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
Bibcode: 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 < rc, θ)
at radii less than a radius rc, 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 > rc, 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 rc ≍
0.96R⊙, giving an estimate of the radius till which the
convective motions are affected by the solar rotation.
Title: Professor Govind Swarup's contribution to Indian science:
the recollections of a non-radio astronomer
Authors: Choudhuri, Arnab Rai
Bibcode: 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.
Title: The meridional circulation of the Sun: Observations, theory
and connections with the solar dynamo
Authors: Choudhuri, Arnab Rai
Bibcode: 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.
Title: A Theoretical Estimate of the Pole-Equator Temperature
Difference and a Possible Origin of the Near-Surface Shear Layer
Authors: Choudhuri, Arnab Rai
Bibcode: 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.
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
Bibcode: 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.
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.
Bibcode: 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.
Title: A New Formula for Predicting Solar Cycles
Authors: Hazra, Gopal; Choudhuri, Arnab Rai
Bibcode: 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.
Title: How the Saha Ionization Equation Was Discovered
Authors: Choudhuri, Arnab Rai
Bibcode: 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.
Title: Flux transport dynamo: From modelling irregularities to
making predictions
Authors: Choudhuri, Arnab Rai
Bibcode: 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.
Title: Predicting a Solar Cycle Before its Onset Using a Flux
Transport Dynamo Model
Authors: Choudhuri, Arnab Rai
Bibcode: 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.
Title: The Sun as a Laboratory for Plasma Physics
Authors: Choudhuri, Arnab Rai
Bibcode: 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).
Title: My Life and My Journey through Solar Physics
Authors: Choudhuri, Arnab Rai
Bibcode: 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.
Title: The Sun's polar magnetic field: datasets, proxies and
theoretical issues
Authors: Choudhuri, Arnab Rai
Bibcode: 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.
Title: Explaining the variation of the meridional circulation with
the solar cycle
Authors: Hazra, Gopal; Choudhuri, Arnab Rai
Bibcode: 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.
Title: A theoretical model of the variation of the meridional
circulation with the solar cycle
Authors: Hazra, Gopal; Choudhuri, Arnab Rai
Bibcode: 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.
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.
Bibcode: 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). (4
data files).
Title: Starspots, stellar cycles and stellar flares: Lessons from
solar dynamo models
Authors: Choudhuri, Arnab Rai
Bibcode: 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.
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.
Bibcode: 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.
Title: The treatment of magnetic buoyancy in flux transport dynamo
models
Authors: Choudhuri, Arnab Rai; Hazra, Gopal
Bibcode: 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.
Title: Presenting the science of the Sun to the general public
Authors: Choudhuri, Arnab Rai
Bibcode: 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/
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
Bibcode: 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.
Title: Book review: Nature's third cycle - a story of sunspots
(Choudhuri)
Authors: Meadows, P.; Choudhuri, A. R.
Bibcode: 2015JBAA..125Q.123M
Altcode:
No abstract at ADS
Title: Communicating the science of the 11-year sunspot cycle to
the general public
Authors: Choudhuri, A. R.
Bibcode: 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.
Title: A Critical Assessment of the Flux Transport Dynamo
Authors: Choudhuri, Arnab Rai
Bibcode: 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.
Title: Flux Transport Dynamos: From Kinematics to Dynamics
Authors: Karak, Bidya Binay; Jiang, Jie; Miesch, Mark S.; Charbonneau,
Paul; Choudhuri, Arnab Rai
Bibcode: 2015sac..book..561K
Altcode:
No abstract at ADS
Title: Flux Transport Dynamos: From Kinematics to Dynamics
Authors: Karak, Bidya Binay; Jiang, Jie; Miesch, Mark S.; Charbonneau,
Paul; Choudhuri, Arnab Rai
Bibcode: 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.
Title: The irregularities of the sunspot cycle and their theoretical
modelling
Authors: Choudhuri, Arnab Rai
Bibcode: 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.
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
Bibcode: 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.
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
Bibcode: 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 ⊙ 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 m 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 m. 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.
Title: Is a Deep One-cell Meridional Circulation Essential for the
Flux Transport Solar Dynamo?
Authors: Hazra, Gopal; Karak, Bidya Binay; Choudhuri, Arnab Rai
Bibcode: 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
Bibcode: 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
Bibcode: 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
Bibcode: 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
Bibcode: 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
> 10 d), using the Kepler satellite by Maehara et al. (2012, Nature,
485, 478) have revealed the existence of superflares (with energy
of 1033-1035 erg). From statistical analyses
of these superflares, it was found that superflares with energy
of 1034 erg occur once in 800 yr, and superflares with
1035 erg occur once in 5000 yr. In this paper, we examine
whether superflares with energy of 1033-1035
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 ×
1023 Mx (= G cm2) within one solar cycle period,
and lead to superflares with an energy of 1034 erg. To
store a total magnetic flux of ∼1024 Mx, necessary for
generating 1035 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 1034
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.
Bibcode: 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
Bibcode: 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
Bibcode: 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
Bibcode: 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
Bibcode: 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
Bibcode: 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
Bibcode: 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 2×1011 cm2 s−1. 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 2.8×1012 cm2 s−1
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
Bibcode: 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
Bibcode: 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
Bibcode: 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
Bibcode: 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
Bibcode: 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
Bibcode: 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
Bibcode: 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.
Bibcode: 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
Bibcode: 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
Bibcode: 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
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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
Bibcode: 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
Bibcode: 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
Bibcode: 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
Bibcode: 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
Bibcode: 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
Bibcode: 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&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
Bibcode: 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
Bibcode: 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 1012cm2s-1), 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
Bibcode: 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
Bibcode: 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
Bibcode: 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.
Bibcode: 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
Bibcode: 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.
Bibcode: 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
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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 & Konar 2002, MNRAS 332, 933; Konar & 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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
Bibcode: 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.
Bibcode: 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
Bibcode: 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
Bibcode: 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
Bibcode: 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
Bibcode: 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
Bibcode: 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
Bibcode: 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.
Bibcode: 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.
Bibcode: 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
Bibcode: 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
Bibcode: 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
Bibcode: 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 105yr, 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
Bibcode: 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.
Bibcode: 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.
Bibcode: 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
Bibcode: 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
Bibcode: 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, &
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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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&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.
Bibcode: 2000BAAS...32..807N
Altcode:
No abstract at ADS
Title: The solar dynamo
Authors: Choudhuri, Arnab Rai
Bibcode: 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
Bibcode: 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
Bibcode: 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.
Bibcode: 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.
Bibcode: 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
Bibcode: 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
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 1995JApAS..16..391D
Altcode:
No abstract at ADS
Title: The solar dynamo with meridional circulation.
Authors: Choudhuri, A. R.; Schussler, M.; Dikpati, M.
Bibcode: 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
Bibcode: 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 betweenBr
andBφ at lower latitudes.
Title: The evolution of the Sun's poloidal field.
Authors: Dikpati, M.; Choudhuri, A. R.
Bibcode: 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
Bibcode: 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.
Bibcode: 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.
Bibcode: 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−1 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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 - 104
G flux tubes with radii of a few hundred km being dragged radially by
upflows of 70 m s-1.
Title: Influence of turbulence on rising flux tubes in the solar
convection zone
Authors: Choudhuri, A. R.; D'Silva, S.
Bibcode: 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.
Bibcode: 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
Bibcode: 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.
Bibcode: 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.
Bibcode: 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
Bibcode: 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.
Bibcode: 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
Bibcode: 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
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Title: Magnetic helicity as a constraint on coronal dissipation.
Authors: Choudhuri, Arnab Rai
Bibcode: 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.
Title: Magnetic Energy Dissipation in Force-free Jets
Authors: Choudhuri, Arnab Rai; Konigl, Arieh
Bibcode: 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.
Title: Force-free Equilibria of Magnetized Jets: Erratum
Authors: Koenigl, Arieh; Choudhuri, Arnab Rai
Bibcode: 1986ApJ...305..954K
Altcode:
No abstract at ADS
Title: The Dynamics of Magnetically Trapped Fluids. I. Implications
for Umbral Dots and Penumbral Grains
Authors: Choudhuri, A. R.
Bibcode: 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.
Title: The Influence of the Coriolis Force on Flux Tubes Rising
through Solar Convection Zone
Authors: Choudhuri, A. R.; Gilman, P. A.
Bibcode: 1986BAAS...18..703C
Altcode:
No abstract at ADS
Title: Force-free Equilibria of Magnetized Jets
Authors: Koenigl, A.; Choudhuri, A. R.
Bibcode: 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.
Title: A model of the polarization position-angle swings in BL
Lacertae objects.
Authors: Konigl, A.; Choudhuri, A. R.
Bibcode: 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.
Title: The dynamics of magnetically-trapped fluids: 1. Implications
for umbral dots and penumbral grains
Authors: Choudhuri, Arnab Rai
Bibcode: 1985PhDT.......173C
Altcode:
No abstract at ADS
Title: The effect of closed boundary conditions on a stationary dynamo
Authors: Choudhuri, A. R.
Bibcode: 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.
Title: The Effect of Closed Boundary Conditions on the Solar Dynamo
Authors: Choudhuri, A. R.
Bibcode: 1983BAAS...15..994C
Altcode:
No abstract at ADS
Title: Nonaxisymmetric equilibria of magnetized jets.
Authors: Königl, A.; Choudhuri, A. R.
Bibcode: 1983BAAS...15..944K
Altcode:
No abstract at ADS
Title: Monaxisymmetric Equilibria of Magnetized Jets
Authors: Königl, A.; Choudhuri, A. R.
Bibcode: 1983BAAS...15Q.944K
Altcode:
No abstract at ADS
Title: Basic Magmatism in Guiana and Continental Drift
Authors: Choudhuri, A.; Milner, M. W.
Bibcode: 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
Bas1.