model, it was assumed that the flow pattern
  remained stationary over time. However, as the surface magnetic field
  weakens, the accretion takes place over a wider region around the
  pole, making the flow more radial and isotropic. In the present work,
  we extend this two-dimensional model to include the time dependence of
  the flow of the accreted material. The final radial flow is found to
  be less efficient in screening the magnetic field compared with the
  initial tangential flow. After an initial phase of rapid decay, the
  magnetic field slowly reaches an asymptotic value when the accretion
  becomes nearly isotropic and radial. Assuming the initial extent of
  the polar cap to be ~5°-10°, a simple geometric argument suggests
  that the magnetic field should decay by 3-4 orders of magnitude before
  stabilizing to an asymptotic value, consistent with the magnetic fields
  observed in millisecond pulsars.

---------------------------------------------------------
Title: Why do millisecond pulsars have weaker magnetic fields compared
    to ordinary pulsars?
Authors: Choudhuri, Arnab Rai; Konar, Sushan
2004CSci...86..444C    Altcode: 2003astro.ph.11031C
  Millisecond pulsars, with magnetic fields weaker by three to four
  orders compared to those of ordinary pulsars, are presumed to be neutron
  stars spun up by binary accretion. We expect the magnetic field to get
  screened by the accreted material. Our simulation of this screening
  mechanism shows that the field decreases by a purely geometric factor
  sin-7/2 qP,i before freezing to an asymptotic value, where qP,i is
  the initial angular width of the polar cap. If qP,i lies in the range
  5-10 °, then the magnetic field diminution factor turns out to be ~
  103-104, in conformity with observational data.

---------------------------------------------------------
Title: Full Sphere Axisymmetric Simulations of the Solar Dynamo
Authors: Nandy, Dibyendu; Chatterjee, Piyali; Choudhuri, Arnab Rai
2004IAUS..223..133N    Altcode: 2005IAUS..223..133N
  We explore a full sphere (2D axisymmetric) kinematic solar dynamo
  model based on the Babcock-Leighton idea that the poloidal field is
  generated in the surface layers from the decay of tilted bipolar solar
  active regions. This model incorporates the helioseismically deduced
  solar rotation profile and an algorithm for buoyancy motivated from
  simulations of flux tube dynamics. A prescribed deep meridional
  circulation plays an important role in the advection of magnetic
  flux. We specifically address the parity issue and show that -
  contrary to some recent claims - the Babcock-Leighton dynamo can
  reproduce solar-like dipolar parity if certain reasonable conditions
  are satisfied in the solar interior, the most important requirement
  being that the poloidal field of the two hemispheres be efficiently
  coupled across the equator.

---------------------------------------------------------
Title: The Origin of Helicity in Solar Active Regions
Authors: Choudhuri, Arnab Rai; Chatterjee, Piyali; Nandy, Dibyendu
2004IAUS..223...45C    Altcode: 2005IAUS..223...45C; 2004astro.ph..6598C
  We present calculations of helicity based on our solar dynamo model
  and show that the results are consistent with observational data.

---------------------------------------------------------
Title: On Solar Radius Variation with Magnetic Field
Authors: Choudhuri, Arnab Rai; Chatterjee, Piyali
2003astro.ph.11028C    Altcode:
  In response to the claim by Dziembowski et al. (2001) that the solar
  radius decreases with magnetic activity at the rate of 1.5 km/yr, we
  consider the theoretical question whether a radius variation is expected
  with the solar cycle. If the radius variation is caused by the magnetic
  pressure of toroidal flux tubes at the bottom of the convection zone,
  then the dynamo model of Nandy and Choudhuri (2002) would suggest a
  radius decrease with magnetic activity, in contrast to other dynamo
  models which would suggest a radius increase. However, the radius
  decrease is estimated to be only of the order of hundreds of metres.

---------------------------------------------------------
Title: On the Connection Between Mean Field Dynamo Theory and
    Flux Tubes
Authors: Choudhuri, Arnab Rai
2003SoPh..215...31C    Altcode: 2002astro.ph.11591C
  Mean field dynamo theory deals with various mean quantities and does
  not directly throw any light on the question of existence of flux
  tubes. We can, however, draw important conclusions about flux tubes
  in the interior of the Sun by combining additional arguments with
  the insights gained from solar dynamo solutions. The polar magnetic
  field of the Sun is of order 10 G, whereas the toroidal magnetic
  field at the bottom of the convection zone has been estimated to be
  100 000 G. Simple order-of-magnitude estimates show that the shear
  in the tachocline is not sufficient to stretch a 10 G mean radial
  field into a 100 000 G mean toroidal field. We argue that the polar
  field of the Sun must get concentrated into intermittent flux tubes
  before it is advected to the tachocline. We estimate the strengths
  and filling factors of these flux tubes. Stretching by shear in the
  tachocline is then expected to produce a highly intermittent magnetic
  configuration at the bottom of the convection zone. The meridional
  flow at the bottom of the convection zone should be able to carry
  this intermittent magnetic field equatorward, as suggested recently
  by Nandy and Choudhuri (2002). When a flux tube from the bottom of
  the convection zone rises to a region of pre-existing poloidal field
  at the surface, we point out that it picks up a twist in accordance
  with the observations of current helicities at the solar surface.

---------------------------------------------------------
Title: Insights on Turbulent Flows
Authors: Nandy, D.; Choudhuri, A. R.
2003PADEU..13...21N    Altcode:
  Turbulent flows in the interior of the Sun, both at small and large
  scales, are believed to feed and sustain the solar hydromagnetic dynamo
  that generates the solar cycle. The solar cycle itself strikingly
  manifests in a 11-year periodic variation in the number of sunspots
  seen on the solar surface. Sunspots are regions of concentrated
  magnetic fields, occurring at low latitudes on the solar surface and are
  believed to be tracers of the underlying dynamo mechanism. An important
  ingredient in recent models of the dynamo mechanism is the meridional
  flow of material, which is believed to originate from turbulent stresses
  in the solar convection zone. This meridional circulation is observed
  to be poleward in the outer 15% of the Sun and must be balanced by an
  equatorward counterflow in the interior. The nature and exact location
  of this counterflow, however, is unknown. We discuss here results from
  a dynamo model that reproduces the correct latitudinal distribution
  of sunspots and show that this requires a meridional counterflow
  of material that penetrates much deeper than hitherto believed --
  into the radiative layers below the convection zone. We comment on
  the viability of such a deep counterflow of material and discuss its
  implications for turbulent convection and elemental abundance in the
  Sun and related stellar atmospheres.

---------------------------------------------------------
Title: The solar dynamo as a model of the solar cycle
Authors: Choudhuri, A. R.
2003dysu.book..103C    Altcode:
  It is believed that the Sun's magnetic field is produced by the dynamo
  process, which involves nonlinear interactions between the solar plasma
  and the magnetic field. Summarising the main charateristics of solar
  magnetic field, the basic ideas of dynamo theory are presented and
  its current status is discussed.

---------------------------------------------------------
Title: Diamagnetic screening of the magnetic field in accreting
    neutron stars
Authors: Konar, Sushan; Choudhuri, Arnab Rai
2002BASI...30..697K    Altcode: 2002astro.ph..2248K
  A possible mechanism for screening of the surface magnetic field of an
  accreting neutron star, by the accreted material, is investigated. In
  particular, we investigate the nature of the evolution of the internal
  field configuration in the case of a) a polar cap accretion and b)
  a spherical accretion.

---------------------------------------------------------
Title: Solar dynamo models with realistic internal rotation
Authors: Choudhuri, Arnab Rai; Nandy, Dibyendu
2002ESASP.505...91C    Altcode: 2002IAUCo.188...91C; 2002solm.conf...91C
  Solar dynamo models based on differetial rotation inferred from
  helioseismology tend to produce rather strong magnetic activity at high
  solar latitudes, in contrast to the observed fact that sunspots appear
  at low latitudes. We show that a meridional circulation penetrating
  below the tachocline can solve this problem.

---------------------------------------------------------
Title: Diamagnetic screening of the magnetic field in accreting
    neutron stars
Authors: Choudhuri, Arnab Rai; Konar, Sushan
2002MNRAS.332..933C    Altcode: 2001astro.ph..8229C
  A possible mechanism for screening of the surface magnetic field of an
  accreting neutron star, by the accreted material, is investigated. We
  model the material flow in the surface layers of the star by an
  assumed two-dimensional velocity field satisfying all the physical
  requirements. Using this model velocity we find that, in the absence
  of magnetic buoyancy, the surface field is screened (i.e. there
  is submergence of the field by advection) within the time-scale
  of material flow of the top layers. On the other hand, if magnetic
  buoyancy is present, the screening happens over a time-scale that is
  characteristic of the slower flow of the deeper (and hence, denser)
  layers. For accreting neutron stars, this longer time-scale turns out
  to be about 10<SUP>5</SUP>yr, which is of a similar order of magnitude
  to the accretion time-scale of most massive X-ray binaries.

---------------------------------------------------------
Title: Explaining the Latitudinal Distribution of Sunspots with Deep
    Meridional Flow
Authors: Nandy, Dibyendu; Choudhuri, Arnab Rai
2002Sci...296.1671N    Altcode:
  Sunspots, dark magnetic regions occurring at low latitudes on the Sun's
  surface, are tracers of the magnetic field generated by the dynamo
  mechanism. Recent solar dynamo models, which use the helioseismically
  determined solar rotation, indicate that sunspots should form at high
  latitudes, contrary to observations. We present a dynamo model with
  the correct latitudinal distribution of sunspots and demonstrate that
  this requires a meridional flow of material that penetrates deeper
  than hitherto believed, into the stable layers below the convection
  zone. Such a deep material flow may have important implications for
  turbulent convection and elemental abundance in the Sun and similar
  stars.

---------------------------------------------------------
Title: On the absence of sunspots at high solar latitudes and
    associated constraints on the meridional flow in the solar interior
Authors: Nandy, D.; Choudhuri, A. R.
2002AAS...200.8901N    Altcode: 2002BAAS...34..791N
  Sunspots -- dark magnetic regions -- occur at low latitudes on the Sun's
  surface and are believed to be tracers of the magnetic field generated
  in the interior by the dynamo mechanism. An important ingredient
  in recent models of this dynamo mechanism is the meridional flow of
  material, which is observed to be poleward on the Sun's surface and must
  be balanced by an equatorward counterflow in the interior. The nature
  and exact location of this counterflow, however, is unknown. Recent
  solar dynamo models, which use the helioseismically determined internal
  rotation of the Sun and confine the counterflow within the convection
  zone (as classical theories of solar convection would suggest),
  indicate that sunspots should form at higher latitudes, contrary to
  observations. Here we present a solar dynamo model with the correct
  latitudinal distribution of sunspots and show that this requires a
  counterflow that penetrates much deeper than hitherto believed - into
  the stable layers below the convection zone. The existence of such
  a deep counterflow of material may have important implications for
  turbulent convection and elemental abundance in the Sun and related
  stellar atmospheres.

---------------------------------------------------------
Title: Diamagnetic Screening of the Magnetic Field of an Accreting
    Neutron Star
Authors: Konar, S.; Choudhuri, A.
2002cosp...34E.721K    Altcode: 2002cosp.meetE.721K
  A possible mechanism for screening of the surface magnetic field of an
  accreting neutron star, by the accreted material, is investigated. In
  particular, we investigate the nature of the evolution of the internal
  field configuration in the case of a) a polar cap accretion and b)
  a spherical accretion.

---------------------------------------------------------
Title: The Orientational Relaxation of Bipolar Active Regions
Authors: Longcope, Dana; Choudhuri, Arnab Rai
2002SoPh..205...63L    Altcode:
  In the mean, bipolar active regions are oriented nearly toroidally,
  according to Hale's polarity law, with a latitude-dependent tilt known
  as Joy's Law. The tilt angles of individual active regions deviate
  from this mean behavior and change over time. It has been found that on
  average the change is toward the mean angle at a rate characteristic of
  4.37 days (Howard, 1996). We show that this orientational relaxation
  is consistent with the standard model of flux tube emergence from
  a deep dynamo layer. Under this scenario Joy's law results from the
  Coriolis effect on the rising flux tube (D'Silva and Choudhuri, 1993),
  and departures from it result from turbulent buffeting of the tubes
  (Longcope and Fisher, 1996). We show that relaxation toward Joy's
  angle occurs because the turbulent perturbations relax on shorter
  time scales than the perturbations from the Coriolis force. The
  turbulent perturbations relax more rapidly because they are localized
  to the topmost portion of the convection zone while the Coriolis
  perturbations are more widely distributed. If a fully-developed active
  region remains connected to the strong toroidal magnetic field at
  the base of the convection zone, its tilt will eventually disappear,
  leaving it aligned perfectly toroidally. On the other hand, if the flux
  becomes disconnected from the toroidal field the bipole will assume a
  tilt indicative of the location of disconnection. We compare models
  which are connected and disconnected from the toroidal field. Only
  those disconnected at points very deep in the convection zone are
  consistent with observed time scale of orientational relaxation.

---------------------------------------------------------
Title: Toward a Mean Field Formulation of the Babcock-Leighton Type
    Solar Dynamo. I. α-Coefficient versus Durney's Double-Ring Approach
Authors: Nandy, Dibyendu; Choudhuri, Arnab Rai
2001ApJ...551..576N    Altcode: 2001astro.ph..7466N
  We develop a model of the solar dynamo in which, on the one hand, we
  follow the Babcock-Leighton approach to include surface processes,
  such as the production of poloidal field from the decay of active
  regions, and, on the other hand, we attempt to develop a mean field
  theory that can be studied in quantitative detail. One of the main
  challenges in developing such models is to treat the buoyant rise
  of the toroidal field and the production of poloidal field from it
  near the surface. A previous paper by Choudhuri, Schüssler, &amp;
  Dikpati in 1995 did not incorporate buoyancy. We extend this model by
  two contrasting methods. In one method, we incorporate the generation
  of the poloidal field near the solar surface by Durney's procedure
  of double-ring eruption. In the second method, the poloidal field
  generation is treated by a positive α-effect concentrated near the
  solar surface coupled with an algorithm for handling buoyancy. The
  two methods are found to give qualitatively similar results.

---------------------------------------------------------
Title: The Current Status of Kinematic Solar Dynamo Models
Authors: Choudhuri, A. R.
2000JApA...21..373C    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: The Role of Magnetic Buoyancy in a Babcock-Leighton Type
    Solar Dynamo
Authors: Nandy, D.; Choudhuri, A. R.
2000JApA...21..381N    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Incorporating magnetic buoyancy in solar dynamo models:
    New results, problems -- and their possible solutions.
Authors: Nandy, D.; Choudhuri, A. R.
2000SPD....31.0134N    Altcode:
  There have been traditionally two kinds of approaches
  to understand the origin of the solar magnetic cycle: the
  Parker--Steenbeck--Krause--Rädler approach and the Babcock--Leighton
  approach. It seems at present that the most promising models of the
  solar dynamo are those which incorporate the best features of both
  these traditional approaches. One of the uncertainties in these hybrid
  models (Choudhuri et. al. 1995) lies in the treatment of magnetic
  buoyancy within a mean field framework, a subject which has rarely been
  explored in the past (Durney 1997). We study this problem by exploring
  possible ways of incorporating magnetic buoyancy in a dynamo code --
  to simulate the formation and subsequent decay of sunspots and the
  recycling of fields, with magnetic buoyancy as an important player in
  the flux-transport process. The results as well as some problems faced
  by such new generation of dynamo models, will be discussed. References:
  Durney B. R., 1997, ApJ 486, 1065 Choudhuri A. R., Schussler M.,
  Dikpati M., 1995, A&amp;A 303, L29 Nandi D., Choudhuri A. R., 2000,
  submitted to ApJ

---------------------------------------------------------
Title: Incorporating magnetic buoyancy in solar dynamo models:
    new results, problems - and their possible solutions.
Authors: Nandi, D.; Choudhuri, A. R.
2000BAAS...32..807N    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: The solar dynamo
Authors: Choudhuri, Arnab Rai
1999CSci...77.1475C    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: On the large-scale diffuse magnetic field of the Sun -
    II. The Contribution of Active Regions
Authors: Choudhuri, Arnab Rai; Dikpati, Mausumi
1999SoPh..184...61C    Altcode:
  Dikpati and Choudhuri (1994, 1995) developed a model for the poleward
  migration of the weak diffuse magnetic field on the Sun's surface. This
  field was identified with the poloidal component produced by the solar
  dynamo operating at the base of the convection zone, and its evolution
  was studied by considering the effects of meridional circulation and
  turbulent diffusion. The earlier model is extended in this paper by
  incorporating the flux from, the decay of tilted active regions near
  the solar surface as an additional source of the poloidal field. This
  extended model can now explain various low-latitude features in the
  time-latitude diagram of the weak diffuse fields. These low-latitude
  features could not be accounted for in the earlier model, which was very
  successful in modeling the behavior at high latitudes. The time-latitude
  diagrams show that regions of a particular polarity often have `tongues'
  of opposite polarity. Such tongues can be produced in the theoretical
  model by incorporating fluctuations in the source term arising out of
  the decaying active regions.

---------------------------------------------------------
Title: The physics of fluids and plasmas : an introduction for
    astrophysicists /
Authors: Choudhuri, Arnab Rai
1998pfp..book.....C    Altcode:
  Introduction; Part I. Neutral Fluids: 2. Boltzmann equation; 3. March
  towards hydrodynamics; 4. Properties of ideal fluids; 5. Viscous
  flows; 6. Gas dynamics; 7. Linear theory of waves and instabilities;
  8. Turbulence; 9. Rotation and hydrodynamics; Part II. Plasmas:
  10. Plasma orbit theory; 11. Dynamics of many charged particles;
  12. Collisionless processes in plasmas; 13. Collisional processes and
  the one-fluid model; 14. Basic magnetohydrodynamics; 15. Theory of
  magnetic topologies; 16. Dynamo theory; Appendices: A. Useful vector
  relations; B. Integrals in kinetic theory; C. Formulae and equations in
  cylindrical and spherical coordinates; D. Values of various quantities;
  E. Basic parameters pertaining to plasmas; Suggestions for further
  reading; References.

---------------------------------------------------------
Title: Coronal heating by magnetic kink waves
Authors: Banerjee, D.; Dikpati, M.; Choudhuri, A. R.
1998ASPC..138...15B    Altcode: 1998stas.conf...15B
  No abstract at ADS

---------------------------------------------------------
Title: (Erratum) The solar dynamo with meridional circulation.
Authors: Choudhuri, A. R.; Schuessler, M.; Dikpati, M.
1997A&A...319..362C    Altcode:
  Erratum to Astron. Astrophys. 303, L29 (1995).

---------------------------------------------------------
Title: Magnetic fields in the sun's interior: What do we know
    about them?
Authors: Choudhuri, Arnab Rai
1996BASI...24..219C    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: On the Out of Phase Appearance of Large-Scale Diffuse Magnetic
    Field of the Sun with Respect to Sunspots
Authors: Dikpati, Mausumi; Choudhuri, Arnab Rai
1996Ap&SS.243..169D    Altcode: 1996IAUCo.154..169D
  We assume the large-scale diffuse magnetic field of the Sun to originate
  from the poloidal component of a dynamo operating at the base of
  the convection zone, whereas the sunspots are due to the toroidal
  component. The evolution of the poloidal component is studied to model
  the poleward migration of the diffuse field seen on the solar surface
  and the polar reversal at the time of sunspots maxima (Dikpati and
  Choudhuri 1994, 1995).

---------------------------------------------------------
Title: The Evolutton Of The Magnetic Structure of the Solar Corona
    With The Solar Cycle
Authors: Dikpati, Mausumi; Choudhuri, Arnab Rai; Venkatakrishnan, P.
1996ASPC...95..309D    Altcode: 1996sdit.conf..309D
  No abstract at ADS

---------------------------------------------------------
Title: Energy Transport to the Solar Corona by Magnetic Kink Waves
Authors: Choudhuri, A. R.; Dikpati, M.; Banerjee, D.
1995JApAS..16..390C    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: The Evolution of Weak Magnetic Fields of the Sun in Relation
    to Dynamo Theory
Authors: Dikpati, M.; Choudhuri, A. R.
1995JApAS..16..391D    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: The solar dynamo with meridional circulation.
Authors: Choudhuri, A. R.; Schussler, M.; Dikpati, M.
1995A&A...303L..29C    Altcode:
  We show that meridional circulation can have a profound influence
  on dynamo models for the solar cycle. Motivated by the observed tilt
  angles of sunspot groups we assume that the generation of the poloidal
  field takes place near the surface, while a shear layer of radial
  differential rotation produces the toroidal field at the bottom of
  the convection zone. Both layers are coupled by a circulation with a
  poleward directed flow in the upper part and an equatorward flow in the
  deep layers of the convection zone. The circulation forces the toroidal
  field belts (which are responsible for the surface activity) to move
  equatorward. This leads to butterfly diagrams in qualitative agreement
  with the observations, even if the dynamo wave would propagate poleward
  in the absence of circulation. This result opens the possibility to
  construct models for the solar cycle which are based on observational
  data (tilt angles, differential rotation, and meridional circulation).

---------------------------------------------------------
Title: On the Large-Scale Diffuse Magnetic Field of the Sun
Authors: Dikpati, Mausumi; Choudhuri, Arnab Rai
1995SoPh..161....9D    Altcode:
  Although the sunspots migrate towards the equator, the large-scale
  weak diffuse magnetic fields of the Sun migrate poleward with the solar
  cycle, the polar field reversing at the time of the sunspot maxima. We
  apply the vector model of Dikpati and Choudhuri (1994, Paper I) to fit
  these observations. The dynamo layer at the base of the convection zone
  is taken to be the source of the diffuse field, which is then evolved
  in the convection zone subject to meridional circulation and turbulent
  diffusion. We find that the longitudinally averaged observational
  data can be fitted reasonably well both for positive and negative
  values of theα-effect by adjusting the subsurface meridional flow
  suitably. The model will be extended in a future paper to include the
  decay of active regions as an extra source of the diffuse field, which
  may be necessary to explain the probable phase lag betweenB<SUB>r</SUB>
  andB<SUB>φ</SUB> at lower latitudes.

---------------------------------------------------------
Title: The evolution of the Sun's poloidal field.
Authors: Dikpati, M.; Choudhuri, A. R.
1994A&A...291..975D    Altcode:
  We present here a model to explain how the weak large-scale diffuse
  magnetic fields of the Sun migrate poleward in contrast to the sunspots
  which migrate equatorward with the progress of the solar cycle. We
  study the evolution of the Sun's poloidal field in the convection
  zone by assuming that it is produced by an equatorward-propagating
  dynamo wave at the base of the convection zone and is then subject to
  turbulent diffusion and a meridional circulation with a poleward surface
  flow. The magnetic fieldlines in the lower part of the convection zone
  first move towards the equator where they are pushed upward by the
  upwelling meridional flow there to form magnetic bubbles by joining with
  their opposite hemisphere counterparts. After reaching the surface,
  these bubbles drift to higher latitudes with the poleward meridional
  flow. Our model incorporates the three-dimensional vector character of
  the magnetic field, whereas the previous flux transport models treated
  the magnetic field as a scalar on the two-dimensional solar surface.

---------------------------------------------------------
Title: Energy Transport to the Solar Corona by Magnetic Kink Waves
Authors: Choudhuri, Arnab R.; Dikpati, Mausumi; Banerjee, Dipankar
1993ApJ...413..811C    Altcode:
  We show that the magnetic kink waves generated by the motions of
  photospheric footpoints of the coronal flux tubes can supply adequate
  energy for heating the quiet corona, provided there are occasional rapid
  motions of these footpoints as found in recent observations. Choudhuri
  et al. (1992) modeled the solar corona as isothermal atmosphere and
  showed that these rapid motions are much more efficient for transporting
  energy compared to the slow footpoint motions taking place most of
  the time. We extend these calculations for a two-layer atmosphere,
  with the lower layer having chromospheric thickness and temperature,
  and the upper layer having coronal temperature. Even in the presence of
  such a temperature jump we find that the rapid footpoint motions are
  still much more efficient for transporting energy to the corona and
  the estimated energy flux is sufficient for quiet coronal heating. We
  discuss the general problem of the propagation of kink pulses in
  a two-layer atmosphere for different possible values of the basic
  parameters. We find a fairly complicated behavior which could not be
  anticipated from the analysis of a pure Fourier mode.

---------------------------------------------------------
Title: A theoretical model for tilts of bipolar magnetic regions
Authors: D'Silva, S.; Choudhuri, A. R.
1993A&A...272..621D    Altcode:
  Joy's law (Hale et al. 1919) states that bipolar magnetic regions (BMRs)
  are inclined to the latitudinal line, with the p-spot (preceding spot)
  of the BMR closer to the equator and the tilt angle increasing with
  latitude. It is believed that the solar dynamo operates in the overshoot
  region just below the convection zone and the BMRs are produced by
  the flux loops rising from there due to magnetic buoyancy. These
  rising loops are expected to be twisted by the Coriolis force so that
  they eventually emerge on the solar surface with a tilt. We extend
  the numerical calculations of Choudhuri (1989) to study the tilts
  produced on the rising flux loops by the Coriolis force. We find
  that the theoretically calculated tilts match the observations only
  if the magnetic field of the flux loops lies in the range between 60
  and 160 kG. For such flux loops, the tilt has the correct magnitude
  and also varies correctly with the latitude. If the magnetic fields
  were stronger than 160 kG, then Coriolis force is much weaker than
  magnetic buoyancy and is only able to produce tilts which are very
  small in overall magnitude, though they still vary correctly with
  latitude. On the other hand, if the fields were smaller than 60 kG, then
  the Coriolis force would have been so overpowering that the flux loops
  would move parallel to the rotation axis as found earlier (Choudhuri
  1989). Such flux loops appear only in high latitudes and do not obey
  Joy's law. On changing the drag on the flux tube, these conclusions
  are not changed. If we change the footpoint separation of the flux
  loop, then we find that magnetic tension may halt the rise of the flux
  loop if the footpoint separation is below a critical value. However,
  for flux tubes which are able to reach the surface, the range from 60
  to 160 kG for the magnetic field still approximately holds. Thus our
  calculations seem to rule out either equipartition fields (about 10 kG)
  or very strong megagauss fields.

---------------------------------------------------------
Title: Implications of Rapid Footpoint Motions of Photospheric Flux
    Tubes for Coronal Heating
Authors: Choudhuri, Arnab R.; Auffret, Herve; Priest, Eric R.
1993SoPh..143...49C    Altcode:
  Some recent observations at Pic-du-Midi (Mulleret al., 1992a) suggest
  that the photospheric footpoints of coronal magnetic field lines
  occasionally move rapidly with typical velocities of the order 3 km
  s<SUP>−1</SUP> for about 3 or 4 min. We argue that such occasional
  rapid footpoint motions could have a profound impact on the heating of
  the quiet corona. Qualitative estimates indicate that these occasional
  rapid motions can account for the entire energy flux needed to heat the
  quiet corona. We therefore carry out a mathematical analysis to study
  in detail the response of a vertical thin flux tube to photospheric
  footpoint motions in terms of a superposition of linear kink modes
  for an isothermal atmosphere. We find the resulting total energy that
  is asymptotically injected into an isothermal atmosphere (i.e., an
  atmosphere without any back reflection). By using typical parameter
  values for fast and slow footpoint motions, we show that, even if the
  footpoints spend only 2.5% of the time undergoing rapid motions, still
  these rapid motions could be more efficient in transporting energy to
  the corona than the slow motions that take place most of the time.

---------------------------------------------------------
Title: The Cluster Model of Sunspots
Authors: Choudhuri, Arnab R.
1992ASIC..375..243C    Altcode: 1992sto..work..243C
  The cluster model of sunspots proposed by Parker (1979) is examined
  in the light of recent theoretical and observational developments. In
  particular, attention is given to the original arguments for the cluster
  model, fine structure of sunspots, growth and decay of sunspots,
  and waves and oscillations in sunspots. The importance of defining
  the regime of validity of the cluster model is emphasized.

---------------------------------------------------------
Title: Stochastic fluctuations of the solar dynamo
Authors: Choudhuri, A. R.
1992A&A...253..277C    Altcode:
  The solar dynamo equations are presently solved in a slab with a
  single mode, taking the alpha-coefficient to be constant in space, but
  fluctuating stochastically in time with given amplitude and correlation
  time. Choudhuri's (1990) contention that the solar dynamo is of
  the alpha-squared(omega) type is reinforced by the solar behavior's
  qualitative agreement with such a dynamo's level of irregularities,
  which increase with rising amplitude or correlation time of stochastic
  fluctuations.

---------------------------------------------------------
Title: The Effect of Kelvin-Helmholtz Instability on Rising Flux
    Tubes in the Convection Zone
Authors: D'Silva, Sydney Z.; Choudhuri, Arnab R.
1991SoPh..136..201D    Altcode:
  If the solar dynamo operates at the bottom of the convection zone,
  then the magnetic flux created there has to rise to the surface. When
  the convection zone is regarded as passive, the rising flux is
  deflected by the Coriolis force to emerge at rather high latitudes,
  poleward of typical sunspot zones (Choudhuri and Gilman, 1987;
  Choudhuri, 1989). Choudhuri and D'Silva (1990) included the effects
  of convective turbulence on the rising flux through (a) giant cell
  drag and (b) momentum exchange by small-scale turbulence. The momentum
  exchange mechanism could enable flux tubes of radii not more than a few
  hundred km to emerge radially at low latitudes, but the giant cell drag
  mechanism required unrealistically small flux tube radii (a few meters
  for a reasonable giant cell upflow) to counteract the Coriolis force. We
  now include the additional effect of Kelvin-Helmholtz instability in a
  symmetrical flux ring caused by the azimuthal flow induced during its
  rise. The azimuthal flow crosses the threshold for the instability only
  if there is a giant cell upflow to drag the flux tubes appreciably. In
  the absence of such a drag, as in the case of a passive convection
  zone or in the case of momentum exchange by small-scale turbulence, the
  azimuthal velocity never becomes large enough to cause the instability,
  leaving the results of the previous calculations unaltered. The giant
  cell drag, aided by Kelvin-Helmholtz instability, however, becomes
  now a viable mechanism for curbing the Coriolis force - 10<SUP>4</SUP>
  G flux tubes with radii of a few hundred km being dragged radially by
  upflows of 70 m s<SUP>-1</SUP>.

---------------------------------------------------------
Title: Influence of turbulence on rising flux tubes in the solar
    convection zone
Authors: Choudhuri, A. R.; D'Silva, S.
1990A&A...239..326C    Altcode:
  The role of turbulence in facilitating the flux tubes generated at low
  solar latitudes at the bottom of the convection zone to emerge at the
  typical sunspot latitudes is investigated. It is found that large scale
  turbulence on the scale of the giant cells cannot dominate the Coriolis
  force, since such domination would require either an unreasonably large
  updraft velocity in the giant cells or an unreasonably small flux tube
  radii. On the other hand, small-scale turbulence can suppress the
  Coriolis force by exchanging angular momentum between the flux tube
  and the surroundings, provided the flux tubes have radii smaller than
  a few hundred km.

---------------------------------------------------------
Title: A correction to Spruit's equation for the dynamics of thin
    flux tubes
Authors: Choudhuri, A. R.
1990A&A...239..335C    Altcode:
  It is pointed out that a term was overlooked in the derivation of
  the equation of motion for a thin flux tube by Spruit (1981). The
  correction to be applied in an inertial frame and in a rotating frame
  are discussed. This correction makes the formulation self-consistent,
  though it does not invalidate the qualitative results obtained by
  various investigators who have used Spruit's equation.

---------------------------------------------------------
Title: On the Possibility of an alpha 2 omega --Type Dynamo in a
    Thin Layer inside the Sun
Authors: Choudhuri, Arnab Rai
1990ApJ...355..733C    Altcode:
  If the solar dynamo operates in a thin layer of 10,000-km thickness
  at the interface between the convection zone and the radiative core,
  using the facts that the dynamo should have a period of 22 years and
  a half-wavelength of 40 deg in the theta-direction, it is possible
  to impose restrictions on the values which various dynamo parameters
  are allowed to have. It is pointed out that the dynamo should be of
  alpha-sq omega nature, and kinematical calculations are presented
  for free dynamo waves and for dynamos in thin rectangular slabs with
  appropriate boundary conditions. An alpha-sq omega dynamo is expected
  to produce a significant poloidal field which does not leak to the
  solar surface. It is found that the turbulent diffusity eta and
  alpha-coefficient are restricted to values within about a factor of
  10, the median values being eta of about 10 to the 10th sq cm/sec and
  alpha of about 10 cm/sec. On the basis of mixing length theory, it is
  pointed out that such values imply a reasonable turbulent velocity of
  the order 30 m/s, but rather small turbulent length scales like 300 km.

---------------------------------------------------------
Title: Locating the Seat of the Solar Dynamo
Authors: Choudhuri, Arnab R.
1990IAUS..142...51C    Altcode:
  The hypothesis that the solar dynamo operates in a thin layer at
  the bottom of the convection zone is addressed. Recent work on the
  question whether the magnetic flux can be made to emerge at sunspot
  latitudes is reviewed. It is concluded that this hypothesis can fit
  the observational facts only if there is turbulence with a length
  scale of a few hundred kilometers in and around the dynamo region.

---------------------------------------------------------
Title: Effect of Turbulence on Emerging Magnetic Flux Tubes in the
    Convection Zone
Authors: D'Silva, S.; Choudhuri, A. R.
1990IAUS..142...60D    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: The Evolution of Loop Structures in Flux Rings Within the
    Solar Convection Zone
Authors: Choudhuri, Arnab Rai
1989SoPh..123..217C    Altcode:
  Choudhuri and Gilman (1987) considered certain implications of the
  hypothesis that the magnetic flux within the Sun is generated at the
  bottom of the convection zone and then rises through it. Taking flux
  rings symmetric around the rotation axis and using reasonable values
  of different parameters, they found that the Coriolis force deflects
  these flux rings into trajectories parallel to the rotation axis so
  that they emerge at rather high latitudes. This paper looks into the
  question of whether the action of the Coriolis force is subdued when
  the initial configuration of the flux ring has non-axisymmetries in the
  form of loop structures. The results depend dramatically on whether the
  flux ring with the loops lies completely within the convection zone
  or whether the lower parts of it are embedded in the stable layers
  underneath the convection zone. In the first case, the Coriolis force
  supresses the non-axisymmetric perturbations so that the flux ring
  tends to remain symmetric and the trajectories are very similar to
  those of Choudhuri and Gilman (1987). In the second case, however, the
  lower parts of the flux ring may remain anchored underneath the bottom
  of the convection zone, but the upper parts of the loops still tend to
  move parallel to the rotation axis and emerge at high latitudes. Thus
  the problem of the magnetic flux not being able to come out at the
  sunspot latitudes still persists after the non-axisymmetries in the
  flux rings are taken into account.

---------------------------------------------------------
Title: The Possible Role of Meridional Flows in Suppressing Magnetic
    Buoyancy
Authors: van Ballegooijen, A. A.; Choudhuri, A. R.
1988ApJ...333..965V    Altcode:
  The equation of motion for a toroidal flux ring in a stellar
  convective envelope is derived, and the equilibrium of such a ring
  is considered. Necessary conditions for the stability of toroidal
  flux rings are derived, and results of stability calculations for a
  particular model of the meridional flow are presented. The motions of
  the flux rings when the rings are far from their equilibrium position
  or when equilibrium does not exist are considered. The results confirm
  the linear stability analysis, and show that in the absence of stable
  equilibrium, the rings move toward the solar surface along a trajectory
  which is parallel to the rotation axis. It is expected that viscosity
  will tend to reduce the rotational velocity difference between the
  flux ring and its surroundings, thus reducing the Coriolis force and
  altering the equilibrium. The storage time of toroidal flux rings is
  estimated, and some implications for the sun are discussed.

---------------------------------------------------------
Title: On the coalescence of twisted flux tubes
Authors: Choudhuri, Arnab Rai
1988GApFD..40..261C    Altcode:
  We study the problem of the coalescence of twisted flux tubes by
  assuming that the azimuthal field lines reconnect at a current sheet
  during the coalescence process and everywhere else the magnetic field is
  frozen in the fluid. We derive relations connecting the topology of the
  coalesced flux tube with the topologies of the initial flux tubes, and
  then obtain a structure equation for calculating the field configuration
  of the coalesced flux tube from the given topology. Some solutions for
  the two extreme cases of low- plasma and high- plasma are discussed. The
  coalesced flux tube has less twist than the initial flux tube. Magnetic
  helicity is found to be exactly conserved during the coalescence, but
  the assumptions in the model put a constraint on the energy dissipation
  so that we do not get a relaxation to the minimum-energy Taylor state in
  the low- case. It is pointed out that the structure equation connecting
  the topology and the equilibrium configuration is quite general and
  can be of use in many two-dimensional flux tube problems.

---------------------------------------------------------
Title: Theoretical modelling of the fine structures in sunspots.
Authors: Choudhuri, Arnab Rai
1987NASCP2483..105C    Altcode: 1987tphr.conf..105C
  Until a decade ago most solar physicists thought of a sunspot as the
  upper end of a giant flux tube floating vertically. The existence of
  umbral dots and penumbral grains has been known for several decades. On
  the basis of available observations, they seem to be regions of
  photospheric intensity with upflowing gas motion and magnetic fields
  much weaker than in the surrounding sunspot surface. It has also
  been suggested that the differences in the appearances of umbral dots
  and granular cells are caused by the highly nonlinear nature of the
  convection problem in the presense of strong magnetic fields. The main
  ideas are presented here without any equations. It can be shown that
  a pocket of field free gas surrounded by a vertical magnetic field in
  the presence of gravity takes up the shape of a tapering column ending
  at a vertex at the top. Some convection is expected to take place in
  the trapped field free gas, whereas the magnetic field around it makes
  those regions stable against convection. Eventually the apex of the
  tapering column reaches the photospheric surface where the bulging
  of the magnetic field makes the field no longer able to close on the
  field free gas and trap it underneath.

---------------------------------------------------------
Title: The Coalescence of Twisted Magnetic Flux Tubes
Authors: Choudhuri, A. R.
1987BAAS...19R.939C    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: The Influence of the Coriolis Force on Flux Tubes Rising
    through the Solar Convection Zone
Authors: Choudhuri, Arnab Rai; Gilman, Peter A.
1987ApJ...316..788C    Altcode:
  In order to study the effect of the Coriolis force due to solar
  rotation on rising magnetic flux, the authors consider a flux ring,
  azimuthally symmetric around the rotation axis, starting from rest
  at the bottom of the convection zone, and then follow the trajectory
  of the flux ring as it rises. If it is assumed that the flux ring
  remains azimuthally symmetric during its ascent, then the problem
  can be described essentially in terms of two parameters: the value
  of the initial magnetic field in the ring when it starts, and the
  effective drag experienced by it. For field strengths at the bottom
  of the convection zone of order 10,000 G or less, it is found that
  the Coriolis force plays a dominant role and flux rings starting from
  low latitudes at the bottom are deflected and emerge at latitudes
  significantly poleward of sunspot zones.

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Title: Magnetic helicity as a constraint on coronal dissipation.
Authors: Choudhuri, Arnab Rai
1986NASCP2442..451C    Altcode: 1986copp.nasa..451C
  The Taylor hypothesis has provided a model for the relaxed magnetic
  configurations of not only laboratory plasmas, but also of astrophysical
  plasmas. However, energy dissipation is possible only for systems which
  depart from a strict Taylor state, and hence a parameter describing
  that departure must be introduced, when the Taylor hypothesis is used
  to estimate the dissipation. An application of the Taylor hypothesis to
  the problem of coronal heating provides an insight into this difficult
  problem. When particular sorts of footpoint motions put energy and
  helicity in the corona, the conservation of helicity puts a constraint
  on how much of the energy can be dissipated. However, on considering
  a random distribution of footpoint motions, this constraint gets
  washed away, and the Taylor hypothesis is probably not going to play
  any significant role in the actual calculation of relevant physical
  quantities in the coronal heating problem.

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Title: Magnetic Energy Dissipation in Force-free Jets
Authors: Choudhuri, Arnab Rai; Konigl, Arieh
1986ApJ...310...96C    Altcode:
  It is shown that a magnetic-pressure-dominated, supersonic jet which
  expands (or contracts) in response to variations in the confining
  external pressure can dissipate magnetic energy through field-line
  reconnection as it relaxes to a minimum-energy configuration. In order
  for a continuous dissipation to take place, the effective reconnection
  time must be a fraction ɛ ⪉ 1 of the expansion time. The amount of
  energy dissipation is calculated, and it is concluded that magnetic
  energy dissipation could, in principle, power the observed synchrotron
  emission in extragalactic radio jets such as NGC 6251. However, this
  mechanism is only viable if the reconnection time is substantially
  shorter than the nominal resistive tearing time in the jet.

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Title: Force-free Equilibria of Magnetized Jets: Erratum
Authors: Koenigl, Arieh; Choudhuri, Arnab Rai
1986ApJ...305..954K    Altcode:
  No abstract at ADS

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Title: The Dynamics of Magnetically Trapped Fluids. I. Implications
    for Umbral Dots and Penumbral Grains
Authors: Choudhuri, A. R.
1986ApJ...302..809C    Altcode:
  A study of the magnetohydrodynamic system in which a nonmagnetized fluid
  in a gravitational field is surrounded by a fluid carrying a vertical
  magnetic field is presented. It is pointed out that this study can throw
  some light on the fine-structural features of a sunspot. The equilibrium
  configuration of the field-free fluid is a tapering column ending at an
  apex. The regions away form the apex can be studied by the slender flux
  tube approximation. A scheme developed to treat the apex indicates that,
  just below the apex, the radius of the tapering column opens up with
  a 3/2 power dependence on the depth below the apex. If the internal
  pressure of the field-free fluid is increased, the apex rises, and a
  static equilibrium may not be possible beyond a limit if the magnetic
  pressure drops quickly above a certain height. The nature of steady-flow
  solutions beyond this limit is investigated. Under conditions inside
  a sunspot, a column of field-free gas is found to rise with a velocity
  of about 100 km/hr. If umbral dots and penumbral grains are interpreted
  as regions where the field-free gas ultimately emerges, a very natural
  explanation of most of their observed properties is obtained.

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Title: The Influence of the Coriolis Force on Flux Tubes Rising
    through Solar Convection Zone
Authors: Choudhuri, A. R.; Gilman, P. A.
1986BAAS...18..703C    Altcode:
  No abstract at ADS

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Title: Force-free Equilibria of Magnetized Jets
Authors: Koenigl, A.; Choudhuri, A. R.
1985ApJ...289..173K    Altcode:
  Force-free equilibrium configurations of magnetic-pressure-dominated
  magnetized supersonic jets confined by slowly varying external pressure
  are investigated analytically. For the case where internal dissipation
  mechanisms are active, the lowest-energy field configuration is found
  to be the superposition of an axisymmetric mode and a helical mode with
  a wavelength equal to 5 times the jet radius, and the pressure below
  which the nonaxisymmetric mode becomes energetically favorable is given
  as 2700 times the product of the 4th power of the magnetic helicity
  per unit length and the -6th power of the magnetic flux. A model of
  the total and polarized emission of such a configuration is developed
  and applied to the extended well-collimated astronomically resolved
  jet NGC 6251. The model is shown to reproduce significant features
  such as transverse oscillations of the ridge line, width oscillations
  and emission knots, the projected magnetic-field configuration,
  oscillations of the degree of polarization, and the distribution of
  the Faraday rotation measure.

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Title: A model of the polarization position-angle swings in BL
    Lacertae objects.
Authors: Konigl, A.; Choudhuri, A. R.
1985ApJ...289..188K    Altcode:
  The polarization position-angle swings that have been measured in
  a number of BL Lacertae objects and highly variable quasars are
  interpreted in terms of shock waves which illuminate (by enhanced
  synchrotron radiation) successive transverse cross sections
  of a magnetized, relativistic jet. The jet is assumed to have a
  nonaxisymmetric magnetic field configuration of the type discussed in
  the companion paper on the equilibria of force-free jets. For a jet
  that is viewed at a small angle to the axis, the passage of a shock
  will give rise to an apparent rotation of the polarization position
  angle whose amplitude can be substantially larger than 180 deg. The
  effects of freely propagating shocks are compared with those of bow
  shocks which form in front of dense obstacles in the jet, and specific
  applications to 0727 - 115 and BL Lacertae are considered. In the case
  of 0727 - 115, it is pointed out that the nonuniformity of the swing
  rate and the apparent oscillations of the degree of polarization could
  be a consequence of relativistic aberration.

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Title: The dynamics of magnetically-trapped fluids: 1. Implications
    for umbral dots and penumbral grains
Authors: Choudhuri, Arnab Rai
1985PhDT.......173C    Altcode:
  No abstract at ADS

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Title: The effect of closed boundary conditions on a stationary dynamo
Authors: Choudhuri, A. R.
1984ApJ...281..846C    Altcode:
  One of two boundary conditions generally assumed in solutions
  of the dynamo equation is related to the disappearance of the
  azimuthal field at the boundary. Parker (1984) points out that
  for the realization of this condition the field must escape freely
  through the surface. Escape requires that the field be detached from
  the gas in which it is embedded. In the case of the sun, this can be
  accomplished only through reconnection in the tenuous gas above the
  visible surface. Parker concludes that the observed magnetic activity
  on the solar surface permits at most three percent of the emerging flux
  to escape. He arrives at the conclusion that, instead of B(phi) = 0,
  the partial derivative of B(phi) to r is equal to zero. The present
  investigation is concerned with the effect of changing the boundary
  condition according to Parker's conclusion. Implications for the solar
  convection zone are discussed.

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Title: The Effect of Closed Boundary Conditions on the Solar Dynamo
Authors: Choudhuri, A. R.
1983BAAS...15..994C    Altcode:
  No abstract at ADS

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Title: Nonaxisymmetric equilibria of magnetized jets.
Authors: Königl, A.; Choudhuri, A. R.
1983BAAS...15..944K    Altcode:
  No abstract at ADS

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Title: Monaxisymmetric Equilibria of Magnetized Jets
Authors: Königl, A.; Choudhuri, A. R.
1983BAAS...15Q.944K    Altcode:
  No abstract at ADS

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Title: Basic Magmatism in Guiana and Continental Drift
Authors: Choudhuri, A.; Milner, M. W.
1971NPhS..232..154C    Altcode: 1971Natur.232..154C
  IN this article we discuss some features of basic magmatism on
  the Guiana Shield that are relevant to the recent article by Le
  Bas<SUP>1</SUP>.