Author name code: nandy
ADS astronomy entries on 2022-09-14
author:"Nandy, Dibyendu" 
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Title: A Comparative Analysis of Machine-learning Models for Solar
Flare Forecasting: Identifying High-performing Active Region Flare
    Indicators
Authors: Sinha, Suvadip; Gupta, Om; Singh, Vishal; Lekshmi, B.;
   Nandy, Dibyendu; Mitra, Dhrubaditya; Chatterjee, Saikat; Bhattacharya,
   Sourangshu; Chatterjee, Saptarshi; Srivastava, Nandita; Brandenburg,
   Axel; Pal, Sanchita
Bibcode: 2022ApJ...935...45S
Altcode: 2022arXiv220405910S
  Solar flares create adverse space weather impacting space- and
  Earth-based technologies. However, the difficulty of forecasting
  flares, and by extension severe space weather, is accentuated by the
  lack of any unique flare trigger or a single physical pathway. Studies
  indicate that multiple physical properties contribute to active region
  flare potential, compounding the challenge. Recent developments in
  machine learning (ML) have enabled analysis of higher-dimensional data
  leading to increasingly better flare forecasting techniques. However,
  consensus on high-performing flare predictors remains elusive. In the
  most comprehensive study to date, we conduct a comparative analysis of
  four popular ML techniques (k nearest neighbors, logistic regression,
  random forest classifier, and support vector machine) by training these
  on magnetic parameters obtained from the Helioseismic and Magnetic
  Imager on board the Solar Dynamics Observatory for the entirety of solar
  cycle 24. We demonstrate that the logistic regression and support vector
  machine algorithms perform extremely well in forecasting active region
  flaring potential. The logistic regression algorithm returns the highest
  true skill score of 0.967 ± 0.018, possibly the highest classification
  performance achieved with any strictly parametric study. From a
  comparative assessment, we establish that magnetic properties like
  total current helicity, total vertical current density, total unsigned
  flux, R_VALUE, and total absolute twist are the top-performing flare
  indicators. We also introduce and analyze two new performance metrics,
  namely, severe and clear space weather indicators. Our analysis
  constrains the most successful ML algorithms and identifies physical
  parameters that contribute most to active region flare productivity.

Title: Long-term forcing of Sun's coronal field, open flux and
    cosmic ray modulation potential during grand minima, maxima and
    regular activity phases by the solar dynamo mechanism
Authors: Dash, Soumyaranjan; Nandy, Dibyendu; Usoskin, Ilya
Bibcode: 2022arXiv220812103D
Altcode:
  Magnetic fields generated in the Sun's interior by the solar dynamo
  mechanism drive solar activity over a range of time-scales. While
  space-based observations of the Sun's corona exist only for few decades,
  direct sunspot observations exist for a few centuries, solar open flux
  and cosmic ray flux variations can be reconstructed through studies of
  cosmogenic isotopes over thousands of years. While such reconstructions
  indicate the presence of extreme solar activity fluctuations in
  the past, causal links between millennia scale dynamo activity,
  consequent coronal field, solar open flux and cosmic ray modulation
  remain elusive. By utilizing a stochastically forced solar dynamo model
  we perform long-term simulations to illuminate how the dynamo generated
  magnetic fields govern the structure of the solar corona and the state
  of the heliosphere -- as indicated by variations in the open flux
  and cosmic ray modulation potential. We establish differences in the
  nature of the large-scale structuring of the solar corona during grand
  maximum, minimum, and regular solar activity phases and simulate how the
  open flux and cosmic ray modulation potential varies over time scales
  encompassing these different phases of solar activity. We demonstrate
  that the power spectrum of simulated and reconstructed solar open flux
  are consistent with each other. Our study provides the theoretical
  basis for interpreting long-term solar cycle variability based on
  reconstructions relying on cosmogenic isotopes and connects solar
  internal variations to the forcing of the state of the heliosphere.

Title: Adaptive hyperspectral imaging using structured illumination
    in a spatial light modulator-based interferometer
Authors: Chandra, Amar Deo; Karmakar, Mintu; Nandy, Dibyendu;
   Banerjee, Ayan
Bibcode: 2022OExpr..3019930C
Altcode: 2022arXiv220410587D
  We develop a novel hyperspectral imaging system using structured
  illumination in an SLM-based Michelson interferometer. In our design,
  we use a reflective SLM as a mirror in one of the arms of a Michelson
  interferometer, and scan the interferometer by varying the phase across
  the SLM display. For achieving the latter, we apply a checkerboard
  phase mask on the SLM display where the gray value varies between 0-255,
  thereby imparting a dynamic phase of up to 262° to the incident light
  beam. We couple a supercontinuum source into the interferometer in order
  to mimic an astronomical object such as the Sun, and choose a central
  wavelength of 637.4 nm akin to the strong emission line of Fe X present
  in the solar spectrum. We use a bandwidth of 30 nm, and extract fringes
  corresponding to a spectral resolution of 3.8 nm which is limited by
  the reflectivity of the SLM. We also demonstrate a maximum wavelength
  tunability of ~8 nm by varying the phase over the phase mask with a
  spectral sampling of around 0.03 nm between intermediate fringes. The
  checkerboard phase mask can be adapted close to real time on time-scales
  of a few tens of milliseconds to obtain spectral information for other
  near-contiguous wavelengths. The compactness, potential low cost, low
  power requirements, real-time tunability and lack of moving mechanical
  parts in the setup implies that it can have very useful applications in
  settings which require near real-time, multi-wavelength spectroscopic
  applications, and is especially relevant in space astronomy.

Title: A magnetic cloud prediction model for forecasting space
    weather relevant properties of Earth-directed coronal mass ejections
Authors: Pal, Sanchita; Nandy, Dibyendu; Kilpua, Emilia K J
Bibcode: 2022arXiv220305231P
Altcode:
  Coronal Mass Ejections (CMEs) are energetic storms in the Sun
  that result in the ejection of large-scale magnetic clouds
  (MCs) in interplanetary space that contain enhanced magnetic
  fields with coherently changing field direction. The severity of
  geomagnetic perturbations depends on the direction and strength of
  the interplanetary magnetic field (IMF), as well as the speed and
  duration of passage of the storm. The coupling between the heliospheric
  environment and Earth's magnetosphere is the strongest when the IMF
  direction is persistently southward for a prolonged period. Predicting
  the magnetic profile of such Earth-directed CMEs is crucial for
  estimating their geomagnetic impact. We aim to build upon and integrate
  diverse techniques towards development of a comprehensive magnetic cloud
  prediction (MCP) model that can forecast the magnetic field vectors,
  Earth-impact time, speed and duration of passage of solar storms. A
  novelty of our scheme is the ability to predict the passage duration of
  the storm without recourse to computationally intensive, time-dependent
  dynamical equations. Our methodology is validated by comparing the
  MCP model output with observations of ten MCs at 1 AU. In our sample,
  we find that eight MCs show a root mean square deviation of less than
  0.1 between predicted and observed magnetic profiles and the passage
  duration of seven MCs fall within the predicted range. Based on the
  success of this approach, we conclude that predicting the near-Earth
  properties of MCs based on analysis and modelling of near-Sun CME
  observations is a viable endeavor with potential benefits for space
  weather assessment.

Title: Simulating Magnetic Switchback Structures in the near-Sun
    Solar Wind Observed by Parker Probe
Authors: Roddanavar, Arpita; Nandy, Dibyendu
Bibcode: 2021AGUFMSH15C2049R
Altcode:
  The Suns atmosphere, the Corona drives solar plasma winds in to the
  interplanetary medium that consist of charged particles embedded with
  magnetic fields. The solar wind assumes the shape of a spiral moving
  radially outwards from the Sun which is known as the Parker Spiral. The
  Parker Solar Probe Mission has observed that the magnetic fields in
  the near-Sun solar wind undergo magnetic polarity inversions. These
  deviations are termed magnetic switchbacks. Magnetic switchbacks are
  also accompanied by variations in the solar wind plasma parameters. We
  present proof of concept simulations to test a hypothesis for the
  origin of magnetic switchback structures based on vortical turbulence
  in the solar wind and present our preliminary results here.

Title: The Roles of Intrinsic and Imposed Magnetospheres in Shielding
    Planetary Atmospheres
Authors: Basak, Arnab; Nandy, Dibyendu
Bibcode: 2021AGUFMSM55C1792B
Altcode:
  We present results of three-dimensional compressible magnetohydrodynamic
  (MHD) simulations of the interactions between stellar wind and
  planets with different magnetospheric strengths, using the Star-Planet
  Interaction Module (CESSI-SPIM) developed at CESSI, IISER Kolkata. A
  gravitationally stratified planetary atmosphere is considered which is
  in hydrostatic equilibrium with the ambient medium, while the stellar
  wind follows magnetized shock conditions. The module is benchmarked
  using the observations from specific spacecraft orbits of Mars Global
  Surveyor (MGS) and Mars Atmosphere and Volatile EvolutioN (MAVEN)
  missions. The magnetopause stand-off distance is found to be the
  greatest for a strong intrinsic magnetosphere and the least for an
  imposed magnetosphere, implying that the stellar wind penetration is the
  maximum for a non-magnetized planet, resulting in higher atmospheric
  loss. This indicates that although the imposed magnetosphere protects
  the planetary atmosphere from the stellar wind, its level of shielding
  is not as effective as that of intrinsic magnetosphere. The study is
  important from the perspective of planetary habitability in solar and
  exoplanetary systems. [Basak &amp; Nandy, MNRAS 502, 3569 (2021)]

Title: Understanding the Global Coronal Magnetic Fields using
    Data-constrained Magnetohydrodynamic Model
Authors: Dash, Soumyaranjan; Nandy, Dibyendu; Vaidya, Bhargav
Bibcode: 2021AGUFMSH15G2085D
Altcode:
  Coronal magnetic field evolution modulates our space environment
  via coronal mass ejections, flares, and changes in solar wind
  conditions. However, routine observations of magnetic field in
  the optically thin solar corona are not yet well-developed. Hence
  understanding the coronal magnetic field distribution using
  data-constrained global magnetohydrodynamic (MHD) models is of paramount
  importance nowadays. There are several factors which can drive the
  evolution e.g. flux emergence, photospheric flows, stratification in
  thermodynamic variables, solar wind conditions. Simulations using MHD
  models will help us generate the global magnetic field distribution,
  which can be constrained using total solar eclipse observations. Apart
  from this, such models have the potential to generate polarization
  characteristics utilizing the model output, which will help in better
  interpretation of data from future solar missions like PUNCH. We discuss
  the magnetic field distribution and polarization characteristics
  for past solar eclipses based on MHD simulations for global solar
  corona using a variety of approaches based on data-driven surface
  flux transport models, potential field source surface models and full
  MHD models.

Title: Subsurface Plasma Flows and the Flare Productivity of Solar
    Active Regions
Authors: Biji, Lekshmi; Jain, Kiran; Komm, Rudolf; Nandy, Dibyendu
Bibcode: 2021AGUFMSH54A..07B
Altcode:
  Highly energetic solar events such as solar flares and Coronal
  Mass Ejections (CMEs) can lead to extreme space weather. Hence, it
  is essential to understand their physical drivers and explore what
  governs their occurrence and intensity. By using the near-surface
  velocities derived by the ring-diagram analysis of active region
  patches using Global Oscillation Network Group (GONG) Doppler
  velocity measurements, we seek to explore the connection between
  subsurface flow properties and solar flares. The temporal evolution
  of vorticity and kinetic helicity of flaring and non-flaring active
  regions is investigated. The integrated vorticity, kinetic and current
  helicities, and magnetic flux one day prior to the flare are observed
  to be correlated with the integrated flare intensity. We show that
  active regions with strong subsurface vorticity and kinetic helicity
  tend to generate high intensity flares. We hypothesize that this is
  achieved via energy injection into subsurface magnetic flux systems
  by helical plasmas flows.

Title: Estimating magnetospheric currents and geoeffectiveness of
    interplanetary CMEs with magnetohydrodynamic simulations
Authors: ROY, Souvik; Nandy, Dibyendu
Bibcode: 2021AGUFMSM41A..08R
Altcode:
  The high energetic plasma and the embedded magnetic field of coronal
  mass ejections interact with planetary magnetospheres giving rise
  to transient perturbations such as geomagnetic storms. Predicting
  the geomagnetic impact of such interplanetary coronal mass ejections
  (ICME) is of utmost importance for the protection of our technological
  infrastructure that is affected by space weather. We use 3D compressible
  magnetohydrodynamic simulation of a star-planet system to model and
  study an ICME-Earth interaction event of 20th November 2003. In the
  modelled interaction, we observe a change in magnetopause shape and
  stand-off distance on ICME impact, day and night side reconnections and
  induction of high currents in the magnetosphere. We also notice the
  formation of a ring of strong equatorial current around the Earth,
  leading to a reduction of the geomagnetic field. We calculate the
  simulated reduction in the magnetic field and compare that to the
  observed geomagnetic indices in order to establish a predictive approach
  for geomagnetic storms. These simulations are expected to illuminate
  the physical processes that result in space weather impacts of stellar
  magnetic storms in planetary and exoplanetary systems.

Title: Impact of Anomalous Active Regions on the Large Scale Magnetic
    Fields of the Solar Cycle
Authors: Pal, Shaonwita; Nandy, Dibyendu; Bhowmik, Prantika; Dash,
   Soumyaranjan; Mahajan, Sushant; Munoz-Jaramillo, Andres
Bibcode: 2021AGUFMSH55D1878P
Altcode:
  Emergence of anomalous bipolar magnetic regions (combinations of
  anti-hale and anti-joy regions) on the solar surface can influence cycle
  to cycle variability and irregularities. We perform a comprehensive
  analysis of the dipole moment and polar field build up due to
  the appearance of anomalous active regions on the solar surface
  using a solar surface flux transport model. Our aim is to study the
  differences and the similarities between these anomalous regions and
  their effect in global solar cycle dynamics. Although these regions
  appear in small numbers, if they carry significant flux, they are
  found to significantly impact the polar field strength and thereby,
  the amplitude of future cycles.

Title: Solar evolution and extrema: current state of understanding
    of long-term solar variability and its planetary impacts
Authors: Nandy, Dibyendu; Martens, Petrus C. H.; Obridko, Vladimir;
   Dash, Soumyaranjan; Georgieva, Katya
Bibcode: 2021PEPS....8...40N
Altcode:
  The activity of stars such as the Sun varies over timescales
  ranging from the very short to the very long—stellar and planetary
  evolutionary timescales. Experience from our solar system indicates that
  short-term, transient events such as stellar flares and coronal mass
  ejections create hazardous space environmental conditions that impact
  Earth-orbiting satellites and planetary atmospheres. Extreme events
  such as stellar superflares may play a role in atmospheric mass loss
  and create conditions unsuitable for life. Slower, long-term evolutions
  of the activity of Sun-like stars over millennia to billions of years
  result in variations in stellar wind properties, radiation flux, cosmic
  ray flux, and frequency of magnetic storms. This coupled evolution of
  star-planet systems eventually determines planetary and exoplanetary
  habitability. The Solar Evolution and Extrema (SEE) initiative of the
  Variability of the Sun and Its Terrestrial Impact (VarSITI) program of
  the Scientific Committee on Solar-Terrestrial Physics (SCOSTEP) aimed
  to facilitate and build capacity in this interdisciplinary subject
  of broad interest in astronomy and astrophysics. In this review, we
  highlight progress in the major themes that were the focus of this
  interdisciplinary program, namely, reconstructing and understanding
  past solar activity including grand minima and maxima, facilitating
  physical dynamo-model-based predictions of future solar activity,
  understanding the evolution of solar activity over Earth's history
  including the faint young Sun paradox, and exploring solar-stellar
  connections with the goal of illuminating the extreme range of activity
  that our parent star—the Sun—may have displayed in the past,
  or may be capable of unleashing in the future.

Title: Solar Cycle Evolution of Filaments over a Century:
    Investigations with the Meudon and McIntosh Hand-drawn Archives
Authors: Mazumder, Rakesh; Chatterjee, Subhamoy; Nandy, Dibyendu;
   Banerjee, Dipankar
Bibcode: 2021ApJ...919..125M
Altcode: 2021arXiv210604320M
  Hand-drawn synoptic maps from the Meudon Observatory (1919 onwards)
  and the McIntosh archive (1967 onwards) are two important sources of
  long-term, manually recorded filament observations. In this study,
  we calibrate the Meudon maps and subsequently identify filaments
  through an automated method. We extract physical parameters from this
  filament database and perform a comparative study of their long-term
  evolution focusing on the cotemporal period of the McIntosh and
  Meudon observations. The spatiotemporal evolution of filaments
  manifests in the form of a filament butterfly diagram, further
  indicating that they are intimately related to the large-scale solar
  cycle. Physical descriptors such as the number and length of filaments,
  which are tracers of the solar surface magnetic field, have cycles
  which are phase locked with the ~11 yr sunspot cycle. The tilt-angle
  distribution of filaments-both near to or distant from active region
  locations-indicates that their origin is due to either large-scale
  surface magnetic field or inter-active-region field evolution. This
  study paves the way for constructing a composite series of hand-drawn
  filament data with minimal gaps stretching over the time span of solar
  filament observations up to a century. On the one hand, this would
  serve as a useful constraint for models of magnetic field emergence
  and evolution on the Sun's surface over multiple solar cycles, and
  on the other hand, this filament database may be used to guide the
  reconstruction of filament/prominence associated eruptive events before
  the space age.

Title: Stellar mid-life crisis: subcritical magnetic dynamos of
    solar-like stars and the breakdown of gyrochronology
Authors: Tripathi, Bindesh; Nandy, Dibyendu; Banerjee, Soumitro
Bibcode: 2021MNRAS.506L..50T
Altcode: 2018arXiv181205533T
  Recent observations have revealed the surprising breakdown of
  stellar gyrochronology relations at about the age of the Sun hinting
  the middle-aged, solar-like stars transition to a magnetically
  inactive future. We provide a theoretical basis for these intriguing
  observations inspired by simulations with a mathematical-dynamo model
  that can explore long-term solar cycle fluctuations. We reproduce
  the observed bimodal distribution of sunspot numbers, but only for
  subcritical dynamos. Based on a bifurcation analysis, we argue that the
  ageing of solar-like stars makes the magnetically weak dynamo regime
  readily accessible. Weak magnetic field production in this regime
  compromises wind-driven angular momentum losses, thus disrupting the
  hegemony of magnetic braking on stellar rotational spin-down. This
  hypothesis of subcritical magnetic dynamos of solar-like stars
  provides a self-consistent, unifying physical basis for a diversity
  of solar-stellar phenomena such as why stars beyond their mid-life
  do not spin-down as fast as in their youth, the break-down of stellar
  gyrochronology relations, the observed bimodal distribution of long-term
  sunspot observations, and recent findings suggesting that the Sun may
  be transitioning to a magnetically inactive future.

Title: Modelling the Impact of Magnetic Storms on Planetary
    Environments
Authors: Roy, Souvik; Nandy, Dibyendu
Bibcode: 2021EGUGA..23.8863R
Altcode:
  Coronal mass ejections (CMEs), large scale transient eruptions observed
  in the Sun, are thought to also be spawned by other magnetically
  active stars. The magnetic flux ropes intrinsic to these storms, and
  associated high-speed plasma ejecta perturb planetary environments
  creating hazardous conditions. To understand the physics of CME
  impact and consequent perturbations in planetary environments, we
  use 3D compressible magnetohydrodynamic simulation of a star-planet
  module (CESSI-SPIM) developed at CESSI, IISER Kolkata based on the
  PLUTO code architecture. We explore magnetohydrodynamic processes
  such as the formation of a bow-shock, magnetopause, magnetotail,
  planet-bound current sheets and atmospheric mass loss as a consequence
  of magnetic-storm-planetary interactions. Specifically, we utilize
  a realistic, twisted flux rope model for our CME, which leads
  to interesting dynamics related to helicity injection into the
  magnetosphere. Such studies will help us understand how energetic
  magnetic storms from host stars impact magnetospheres and atmospheres
  with implications for planetary and exoplanetary habitability.

Title: Modelling the imposed magnetospheres of Mars-like exoplanets:
    star-planet interactions and atmospheric losses
Authors: Basak, Arnab; Nandy, Dibyendu
Bibcode: 2021MNRAS.502.3569B
Altcode: 2021MNRAS.tmp..252B
  Based on 3D compressible magnetohydrodynamic simulations, we explore
  the interactions between the magnetized wind from a solar-like star
  and a Mars-like planet - with a gravitionally stratified atmosphere
  - that is either non-magnetized or hosts a weak intrinsic dipolar
  field. The primary mechanism for the induction of a magnetosphere around
  a non-magnetized conducting planet is the pile-up of stellar magnetic
  fields in the day-side region. The magnetopause stand-off distance
  decreases as the strength of the planetary dipole field is lowered and
  saturates to a minimum value for the case of a planet with no magnetic
  field. Global features such as bow shock, magnetosheath, magnetotail,
  and strong current sheets are observed in the imposed magnetosphere. We
  explore variations in atmospheric mass loss rates for different stellar
  wind strengths to understand the impact of stellar magnetic activity and
  plasma winds - and their evolution - on (exo)planetary habitability. In
  order to simulate a case analogous to the present-day Mars, a planet
  without atmosphere is considered. Our simulations are found to be
  in good agreement with observational data from Mars Global Surveyor
  and Mars Atmosphere and Volatile EvolutioN missions and is expected
  to complement observations from the Emirates (Hope) Mars Mission,
  China's Tianwen-1 and NASA's Mars 2020 Perseverance mission.

Title: Modelling the solar wind forced Martian environment
Authors: Basak, Arnab; Nandy, Dibyendu
Bibcode: 2021csss.confE...2B
Altcode:
  Since the halting of the Martian dynamo, the constant bombardment
  of solar wind particles on the planet led to the erosion of a
  significant portion of its atmosphere due to the lack of proper
  magnetic shielding. Several missions devoted to the exploration of the
  "red planet", such as Phobos, Mars Global Surveyor, Mars Atmosphere
  and Volatile Evolution, InSight, etc., provide vital information
  about the Martian environment which is important from the aspect
  of planetary habitability. We present results of three dimensional
  compressible magnetohydrodynamic simulations of solar wind and Mars
  interaction, using the Star Planet Interaction Module (CESSI-SPIM)
  developed at CESSI, IISER Kolkata. We elucidate mechanisms that lead
  to the formation of an imposed magnetosphere around Mars and provide
  a theoretical viewpoint on the interaction of stellar winds with
  non-magnetized planets with/without atmospheres. The results are found
  to be in agreement with observational data from various missions. The
  above study is not only relevant for planets and moons in our solar
  system but also provides important insight for the exploration of
  habitable planets in extrasolar systems.

Title: Progress in Solar Cycle Predictions: Sunspot Cycles 24-25
    in Perspective
Authors: Nandy, Dibyendu
Bibcode: 2021SoPh..296...54N
Altcode: 2020arXiv200901908N
  The dynamic activity of the Sun—sustained by a magnetohydrodynamic
  dynamo mechanism working in its interior—modulates the
  electromagnetic, particulate, and radiative environment in space. While
  solar activity variations on short timescale create space weather,
  slow long-term modulation forms the basis of space climate. Space
  weather impacts diverse space-reliant technologies while space
  climate influences planetary atmospheres and climate. Having prior
  knowledge of the Sun's activity is important in these contexts. However,
  forecasting solar-stellar magnetic activity has remained an outstanding
  challenge. In this review, predictions for Sunspot Cycle 24 and the
  upcoming Solar Cycle 25 are summarized, and critically assessed. The
  analysis demonstrates that while predictions based on diverse techniques
  disagree across Solar Cycles 24-25, physics-based predictions for Solar
  Cycle 25 have converged and indicates a weak to moderate-weak sunspot
  cycle. I argue that this convergence in physics-based predictions is
  indicative of progress in the fundamental understanding of solar cycle
  predictability. Based on this understanding, resolutions to several
  outstanding questions related to solar cycle predictions are discussed;
  these questions include: is it possible to predict the solar cycle, what
  is the best proxy for predictions, how early can we predict the solar
  cycle and how many cycles into the future can we predict relying on our
  current understanding? Based on our analysis, we also suggest a rigorous
  pathway towards generating and disseminating a "consensus forecast"
  by any solar cycle prediction panels tasked with such a challenge.

Title: Magnetohydrodynamical Understanding of the Interactions
    Between Coronal Mass Ejections and Earth's Magnetosphere.
Authors: ROY, S.; Nandy, D.
Bibcode: 2020AGUFMSM0510004R
Altcode:
  Coronal mass ejections (CMEs), the large scale transient eruptions from
  the Sun, interact with the Earth's magnetosphere while travelling into
  the heliosphere. The energetic interplanetary CME (ICME) at 1AU not only
  creates geomagnetic storms and disrupts the magnetic field structure
  around the Earth but also impacts the plasma environment, causes
  strong aurorae, and disturbs the radio and electrical transmission
  massively. We use 3D compressible magnetohydrodynamic simulation of
  a star-planet system and study the interesting magnetohydrodynamic
  processes like bow-shock, magnetopause, magnetotail, planet-bound
  current sheets, magnetic reconnections, atmospheric mass loss as
  well as particle injection, etc., when an ICME flux rope crosses
  the Earth at 1 AU. We use the uniformly twisted force-free flux rope
  model proposed by Gold and Hoyle in 1960 to initiate the ICME and vary
  the flux rope properties using actual observational data. We observe
  a change in magnetopause's shape and the stand-off distance to the
  magnetopause. We notice twist helicity injection inside the magnetotail
  current system. We discover comparative increment in both the rates
  of atmospheric mass out-flow and solar wind in-flow in the vicinity
  of Earth during the geo-storm. Such studies will help us understand
  how energetic magnetic storms from a host star impact planetary
  magnetospheres and atmospheres with implications for planetary and
  exoplanetary habitability.

Title: Flux Erosion of Magnetic Clouds by Reconnection With the
    Sun's Open Flux
Authors: Pal, S.; Dash, S.; Nandy, D.
Bibcode: 2020AGUFMSH041..07P
Altcode:
  Magnetic clouds (MCs) are flux rope magnetic structures forming
  a subset of solar coronal mass ejections, which have significant
  space weather impacts. The geoeffectiveness of MCs depends on their
  properties, which evolve during their interplanetary passage. Based
  on an analysis of observations spanning two solar cycles, we establish
  that MCs interacting with the ambient solar wind magnetic field (i.e.,
  heliospheric open flux) lose a substantial amount of their initial
  magnetic flux via magnetic reconnection, which, in some cases, reduce
  their geoeffectiveness. We find a linear correlation between the eroded
  flux of MCs and solar open flux which is consistent with the scenario
  that MC erosion is mediated via the local heliospheric magnetic field
  draping around an MC during its interplanetary propagation. The solar
  open flux is governed by the sunspot cycle. This work therefore uncovers
  a hitherto unknown pathway for solar cycle modulation of the properties
  of MCs.

Title: COSPAR International Space Weather Action Teams: Addressing
    Challenges Across the Field of Space Weather.
Authors: Kuznetsova, M. M.; Belehaki, A.; Bisi, M. M.; Bruinsma, S.;
   Fung, S. F.; Glover, A.; Grande, M.; Guo, J.; Jun, I.; Linker, J.;
   Mann, I. R.; Masson, A.; Mendoza, A. M. M.; Murray, S. A.; Nandy, D.;
   Opgenoorth, H. J.; Pevtsov, A. A.; Plainaki, C.; Reiss, M.; Sutton,
   E. K.; Temmer, M.; Usoskin, I. G.; Yao, Z.; Yardley, S.; Zheng, Y.
Bibcode: 2020AGUFMSH0030022K
Altcode:
  Advanced predictions of space weather impacts require improved
  understanding and modeling capabilities of coupled chains of space
  environment processes. It is necessary to assemble parts of the
  source-to-impact puzzle by identifying, addressing and solving
  problems focused on specific physical domains, and then to connect
  all validated solutions from space weather origins on the sun to
  impacts on coupled geospace system, humans and technologies. To
  address the need for multi-disciplinary international space weather
  research community connecting experts in space weather phenomena
  across all domains and experts in space environment impact,
  the COSPAR Panel on Space Weather facilitated establishment of
  a network of International Space Weather Action Teams (ISWAT, <A
  href="https://www.iswat-cospar.org">https://www.iswat-cospar.org</A>,
  @IswatCosparOrg). ISWAT serves as a global hub for community coordinated
  topical collaborations focused on different aspects of space weather
  including advancing understanding, assessment and improvement of
  modeling capabilities, transitioning advances in research to operations,
  optimized utilization of available observations, and generating inputs
  to future instrumentation deployment. Action teams are building
  blocks of ISWAT initiative. ISWAT action teams are organized into
  domain-based ISWAT clusters. Action teams are working in coordinated
  effort across physical domain and across borders. The primary ISWAT
  goal is to advance space weather predictive capabilities based on best
  science available. The ISWAT currently includes more than 250 active
  participants and more than 50 action teams. The presentation will
  overview the outcome from the COSPAR ISWAT Inaugural Working Meeting
  in February 2020, highlight recent progress in advancing physics-based
  predictive capabilities and discuss plans for transforming COSPAR space
  weather Roadmap into a living document maintained by the community.

Title: A Physics-based Prediction of Solar Cycle 25 and Reconstruction
    of Century-Scale Solar Activity
Authors: Nandy, D.; Bhowmik, P.
Bibcode: 2020AGUFMSH053..06N
Altcode:
  The activity of the Sun generates space weather and forces the
  atmosphere and climate of planets. The basis of this dynamic solar
  activity is its magnetic cycle, which is manifest in variations of
  sunspots observed on the solar surface. These sunspots are generated
  by complex interactions of plasma flows and magnetic fields in the
  solar interior via the magnetohydrodynamic dynamo mechanism. While
  predictions of the future state of the solar cycle, including its
  strength and timing are critical to space weather awareness and
  mitigation strategies, making accurate physical model based predictions
  remain a challenging task. In this talk, I shall discuss the advances
  made in understanding the physics of solar cycle predictions in the
  last decade. I shall also discuss a prediction of the strength and
  timing of solar cycle 25 which is based on a methodology of coupling a
  surface flux transport model to a solar dynamo model; this methodology
  explains well the last century of solar activity and provides new
  insights in to the inner workings of the solar dynamo mechanism.

Title: Dynamics of the Sun's Polar Field: Possible Insights from
    Out of the Ecliptic Observations
Authors: Dash, S.; Nandy, D.; Pal, S.
Bibcode: 2020AGUFMSH014..06D
Altcode:
  The solar polar fields play a crucial role in sustaining the solar
  dynamo mechanism. Observations and computational modelling indicates
  that the surface dynamics of active regions mediated via plasma flows,
  which leads to the polar field build-up, is a fundamental aspect of the
  solar dynamo mechanism; in fact, this surface mediated flux dynamics
  also known as the Babcock-Leighton mechanism, is now implicated to
  be the leading contributor to the Sun's polar field, and thus also
  provides a window to the future solar cycle. This emergent understanding
  is one of the major drivers of novel mission concepts aiming to image
  the solar high latitudes from out of the ecliptic locations. Based on
  numerical simulations of the near-surface magnetic field dynamics and
  the Sun's polar landscape, here we discuss what new insights may be
  provided by out-of-the ecliptic solar physics missions.

Title: Metis - Solar Orbiter Topical Team on "Modelling of CME
    propagation/evolution in corona and solar wind in connection with
    Space Weather"
Authors: Bemporad, A.; Banerjee, D.; Berlicki, A.; Biondo, R.; Boe,
   B.; Calchetti, D.; Capuano, G.; De Leo, Y.; Del Moro, D.; Feng, L.;
   Foldes, R.; Frassati, F.; Frazin, R. A.; Giovannelli, L.; Giunta,
   A. S.; Heinzel, P.; Ippolito, A.; Janvier, M.; Jerse, G.; Kilpua,
   K. E. J.; Laurenza, M.; Lloveras, D.; Magdalenic, J.; Mancuso, S.;
   Messerotti, M.; Mierla, M.; Nandy, D.; Napoletano, G.; Nuevo, F.;
   Pagano, P.; Pinto, R.; Plainaki, C.; Reale, F.; Romoli, M.; Rodriguez,
   L.; Slemer, A.; Spadaro, D.; Susino, R.; Stangalini, M.; Vainio,
   R. O.; Valori, G.; Vásquez, A. M.; West, M. J.
Bibcode: 2020AGUFMSH0360027B
Altcode:
  Despite the current availability of multi-spacecraft observations of
  Coronal Mass Ejections (CMEs) and their interplanetary counterpart
  (ICMEs), at present we still don't understand which physical phenomena
  are driving their expansion and propagation phases. This also limits
  our understanding on how CMEs (observed with remote sensing data)
  become ICMEs (observed in situ), how they interact with the background
  solar wind, and how their final geo-effectiveness can be modified
  during their interplanetary evolution. Such problems match some of
  the scientific objectives of the Solar Orbiter Science Activity Plan
  and of the Metis coronagraph. Thanks to its multi-channel capability,
  Metis (acquiring images in the visible light and at the same time in
  the UV HI Lyman-alpha emission) will really provide an unprecedented
  view of CMEs and in particular of their thermodynamic evolution. At
  closest approaches to the Sun (in the nominal mission), Metis will
  acquire high spatial resolution and/or temporal cadence multi-channel
  images of CMEs. Farther from the Sun, Metis will shed light on the
  early Interplanetary propagation of CMEs. Later on (in the extended
  mission) Metis will observe for the first time the CME/ICME propagation
  out-of-ecliptic. These novelties will be combined with the unique
  vantage point that will be offered by the Solar Orbiter spacecraft,
  and supported with valuable data acquired by other on-board remote
  sensing (e.g. SPICE, EUI, SoloHI) and in situ (e.g. EPD, MAG,
  SWA, RPW) instruments. In this contribution we present the ongoing
  activities of the Metis Topical Team on "CME/ICME propagation", (<A
  href="http://metis.oato.inaf.it/topical_teams.html">http://metis.oato.inaf.it/topical_teams.html</A>),
  an international working group recently established and gathering
  scientists from different countries, experts of both in-situ and remote
  sensing observations, as well as numerical simulations, and we summarize
  the main science objectives discussed during the last months.

Title: Solar Cycle Variation of Meridional Flow
Authors: Biji, L.; Nandy, D.
Bibcode: 2020AGUFMSH0020002B
Altcode:
  The solar meridional flow plays a crucial role in driving the solar
  dynamo. We study the near-surface hemispherical asymmetries in the
  meridional flow over 18 years using the velocities obtained from the
  Global Oscillation Network Group (GONG) and Helioseismic and Magnetic
  Imager (HMI) ring-diagram pipelines. Our analysis shows that the
  hemispherical asymmetry in the flow is strongly related to the solar
  cycle asymmetry. We observe a time-delayed anti-correlation with the
  asymmetry in meridional flow preceding the asymmetry in the sunspot
  cycle. We investigate the relationship between sunspot tilt angles and
  plasma inflows, and uncover the first observational evidence of the
  hypothesized non-linear quenching in the Babcock Leighton poloidal field
  generation mechanism mediated by surface plasma flows. Our observations
  illuminate feedback processes between magnetic fields and plasma flows
  and set new constraints on magneto-hydrodynamic dynamo models of the
  solar cycle.

Title: Magnetohydrodynamic Simulations of the Solar Wind Interaction
    with Mars
Authors: Basak, A.; Nandy, D.
Bibcode: 2020AGUFMSM0530004B
Altcode:
  Present day Mars is known to be a planet devoid of any intrinsic global
  dipolar field due to the absence of a "once active" dynamo process
  that existed billions of years ago. The lack of proper magnetic
  shielding led to the erosion of the Martian atmosphere mediated by
  solar wind interactions, thereby impacting the habitability of the
  planet. Since the dawn of space age, there have been numerous missions
  devoted to the exploration of the Martian environment to understand
  its evolution and current conditions. In the present study, we carry
  out three dimensional compressible magnetohydrodynamic simulations
  of the interactions between solar wind and a Mars-like planet using
  the Star Planet Interaction Module (CESSI-SPIM) developed at CESSI,
  IISER Kolkata. We ignore the presence of remnant crustal fields and take
  precautions for not allowing any undesirable numerical outflows from the
  planet by using gravitational stratification. The mechanism of formation
  of an imposed magnetosphere around the planet is presented in detail
  with emphasis on magnetic reconnections that lead to the same. The
  pile up of stellar magnetic field on the day side of the planet, which
  is responsible for the induction of magnetosphere, may be inhibited
  if the planet hosts even a weak dipolar field. The structures of the
  bow shock, magnetotail and magnetic pile up region are found to be in
  good agreement with observational data from missions such as Phobos,
  Mars Global Surveyor (MGS) and Mars Atmosphere and Volatile Evolution
  (MAVEN). The results presented are not only applicable to planets and
  moons in our solar system but also provide important insight for the
  exploration of habitable planets in far out exoplanetary systems.

Title: Does the mean-field α effect have any impact on the memory
    of the solar cycle?
Authors: Hazra, Soumitra; Brun, Allan Sacha; Nandy, Dibyendu
Bibcode: 2020A&A...642A..51H
Altcode: 2020arXiv200302776H
  Context. Predictions of solar cycle 24 obtained from advection-dominated
  and diffusion-dominated kinematic dynamo models are different if
  the Babcock-Leighton mechanism is the only source of the poloidal
  field. Some previous studies argue that the discrepancy arises
  due to different memories of the solar dynamo for advection- and
  diffusion-dominated solar convection zones. <BR /> Aims: We aim
  to investigate the differences in solar cycle memory obtained from
  advection-dominated and diffusion-dominated kinematic solar dynamo
  models. Specifically, we explore whether inclusion of Parker's
  mean-field α effect, in addition to the Babcock-Leighton mechanism,
  has any impact on the memory of the solar cycle. <BR /> Methods: We
  used a kinematic flux transport solar dynamo model where poloidal
  field generation takes place due to both the Babcock-Leighton
  mechanism and the mean-field α effect. We additionally considered
  stochastic fluctuations in this model and explored cycle-to-cycle
  correlations between the polar field at minima and toroidal field
  at cycle maxima. <BR /> Results: Solar dynamo memory is always
  limited to only one cycle in diffusion-dominated dynamo regimes
  while in advection-dominated regimes the memory is distributed
  over a few solar cycles. However, the addition of a mean-field α
  effect reduces the memory of the solar dynamo to within one cycle in
  the advection-dominated dynamo regime when there are no fluctuations
  in the mean-field α effect. When fluctuations are introduced in the
  mean-field poloidal source a more complex scenario is evident, with very
  weak but significant correlations emerging across a few cycles. <BR
  /> Conclusions: Our results imply that inclusion of a mean-field α
  effect in the framework of a flux transport Babcock-Leighton dynamo
  model leads to additional complexities that may impact memory and
  predictability of predictive dynamo models of the solar cycle.

Title: Prediction of Sunspot Cycle 25: Based on a Century-scale
    Data-driven Magnetic Field Simulations
Authors: Bhowmik, P.; Nandy, D.
Bibcode: 2020SPD....5120902B
Altcode:
  Solar variability governs the electromagnetic, radiative, and
  particulate environment in the heliosphere creating hazardous weather
  in space through eruptive events such as solar flares, coronal mass
  ejections. Moreover, modulation in solar output in terms of solar
  irradiance defines space climate. Both the short and long-term solar
  variabilities are closely associated with and mostly dominated by
  the sunspot cycle. Thus, in the context of space weather studies,
  predicting the sunspot cycle has gained a significant impetus in recent
  times. However, scientific studies have shown that the intrinsic
  stochastic nature of the solar convection zone limits the range of
  predictability to half a solar cycle, and, the dipolar field during
  the cycle minimum is one of the best precursors for sunspot cycle
  prediction. In comparison, we have devised a methodology by combining an
  observational data-driven surface flux transport model and an interior
  dynamo model to extend the prediction time window to decadal scale. This
  new methodology has been validated by performing a century-scale
  data-driven simulation which reproduced the past observation quite
  successfully — the first of its kind. Subsequently, we employ this
  technique for predicting sunspot cycle 25 while considering various
  possible uncertainties. Our ensemble forecast indicates cycle 25 would
  be similar or slightly stronger than the current cycle and peak around
  2024. Sunspot cycle 25 may thus reverse the persistent weakening
  trend in solar activity which has led to speculation of an imminent
  Maunder-like grand minimum and cooling of global climate.

Title: A model-free, data-based forecast for sunspot cycle 25
Authors: Espuña-Fontcuberta, Aleix; Chatterjee, Saikat; Mitra,
   Dhrubaditya; Nandy, Dibyendu
Bibcode: 2020arXiv200512166E
Altcode:
  The dynamic activity of the Sun, governed by its cycle of sunspots
  -- strongly magnetized regions that are observed on its surface
  -- modulate our solar system space environment creating space
  weather. Severe space weather leads to disruptions in satellite
  operations, telecommunications, electric power grids and air-traffic on
  polar routes. Forecasting the cycle of sunspots, however, has remained
  a challenging problem. We use reservoir computing -- a model-free,
  neural--network based machine-learning technique -- to forecast the
  upcoming solar cycle, sunspot cycle 25. The standard algorithm forecasts
  that solar cycle 25 is going to last about ten years, the maxima is
  going to appear in the year 2024 and the maximum number of sunspots
  is going to be 113 ($\pm15$). We also develop a novel variation of the
  standard algorithm whose forecasts for duration and peak timing matches
  that of the standard algorithm, but whose peak amplitude forecast is 124
  ($\pm2$) -- within the upper bound of the standard reservoir computing
  algorithm. We conclude that sunspot cycle 25 is likely to be a weak,
  lower than average solar cycle, somewhat similar in strength to sunspot
  cycle 24.

Title: Flux Erosion of Magnetic Clouds by Reconnection With the
    Sun's Open Flux
Authors: Pal, Sanchita; Dash, Soumyaranjan; Nandy, Dibyendu
Bibcode: 2020GeoRL..4786372P
Altcode: 2021arXiv210305990P
  Magnetic clouds (MCs) are flux rope magnetic structures forming
  a subset of solar coronal mass ejections, which have significant
  space weather impacts. The geoeffectiveness of MCs depends on their
  properties, which evolve during their interplanetary passage. Based
  on an analysis of observations spanning two solar cycles, we establish
  that MCs interacting with the ambient solar wind magnetic field (i.e.,
  heliospheric open flux) lose a substantial amount of their initial
  magnetic flux via magnetic reconnection, which, in some cases, reduce
  their geoeffectiveness. We find a linear correlation between the eroded
  flux of MCs and solar open flux which is consistent with the scenario
  that MC erosion is mediated via the local heliospheric magnetic field
  draping around an MC during its interplanetary propagation. The solar
  open flux is governed by the sunspot cycle. This work therefore uncovers
  a hitherto unknown pathway for solar cycle modulation of the properties
  of MCs.

Title: Sunspot Cycle 25 is Brewing: Early Signs Herald its Onset
Authors: Nandy, Dibyendu; Bhatnagar, Aditi; Pal, Sanchita
Bibcode: 2020RNAAS...4...30N
Altcode:
  No abstract at ADS

Title: Prediction of the Sun's Coronal Magnetic Field and
    Forward-modeled Polarization Characteristics for the 2019 July 2
    Total Solar Eclipse
Authors: Dash, Soumyaranjan; Bhowmik, Prantika; Athira, B. S.; Ghosh,
   Nirmalya; Nandy, Dibyendu
Bibcode: 2020ApJ...890...37D
Altcode: 2019arXiv190610201D
  On 2019 July 2 a total solar eclipse—visible across parts of the
  Southern Pacific Ocean, Chile, and Argentina—enabled observations
  of the Sun's corona. The structure and emission characteristics of the
  corona are determined by underlying magnetic fields, which also govern
  coronal heating and solar eruptive events. However, coronal magnetic
  field measurements remain an outstanding challenge. Coronal magnetic
  field models serve an important purpose in this context. Earlier work
  has demonstrated that the large-scale coronal structure is governed
  by surface flux evolution and memory buildup, which allows for
  its prediction on solar rotational timescales. Utilizing this idea
  and based upon a 51 day forward run of a predictive solar surface
  flux transport model and a potential field source surface model, we
  predict the coronal structure of the 2019 July 2 solar eclipse. We
  also forward model the polarization characteristics of the coronal
  emission. Our prediction of two large-scale streamer structures and
  their locations on the east and west limbs of the Sun match eclipse
  observations reasonably well. We demonstrate that the Sun's polar fields
  strongly influence the modeled corona, concluding that accurate polar
  field observations are critical. This study is relevant for coronal
  magnetometry initiatives envisaged with the Daniel K. Inouye Solar
  Telescope, Coronal Multichannel Polarimeter and upcoming space-based
  instruments such as Solar Orbiter, Solar Ultraviolet Imaging Telescope
  and the Variable Emission Line Coronagraph on board the Indian Space
  Research Organisation's Aditya-L1 space mission.

Title: Subcritical Magnetic Dynamos of Middle-aged Sun-like Stars
    Reconcile Solar-Stellar Activity Observation
Authors: Tripathi, Bindesh; Nandy, Dibyendu; Banerjee, Soumitro
Bibcode: 2020APS..DPPZ07009T
Altcode:
  Long-term solar magnetic activity reconstructions indicate the solar
  dynamo operates in two distinct - grand minimum and regular activity
  - modes. By employing bifurcation analysis of a physically-motivated
  time delay dynamo model, we establish this to be a direct consequence
  of dynamo hysteresis. We reproduce the observed bimodal distribution
  of sunspots, but only for subcritical dynamos. We also demonstrate
  how the Sun can enter into the grand minima episodes and recover from
  it. A theoretical framework consistent with these findings explain
  confounding observations of an abrupt midlife transition in stellar
  activity, characterized by reduced angular momentum loss rates and
  breakdown of gyrochronology relations. Our study indicates that an
  evolving dynamo bridges a diversity of phenomena in Sun-like stars
  across their lifetime. <P />Supported by the Center of Excellence in
  Space Sciences India, MHRD, India; St. Xavier's College, Nepal.

Title: Single-shot measurement of the space-varying polarization state
    of light through interferometric quantification of the geometric phase
Authors: B S, Athira; Pal, Mandira; Mukherjee, Sounak; Mishra,
   Jatadhari; Nandy, Dibyendu; Ghosh, Nirmalya
Bibcode: 2020PhRvA.101a3836B
Altcode:
  A light beam carrying a spatially varying state of polarization
  generates a space-varying Pancharatnam-Berry geometric phase while
  propagating through a homogeneous anisotropic medium. We show that
  determination of such a space-varying geometric phase provides a
  unique way to quantify the space-varying polarization state of light
  using a single-shot interferometric measurement. We demonstrate this
  concept in a Mach-Zehnder interferometric arrangement using a linearly
  polarized reference light beam, where full information on the spatially
  varying polarization state is successfully recovered by quantifying the
  space-varying geometric phase and the contrast of interference. The
  proposed method enables single-shot measurement of any space-varying
  polarization state of light from the measured interference pattern with
  a polarized reference beam. This approach shows considerable potential
  for instantaneous mapping of complex space-varying polarization of
  light in diverse applications, such as astronomy, biomedical imaging,
  and nanophotonics, where high precision and near real-time measurement
  of spatial polarization patterns are desirable.

Title: International Scientific Coordination on Space Weather:
    A COSPAR Panel on Space Weather Perspective
Authors: Bisi, M. M.; Kuznetsova, M. M.; Temmer, M.; Opgenoorth, H. J.;
   Belehaki, A.; Bruinsma, S.; Glover, A.; Heynderickx, D.; Linker, J.;
   Mann, I. R.; Murray, S. A.; Nandy, D.
Bibcode: 2019AGUFMSM31C3543B
Altcode:
  The understanding and prediction of space-weather phenomena and
  their respective impact(s) on society have been widely-acknowledged
  as an international challenge and something that requires a global
  coordination and focus. In order to address this need to form
  more-formal worldwide collaboration and coordination, and to maximise
  return on such efforts (particularly scientifically), the Committee
  on Space Research (COSPAR) Panel on Space Weather (PSW) has created a
  network of International Space Weather Action Teams (ISWATs). <P />The
  COSPAR PSW ISWAT initiative is capitalising on established efforts by
  engaging existing national and international "teams" and "facilitates"
  to form individual ISWATs that are being grouped into clusters
  by domains/themes related to different aspects of solar/coronal,
  heliospheric, ionospheric/atmospheric, and planetary space-weather
  phenomena. The initiative also includes overarching themes such as
  dealing with large data sets and model/scientific validations. The
  ISWAT initiative places a strong encouragement for scientists to go
  beyond their funding borders to form ISWATs better suited to address
  challenges that one individual or small group/team may not be able to
  address alone. <P />The ISWAT initiative serves as a global hub for
  community coordinated topical focused collaborations and as a global
  community voice for the next generation of both scientific and strategic
  planning - this includes an update of the COSPAR/ILWS space weather
  scientific roadmap (to transform the roadmap into a living document)
  and to potentially provide an operational roadmap in parallel. <P
  />This presentation will re-introduce the ISWAT initiative, review
  its current status and plans for community-wide campaigns, highlight
  the overarching current plans for PSW, and place a focus on two key
  space-weather areas: the ambient heliosphere/background solar wind
  (designated as ISWAT theme H1) and CME structure, evolution and
  propagation through heliosphere (designated as ISWAT theme H2).

Title: Showcasing the just released ISWAT website
(http://www.iswat-cospar.org) built with a content management platform
    to serve as an online presence for the ISWAT (International Space
    Weather Action Teams) - community driven effort hosted by the COSPAR
    Panel on Space Weather.
Authors: Mendoza, A. M. M.; Kuznetsova, M.; Opgenoorth, H. J.;
   Belehaki, A.; Bisi, M. M.; Bruinsma, S.; Heynderickx, D.; Linker,
   J.; Mann, I. R.; Murray, S. A.; Nandy, D.; Temmer, M.
Bibcode: 2019AGUFMSM31C3181M
Altcode:
  We will showcase the just released ISWAT website (<A
  href="http://www.iswat-cospar.org/">http://www.iswat-cospar.org</A>)
  built with a content management platform to serve as an online
  presence for the <P />ISWAT (International Space Weather Action
  Teams) - community driven effort hosted by the COSPAR Panel on Space
  Weather. <P />The website was created to represent ISWAT overarching
  goal to serve as a global hub for topical collaborations and focused
  on different aspects of space weather. <P />The homepage main's ISWAT
  image menu shows ISWAT clusters that cover Solar (S), Heliosphere
  (H) and Geospace (G) domains. Each cluster (S1-S3, H1-H4, G1-G3)
  <P />shown in the image is links to dedicated webpages that contain
  information about cluster goals and links to entry pages of registered
  action teams. <P />The "Join ISWAT" link contains 2 interactive forms
  for joining ISWAT mailing list and for registration of established and
  emerging international teams focused on different <P />aspects of space
  weather. After the registration is confirmed by cluster moderator a link
  to a new team entry page is added to a submitted cluster site. A team
  start entry <P />team page will contain information submitted during
  registration that may include a link to an external team page as an
  option. Another interactive form to join a <P />registered ISWAT team
  will be added in the near future. <P />Future planned additions include
  a Forum to create threaded discussion boards to encourage discussions
  on global coordination of space weather and invite community inputs
  to global space weather roadmap updates. <P />The website will be
  eventually maintained and facilitated by the COSPAR Panel on Space
  Weather Chairs/Vice-chairs, ISWAT cluster moderators, and ISWAT team
  <P />representatives.

Title: International Scientific Coordination on Space Weather:
    A COSPAR Panel on Space Weather Perspective
Authors: Kuznetsova, M.; Bisi, M. M.; Kusano, K.; Fuller-Rowell,
   T. J.; Mann, I.; Belehaki, A.; Minow, J. I.; Munoz-Jaramillo, A.;
   Masson, A.; Bruinsma, S.; Bisi, M. M.; Kuznetsova, M. M.; Temmer, M.;
   Opgenoorth, H. J.; Belehaki, A.; Bruinsma, S.; Glover, A.; Heynderickx,
   D.; Linker, J.; Mann, I. R.; Murray, S. A.; Nandy, D.
Bibcode: 2019AGUFMSM31C3543K
Altcode:
  The understanding and prediction of space-weather phenomena and
  their respective impact(s) on society have been widely-acknowledged
  as an international challenge and something that requires a global
  coordination and focus. In order to address this need to form
  more-formal worldwide collaboration and coordination, and to maximise
  return on such efforts (particularly scientifically), the Committee
  on Space Research (COSPAR) Panel on Space Weather (PSW) has created a
  network of International Space Weather Action Teams (ISWATs). <P />The
  COSPAR PSW ISWAT initiative is capitalising on established efforts by
  engaging existing national and international "teams" and "facilitates"
  to form individual ISWATs that are being grouped into clusters
  by domains/themes related to different aspects of solar/coronal,
  heliospheric, ionospheric/atmospheric, and planetary space-weather
  phenomena. The initiative also includes overarching themes such as
  dealing with large data sets and model/scientific validations. The
  ISWAT initiative places a strong encouragement for scientists to go
  beyond their funding borders to form ISWATs better suited to address
  challenges that one individual or small group/team may not be able to
  address alone. <P />The ISWAT initiative serves as a global hub for
  community coordinated topical focused collaborations and as a global
  community voice for the next generation of both scientific and strategic
  planning - this includes an update of the COSPAR/ILWS space weather
  scientific roadmap (to transform the roadmap into a living document)
  and to potentially provide an operational roadmap in parallel. <P
  />This presentation will re-introduce the ISWAT initiative, review
  its current status and plans for community-wide campaigns, highlight
  the overarching current plans for PSW, and place a focus on two key
  space-weather areas: the ambient heliosphere/background solar wind
  (designated as ISWAT theme H1) and CME structure, evolution and
  propagation through heliosphere (designated as ISWAT theme H2).

Title: The origin of parity changes in the solar cycle
Authors: Hazra, Soumitra; Nandy, Dibyendu
Bibcode: 2019MNRAS.489.4329H
Altcode: 2019MNRAS.tmp.2131H; 2019arXiv190606780H
  Although sunspots have been systematically observed on the Sun's surface
  over the last four centuries, their magnetic properties have been
  revealed and documented only since the early 1900s. Sunspots typically
  appear in pairs of opposite magnetic polarities which have a systematic
  orientation. This polarity orientation is opposite across the equator
  - a trend that has persisted over the last century. Taken together
  with the configuration of the global poloidal field of the Sun - this
  phenomena is consistent with the dipolar parity state of an underlying
  magnetohydrodynamic dynamo. Although transient hemispheric asymmetry
  in sunspot emergence is observed, a global parity shift has never been
  observed. We simulate hemispheric asymmetry through introduction of
  random fluctuations in a computational dynamo model of the solar cycle
  and demonstrate that changes in parity are indeed possible in long-term
  simulations covering thousands of years. Quadrupolar modes are found to
  exist over significant fraction of the simulated time. In particular,
  we find that a parity shift in the underlying nature of the sunspot
  cycle is more likely to occur when sunspot activity dominates in any
  one hemisphere for a time which is significantly longer than the cycle
  period. We establish causal pathways connecting hemispheric asymmetry
  to parity flips mediated via a decoupling of the dynamo cycle period
  across the two solar hemispheres. Our findings indicate that the solar
  cycle may have resided in quadrupolar parity states in the past, and
  provides a possible pathway for predicting parity flips in the future.

Title: Hemispheric asymmetry in meridional flow and the sunspot cycle
Authors: Lekshmi, B.; Nandy, Dibyendu; Antia, H. M.
Bibcode: 2019MNRAS.489..714L
Altcode:
  Magnetohydrodynamic dynamo modelling shows that the large-scale
  solar meridional plasma flow plays an important role in governing the
  dynamics of the sunspot cycle. Observations indicate that meridional
  flow velocities at each solar latitude and depth vary over time and are
  asymmetric across the equator. Here, using helioseismic observations
  we explore the temporal variation in the hemispherical asymmetry of
  near-surface residual (time-varying) component of the Sun's meridional
  flow velocity. The meridional flow velocities obtained from Global
  Oscillation Network Group (GONG) and Helioseismic and Magnetic Imager
  (HMI) onboard Solar Dynamics Observatory (SDO) ring-diagram pipelines
  are used in this work. Our data set covers the declining phase of
  cycle 23 and cycle 24 (from July 2001 till December 2018) and the
  flow velocities are poleward for the observed depth range. We observe
  a time delayed anticorrelation between the hemispherical asymmetry in
  near-surface meridional flow velocities and the sunspot cycle quantified
  in terms of magnetic flux and sunspot number. Interestingly, asymmetry
  in meridional flow velocity precedes the asymmetry in sunspot cycle by
  3.1-3.5 yr. We propose that meridional flow asymmetry is a precursor
  of asymmetry in hemispherical cycle strength. The symmetric component
  of meridional flow is observed to be positively correlated with the
  corresponding symmetric components of the magnetic cycle, also with a
  time delay. Our analysis sets important constraints on theories for the
  origin of meridional plasma flow asymmetries and its temporal variations
  and is relevant for understanding the role of plasma flux transport
  processes in determining hemispheric asymmetry in the sunspot cycle.

Title: Solar Filament Eruptions as Precursors to Flare-CME Events:
    Establishing the Temporal Connection
Authors: Sinha, Suvadip; Srivastava, Nandita; Nandy, Dibyendu
Bibcode: 2019ApJ...880...84S
Altcode:
  Elongated structures on the Sun’s surface known as filaments are known
  to have a connection with energetic events of space weather consequence
  (flares and coronal mass ejections (CMEs)). In this work, we explore
  the connection between the eruptive dynamics of filaments and the
  initiation of solar flares and CMEs. We estimate the filament eruption
  start time by tracking the filament throughout its eruption phase. We
  define the filament eruption start time as the time from which the
  filament area starts to decrease as observed in Hα images. A total of
  33 eruptive filament events are reported in this study, out of which 73%
  are CME associated and 76% are related to solar flares. We find a good
  correlation between area decay rate of the quiescent filaments and the
  speed of the associated CMEs with a correlation coefficient of 0.75. By
  analyzing the time delay of the extreme ultraviolet brightening of solar
  flares relative to the start time of associated filament eruption,
  we show that in 83% of cases, filament eruption precedes the flare
  brightening, which indicates that eruptive filaments can be considered
  as one of the precursors for the occurrence of a solar flare. Finally,
  we study the time delay of the CME onset from the time of initiation of
  the filament eruption process and show that for most of the cases, CMEs
  occur within 2 hr from the start time of the filament eruptions. This
  study would be useful for space weather assessment and characterization
  based on automated trackers of solar filament dynamics.

Title: Modeling Star-Planet Interactions in Far-out Planetary and
    Exoplanetary Systems
Authors: Bharati Das, Srijan; Basak, Arnab; Nandy, Dibyendu; Vaidya,
   Bhargav
Bibcode: 2019ApJ...877...80B
Altcode: 2019ApJ...877...80D; 2018arXiv181207767B
  The magnetized wind from a host star plays a vital role in shaping the
  magnetospheric configuration of the planets it harbors. We carry out
  three-dimensional (3D) compressible magnetohydrodynamic simulations
  of the interactions between magnetized stellar winds and planetary
  magnetospheres corresponding to a far-out star-planet system, with
  and without planetary dipole obliquity. We identify the pathways
  that lead to the formation of a dynamical steady-state magnetosphere
  and find that magnetic reconnection plays a fundamental role in the
  process. The magnetic energy density is found to be greater on the
  nightside than on the dayside, and the magnetotail is comparatively
  more dynamic. It is found that stellar wind plasma injection into the
  inner magnetosphere is possible through the magnetotail. We further
  study magnetospheres with extreme tilt angles, keeping in perspective
  the examples of Uranus and Neptune. High dipole obliquities may also
  manifest due to polarity excursions during planetary field reversals. We
  find that global magnetospheric reconnection sites change for large
  planetary dipole obliquity, and more complex current sheet structures
  are generated. We discuss the implications of these findings for
  atmospheric erosion, the introduction of stellar and interplanetary
  species that modify the composition of the atmosphere, auroral activity,
  and magnetospheric radio emission. This study is relevant for exploring
  star-planet interactions and its consequence on atmospheric dynamics
  and habitability in solar system planets and exoplanets.

Title: Prediction of the Sun’s Corona for the Total Solar Eclipse
    on 2019 July 2
Authors: Dash, Soumyaranjan; Bhowmik, Prantika; Nandy, Dibyendu
Bibcode: 2019RNAAS...3...86D
Altcode: 2019RNAAS...3f..86D
  No abstract at ADS

Title: Work Done by Lorentz Force Drives Solar-Stellar Magnetic Cycles
Authors: Mahajan, Sushant Sushil; Nandy, Dibyendu; Martens, Petrus C.
Bibcode: 2019shin.confE.199M
Altcode:
  Our theoretical analysis of the equations of magnetohydrodynamics
  applied to the solar dynamo suggests that the work done by Lorentz force
  is the source of magnetic energy inside the solar convection zone. The
  action of Lorentz Force on poloidal field inside the convection zone
  is expected to leave behind signs of magnetic tension which manifest
  in the form of reduced latitudinal shear in differential rotation. We
  show that these expected signs of magnetic tension are consistent
  with the torsional oscillation profile of the Sun measured by three
  different instruments and hence can be used to locate regions inside
  the Sun where the magnetic field in sunspots originates.

Title: A 3D kinematic Babcock Leighton solar dynamo model sustained
    by dynamic magnetic buoyancy and flux transport processes
Authors: Kumar, Rohit; Jouve, Laurène; Nandy, Dibyendu
Bibcode: 2019A&A...623A..54K
Altcode: 2019arXiv190104251K
  Context. Magnetohydrodynamic interactions between plasma flows
  and magnetic fields is fundamental to the origin and sustenance
  of the 11-year sunspot cycle. These processes are intrinsically
  three-dimensional (3D) in nature. <BR /> Aims: Our goal is to
  construct a 3D solar dynamo model that on the one hand captures
  the buoyant emergence of tilted bipolar sunspot pairs, and on the
  other hand produces cyclic large-scale field reversals mediated via
  surface flux-transport processes - that is, the Babcock-Leighton
  mechanism. Furthermore, we seek to explore the relative roles of
  flux transport by buoyancy, advection by meridional circulation, and
  turbulent diffusion in this 3D dynamo model. <BR /> Methods: We perform
  kinematic dynamo simulations where the prescribed velocity field is
  a combination of solar-like differential rotation and meridional
  circulation, along with a parametrized turbulent diffusivity. We
  use a novel methodology for modeling magnetic buoyancy through
  field-strength-dependent 3D helical up-flows that results in the
  formation of tilted bipolar sunspots. <BR /> Results: The bipolar
  spots produced in our simulations participate in the process of
  poloidal-field generation through the Babcock-Leighton mechanism,
  resulting in self-sustained and periodic large-scale magnetic field
  reversal. Our parameter space study varying the amplitude of the
  meridional flow, the convection zone diffusivity, and parameters
  governing the efficiency of the magnetic buoyancy mechanism reveal
  their relative roles in determining properties of the sunspot cycle
  such as amplitude, period, and dynamical memory relevant to solar cycle
  prediction. We also derive a new dynamo number for the Babcock-Leighton
  solar dynamo mechanism which reasonably captures our model dynamics. <BR
  /> Conclusions: This study elucidates the relative roles of different
  flux-transport processes in the Sun's convection zone in determining
  the properties and physics of the sunspot cycle and could potentially
  lead to realistic, data-driven 3D dynamo models for solar-activity
  predictions and exploration of stellar magnetism and starspot formation
  in other stars.

Title: Prediction of the strength and timing of sunspot cycle 25
    reveal decadal-scale space environmental conditions
Authors: Bhowmik, Prantika; Nandy, Dibyendu
Bibcode: 2018NatCo...9.5209B
Altcode: 2019arXiv190904537B
  The Sun's activity cycle governs the radiation, particle
  and magnetic flux in the heliosphere creating hazardous space
  weather. Decadal-scale variations define space climate and force
  the Earth's atmosphere. However, predicting the solar cycle is
  challenging. Current understanding indicates a short window for
  prediction best achieved at previous cycle minima. Utilizing magnetic
  field evolution models for the Sun's surface and interior we perform
  the first century-scale, data-driven simulations of solar activity and
  present a scheme for extending the prediction window to a decade. Our
  ensemble forecast indicates cycle 25 would be similar or slightly
  stronger than the current cycle and peak around 2024. Sunspot cycle
  25 may thus reverse the substantial weakening trend in solar activity
  which has led to speculation of an imminent Maunder-like grand minimum
  and cooling global climate. Our simulations demonstrate fluctuation
  in the tilt angle distribution of sunspots is the dominant mechanism
  responsible for solar cycle variability.

Title: The Association of Filaments, Polarity Inversion Lines, and
Coronal Hole Properties with the Sunspot Cycle: An Analysis of the
    McIntosh Database
Authors: Mazumder, Rakesh; Bhowmik, Prantika; Nandy, Dibyendu
Bibcode: 2018ApJ...868...52M
Altcode: 2018arXiv181002133M
  Filaments and coronal holes, two principal features observed in
  the solar corona, are sources of space weather variations. Filament
  formation is closely associated with polarity inversion lines (PILs)
  on the solar photosphere which separate positive and negative polarities
  of the surface magnetic field. The origin of coronal holes is governed
  by large-scale unipolar magnetic patches on the photosphere from where
  open magnetic field lines extend to the heliosphere. We study the
  properties of filaments, PILs, and coronal holes in solar cycles 20,
  21, 22, and 23 utilizing the McIntosh archive. We detect a prominent
  cyclic behavior of filament length, PIL length, and coronal hole area
  with significant correspondence with the solar magnetic cycle. The
  spatio-temporal evolution of the geometric centers of filaments shows
  a butterfly-like structure and distinguishable poleward migration
  of long filaments during cycle maxima. We identify this rush to
  the poles of filaments to be co-temporal with the initiation of
  polar field reversal as gleaned from Mount Wilson and Wilcox Solar
  Observatory polar field observations, and quantitatively establish
  their temporal correspondence. We analyze the filament tilt angle
  distribution to constrain their possible origins. The majority of the
  filaments exhibit negative and positive tilt angles in the northern
  and the southern hemispheres, respectively, strongly suggesting
  that their formation is governed by the overall large-scale magnetic
  field distribution on the solar photosphere and not by the small-scale
  intra-active region magnetic field configurations. We also investigate
  the hemispheric asymmetry in filaments, PILs, and coronal holes. We
  find that the hemispheric asymmetry in filaments and PILs is positively
  correlated with sunspot area asymmetry, whereas coronal hole asymmetry
  is uncorrelated.

Title: The Extended Solar Cycle: Muddying the Waters of Solar/Stellar
    Dynamo Modeling Or Providing Crucial Observational Constraints?
Authors: Srivastava, Abhishek K.; McIntosh, Scott W.; Arge,
   N.; Banerjee, Dipankar; Dikpati, Mausumi; Dwivedi, Bhola N.;
   Guhathakurta, Madhulika; Karak, B. B.; Leamon, Robert J.; Matthew,
   Shibu K.; Munoz-Jaramillo, Andres; Nandy, D.; Norton, Aimee; Upton,
   L.; Chatterjee, S.; Mazumder, Rakesh; Rao, Yamini K.; Yadav, Rahul
Bibcode: 2018FrASS...5...38S
Altcode: 2018arXiv180707601S
  In 1844 Schwabe discovered that the number of sunspots increased and
  decreased over a period of about 11 years, that variation became known
  as the sunspot cycle. Almost eighty years later, Hale described the
  nature of the Sun's magnetic field, identifying that it takes about 22
  years for the Sun's magnetic polarity to cycle. It was also identified
  that the latitudinal distribution of sunspots resembles the wings of
  a butterfly showing migration of sunspots in each hemisphere that
  abruptly start at mid-latitudes (about ±35(o) ) towards the Sun's
  equator over the next 11 years. These sunspot patterns were shown
  to be asymmetric across the equator. In intervening years, it was
  deduced that the Sun (and sun-like stars) possess magnetic activity
  cycles that are assumed to be the physical manifestation of a dynamo
  process that results from complex circulatory transport processes in
  the star's interior. Understanding the Sun's magnetism, its origin
  and its variation, has become a fundamental scientific objective
  the distribution of magnetism, and its interaction with convective
  processes, drives various plasma processes in the outer atmosphere
  that generate particulate, radiative, eruptive phenomena and shape the
  heliosphere. In the past few decades, a range of diagnostic techniques
  have been employed to systematically study finer scale magnetized
  objects, and associated phenomena. The patterns discerned became
  known as the ``Extended Solar Cycle'' (ESC). The patterns of the ESC
  appeared to extend the wings of the activity butterfly back in time,
  nearly a decade before the formation of the sunspot pattern, and to
  much higher solar latitudes. In this short review, we describe their
  observational patterns of the ESC and discuss possible connections
  to the solar dynamo as we depart on a multi-national collaboration to
  investigate the origins of solar magnetism through a blend of archived
  and contemporary data analysis with the goal of improving solar dynamo
  understanding and modeling.

Title: Dependence of Coronal Mass Ejection Properties on Their
    Solar Source Active Region Characteristics and Associated Flare
    Reconnection Flux
Authors: Pal, Sanchita; Nandy, Dibyendu; Srivastava, Nandita;
   Gopalswamy, Nat; Panda, Suman
Bibcode: 2018ApJ...865....4P
Altcode: 2018arXiv180804144P
  The near-Sun kinematics of coronal mass ejections (CMEs) determine
  the severity and arrival time of associated geomagnetic storms. We
  investigate the relationship between the deprojected speed and
  kinetic energy of CMEs and magnetic measures of their solar sources,
  reconnection flux of associated eruptive events, and intrinsic
  flux-rope characteristics. Our data covers the period 2010-2014 in
  solar cycle 24. Using vector magnetograms of source active regions,
  we estimate the size and nonpotentiality. We compute the total
  magnetic reconnection flux at the source regions of CMEs using
  the post-eruption arcade method. By forward modeling the CMEs,
  we find their deprojected geometric parameters and constrain their
  kinematics and magnetic properties. Based on an analysis of this
  database, we report that the correlation between CME speed and their
  source active region size and global nonpotentiality is weak, but not
  negligible. We find the near-Sun velocity and kinetic energy of CMEs to
  be well correlated with the associated magnetic reconnection flux. We
  establish a statistically significant empirical relationship between
  the CME speed and reconnection flux that may be utilized for prediction
  purposes. Furthermore, we find CME kinematics to be related with the
  axial magnetic field intensity and relative magnetic helicity of their
  intrinsic flux ropes. The amount of coronal magnetic helicity shed by
  CMEs is found to be well correlated with their near-Sun speeds. The
  kinetic energy of CMEs is well correlated with their intrinsic magnetic
  energy density. Our results constrain processes related to the origin
  and propagation of CMEs and may lead to better empirical forecasting
  of their arrival and geoeffectiveness.

Title: The solar dynamo as an interplay of rotational shear and
    magnetic field
Authors: Mahajan, Sushant Sushil; Nandy, Dibyendu; Martens, Petrus C.
Bibcode: 2018shin.confE.154M
Altcode:
  No abstract at ADS

Title: Asymmetry in Solar Torsional Oscillation and the Sunspot Cycle
Authors: Lekshmi, B.; Nandy, Dibyendu; Antia, H. M.
Bibcode: 2018ApJ...861..121L
Altcode: 2018arXiv180703588B; 2018ApJ...861..121B; 2018arXiv180703588L
  Solar torsional oscillations are migrating bands of slower- and
  faster-than-average rotation, which are strongly related to the Sun’s
  magnetic cycle. We perform a long-term study (16 yr) of hemispherical
  asymmetry in solar torsional oscillation velocity using helioseismic
  data for the first time. We study the north-south asymmetry in the
  velocity using the zonal flow velocities obtained by ring diagram
  analysis of the Global Oscillation Network Group (GONG) Doppler
  images. We find significant hemispherical asymmetry in the torsional
  oscillation velocity and explore its variation with respect to depth,
  time, and latitude. We also calculate the hemispherical asymmetry in
  the surface velocity measurements from the Mount Wilson Observatory
  and the zonal flow velocities obtained from the Helioseismic and
  Magnetic Imager ring diagram pipeline. These asymmetries are found to
  be consistent with the asymmetry obtained from GONG observations. We
  show that the asymmetry in near-surface torsional oscillation velocity
  is correlated with the asymmetry in magnetic flux and sunspot number
  at the solar surface, with the velocity asymmetry preceding the flux
  and sunspot number asymmetries. We speculate that the asymmetry in
  torsional oscillation velocity may help in predicting the hemispherical
  asymmetry in sunspot cycles.

Title: Origin and Recovery from Grand Solar Minima in a Time Delay
    Dynamo Model with Magnetic Noise as an Additional Poloidal Source
Authors: Tripathi, Bindesh; Nandy, Dibyendu; Banerjee, Soumitro
Bibcode: 2018arXiv180411350T
Altcode:
  We explore a reduced Babcock-Leighton (BL) dynamo model based on delay
  differential equations using numerical bifurcation analysis. This model
  reveals hysteresis, seen in the recent mean-field dynamo model and
  the direct numerical simulations of turbulent dynamos. The BL model
  with 'magnetic noise' as an additional weak-source of the poloidal
  field recovers the solar cycle every time from grand minima, which
  BL source alone cannot do. The noise-incorporated model exhibits a
  bimodal distribution of toroidal field energy confirming two modes
  of solar activity. It also shows intermittency and reproduces phase
  space collapse, an experimental signature of the Maunder Minimum. The
  occurrence statistics of grand minima in our model agree reasonably
  well with the observed statistics in the reconstructed sunspot
  number. Finally, we demonstrate that the level of magnetic noise
  controls the duration of grand minima and even has a handle over its
  waiting period, suggesting a triggering effect of grand minima by the
  noise and thus shutting down the global dynamo. Therefore, we conclude
  that the 'magnetic noise' due to small-scale turbulent dynamo action
  (or other sources) plays a vital role even in Babcock-Leighton dynamo
  models.

Title: Torsional Oscillations in the Suns rotation contribute to
    the Waldmeier-effect in Solar Cycles
Authors: Mahajan, Sushant S.; Nandy, Dibyendu; Antia, H. M.; Dwivedi,
   B. N.
Bibcode: 2018arXiv180307758M
Altcode:
  Temporal variations in the Suns internal velocity field with
  a periodicity of about 11 years have been observed over the
  last four decades. The period of these torsional oscillations
  and their latitudinal propagation roughly coincides with the
  period and equatorward propagation of sunspots which originate
  from a magnetohydrodynamic dynamo mechanism operating in the Suns
  interior. While the solar differential rotation plays an important
  role in this dynamo mechanism by inducting the toroidal component of
  magnetic field, the impact of torsional oscillations on the dynamo
  mechanism and hence the solar cycle is not well understood. Here, we
  include the observed torsional oscillations into a flux transport dynamo
  model of the solar cycle to investigate their effect. We find that the
  overall amplitude of the solar cycle does not change significantly on
  inclusion of torsional oscillations. However, all the characteristics of
  the Waldmeier effect in the sunspot cycle are qualitatively reproduced
  by varying only the amplitude of torsional oscillations. The Waldmeier
  effect, first noted in 1935, includes the important characteristic
  that the amplitude of sunspot cycles is anti-correlated to their rise
  time; cycles with high initial rise rate tend to be stronger. This has
  implications for solar cycle predictions. Our results suggest that
  the Waldmeier effect could be a plausible outcome of cycle to cycle
  modulation of torsional oscillations and provides a physical basis for
  sunspot cycle forecasts based on torsional oscillation observations. We
  also provide a theoretical explanation based on the magnetic induction
  equation thereby connecting two apparently disparate phenomena.

Title: A Magnetofrictional model for the solar corona
Authors: Dash, Soumyaranjan; Nandy, Dibyendu
Bibcode: 2018IAUS..340...87D
Altcode:
  Regular reconstruction of global solar corona constrained by
  observational data is required to monitor the space weather
  variations. We develop a model for simulating the global coronal
  magnetic field using magnetofrictional approach. Here we perform
  simulations to study the evolution of the magnetic field associated
  with a bipolar active region in response to photospheric flows.

Title: The activity evolution of Solar-like stars with age and its
    planetary impact
Authors: Das, Srijan Bharati; Basak, Arnab; Nandy, Dibyendu
Bibcode: 2018IAUS..340..240D
Altcode:
  The age-dependent activity of a star dictates the extent of its
  planetary impact. We study the interaction of the stellar wind
  produced by Solar-like stars with the magnetosphere of Earth-like
  planets using three dimensional (3D) magnetohydrodynamic (MHD)
  simulations. The numerical simulations reveal important features of
  star-planet interaction e.g. bow-shock, magnetopause, magnetotail,
  etc. Interesting phenomena such as particle injection into the planetary
  atmosphere as well as atmospheric mass loss are also observed which
  are instrumental in determining the atmospheric retention by the planet.

Title: Study of starspots in fully convective stars using three
    dimensional MHD simulations
Authors: Basak, Arnab; Nandy, Dibyendu
Bibcode: 2018IAUS..340..303B
Altcode:
  Concentrated magnetic structures such as sunspots and starspots play
  a fundamental role in driving solar and stellar activity. However, as
  opposed to the sun, observations as well as numerical simulations have
  shown that stellar spots are usually formed as high-latitude patches
  extended over wide areas. Using a fully spectral magnetohydrodynamic
  (MHD) code, we simulate polar starspots produced by self-consistent
  dynamo action in rapidly rotating convective shells. We carry out high
  resolution simulations and investigate various properties related to
  stellar dynamics which lead to starspot formation.

Title: Properties of Coronal Holes in Solar Cycle 21-23 using
    McIntosh archive
Authors: Mazumder, Rakesh; Bhowmik, Prantika; Nandy, Dibyendu
Bibcode: 2018IAUS..340..187M
Altcode:
  We study the properties of coronal holes during solar cycle 21-23 from
  the McIntosh archive. In the spatial distribution of coronal hole area
  we find that there is a sharp increase in coronal hole area at high
  latitude in agreement with expected open flux configuration there. In
  overall spatiotemporal distribution of coronal hole centroids, we
  find the dominance of high latitude coronal holes except for the
  maximum of the solar cycle, when coronal holes mostly appear in low
  latitudes. This is in agreement with the expected solar cycle evolution
  of surface magnetic flux.

Title: Asymmetry in Solar Torsional Oscillation
Authors: Lekshmi, B.; Nandy, Dibyendu; Antia, H. M.
Bibcode: 2018IAUS..340...11L
Altcode:
  Solar torsional oscillations are migrating bands of slower and faster
  than average rotation, which are thought to be related to the Sun's
  magnetic cycle. We perform the first long-term study (16 years)
  of hemispherical asymmetry in solar torsional oscillation velocity
  using helioseismic data. We explore the spatial and temporal variation
  of North-South asymmetry using zonal flow velocities obtained from
  ring diagram analysis of the Global Oscillation Network Group (GONG)
  Doppler images. We find a strong correlation between the asymmetries
  of near-surface torsional oscillation with magnetic flux and sunspot
  number, with the velocity asymmetry preceding in both the cases. We
  speculate that the asymmetry in torsional oscillation velocity may
  help in predicting the hemispherical asymmetry in the sunspot cycle.

Title: The Large-scale Coronal Structure of the 2017 August 21 Great
American Eclipse: An Assessment of Solar Surface Flux Transport
    Model Enabled Predictions and Observations
Authors: Nandy, Dibyendu; Bhowmik, Prantika; Yeates, Anthony R.;
   Panda, Suman; Tarafder, Rajashik; Dash, Soumyaranjan
Bibcode: 2018ApJ...853...72N
Altcode:
  On 2017 August 21, a total solar eclipse swept across the contiguous
  United States, providing excellent opportunities for diagnostics of the
  Sun’s corona. The Sun’s coronal structure is notoriously difficult
  to observe except during solar eclipses; thus, theoretical models must
  be relied upon for inferring the underlying magnetic structure of the
  Sun’s outer atmosphere. These models are necessary for understanding
  the role of magnetic fields in the heating of the corona to a million
  degrees and the generation of severe space weather. Here we present a
  methodology for predicting the structure of the coronal field based
  on model forward runs of a solar surface flux transport model,
  whose predicted surface field is utilized to extrapolate future
  coronal magnetic field structures. This prescription was applied to
  the 2017 August 21 solar eclipse. A post-eclipse analysis shows good
  agreement between model simulated and observed coronal structures and
  their locations on the limb. We demonstrate that slow changes in the
  Sun’s surface magnetic field distribution driven by long-term flux
  emergence and its evolution governs large-scale coronal structures
  with a (plausibly cycle-phase dependent) dynamical memory timescale
  on the order of a few solar rotations, opening up the possibility for
  large-scale, global corona predictions at least a month in advance.

Title: An Early Prediction of Sunspot Cycle 25
Authors: Nandy, D.; Bhowmik, P.
Bibcode: 2017AGUFMSH21A2639N
Altcode:
  The Sun's magnetic activity governs our space environment, creates
  space weather and impacts our technologies and climate. With increasing
  reliance on space- and ground-based technologies that are subject to
  space weather, the need to be able to forecast the future activity of
  the Sun has assumed increasing importance. However, such long-range,
  decadal-scale space weather prediction has remained a great challenge
  as evident in the diverging forecasts for solar cycle 24. Based
  on recently acquired understanding of the physics of solar cycle
  predictability, we have devised a scheme to extend the forecasting
  window of solar cycles. Utilizing this we present an early forecast
  for sunspot cycle 25 which would be of use for space mission planning,
  satellite life-time estimates, and assessment of the long-term impacts
  of space weather on technological assets and planetary atmospheres.

Title: A Sun-to-Earth Analysis of Magnetic Helicity of the 2013
    March 17-18 Interplanetary Coronal Mass Ejection
Authors: Pal, Sanchita; Gopalswamy, Nat; Nandy, Dibyendu; Akiyama,
   Sachiko; Yashiro, Seiji; Makela, Pertti; Xie, Hong
Bibcode: 2017ApJ...851..123P
Altcode: 2017arXiv171201114P
  We compare the magnetic helicity in the 2013 March 17-18 interplanetary
  coronal mass ejection (ICME) flux rope at 1 au and in its solar
  counterpart. The progenitor coronal mass ejection (CME) erupted on
  2013 March 15 from NOAA active region 11692 and is associated with
  an M1.1 flare. We derive the source region reconnection flux using
  the post-eruption arcade (PEA) method that uses the photospheric
  magnetogram and the area under the PEA. The geometrical properties of
  the near-Sun flux rope is obtained by forward-modeling of white-light
  CME observations. Combining the geometrical properties and the
  reconnection flux, we extract the magnetic properties of the CME
  flux rope. We derive the magnetic helicity of the flux rope using its
  magnetic and geometric properties obtained near the Sun and at 1 au. We
  use a constant-α force-free cylindrical flux rope model fit to the
  in situ observations in order to derive the magnetic and geometric
  information of the 1 au ICME. We find a good correspondence in both
  amplitude and sign of the helicity between the ICME and the CME,
  assuming a semi-circular (half torus) ICME flux rope with a length of
  π au. We find that about 83% of the total flux rope helicity at 1 au
  is injected by the magnetic reconnection in the low corona. We discuss
  the effect of assuming flux rope length in the derived value of the
  magnetic helicity. This study connecting the helicity of magnetic flux
  ropes through the Sun-Earth system has important implications for the
  origin of helicity in the interplanetary medium and the topology of
  ICME flux ropes at 1 au and hence their space weather consequences.

Title: Living Around Active Stars
Authors: Nandy, D.; Valio, A.; Petit, P.
Bibcode: 2017IAUS..328.....N
Altcode:
  No abstract at ADS

Title: A Data Driven, Zero-Dimensional Time Delay Model with Radiative
    Forcing for Simulating Global Climate
Authors: Tarafder, Rajashik; Nandy, Dibyendu
Bibcode: 2017arXiv170908860T
Altcode:
  Several complicated non-linear models exist which simulate the physical
  processes leading to fluctuations in global climate. Some of these
  more advanced models use observations to constrain various parameters
  involved. However, they tend to be very computationally expensive. Also,
  the exact physical processes that affect the climate variations have
  not been completely comprehended. Therefore, to obtain an insight
  into global climate, we have developed a physically motivated reduced
  climate model. The model utilizes a novel mathematical formulation
  involving a non-linear delay differential equation to study temperature
  fluctuations when subjected to imposed radiative forcing. We have
  further incorporated simplified equations to test the effect of
  speculated mechanisms of climate forcing and evaluated the extent of
  their influence. The findings are significant in our efforts to predict
  climate change and help in policy framing necessary to tackle it.

Title: Strong Hemispheric Asymmetry can Trigger Parity Changes in
    the Sunspot Cycle
Authors: Hazra, Soumitra; Nandy, Dibyendu
Bibcode: 2017SPD....4830604H
Altcode:
  Although sunspots have been systematically observed on the Sun’s
  surface over the last four centuries, their magnetic properties have
  been revealed and documented only since the early 1900s. Sunspots
  typically appear in pairs of opposite magnetic polarity which have
  a systematic orientation. This polarity orientation is opposite
  across the equator - a trend that has persisted over the last century
  since observations of sunspot magnetic fields exist. Taken together
  with the configurationof the global poloidal field of the Sun - that
  governs the heliospheric open flux and cosmic ray flux at Earth - this
  phenomena is consistent with the dipolar parity state of an underlying
  magnetohydrodynamic dynamo mechanism. Although, hemispheric asymmetry
  in the emergence of sunspots is observed in the Sun, a parity shift
  has never been observed. We simulate hemispheric asymmetry through
  introduction of random fluctuations in a computational dynamo model
  of the solar cycle and demonstrate that changes in parity are indeed
  possible over long time-scales. In particular, we find that a parity
  shift in the underlying nature of the sunspot cycle is more likely
  to occur when sunspot activity dominates in any one hemisphere for
  a time which is significantly longer compared to the sunspot cycle
  period. Our simulations suggest that the sunspot cycle may have resided
  in quadrupolar parity states in the distant past, and provides a
  possible pathway for predicting parity flips in the future.

Title: The Solar Ultraviolet Imaging Telescope on-board Aditya-L1
Authors: Tripathi, Durgesh; Ramaprakash, A. N.; Khan, Aafaque;
   Ghosh, Avyarthana; Chatterjee, Subhamoy; Banerjee, Dipankar; Chordia,
   Pravin; Gandorfer, Achim; Krivova, Natalie; Nandy, Dibyendu; Rajarshi,
   Chaitanya; Solanki, Sami K.
Bibcode: 2017CSci..113..616T
Altcode: 2022arXiv220407732T
  The Solar Ultraviolet Imaging Telescope (SUIT) is an instrument
  onboard the Aditya-L1 mission of ISRO that will measure and monitor
  the solar radiation emitted in the near-ultraviolet wavelength range
  (200-400 nm). SUIT will simultaneously map the photosphere and the
  chromosphere of the Sun using 11 filters sensitive to different
  wavelengths and covering different heights in the solar atmosphere
  and help us understand the processes involved in the transfer of
  mass and energy from one layer to the other. SUIT will also allow us
  to measure and monitor spatially resolved solar spectral irradiance
  that governs the chemistry of oxygen and ozone in the stratosphere of
  Earth's atmosphere. This is central to our understanding of the Sun
  climate relationship.

Title: Solar Surface Magnetic Field Simulation Enabled Prediction
    of the Large-Scale Coronal Structure of the 21 August 2017 Great
American Eclipse: An Assessment of Model Predictions and Observations
Authors: Nandy, Dibyendu; Bhowmik, Prantika; Yeates, Anthony R.;
   Panda, Suman; Tarafder, Rajashik; Dash, Soumyaranjan
Bibcode: 2017arXiv170805996N
Altcode:
  On 21 August 2017 a total solar eclipse swept across the contiguous
  United States providing excellent opportunities for diagnostics of
  the Sun's corona. The Sun's coronal structure is notoriously difficult
  to observe except during solar eclipses; thus theoretical models must
  be relied upon for inferring the underlying magnetic structure of the
  Sun's outer atmosphere. These models are necessary for understanding
  the role of magnetic fields in the heating of the corona to a million
  degrees and generation of severe space weather. Here we present a
  methodology for predicting the structure of the coronal field based on
  long-term surface flux transport simulations whose output is utilized to
  extrapolate the coronal magnetic field structures. This prescription was
  applied to the 21 August 2017 solar eclipse. Post-eclipse analysis shows
  good agreement between model simulated and observed coronal structures
  and their locations on the limb. We demonstrate that slow changes in
  the Sun's surface magnetic field distribution driven by long-term
  flux emergence and evolution govern large-scale coronal structures
  with a (plausibly cycle-phase dependent) dynamical memory timescale
  on the order of few solar rotations -- opening up the possibility of
  large-scale, global corona predictions at least a month in advance.

Title: The Solar-Stellar Connection
Authors: Brun, A. S.; García, R. A.; Houdek, G.; Nandy, D.;
   Pinsonneault, M.
Bibcode: 2017hdsi.book..309B
Altcode:
  No abstract at ADS

Title: Grand Minima in the Light of Kinematic Flux Transport Solar
    Dynamo Model
Authors: Hazra, S.; Nandy, D.
Bibcode: 2016AGUFMSH43D2590H
Altcode:
  Fluctuations in solar magnetic output,including episodes of grand minima
  such as maunder minima are studied using kinematic flux transport dynamo
  model. We show that cycle to cycle variation and maunder like grand
  minima is naturally produced by introduction of stochastic fluctuation
  in the poloidal field generation mechanism. Our anyalisis shows that
  Babcock-Leighton alpha effect alone can not restart the solar cycle
  once it gets into a grand minimum. An additional alpha effect capable
  of working on weak toroidal magnetic fields - such as that driven
  by helical turbulence - is necessary to recover the solar cycle. Our
  result demonstrates the importance of small scale alpha effect and shows
  how self consistent entry and exit from grand minima like episodes is
  possible. We also investigate the condition which triggers the grand
  minima like episode and find that if the amplitude of the poloidal
  field at the minima of the previous solar cycle drops near about 60 %
  from the mean value of peak poloidal fields, then solar cycle entering
  into grand minima like episode. Finally we address the question whether
  it is possible to predict the onset of grand minima like episodes.

Title: A Proposed Paradigm for Solar Cycle Dynamics Mediated via
    Turbulent Pumping of Magnetic Flux in Babcock-Leighton-type Solar
    Dynamos
Authors: Hazra, Soumitra; Nandy, Dibyendu
Bibcode: 2016ApJ...832....9H
Altcode: 2016arXiv160808167H
  At present, the Babcock-Leighton flux transport solar dynamo models
  appear to be the most promising models for explaining diverse
  observational aspects of the sunspot cycle. The success of these
  flux transport dynamo models is largely dependent upon a single-cell
  meridional circulation with a deep equatorward component at the base
  of the Sun’s convection zone. However, recent observations suggest
  that the meridional flow may in fact be very shallow (confined to the
  top 10% of the Sun) and more complex than previously thought. Taken
  together, these observations raise serious concerns on the validity of
  the flux transport paradigm. By accounting for the turbulent pumping
  of magnetic flux, as evidenced in magnetohydrodynamic simulations of
  solar convection, we demonstrate that flux transport dynamo models
  can generate solar-like magnetic cycles even if the meridional flow
  is shallow. Solar-like periodic reversals are recovered even when
  meridional circulation is altogether absent. However, in this case,
  the solar surface magnetic field dynamics does not extend all the way
  to the polar regions. Very importantly, our results demonstrate that
  the Parker-Yoshimura sign rule for dynamo wave propagation can be
  circumvented in Babcock-Leighton dynamo models by the latitudinal
  component of turbulent pumping, which can generate equatorward
  propagating sunspot belts in the absence of a deep, equatorward
  meridional flow. We also show that variations in turbulent pumping
  coefficients can modulate the solar cycle amplitude and periodicity. Our
  results suggest the viability of an alternate magnetic flux transport
  paradigm—mediated via turbulent pumping—for sustaining solar-stellar
  dynamo action.

Title: The Solar Ultraviolet Imaging Telescope onboard Aditya-L1
Authors: Ghosh, Avyarthana; Chatterjee, Subhamoy; Khan, Aafaque R.;
   Tripathi, Durgesh; Ramaprakash, A. N.; Banerjee, Dipankar; Chordia,
   Pravin; Gandorfer, Achim M.; Krivova, Natalie; Nandy, Dibyendu;
   Rajarshi, Chaitanya; Solanki, Sami K.; Sriram, S.
Bibcode: 2016SPIE.9905E..03G
Altcode:
  The Solar Ultraviolet Imaging Telescope (SUIT) is an instrument onboard
  the Aditya-L1 spacecraft, the first dedicated solar mission of the
  Indian Space Research Organization (ISRO), which will be put in a
  halo orbit at the Sun-Earth Langrage point (L1). SUIT has an off-axis
  Ritchey-Chrétien configuration with a combination of 11 narrow and
  broad bandpass filters which will be used for full-disk solar imaging
  in the Ultravoilet (UV) wavelength range 200-400 nm. It will provide
  near simultaneous observations of lower and middle layers of the solar
  atmosphere, namely the Photosphere and Chromosphere. These observations
  will help to improve our understanding of coupling and dynamics of
  various layers of the solar atmosphere, mechanisms responsible for
  stability, dynamics and eruption of solar prominences and Coronal Mass
  ejections, and possible causes of solar irradiance variability in the
  Near and Middle UV regions, which is of central interest for assessing
  the Sun's influence on climate.

Title: The Impact Of Torsional Oscillations On The Solar Cycle:
    The Waldmeier-effect As An Outcome
Authors: Mahajan, Sushant S.; Nandy, Dibyendu; Dwivedi, Bhola N.;
   Antia, H. M.
Bibcode: 2016SPD....47.0718M
Altcode:
  Temporal variations in the Sun’s internal velocity field with
  a periodicity of about 11 years have been observed in the last
  three decades. The period of these torsional oscillations and their
  latitudinal propagation roughly coincide with the period and equatorward
  propagation of sunspots which originate from a magnetohydrodynamic
  dynamo mechanism operating in the Sun’s interior. While the solar
  differential rotation plays an important role in this dynamo mechanism
  by inducting the toroidal component of magnetic field, the impact of
  torsional oscillations on the dynamo mechanism - and hence the solar
  cycle - is not well understood. Here, we include the observed torsional
  oscillations into a flux transport dynamo model of the solar cycle
  to inves- tigate their effect. Although the overall amplitude of the
  solar cycle does not change significantly on inclusion of torsional
  oscillations we find that all the characteristics of the Waldmeier
  effect inthe sunspot cycle are qualitatively reproduced by varying
  only the amplitude of torsional oscillations. The Waldmeier effect,
  first noted in 1935, includes the important characteristic that the
  amplitude of sunspot cycles is anti-correlated to their rise time;
  cycles with high initial rise rate tend to be stronger. This has
  implications for solar cycle predictions. Our result suggests that the
  Waldmeier effect is a plausible outcome of cycle-to-cycle modulation
  of torsional oscillations and provides a physical basis for sunspot
  cycle forecasts based on torsional oscillation observations.

Title: The Solar-Stellar Connection
Authors: Brun, A. S.; García, R. A.; Houdek, G.; Nandy, D.;
   Pinsonneault, M.
Bibcode: 2015SSRv..196..303B
Altcode: 2014SSRv..tmp...54B; 2015arXiv150306742B
  We discuss how recent advances in observations, theory and numerical
  simulations have allowed the stellar community to progress in its
  understanding of stellar convection, rotation and magnetism and to
  assess the degree to which the Sun and other stars share similar
  dynamical properties. Ensemble asteroseismology has become a reality
  with the advent of large time domain studies, especially from space
  missions. This new capability has provided improved constraints
  on stellar rotation and activity, over and above that obtained via
  traditional techniques such as spectropolarimetry or CaII H&amp;K
  observations. New data and surveys covering large mass and age ranges
  have provided a wide parameter space to confront theories of stellar
  magnetism. These new empirical databases are complemented by theoretical
  advances and improved multi-D simulations of stellar dynamos. We trace
  these pathways through which a lucid and more detailed picture of
  magnetohydrodynamics of solar-like stars is beginning to emerge and
  discuss future prospects.

Title: Erratum: Erratum to: The Solar-Stellar Connection
Authors: Brun, A. S.; García, R. A.; Houdek, G.; Nandy, D.;
   Pinsonneault, M.
Bibcode: 2015SSRv..196..357B
Altcode: 2015SSRv..tmp...30B
  No abstract at ADS

Title: a Roadmap to Advance Understanding of the Science of Space
    Weather
Authors: Schrijver, K.; Kauristie, K.; Aylward, A.; De Nardin, C. M.;
   Gibson, S. E.; Glover, A.; Gopalswamy, N.; Grande, M.; Hapgood, M. A.;
   Heynderickx, D.; Jakowski, N.; Kalegaev, V. V.; Lapenta, G.; Linker,
   J.; Liu, S.; Mandrini, C. H.; Mann, I. R.; Nagatsuma, T.; Nandy, D.;
   Obara, T.; O'Brien, T. P., III; Onsager, T. G.; Opgenoorth, H. J.;
   Terkildsen, M. B.; Valladares, C. E.; Vilmer, N.
Bibcode: 2015AGUFMSH12A..01S
Altcode:
  There is a growing appreciation that the environmental conditions that
  we call space weather impact the technological infrastructure that
  powers the coupled economies around the world. With that comes the need
  to better shield society against space weather by improving forecasts,
  environmental specifications, and infrastructure design. A COSPAR/ILWS
  team recently completed a roadmap that identifies the scientific focus
  areas and research infrastructure that are needed to significantly
  advance our understanding of space weather of all intensities and of
  its implications and costs for society. This presentation provides a
  summary of the highest-priority recommendations from that roadmap.

Title: Radiative properties of few F- and Cl- like alkali and
    alkaline-earth metal ions
Authors: Nandy, D. K.; Singh, Sukhjit; Sahoo, B. K.
Bibcode: 2015MNRAS.452.2546N
Altcode:
  We present high-accuracy calculations of radiative properties such as
  oscillator strengths and transition probabilities, of the allowed ns
  <SUP>2</SUP>S<SUB>1/2</SUB> → np <SUP>2</SUP>P<SUB>1/2, 3/2</SUB>
  transitions and of the forbidden np <SUP>2</SUP>P<SUB>1/2</SUB>
  → np <SUP>2</SUP>P<SUB>3/2</SUB> transitions in the F- and Cl-like
  alkali and alkaline-earth ions with the ground state principal quantum
  number n of the respective ion. For this purpose, we have employed the
  Dirac-Fock, relativistic second-order many-body perturbation theory and
  an all-order perturbative relativistic method in the coupled-cluster
  (CC) theory framework. To test the validity of these methods for
  giving accurate results, we first evaluated the ionization potentials
  in the creation processes of these ions and compare them with their
  experimental values listed in the National Institute of Science
  and Technology data base. Moreover, both the allowed and forbidden
  transition amplitudes are estimated using the above three methods and
  a comparative analysis is made to follow-up the electron correlation
  trends in order to demonstrate the need of using a sophisticated method
  like the CC theory for their precise determination. For astrophysical
  use, we provide the most precise values of the transition properties
  by combining the experimental energies, which suppresses uncertainties
  from the calculated energies, using the transition amplitudes from
  the CC method. These data will be useful in the abundance analysis of
  the considered ions in the astronomical objects and for the diagnostic
  processes of astrophysical plasmas.

Title: Understanding space weather to shield society: A global road
    map for 2015-2025 commissioned by COSPAR and ILWS
Authors: Schrijver, Carolus J.; Kauristie, Kirsti; Aylward, Alan D.;
   Denardini, Clezio M.; Gibson, Sarah E.; Glover, Alexi; Gopalswamy,
   Nat; Grande, Manuel; Hapgood, Mike; Heynderickx, Daniel; Jakowski,
   Norbert; Kalegaev, Vladimir V.; Lapenta, Giovanni; Linker, Jon A.;
   Liu, Siqing; Mandrini, Cristina H.; Mann, Ian R.; Nagatsuma, Tsutomu;
   Nandy, Dibyendu; Obara, Takahiro; Paul O'Brien, T.; Onsager, Terrance;
   Opgenoorth, Hermann J.; Terkildsen, Michael; Valladares, Cesar E.;
   Vilmer, Nicole
Bibcode: 2015AdSpR..55.2745S
Altcode: 2015arXiv150306135S
  There is a growing appreciation that the environmental conditions
  that we call space weather impact the technological infrastructure
  that powers the coupled economies around the world. With that comes
  the need to better shield society against space weather by improving
  forecasts, environmental specifications, and infrastructure design. We
  recognize that much progress has been made and continues to be made
  with a powerful suite of research observatories on the ground and
  in space, forming the basis of a Sun-Earth system observatory. But
  the domain of space weather is vast - extending from deep within the
  Sun to far outside the planetary orbits - and the physics complex
  - including couplings between various types of physical processes
  that link scales and domains from the microscopic to large parts
  of the solar system. Consequently, advanced understanding of space
  weather requires a coordinated international approach to effectively
  provide awareness of the processes within the Sun-Earth system through
  observation-driven models. This roadmap prioritizes the scientific focus
  areas and research infrastructure that are needed to significantly
  advance our understanding of space weather of all intensities and
  of its implications for society. Advancement of the existing system
  observatory through the addition of small to moderate state-of-the-art
  capabilities designed to fill observational gaps will enable significant
  advances. Such a strategy requires urgent action: key instrumentation
  needs to be sustained, and action needs to be taken before core
  capabilities are lost in the aging ensemble. We recommend advances
  through priority focus (1) on observation-based modeling throughout the
  Sun-Earth system, (2) on forecasts more than 12 h ahead of the magnetic
  structure of incoming coronal mass ejections, (3) on understanding
  the geospace response to variable solar-wind stresses that lead to
  intense geomagnetically-induced currents and ionospheric and radiation
  storms, and (4) on developing a comprehensive specification of space
  climate, including the characterization of extreme space storms to guide
  resilient and robust engineering of technological infrastructures. The
  roadmap clusters its implementation recommendations by formulating
  three action pathways, and outlines needed instrumentation and research
  programs and infrastructure for each of these. An executive summary
  provides an overview of all recommendations.

Title: Forbidden transition properties in the ground-state
    configurations of singly ionized noble gas atoms for stellar and
    interstellar media
Authors: Nandy, D. K.; Sahoo, B. K.
Bibcode: 2015MNRAS.450.1012N
Altcode:
  High-accuracy calculations of the forbidden transition amplitudes for
  the np <SUP>2</SUP>P<SUB>1/2</SUB> → np <SUP>2</SUP>P<SUB>3/2</SUB>
  transitions with the ground-state principal quantum number n in
  singly charged inert gas atoms, which are of astrophysical interest,
  have been carried out using sophisticated relativistic many-body
  methods. Using these amplitudes, the line strengths, oscillator
  strengths and transition probabilities of the above transitions and
  lifetimes of the np <SUP>2</SUP>P<SUB>1/2</SUB> states are estimated
  precisely. Most of these transition wavelengths lie in the infrared
  region, while the corresponding Rn II line is the optical one, and
  they can be observed in the stellar and interstellar media, where
  the abundances of these ions have already been identified. The above
  forbidden transitions can also be very useful for astrophysical plasma
  diagnostics and can guide experiments to measure the lifetimes of the
  above np <SUP>2</SUP>P<SUB>1/2</SUB> states.

Title: The Relationship Between Solar Coronal X-Ray Brightness and
Active Region Magnetic Fields: A Study Using High-Resolution Hinode
    Observations
Authors: Hazra, Soumitra; Nandy, Dibyendu; Ravindra, B.
Bibcode: 2015SoPh..290..771H
Altcode: 2015SoPh..tmp...19H; 2014arXiv1406.1683H
  By using high-resolution observations of nearly co-temporal and
  co-spatial Solar Optical Telescope spectropolarimeter and X-Ray
  Telescope coronal X-ray data onboard Hinode, we revisit the problematic
  relationship between global magnetic quantities and coronal X-ray
  brightness. Co-aligned vector magnetogram and X-ray data were used
  for this study. The total X-ray brightness over active regions is
  well correlated with integrated magnetic quantities such as the
  total unsigned magnetic flux, the total unsigned vertical current,
  and the area-integrated square of the vertical and horizontal
  magnetic fields. On accounting for the inter-dependence of the
  magnetic quantities, we inferred that the total magnetic flux is the
  primary determinant of the observed integrated X-ray brightness. Our
  observations indicate that a stronger coronal X-ray flux is not related
  to a higher non-potentiality of active-region magnetic fields. The data
  even suggest a slightly negative correlation between X-ray brightness
  and a proxy of active-region non-potentiality. Although there are
  small numerical differences in the established correlations, the main
  conclusions are qualitatively consistent over two different X-ray
  filters, the Al-poly and Ti-poly filters, which confirms the strength
  of our conclusions and validate and extend earlier studies that used
  low-resolution data. We discuss the implications of our results and
  the constraints they set on theories of solar coronal heating.

Title: Relativistic calculations of radiative properties and
    fine structure constant varying sensitivity coefficients in the
    astrophysically relevant Zn II, Si IV and Ti IV ions
Authors: Nandy, D. K.; Sahoo, B. K.
Bibcode: 2015MNRAS.447.3812N
Altcode:
  We have carried out calculations of the relativistic sensitivity
  coefficients, oscillator strengths, transition probabilities,
  lifetimes and magnetic dipole hyperfine structure constants for a
  number of low-lying states in the Zn II, Si IV and Ti IV ions which
  are abundant in the distant quasars and various stellar plasmas. These
  spectroscopic data will be very useful for probing temporal variation
  of the fine structure constant (α<SUB>e</SUB>) and in the diagnostic
  processes of some of the astrophysical plasmas. We have employed
  all-order perturbative methods in the relativistic coupled-cluster
  framework using the Dirac-Coulomb Hamiltonian to calculate the atomic
  wavefunctions of the considered ions. Reference states are constructed
  with the V<SUP>N-1</SUP> and V<SUP>N+1</SUP> potentials and then
  the electron-electron correlation effects are taken into account by
  constructing all possible singly and doubly excited configurations,
  involving both the core and valence electrons, from the respective
  reference states. We have also determined one electron affinities and
  ionization potentials of many excited states in these Zn II, Si IV and
  Ti IV ions. Except for a few states we have attained accuracies within 1
  per cent for the energies compared with their experimental values. Our
  calculated sensitivity coefficients are estimated to have similar
  accuracies as of the calculated energies. Furthermore, combining
  our calculated transition matrix elements with the experimental
  wavelengths we evaluate transition probabilities, oscillator strengths
  and lifetimes of some of the excited states in these ions. These results
  are compared with the available data in a few cases and found to be
  in very good agreement among themselves. Using our reported hyperfine
  structure constants due to the dominant magnetic dipole interaction,
  it is possible to determine hyperfine splittings approximately in the
  above considered ions.

Title: Long-term Activity Evolution of the Sun-as-a-Star
Authors: Nandy, D.
Bibcode: 2014spih.confE..16N
Altcode:
  No abstract at ADS

Title: A Stochastically Forced Time Delay Solar Dynamo Model:
    Self-consistent Recovery from a Maunder-like Grand Minimum
    Necessitates a Mean-field Alpha Effect
Authors: Hazra, Soumitra; Passos, Dário; Nandy, Dibyendu
Bibcode: 2014ApJ...789....5H
Altcode: 2013arXiv1307.5751H
  Fluctuations in the Sun's magnetic activity, including episodes of
  grand minima such as the Maunder minimum have important consequences for
  space and planetary environments. However, the underlying dynamics of
  such extreme fluctuations remain ill-understood. Here, we use a novel
  mathematical model based on stochastically forced, non-linear delay
  differential equations to study solar cycle fluctuations in which time
  delays capture the physics of magnetic flux transport between spatially
  segregated dynamo source regions in the solar interior. Using this
  model, we explicitly demonstrate that the Babcock-Leighton poloidal
  field source based on dispersal of tilted bipolar sunspot flux, alone,
  cannot recover the sunspot cycle from a grand minimum. We find that
  an additional poloidal field source effective on weak fields—e.g.,
  the mean-field α effect driven by helical turbulence—is necessary
  for self-consistent recovery of the sunspot cycle from grand minima
  episodes.

Title: Implementation and application of the relativistic
    equation-of-motion coupled-cluster method for the excited states of
    closed-shell atomic systems
Authors: Nandy, D. K.; Singh, Yashpal; Sahoo, B. K.
Bibcode: 2014PhRvA..89f2509N
Altcode: 2014arXiv1405.1936N
  We report the implementation of the equation-of-motion coupled-cluster
  (EOMCC) method in the four-component relativistic framework with
  a spherical atomic potential to generate the excited states from
  a closed-shell atomic configuration. This theoretical development
  will be very useful in carrying out high-precision calculations of
  various atomic properties in many atomic systems. We employ this
  method to calculate the excitation energies of many low-lying states
  in a few Ne-like highly charged ions, such as Cr xv, Fe xvii, Co
  xviii, and Ni xix ions, and compare them against their corresponding
  experimental values to demonstrate the accomplishment of the EOMCC
  implementation. The ions considered are appropriate to substantiate
  accurate inclusion of the relativistic effects in the evaluation
  of atomic properties and are also interesting for astrophysical
  studies. Investigation of the temporal variation of the fine-structure
  constant (α) obtained from astrophysical observations is a modern
  research problem for which we also estimate the α sensitivity
  coefficients in the above ions.

Title: A solar dynamo model driven by mean-field alpha and
Babcock-Leighton sources: fluctuations, grand-minima-maxima, and
    hemispheric asymmetry in sunspot cycles
Authors: Passos, D.; Nandy, D.; Hazra, S.; Lopes, I.
Bibcode: 2014A&A...563A..18P
Altcode: 2013arXiv1309.2186P
  Context. Extreme solar activity fluctuations and the occurrence of
  solar grand minima and maxima episodes, such as the Maunder minimum
  and Medieval maximum are well-established, observed features of the
  solar cycle. Nevertheless, such extreme activity fluctuations and the
  dynamics of the solar cycle during Maunder minima-like episodes remain
  ill understood. <BR /> Aims: We explore the origin of such extreme solar
  activity fluctuations and the role of dual poloidal field sources,
  namely the Babcock-Leighton mechanism and the mean-field α effect
  in the dynamics of the solar cycle. We mainly concentrate on entry
  and recovery from grand minima episodes such as the Maunder minimum
  and the dynamics of the solar cycle, including the structure of solar
  butterfly diagrams during grand minima episodes. <BR /> Methods: We use
  a kinematic solar dynamo model with a novel set-up in which stochastic
  perturbations force two different poloidal sources. We explore different
  regimes of operation of these poloidal sources with distinct operating
  thresholds to identify the importance of each. The perturbations are
  implemented independently in both hemispheres which allows the study
  of the level of hemispheric coupling and hemispheric asymmetry in the
  emergence of sunspots. <BR /> Results: From the simulations performed
  we identify a few different ways in which the dynamo can enter a grand
  minima episode. While fluctuations in any of the α effects can trigger
  intermittency, in keeping with results from a mathematical time-delay
  model we find that the mean-field α effect is crucial for the recovery
  of the solar cycle from a grand minima episode, which a Babcock-Leighton
  source alone fails to achieve. Our simulations also demonstrate many
  types of hemispheric asymmetries, including grand minima and failed
  grand minima where only one hemisphere enters a quiescent state. <BR
  /> Conclusions: We conclude that stochastic fluctuations in two
  interacting poloidal field sources working with distinct operating
  thresholds is a viable candidate for triggering episodes of extreme
  solar activity and that the mean-field α effect capable of working
  on weak, sub-equipartition fields is critical to the recovery of the
  solar cycle following an extended solar minimum. Based on our results,
  we also postulate that solar activity can exhibit significant parity
  shifts and hemispheric asymmetry, including phases when only one
  hemisphere is completely quiescent while the other remains active,
  to, successful grand minima like conditions in both hemispheres.

Title: Spectral properties of a few F-like ions
Authors: Nandy, D. K.; Sahoo, B. K.
Bibcode: 2014A&A...563A..25N
Altcode:
  <BR /> Aims: We intend to provide accurate data for the oscillator
  strengths, transition probabilities, lifetimes, and hyperfine shifts
  of the atomic energy levels in the fluorine (F)- like Ti, V, Cr, Mn,
  Co, Ni, Cu, Zn, and Mo ions for the diagnostic of the astrophysical
  plasma. Furthermore, we propose these ions for probing possible
  variation in the fine structure constant (α<SUB>e</SUB>) by observing
  their transition lines from the distant astronomical objects. <BR
  /> Methods: We have employed an all-order perturbative method in
  the relativistic coupled-cluster framework using the Dirac-Coulomb
  Hamiltonian for calculating the atomic wave functions in the considered
  F-like ions. We adopted the Fock-space formalism to account for the
  correlation effects among the occupied electrons by allowing all
  possible singly and doubly excited configurations in two steps: first
  considering the correlations between the electrons in a closed-shell
  configuration, and second constructing the open-shell configurations
  of the first three low-lying states in the considered atomic systems by
  removing the respective electron from the core orbitals. This procedure
  enormously simplifies the computational complexity and overcomes the
  need to select important configuration state functions in determining
  the atomic state functions in contrast to a truncated configuration
  interaction method. Moreover, corrections from the frequency-independent
  Breit interaction and the lowest order quantum electrodynamic effects
  are incorporated self-consistently. <BR /> Results: We present the
  calculated ionization potential energies of the electrons that are
  detached to form the above considered F-like ions. The correctness of
  our estimated results are ensured by comparing them with the available
  experimental values and other calculations. From these calculations,
  we evaluate the relativistic sensitivity coefficients that we propose
  here to be used for probing possible temporal variation of the fine
  structure constant (α<SUB>e</SUB>) through the precise astrophysical
  observation of the spectral lines of the transitions involving the
  above low-lying states. The accuracies of these sensitivity coefficients
  are appraised from the comparison of our calculated energies with the
  experimental values. We also present the line strengths by calculating
  the reduced matrix elements of the allowed transitions and of the M1 and
  E2 forbidden transitions. By combining our calculated line strengths
  with the observed wavelengths, we determine the oscillator strengths,
  transition probabilities, and lifetimes of the first two excited
  states of the above ions and compare them with the other theoretical
  calculations. The hyperfine splittings of the states are also presented
  by calculating their magnetic dipole and electric quadrupole hyperfine
  structure constants.

Title: Helioseismic Perspective of the Solar Dynamo
Authors: Muñoz-Jaramillo, A.; Martens, P. C. H.; Nandy, D.
Bibcode: 2013ASPC..478..271M
Altcode:
  Helioseismology has been, without a doubt, one of the greatest
  contributors to our understanding of the solar cycle. In particular,
  its results have been critical in the development of solar dynamo
  models, by providing modelers with detailed information about the
  internal, large scale flows of solar plasma. <P />This review will
  give a historical overview of the evolution of our understanding of the
  solar cycle, placing special emphasis on advances driven by helioseismic
  results. We will discuss some of the outstanding modeling issues, and
  discuss how Helioseismology can help push our understanding forward
  during the next decade.

Title: Forecasting the solar activity cycle: new insights
Authors: Nandy, Dibyendu; Karak, Bidya Binay
Bibcode: 2013IAUS..294..439N
Altcode: 2013arXiv1312.7613N
  Having advance knowledge of solar activity is important because the
  Sun's magnetic output governs space weather and impacts technologies
  reliant on space. However, the irregular nature of the solar cycle makes
  solar activity predictions a challenging task. This is best achieved
  through appropriately constrained solar dynamo simulations and as such
  the first step towards predictions is to understand the underlying
  physics of the solar dynamo mechanism. In Babcock-Leighton type dynamo
  models, the poloidal field is generated near the solar surface whereas
  the toroidal field is generated in the solar interior. Therefore a
  finite time is necessary for the coupling of the spatially segregated
  source layers of the dynamo. This time delay introduces a memory in the
  dynamo mechanism which allows forecasting of future solar activity. Here
  we discuss how this forecasting ability of the solar cycle is affected
  by downward turbulent pumping of magnetic flux. With significant
  turbulent pumping the memory of the dynamo is severely degraded and
  thus long term prediction of the solar cycle is not possible; only a
  short term prediction of the next cycle peak may be possible based on
  observational data assimilation at the previous cycle minimum.

Title: EXPLORING SOLAR GRAND MINIMA THROUGH A TIME DELAY DYNAMO MODEL
Authors: Hazra, Soumitra; Nandy, D.
Bibcode: 2013SPD....4440305H
Altcode:
  Fluctuations in solar magnetic output, including episodes of grand
  minima such as the Maunder minimum are studied using a mathematical
  model based on time delay differential equations. Time delays physically
  capture the effect of the finite time required for magnetic flux
  transport between two spatially segregated source regions for toroidal
  and poloidal field creation. We show that cycle to cycle variations
  and Maunder-like grand minima are naturally produced by introduction of
  stochastic fluctuation in the poloidal field generation mechanism. Our
  analysis shows that the Babcock-Leighton mechanism, alone, cannot
  restart the sunspot cycle once it settles into a grand minimum. An
  additional poloidal field source capable of working on weak toroidal
  magnetic fields - such as that driven by helical turbulence (mean-field
  alpha-effect) - is necessary to recover the solar cycle. Our result
  demonstrates the importance of a small-scale alpha-effect in the
  context of the large-scale solar cycle dynamics and shows, how self
  consistent entry and exit from grand minima like episodes are possible.

Title: Long-term solar activity and its implications to the
    heliosphere, geomagnetic activity, and the Earth's climate. Preface
    to the Special Issue on Space Climate
Authors: Mursula, Kalevi; Manoharan, Periasamy; Nandy, Dibyendu;
   Tanskanen, Eija; Verronen, Pekka
Bibcode: 2013JSWSC...3A..21M
Altcode:
  The Sun's long-term magnetic variability is the primary driver of space
  climate. This variability is manifested not only in the long-observed
  and dramatic change of magnetic fields on the solar surface, but also
  in the changing solar radiative output across all wavelengths. The Sun's
  magnetic variability also modulates the particulate and magnetic fluxes
  in the heliosphere, which determine the interplanetary conditions and
  impose significant electromagnetic forces and effects upon planetary
  atmospheres. All these effects due to the changing solar magnetic
  fields are also relevant for planetary climates, including the climate
  of the Earth. The ultimate cause of solar variability, at time scales
  much shorter than stellar evolutionary time scales, i.e., at decadal
  to centennial and, maybe, even millennial or longer scales, has its
  origin in the solar dynamo mechanism. Therefore, in order to better
  understand the origin of space climate, one must analyze different
  proxies of solar magnetic variability and develop models of the solar
  dynamo mechanism that correctly produce the observed properties of the
  magnetic fields. This Preface summarizes the most important findings
  of the papers of this Special Issue, most of which were presented in
  the Space Climate-4 Symposium organized in 2011 in Goa, India.

Title: Use of a time delay dynamo model to obtain solar-like sunspot
    cycles
Authors: Amouzou, E.; Nandy, D.; Muñoz-Jaramillo, A.; Martens, P.
Bibcode: 2013ASInC..10...83A
Altcode:
  Using a delay-differential equation model, we simulate the solar
  dynamo. We find that solar-like dynamo solutions exist in certain
  parameter regimes for which the dynamo number is less than or about
  equal to -3 (|N_D| &gt; 3, N_D &lt; 0) and that sunspot cycle periods of
  11 years can be reproduced with the parameter values set at a magnetic
  diffusivity of η = 3.5 × 10^{12} cm^{2}/s and a total time delay of
  approximately 2.8 yr.

Title: Double ring algorithm of solar active region eruptions within
    the framework of kinematic dynamo model
Authors: Hazra, Soumitra; Nandy, Dibyendu
Bibcode: 2013ASInC..10..115H
Altcode: 2013arXiv1302.3133H
  Recent results indicate that the Babcock-Leighton mechanism for poloidal
  field creation plays an important role in the solar cycle. However,
  modelling this mechanism has not always correctly captured the
  underlying physics. In particular, it has been demonstrated that
  using a spatially distributed near-surface alpha-effect to parametrize
  the Babcock-Leighton mechanism generates results which do not agree
  with observations. Motivated by this, we are developing a physically
  more consistent model of the solar cycle in which we model poloidal
  field creation by the emergence and flux dispersal of double-ring
  structures. Here, we present preliminary results from this new dynamo
  model.

Title: Turbulent Pumping of Magnetic Flux Reduces Solar Cycle Memory
    and thus Impacts Predictability of the Sun's Activity
Authors: Karak, Bidya Binay; Nandy, Dibyendu
Bibcode: 2012ApJ...761L..13K
Altcode: 2012arXiv1206.2106K
  Prediction of the Sun's magnetic activity is important because of its
  effect on space environment and climate. However, recent efforts to
  predict the amplitude of the solar cycle have resulted in diverging
  forecasts with no consensus. Yeates et al. have shown that the dynamical
  memory of the solar dynamo mechanism governs predictability, and
  this memory is different for advection- and diffusion-dominated solar
  convection zones. By utilizing stochastically forced, kinematic dynamo
  simulations, we demonstrate that the inclusion of downward turbulent
  pumping of magnetic flux reduces the memory of both advection- and
  diffusion-dominated solar dynamos to only one cycle; stronger pumping
  degrades this memory further. Thus, our results reconcile the diverging
  dynamo-model-based forecasts for the amplitude of solar cycle 24. We
  conclude that reliable predictions for the maximum of solar activity
  can be made only at the preceding minimum—allowing about five years
  of advance planning for space weather. For more accurate predictions,
  sequential data assimilation would be necessary in forecasting models
  to account for the Sun's short memory.

Title: All Quiet on the Solar Front: Origin and Heliospheric
    Consequences of the Unusual Minimum of Solar Cycle 23
Authors: Nandy, D.; Muñoz-Jaramillo, A.; Martens, P. C. H.
Bibcode: 2012SunGe...7...17N
Altcode:
  The magnetic activity of the Sun shapes the heliospheric space
  environment through modulation of the solar wind, interplanetary
  magnetic field, cosmic ray flux and solar irradiance. Sunspots -
  strongly magnetized regions on the solar surface - also spawns solar
  storms such as flares and coronal mass ejections which generate severe
  space weather affecting space-based technologies. The Sun's magnetic
  output varies in a cyclic manner going through phases of maximum and
  minimum activity. Following solar cycle 23 the Sun entered a prolonged
  and unusually long minimum with a large number of days without sunspots
  that was unprecedented in the space age. This long phase of very low
  solar activity resulted in record high cosmic ray flux at Earth, weak
  solar wind speeds and low interplanetary magnetic field. We provide an
  overview of this peculiar solar minimum, critically explore theories
  for its origin and argue that the unusual conditions in the heliosphere
  that we experienced during this minimum eventually originated in solar
  internal dynamics.

Title: Modeling the solar cycle: what the future holds
Authors: Nandy, Dibyendu
Bibcode: 2012IAUS..286...54N
Altcode: 2011arXiv1111.5352N
  Stellar magnetic fields are produced by a magnetohydrodynamic dynamo
  mechanism working in their interior - which relies on the interaction
  between plasma flows and magnetic fields. The Sun, being a well-observed
  star, offers an unique opportunity to test theoretical ideas and
  models of stellar magnetic field generation. Solar magnetic fields
  produce sunspots, whose number increases and decreases with a 11 year
  periodicity - giving rise to what is known as the solar cycle. Dynamo
  models of the solar cycle seek to understand its origin, variation
  and evolution with time. In this review, I summarize observations of
  the solar cycle and describe theoretical ideas and kinematic dynamo
  modeling efforts to address its origin. I end with a discussion on
  the future of solar cycle modeling - emphasizing the importance of
  a close synergy between observational data assimilation, kinematic
  dynamo models and full magnetohydrodynamic models of the solar interior.

Title: The Solar Cycle: From Understanding to Forecasting
Authors: Nandy, Dibyendu
Bibcode: 2012AAS...22030001N
Altcode:
  Solar and stellar magnetic cycles are born out of a magnetohydrodynamic
  dynamo mechanism involving interactions between internal plasma flows
  and magnetic fields. The Sun offers a unique opportunity of exploring
  this dynamo mechanism in detail. The solar cycle is manifested as a
  periodic variation in the number of sunspots. This magnetic activity
  spawns severe space weather which can adversely affect technologies
  exposed to environmental conditions in space. It is also thought that
  slower, long-term variations in the Sun's magnetic activity influence
  planetary climates such as that of the Earth. Understanding the physical
  processes that generate the solar cycle is therefore of fundamental
  importance. While it is expected that this understanding should also
  lead to reliable predictive capabilities, unfortunately, forecasts for
  the amplitude of the (ongoing) solar cycle 24 have not converged. In
  this talk, after providing an introduction to solar dynamo theory, I
  will review our current state of understanding and critically discuss
  the underlying physics of solar cycle predictability.

Title: The Sun Has A Short Memory: Turbulent Pumping Of Magnetic
    Flux Reduces Solar Cycle Memory And Precludes Long-term Predictions
Authors: Nandy, Dibyendu; Karak, B. B.
Bibcode: 2012AAS...22052118N
Altcode:
  Predicting the activity of the Sun is important because of its effect
  on space environmental conditions and climate. However, recent efforts
  to predict the amplitude of the solar cycle have resulted in diverging
  forecasts with no consensus. It is understood that the dynamical
  memory of the solar dynamo mechanism governs predictability and this
  memory is different for advection- and diffusion-dominated solar
  convection zones. By utilizing stochastically forced, kinematic dynamo
  simulations, we demonstrate that the inclusion of downward turbulent
  pumping of magnetic flux reduces the memory of both advection- and
  diffusion-dominated solar dynamos to only one cycle; stronger pumping
  degrades this memory further. We conclude that the dynamical memory
  of the solar cycle is short; reliable predictions for the maximum of
  solar activity can be made only at the preceding minimum which explains
  why early forecasts for the maximum of solar cycle 24 have widely
  diverged. Our analysis suggests that for more accurate predictions,
  sequential data assimilation would be necessary in forecasting models
  to account for the Sun's short memory.

Title: Use of a Time Delay Dynamo Model to Obtain Sun-Like Sunspot
    Cycles
Authors: Amouzou, Ernest C.; Nandy, D.; Munoz-Jaramillo, A.; Martens,
   P. C. H.
Bibcode: 2012AAS...22020611A
Altcode:
  Using a time delay-based, simplified dynamo model, we attempted to
  produce results characteristic of the Sun when the parameters are
  set to solar values. We found that dynamo solutions exist for dynamo
  numbers less than or about equal to -3 (|ND| &gt; 3,ND &lt; 0) and that
  sunspot cycle periods of the same order of magnitude of the 11-year
  sunspot cycle can be obtained when the diffusive time scale and the
  total time delay are both about four years.

Title: Sc III spectral properties of astrophysical interest
Authors: Nandy, D. K.; Singh, Y.; Sahoo, B. K.; Li, C.
Bibcode: 2011JPhB...44v5701N
Altcode: 2011arXiv1107.5453N
  Transition properties such as oscillator strengths, transition
  rates, branching ratios and lifetimes of many low-lying states in the
  doubly ionized scandium (Sc III) are reported. A relativistic method
  in the coupled-cluster framework has been employed to incorporate
  the electron correlation effects due to the Coulomb interaction to
  all orders by considering all possible singly and doubly excited
  electronic configurations conjointly with the leading order triply
  excited configurations in a perturbative approach. Present results
  are compared with the previously reported results for the transition
  lines of astrophysical interest. In addition, some of the transition
  properties and lifetimes of a few low-lying states are given for the
  first time. The role of the correlation effects in the evaluation of
  the transition strengths are described concisely.

Title: Recent Improvements of Kinematic Models of the Solar Magnetic
    Cycle
Authors: Muñoz-Jaramillo, Andrés; Nandy, Dibyendu; Martens, Petrus
   C. H.
Bibcode: 2011shin.confE...3M
Altcode:
  One of the best tools we have for understanding the origin of
  solar magnetic variability are kinematic dynamo models. During the
  last decade, this type of models has seen a continuous evolution
  and has become increasingly successful at reproducing solar cycle
  characteristics. Unfortunately, most of ingredients that make up
  a kinematic dynamo model remain poorly constrained allowing one to
  obtain solar-like solutions by 'tuning' the input parameters' leading
  to controversy regarding which parameter set is more appropriate. In
  this poster we will revisit two of those ingredients and show how to
  constrain them better by using observational data and theoretical
  considerations. <P />For the turbulent magnetic diffusivity -
  an ingredient which attempts to capture the effect of convective
  turbulence on the large scale magnetic field - we show that combining
  mixing-length theory estimates with magnetic quenching allows us
  to obtain viable magnetic cycles (otherwise impossible) and that the
  commonly used diffusivity profiles can be understood as a spatiotemporal
  average of this process. <P />For the poloidal source - the ingredient
  which closes the cycle by regenerating the poloidal magnetic field -
  we introduce a more realistic way of modeling active region emergence
  and decay and find that this resolves existing discrepancies between
  kinematic dynamo models and surface flux transport simulations. This
  formulation has made possible to study the physical mechanisms leading
  to the extended minimum of cycle 23 and paves the way for future
  coupling between kinematic dynamos and models of the solar corona. <P
  />This work is funded by NASA Living With a Star Grant NNX08AW53G to
  Montana State University/Harvard-Smithsonian Center for Astrophysics
  and the Government of India's Ramanujan Fellowship.

Title: Meridional Surface Flows and the Recent Extended Solar Minimum
Authors: Martens, Petrus C.; Nandy, D.; Munoz-Jaramillo, A.
Bibcode: 2011SPD....42.1705M
Altcode: 2011BAAS..43S.1705M
  Nandy, Munoz, &amp; Martens, have published a kinematic dynamo model
  that successfully reproduces the main characteristics of the recent
  extended solar minimum (Nature 2011, 471, 80). The model depends on
  the solar meridional flow and its return flow along the tachocline
  determining the period and character of the cycle. In particular Nandy
  et al. found that a meridional flow that is fast in the first half
  of the cycle and then slows down around solar maximum, can lead to
  an extended minimum with the characteristics of the recent minimum:
  an extended period without sunspots and weak polar fields. <P />It has
  been pointed out that the observed surface meridional flows over the
  last cycle do not fit the pattern assumed by Nandy et al. Hathaway &amp;
  Rightmire (Science 2010, 327-1350) find that the meridional speed of
  small magnetic surface elements observed by SoHO/MDI decreased around
  solar maximum and has not yet recovered. Basu &amp; Antia (ApJ 2010,
  717, 488) find surface plasma meridional flow speeds that are lower at
  solar maximum 23 than at the surrounding minima, which is different
  from both Hathaway and Nandy. <P />While there is no physical reason
  that solar surface flows -- both differential rotation and meridional
  flow -- would vary in lockstep with flows at greater depth, as the
  large radial gradients near the surface clearly indicate, and while
  Nandy et al. have demonstrated that the deeper flows dominate the net
  meridional mass flow, we find that there is in effect a very satisfying
  agreement between the observational results of Hathaway &amp; Rightmire,
  Basu &amp; Antia, and the model assumptions of Nandy, Munoz, &amp;
  Martens. We present an analytical model that reconciles the first two,
  followed by a hydrodynamical model that demonstrates the consistency of
  these observational results with the model assumptions of Nandy et al.

Title: Understanding the Origin of the Extended Minimum of Sunspot
    Cycle 23
Authors: Munoz-Jaramillo, Andres; Nandy, D.; Martens, P. C. H.
Bibcode: 2011SPD....42.1743M
Altcode: 2011BAAS..43S.1743M
  The minimum of solar cycle 23 was characterized by very weak polar
  field strength and a large number of sunspot-less days that was
  unprecedented in the space age. This has had significant consequences in
  the heliospheric space environment in terms of record-high cosmic-ray
  flux and low levels of solar irradiance - which is the primary natural
  driver of the climate system. During this un-anticipated phase,
  there was some speculation as to whether the solar minimum could lead
  to a Maunder-like grand minimum which coincided with the Little Ice
  Age. Here we present the first consistent explanation of the defining
  characteristics of this unusual minimum based on variations in the
  solar meridional plasma flows, and discuss how our results compare with
  observations. <P />This work is funded by NASA Living With a Star Grant
  NNX08AW53G to Montana State University/Harvard-Smithsonian Center for
  Astrophysics and the Government of India's Ramanujan Fellowship.

Title: The Double-Ring Algorithm: Reconciling Surface Flux Transport
    Simulations and Kinematic Dynamo Models
Authors: Munoz-Jaramillo, Andres; Nandy, D.; Martens, P. C. H.;
   Yeates, A. R.
Bibcode: 2011SPD....42.0205M
Altcode: 2011BAAS..43S.0205M
  The emergence of tilted bipolar active regions and the dispersal of
  their flux, mediated via processes such as diffusion, differential
  rotation and meridional circulation is believed to be responsible
  for the reversal of the Sun's polar field. This process (commonly
  known as the Babcock-Leighton mechanism) is usually modeled as a
  near-surface, spatially distributed α-effect in kinematic mean-field
  dynamo models. However, this formulation leads to a relationship
  between polar field strength and meridional flow speed which is
  opposite to that suggested by physical insight and predicted by
  surface flux-transport simulations. With this in mind, we present
  an improved double-ring algorithm for modeling the Babcock-Leighton
  mechanism based on active region eruption, within the framework of
  an axisymmetric dynamo model. We demonstrate that our treatment of
  the Babcock-Leighton mechanism through double-ring eruption leads to
  an inverse relationship between polar field strength and meridional
  flow speed as expected, reconciling the discrepancy between surface
  flux-transport simulations and kinematic dynamo models. Finally,
  we show how this new formulation paves the way for applications,
  which were not possible before, like understanding the nature of the
  extended minimum of sunspot cycle 23 and direct assimilation of active
  region data. <P />This work is funded by NASA Living With a Star Grant
  NNX08AW53G to Montana State University/Harvard-Smithsonian Center for
  Astrophysics and the Government of India's Ramanujan Fellowship.

Title: The unusual minimum of sunspot cycle 23 caused by meridional
    plasma flow variations
Authors: Nandy, Dibyendu; Muñoz-Jaramillo, Andrés; Martens, Petrus
   C. H.
Bibcode: 2011Natur.471...80N
Altcode: 2013arXiv1303.0349N
  Direct observations over the past four centuries show that the number
  of sunspots observed on the Sun's surface varies periodically, going
  through successive maxima and minima. Following sunspot cycle 23,
  the Sun went into a prolonged minimum characterized by a very weak
  polar magnetic field and an unusually large number of days without
  sunspots. Sunspots are strongly magnetized regions generated by a
  dynamo mechanism that recreates the solar polar field mediated through
  plasma flows. Here we report results from kinematic dynamo simulations
  which demonstrate that a fast meridional flow in the first half of a
  cycle, followed by a slower flow in the second half, reproduces both
  characteristics of the minimum of sunspot cycle 23. Our model predicts
  that, in general, very deep minima are associated with weak polar
  fields. Sunspots govern the solar radiative energy and radio flux,
  and, in conjunction with the polar field, modulate the solar wind, the
  heliospheric open flux and, consequently, the cosmic ray flux at Earth.

Title: A review of Space Climate and an introduction to the papers
    of the JASTP special issue on Space Climate
Authors: Mursula, Kalevi; Marsh, Dan; Nandy, Dibyendu; Usoskin, Ilya
Bibcode: 2011JASTP..73..179M
Altcode:
  No abstract at ADS

Title: Magnetic Quenching of Turbulent Diffusivity: Reconciling
    Mixing-length Theory Estimates with Kinematic Dynamo Models of the
    Solar Cycle
Authors: Muñoz-Jaramillo, Andrés; Nandy, Dibyendu; Martens, Petrus
   C. H.
Bibcode: 2011ApJ...727L..23M
Altcode: 2010arXiv1007.1262M
  The turbulent magnetic diffusivity in the solar convection zone is
  one of the most poorly constrained ingredients of mean-field dynamo
  models. This lack of constraint has previously led to controversy
  regarding the most appropriate set of parameters, as different
  assumptions on the value of turbulent diffusivity lead to radically
  different solar cycle predictions. Typically, the dynamo community
  uses double-step diffusivity profiles characterized by low values of
  diffusivity in the bulk of the convection zone. However, these low
  diffusivity values are not consistent with theoretical estimates based
  on mixing-length theory, which suggest much higher values for turbulent
  diffusivity. To make matters worse, kinematic dynamo simulations cannot
  yield sustainable magnetic cycles using these theoretical estimates. In
  this work, we show that magnetic cycles become viable if we combine the
  theoretically estimated diffusivity profile with magnetic quenching of
  the diffusivity. Furthermore, we find that the main features of this
  solution can be reproduced by a dynamo simulation using a prescribed
  (kinematic) diffusivity profile that is based on the spatiotemporal
  geometric average of the dynamically quenched diffusivity. This bridges
  the gap between dynamically quenched and kinematic dynamo models,
  supporting their usage as viable tools for understanding the solar
  magnetic cycle.

Title: Dynamo models of the solar cycle: current trends and future
    prospects
Authors: Nandy, Dibyendu
Bibcode: 2011ASInC...2...91N
Altcode: 2011arXiv1110.5725N
  The magnetic cycle of the Sun, as manifested in the cyclic appearance
  of sunspots, significantly influences our space environment
  and space-based technologies by generating what is now termed as
  space weather. Long-term variation in the Sun's magnetic output also
  influences planetary atmospheres and climate through modulation of solar
  irradiance. Here, I summarize the current state of understanding of
  this magnetic cycle, highlighting important observational constraints,
  detailing the kinematic dynamo modeling approach and commenting on
  future prospects.

Title: A Double-ring Algorithm for Modeling Solar Active Regions:
    Unifying Kinematic Dynamo Models and Surface Flux-transport
    Simulations
Authors: Muñoz-Jaramillo, Andrés; Nandy, Dibyendu; Martens, Petrus
   C. H.; Yeates, Anthony R.
Bibcode: 2010ApJ...720L..20M
Altcode: 2010arXiv1006.4346M
  The emergence of tilted bipolar active regions (ARs) and the dispersal
  of their flux, mediated via processes such as diffusion, differential
  rotation, and meridional circulation, is believed to be responsible
  for the reversal of the Sun's polar field. This process (commonly
  known as the Babcock-Leighton mechanism) is usually modeled as a
  near-surface, spatially distributed α-effect in kinematic mean-field
  dynamo models. However, this formulation leads to a relationship
  between polar field strength and meridional flow speed which is
  opposite to that suggested by physical insight and predicted by surface
  flux-transport simulations. With this in mind, we present an improved
  double-ring algorithm for modeling the Babcock-Leighton mechanism
  based on AR eruption, within the framework of an axisymmetric dynamo
  model. Using surface flux-transport simulations, we first show that an
  axisymmetric formulation—which is usually invoked in kinematic dynamo
  models—can reasonably approximate the surface flux dynamics. Finally,
  we demonstrate that our treatment of the Babcock-Leighton mechanism
  through double-ring eruption leads to an inverse relationship between
  polar field strength and meridional flow speed as expected, reconciling
  the discrepancy between surface flux-transport simulations and kinematic
  dynamo models.

Title: Empirical Modeling of Radiative versus Magnetic Flux for
    the Sun-as-a-Star
Authors: Preminger, Dora; Nandy, Dibyendu; Chapman, Gary; Martens,
   Petrus C. H.
Bibcode: 2010SoPh..264...13P
Altcode: 2010arXiv1006.4354P; 2010SoPh..tmp...92P
  We study the relationship between full-disk solar radiative flux at
  different wavelengths and average solar photospheric magnetic-flux
  density, using daily measurements from the Kitt Peak magnetograph
  and other instruments extending over one or more solar cycles. We
  use two different statistical methods to determine the underlying
  nature of these flux - flux relationships. First, we use statistical
  correlation and regression analysis and show that the relationships are
  not monotonic for total solar irradiance and for continuum radiation
  from the photosphere, but are approximately linear for chromospheric
  and coronal radiation. Second, we use signal theory to examine the
  flux - flux relationships for a temporal component. We find that
  a well-defined temporal component exists and accounts for some of
  the variance in the data. This temporal component arises because
  active regions with high magnetic-field strength evolve, breaking
  up into small-scale magnetic elements with low field strength, and
  radiative and magnetic fluxes are sensitive to different active-region
  components. We generate empirical models that relate radiative flux to
  magnetic flux, allowing us to predict spectral-irradiance variations
  from observations of disk-averaged magnetic-flux density. In most cases,
  the model reconstructions can account for 85 - 90% of the variability
  of the radiative flux from the chromosphere and corona. Our results
  are important for understanding the relationship between magnetic and
  radiative measures of solar and stellar variability.

Title: Towards better Constrained Kinematic Dynamo Models: Turbulent
    Diffusivity and Diffusivity Quenching
Authors: Munoz-Jaramillo, Andres; Nandy, D.; Martens, P. C. H.
Bibcode: 2010AAS...21640116M
Altcode:
  The turbulent magnetic diffusivity in the Solar Convection Zone
  (SCZ) is one of the most poorly constrained ingredients of mean-field
  dynamo models. This lack of constrain has previously led to controversy
  regarding which set of parameters is more appropriate (yielding better
  solar like solutions) and the generation of radically different cycle
  predictions. Furthermore, due to the relative freedom in the different
  parameters associated with it, more often than not it is used to finely
  tune the dynamo solutions. As of now, the dynamo community seems
  to have settled on double step diffusivity profiles characterized
  by low values of diffusivity inside most of the convection zone;
  notwithstanding that these values of diffusivity are not consistent
  with theoretical considerations based on mixing-length theory, which
  suggest much higher values of turbulent diffusivity. To make matters
  worse, standard kinematic dynamo simulations cannot yield sustainable
  magnetic cycles using theoretical estimates. Here we study how magnetic
  diffusivity quenching can provide a physically meaningful way out of
  this discrepancy and whether standard diffusivity profiles are truly
  a representation of a physical process. This work is funded by NASA
  Living With a Star grant NNG05GE47G.

Title: Are Active Regions as Relevant for the Solar Cycle as we Think?
Authors: Munoz-Jaramillo, Andres; Nandy, D.; Martens, P. C. H.
Bibcode: 2010AAS...21640108M
Altcode: 2010BAAS...41R.858M
  The long and short term variability of the Sun is strongly determined
  by the evolution of the solar magnetic cycle, which is sustained
  through the action of a magneto-hydrodynamic dynamo. In our current
  understanding of the dynamo, the poloidal field (which acts as a
  starting point for the cycle) is recreated through the emergence and
  decay of active regions subjected to the collective effect of meridional
  circulation and turbulent diffusion; a process commonly referred to as
  the Babcock-Leighton mechanism. Dynamo models based on this mechanism
  have been quite successful in reproducing the different properties of
  the solar cycle and have also been used to make predictions of cycle
  24. However, the question of whether the BL mechanism is enough to
  sustain the solar cycle has not yet been addressed quantitatively. By
  including real active region data in our state of the art kinematic
  dynamo model we are able to take the first steps into answering this
  question. <P />This work is funded by NASA Living With a Star grant
  NNG05GE47G.

Title: The Unusual Minimum of Solar Cycle 23 Explained
Authors: Nandy, Dibyendu; Munoz-Jaramillo, A.; Martens, P. C. H.
Bibcode: 2010AAS...21631703N
Altcode: 2010BAAS...41..898N
  The minimum in activity between solar cycle 23 and 24 has been the
  deepest in the space age, with an unusually large number of days
  without sunspots and weak solar dipolar field strength. This has
  had consequences for the heliosphere and planetary atmospheres -
  given the weak solar wind, low solar irradiance and radio flux and
  historically high values of cosmic ray flux that has characterized
  this minimum epoch. The origin of this peculiar minimum has not
  yet been clearly understood. Here we present the first theoretical
  explanation of this deep minimum based on simulations of the solar
  dynamo mechanism - which seeks to explain the origin and variability
  of solar magnetic fields. Our simulations have uncovered a somewhat
  surprising explanation, which however, provides a consistent solution
  to both of the unusual features of this minimum; namely, the long period
  when sunspots were missing and the very weak solar polar field strength.

Title: Comparison of a Global Magnetic Evolution Model with
    Observations of Coronal Mass Ejections
Authors: Yeates, A. R.; Attrill, G. D. R.; Nandy, Dibyendu; Mackay,
   D. H.; Martens, P. C. H.; van Ballegooijen, A. A.
Bibcode: 2010ApJ...709.1238Y
Altcode: 2009arXiv0912.3347Y
  The relative importance of different initiation mechanisms for coronal
  mass ejections (CMEs) on the Sun is uncertain. One possible mechanism is
  the loss of equilibrium of coronal magnetic flux ropes formed gradually
  by large-scale surface motions. In this paper, the locations of flux
  rope ejections in a recently developed quasi-static global evolution
  model are compared with observed CME source locations over a 4.5 month
  period in 1999. Using extreme ultraviolet data, the low-coronal source
  locations are determined unambiguously for 98 out of 330 CMEs. An
  alternative method of determining the source locations using recorded
  Hα events was found to be too inaccurate. Despite the incomplete
  observations, positive correlation (with coefficient up to 0.49) is
  found between the distributions of observed and simulated ejections,
  but only when binned into periods of 1 month or longer. This binning
  timescale corresponds to the time interval at which magnetogram data are
  assimilated into the coronal simulations, and the correlation arises
  primarily from the large-scale surface magnetic field distribution;
  only a weak dependence is found on the magnetic helicity imparted to the
  emerging active regions. The simulations are limited in two main ways:
  they produce fewer ejections, and they do not reproduce the strong
  clustering of observed CME sources into active regions. Due to this
  clustering, the horizontal gradient of radial photospheric magnetic
  field is better correlated with the observed CME source distribution
  (coefficient 0.67). Our results suggest that while the gradual formation
  of magnetic flux ropes over weeks can account for many observed CMEs,
  especially at higher latitudes, there exists a second class of CMEs (at
  least half) for which dynamic active region flux emergence on shorter
  timescales must be the dominant factor. Improving our understanding
  of CME initiation in future will require both more comprehensive
  observations of CME source regions and more detailed magnetic field
  simulations.

Title: Outstanding Issues in Solar Dynamo Theory
Authors: Nandy, D.
Bibcode: 2010ASSP...19...86N
Altcode: 2009arXiv0906.4748N; 2010mcia.conf...86N
  The magnetic activity of the Sun, as manifested in the sunspot
  cycle, originates deep within its convection zone through a dynamo
  mechanism, which involves nontrivial interactions between the plasma
  and the magnetic field in the solar interior. Recent advances in
  magnetohydrodynamic dynamo theory have led us closer towards a
  better understanding of the physics of the solar magnetic cycle. In
  conjunction, helioseismic observations of large-scale flows in the solar
  interior has nowmade it possible to constrain some of the parameters
  used in models of the solar cycle. In the first part of this review,
  I briefly describe this current state of understanding of the solar
  cycle. In the second part, I highlight some of the outstanding issues
  in solar dynamo theory related to the nature of the dynamo α-effect,
  magnetic buoyancy, and the origin of Maunder-like minima in activity. I
  also discuss how poor constraints on key physical processes such
  as turbulent diffusion, meridional circulation, and turbulent flux
  pumping confuse the relative roles of these vis-a-vis magnetic flux
  transport. I argue that unless some of these issues are addressed,
  no model of the solar cycle can claim to be "the standard model,"
  nor can any predictions from such models be trusted; in other words,
  we are still not there yet.

Title: Dynamo Processes
Authors: Nandy, Dibyendu
Bibcode: 2010ASSP...18...35N
Altcode: 2010hepr.book...35N
  Magnetic fields play an important role in defining and modulating the
  space environment within the heliosphere. How these magnetic fields
  originate and evolve in stars such as the Sun or planets such as the
  Earth, is in itself a compelling question - the answer to which is of
  universal importance to astrophysics and space science. It is thought
  that magnetic fields are created through a dynamo process which involves
  complex, non-linear interactions within the magnetized plasma that
  is often encountered in astrophysical systems. In this chapter, after
  briefly discussing the importance of magnetic fields in the heliosphere,
  I provide a gentle introduction to concepts in magnetohydrodynamics,
  describe the mathematical foundation of mean-field dynamo theory,
  and explain the basic physical processes that constitute the dynamo
  mechanism. In doing this, I focus mainly on our star - the Sun -
  which is the primary source of heliospheric magnetic fields and their
  variability.

Title: What do Solar Kinematic Models Tell us About the Current
    Minimum?
Authors: Muñoz-Jaramillo, A.; Nandy, D.; Martens, P. C.
Bibcode: 2009AGUFMSH11A1505M
Altcode:
  In the last three years the sun has reached the most unusual minimum
  in the space age. Although minima as long as this one have happened
  several times in the past, this one has come as a surprise in contrast
  with the previous four who where fairly regular. However, such an event
  is a perfect opportunity to learn more about the solar cycle and the
  processes that drive it. In order to understand this event we turn
  to kinematic dynamo models, which are the best tool we currently have
  for understanding the solar cycle. Although modelers have been aware
  of the role of the different components into setting the period of the
  solar cycle, little work has been done in understanding the nature of
  solar minima. Can kinematic models reproduce such an event with all
  it's signatures? In this study we attempt to address this question
  using our state of the art kinematic dynamo model.

Title: ERRATUM: "Helioseismic Data Inclusion in Solar Dynamo Models"
    <A href="bib_query\?2009ApJ...698..461M">(2009, ApJ, 698, 461)</A>
Authors: Muñoz-Jaramillo, Andrés; Nandy, Dibyendu; Martens, Petrus
   C. H.
Bibcode: 2009ApJ...707.1852M
Altcode:
  No abstract at ADS

Title: Solar Cycle Variations of Coronal Null Points: Implications
    for the Magnetic Breakout Model of Coronal Mass Ejections
Authors: Cook, G. R.; Mackay, D. H.; Nandy, Dibyendu
Bibcode: 2009ApJ...704.1021C
Altcode:
  In this paper, we investigate the solar cycle variation of coronal null
  points and magnetic breakout configurations in spherical geometry, using
  a combination of magnetic flux transport and potential field source
  surface models. Within the simulations, a total of 2843 coronal null
  points and breakout configurations are found over two solar cycles. It
  is found that the number of coronal nulls present at any time varies
  cyclically throughout the solar cycle, in phase with the flux emergence
  rate. At cycle maximum, peak values of 15-17 coronal nulls per day are
  found. No significant variation in the number of nulls is found from
  the rising to the declining phase. This indicates that the magnetic
  breakout model is applicable throughout both phases of the solar
  cycle. In addition, it is shown that when the simulations are used
  to construct synoptic data sets, such as those produced by Kitt Peak,
  the number of coronal nulls drops by a factor of 1/6. The vast majority
  of the coronal nulls are found to lie above the active latitudes and
  are the result of the complex nature of the underlying active region
  fields. Only 8% of the coronal nulls are found to be connected to the
  global dipole. Another interesting feature is that 18% of coronal nulls
  are found to lie above the equator due to cross-equatorial interactions
  between bipoles lying in the northern and southern hemispheres. As
  the majority of coronal nulls form above active latitudes, their
  average radial extent is found to be in the low corona below 1.25 R
  <SUB>sun</SUB> (175, 000 km above the photosphere). Through considering
  the underlying photospheric flux, it is found that 71% of coronal
  nulls are produced though quadrupolar flux distributions resulting
  from bipoles in the same hemisphere interacting. When the number
  of coronal nulls present in each rotation is compared to the number
  of bipoles emerging, a wide scatter is found. The ratio of coronal
  nulls to emerging bipoles is found to be approximately 1/3. Overall,
  the spatio-temporal evolution of coronal nulls is found to follow the
  typical solar butterfly diagram and is in qualitative agreement with
  the observed time dependence of coronal mass ejection source-region
  locations.

Title: Helioseismic Data Inclusion in Solar Dynamo Models
Authors: Muñoz-Jaramillo, Andrés; Nandy, Dibyendu; Martens, Petrus
   C. H.
Bibcode: 2009ApJ...698..461M
Altcode: 2008arXiv0811.3441M
  An essential ingredient in kinematic dynamo models of the solar cycle
  is the internal velocity field within the simulation domain—the
  solar convection zone (SCZ). In the last decade or so, the field of
  helioseismology has revolutionized our understanding of this velocity
  field. In particular, the internal differential rotation of the Sun
  is now fairly well constrained by helioseismic observations almost
  throughout the SCZ. Helioseismology also gives us some information
  about the depth dependence of the meridional circulation in the
  near-surface layers of the Sun. The typical velocity inputs used in
  solar dynamo models, however, continue to be an analytic fit to the
  observed differential rotation profile and a theoretically constructed
  meridional circulation profile that is made to match the flow speed
  only at the solar surface. Here, we take the first steps toward
  the use of more accurate velocity fields in solar dynamo models by
  presenting methodologies for constructing differential rotation and
  meridional circulation profiles that more closely conform to the best
  observational constraints currently available. We also present kinematic
  dynamo simulations driven by direct helioseismic measurements for
  the rotation and four plausible profiles for the internal meridional
  circulation—all of which are made to match the helioseismically
  inferred near-surface depth dependence, but whose magnitudes are made to
  vary. We discuss how the results from these dynamo simulations compare
  with those that are driven by purely analytic fits to the velocity
  field. Our results and analysis indicate that the latitudinal shear in
  the rotation in the bulk of the SCZ plays a more important role, than
  either the tachocline or surface radial shear, in the induction of the
  toroidal field. We also find that it is the speed of the equatorward
  counterflow in the meridional circulation right at the base of the
  SCZ, and not how far into the radiative interior it penetrates, that
  primarily determines the dynamo cycle period. Improved helioseismic
  constraints are expected to be available from future space missions
  such as the Solar Dynamics Observatory and through analysis of more
  long-term continuous data sets from ground-based instruments such as
  the Global Oscillation Network Group. Our analysis lays the basis for
  the assimilation of these helioseismic data within dynamo models to
  make future solar cycle simulations more realistic.

Title: The Unusual Minimum of Cycle 23: Observations and
    Interpretation
Authors: Martens, Petrus C.; Nandy, D.; Munoz-Jaramillo, A.
Bibcode: 2009SPD....40.2403M
Altcode:
  The current minimum of cycle 23 is unusual in its long duration, the
  very low level to which Total Solar Irradiance (TSI) has fallen, and
  the small flux of the open polar fields. The deep minimum of TSI seems
  to be related to an unprecedented dearth of polar faculae, and hence to
  the small amount of open flux. Based upon surface flux transport models
  it has been suggested that the causes of these phenomena may be an
  unusually vigorous meridional flow, or even a deviation from Joy's law
  resulting in smaller Joy angles than usual for emerging flux in cycle
  23. There is also the possibility of a connection with the recently
  inferred emergence in polar regions of bipoles that systematically
  defy Hale's law. <P />Much speculation has been going on as to the
  consequences of this exceptional minimum: are we entering another global
  minimum, is this the end of the 80 year period of exceptionally high
  solar activity, or is this just a statistical hiccup? Dynamo simulations
  are underway that may help answer this question. As an aside it must
  be mentioned that the current minimum of TSI puts an upper limit in the
  TSI input for global climate simulations during the Maunder minimum, and
  that a possible decrease in future solar activity will result in a very
  small but not insignificant reduction in the pace of global warming.

Title: Towards Better Constrained Solar Dynamo Models: The Velocity
    Field And Turbulent Diffusivity Profiles
Authors: Munoz-Jaramillo, Andres; Nandy, D.; Martens, P. C. H.
Bibcode: 2009SPD....40.0405M
Altcode:
  The best tool we have for understanding the origin of solar
  magnetic variability is the kinematic dynamo model. During the
  last decade this type of models have seen a continuous evolution
  and have become increasingly successful at reproducing solar cycle
  characteristics. However, some of the key ingredients used in dynamo
  models remain poorly constrained which allows one to obtain solar-like
  solutions by "tuning" the input parameters. Here we present out
  efforts to better constrain two of the most important ingredients of
  solar dynamo models:: The internal velocity field (meridional flow and
  differential rotation) and the turbulent diffusivity. To accomplish
  this goal, we formulate techniques to assimilate the latest results
  from helioseismology to constrain the velocity fields. We also apply
  mixing length theory to the Solar Model S, in conjunction with magnetic
  quenching of the turbulent diffusivity, to generate more realistic
  effective turbulent diffusivity profiles for kinematic dynamo models. In
  essence therefore, we try to address some of these outstanding issues in
  a first-principle physics based approach, rather than an ad-hoc manner.

Title: Effect of the Magnetic Quenching of the Turbulent Diffusivity
    in a Mean-Field Kinematic Solar Dynamo
Authors: Muñoz-Jaramillo, A.; Nandy, D.; Martens, P. C.
Bibcode: 2008AGUSMSP41A..09M
Altcode:
  The fundamental model used to study the solar dynamo mechanism is
  based on the electromagnetic induction equation coupled with Ohm's
  law. Apart from mean-field or other phenomenological source terms
  (such as a Babcock-Leighton alpha-effect), the resultant dynamo
  equation is composed of two terms: An advection and a diffusion
  term. Depending on the relative importance of these two terms, the
  dynamo can operate either in an advection-dominated or a diffusion
  dominated regime. One of the parameters that determine which of these
  regimes the dynamo operates in is the effective magnetic diffusivity,
  this parameter is expected to be enhanced by convective turbulence
  in stellar convection zones. The diffusivity values can range from
  104 cm2/s in the radiative zone (where there is no turbulence) to
  1012-14 cm2/s in the upper convection zone. The depth dependence of
  this effective diffusivity is not particularly well-constrained and
  most commonly used profiles involve a relatively low diffusivity in
  the convection zone (1010-11 cm2/s) - which makes the dynamo operate
  in the advection-dominated regime. The underlying problem here is
  that these values of diffusivity are not consistent with theoretical
  considerations based on mixing-length theory, which suggest much higher
  values of turbulent diffusivity; this would make the dynamo operate
  in a diffusion-dominated regime. However, a possible solution to this
  inconsistency may be in the quenching effect that strong magnetic
  fields have on turbulence. We have recently developed a kinematic solar
  dynamo based on a novel numerical technique called the exponential-
  propagation method. Using this model, we study magnetic diffusivity
  quenching and discuss how its effect may reconcile the theoretically
  suggested turbulent diffusivity values with the effective diffusivity
  profiles most commonly used in this type of models.

Title: Kinematic Dynamo Models Of The Solar Cycle
Authors: Nandy, D.
Bibcode: 2008AGUSMSP33A..01N
Altcode:
  In the kinematic approach to solar dynamo modeling, one solves for the
  mean magnetic field with given velocity fields such as differential
  rotation and meridional circulation; in addition, key physical processes
  such as turbulent diffusion, magnetic buoyancy and the dynamo α-effect
  have to be parameterized appropriately. Kinematic dynamo models
  perhaps do not capture the full range of complexities of stellar
  convection zones and approximates complex processes through simple
  parameterizations. Nevertheless, the underlying physics of this class
  of models is relatively more transparent, they are computationally less
  demanding and they successfully explain many observed features of the
  solar cycle. In this lecture, I will trace the historical development of
  solar kinematic dynamo models, describe the basic physical ingredients
  and observational inputs necessary to build one, present our current
  state of understanding, and highlight outstanding problems.

Title: Twisted solar active region magnetic fields as drivers of
space weather: Observational and theoretical investigations
Authors: Nandy, Dibyendu; Mackay, Duncan H.; Canfield, Richard C.;
   Martens, P. C. H.
Bibcode: 2008JASTP..70..605N
Altcode:
  The properties and dynamics of magnetic fields on the Sun's photosphere
  and outer layers--notably those within solar active regions--govern
  the eruptive activity of the Sun. These photospheric magnetic
  fields also act as the evolving lower boundary of the Sun-Earth
  coupled system. Quantifying the physical attributes of these magnetic
  fields and exploring the mechanisms underlying their influence on the
  near-Earth space environment are of vital importance for forecasting
  and mitigating adverse space weather effects. In this context, we
  discuss here a novel technique for measuring twist in the magnetic
  field lines of solar active regions that does not invoke the force-free
  field assumption. Twist in solar active regions can play an important
  role in flaring activity and the initiation of CMEs via the kink
  instability mechanism; we outline a procedure for determining this
  solar active region eruptive potential. We also discuss how twist in
  active region magnetic fields can be used as inputs in simulations of
  the coronal and heliospheric fields; specifically, we explore through
  simulations, the formation, evolution and ejection of magnetic flux
  ropes that originate in twisted magnetic structures. The results and
  ideas presented here are relevant for exploring the role of twisted
  solar active region magnetic fields and flux ropes as drivers of space
  weather in the Sun-Earth system.

Title: Magnetic Helicity, Coronal Heating and Solar Flaring Activity:
    A Review of the Role of Active Region Twist
Authors: Nandy, D.
Bibcode: 2008ASPC..383..201N
Altcode:
  Magnetic helicity of solar active regions is quantified by the twist
  and writhe of their underlying flux tubes. The twist component,
  in particular, is believed to be an important determinant of the
  energetics and dynamics of active regions that ultimately lead to
  coronal heating and solar eruptive activity. Here, I review the role
  of active region twist in the context of solar activity --- laying
  emphasis on constraints that are set by observations. The results,
  taken cumulatively, suggest a new paradigm. In this paradigm, the
  distribution and evolution of twist in active region sub-structures
  play a more predominant role in driving solar dynamic activity than
  the magnitude of the global active region twist itself. I discuss
  the implications of these observational constraints for theories of
  stellar coronal heating and solar eruptive activity.

Title: Kinematic properties of solar coronal mass ejections:
    Correction for projection effects in spacecraft coronagraph
    measurements
Authors: Howard, T. A.; Nandy, D.; Koepke, A. C.
Bibcode: 2008JGRA..113.1104H
Altcode:
  One of the main sources of uncertainty in quantifying the kinematic
  properties of coronal mass ejections (CMEs) using coronagraphs is
  the fact that coronagraph images are projected into the sky plane,
  resulting in measurements which can differ significantly from their
  actual values. By identifying solar surface source regions of CMEs
  using X-ray and Hα flare and disappearing filament data, and through
  considerations of CME trajectories in three-dimensional (3-D) geometry,
  we have devised a methodology to correct for the projection effect. We
  outline this method here. The methodology was automated and applied
  to over 10,000 CMEs in the Coordinated Data Analysis Workshop (CDAW)
  (SOHO Large Angle Spectroscopic Coronagraph) catalog spanning 1996-2005,
  in which we could associate 1961 CMEs with an appropriate surface
  event. In the latter subset, deprojected speeds, accelerations, and
  launch angles were determined to study CME kinematics. Our analysis
  of this subset of events reconfirms some important trends, notably
  that previously uncovered solar cycle variation of CME properties are
  preserved, CMEs with greater width have higher speeds, and slower
  CMEs tend to accelerate while faster CMEs tend to decelerate. This
  points out that statistical trends in CME properties, recovered from
  plane-of-sky measurements, may be preserved even in the face of more
  sophisticated 3-D measurements from spacecrafts such as STEREO, if CME
  trajectories are predominantly radial. However, our results also show
  that the magnitude of corrected measurements can differ significantly
  from the projected plane-of-sky measurements on a case-by-case basis
  and that acceleration is more sensitive to the deprojection process
  than speed. Average corrected speed and acceleration tend to be a
  factor of 1.7 and 4.4 higher than their projected values, with mean
  corrected speed and acceleration magnitudes being on the order of 1000
  km/s and 50 m/s<SUP>2</SUP>, respectively. We conclude that while using
  the plane-of-sky measurements may be suitable for studies of general
  trends in a large sample of events, correcting for projection effects
  is mandatory for those investigations which rely on a numerically
  precise determination of the properties of individual CMEs.

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&amp;A 449, 781 789, 2006] study the
  penetration of the wrapped poloidal field into the rising flux tube
  due to turbulent diffusion using a simple 1-d model. They find that
  the extent of penetration of the wrapped field will depend on how
  weak the magnetic field inside the rising flux tube becomes before
  its emergence. They conclude that more detailed observational data
  will throw light on the physical conditions of flux tubes just before
  their emergence to the photosphere.

Title: Exploring the Physical Basis of Solar Cycle Predictions:
    Flux Transport Dynamics and Persistence of Memory in Advection-
    versus Diffusion-dominated Solar Convection Zones
Authors: Yeates, Anthony R.; Nandy, Dibyendu; Mackay, Duncan H.
Bibcode: 2008ApJ...673..544Y
Altcode: 2007arXiv0709.1046Y
  The predictability, or lack thereof, of the solar cycle is governed by
  numerous separate physical processes that act in unison in the interior
  of the Sun. Magnetic flux transport and the finite time delay that it
  introduces, specifically in the so-called Babcock-Leighton models of
  the solar cycle with spatially segregated source regions for the α-
  and Ω-effects, play a crucial rule in this predictability. Through
  dynamo simulations with such a model, we study the physical basis
  of solar cycle predictions by examining two contrasting regimes, one
  dominated by diffusive magnetic flux transport in the solar convection
  zone, the other dominated by advective flux transport by meridional
  circulation. Our analysis shows that diffusion plays an important
  role in flux transport, even when the solar cycle period is governed
  by the meridional flow speed. We further examine the persistence of
  memory of past cycles in the advection- and diffusion-dominated regimes
  through stochastically forced dynamo simulations. We find that in the
  advection-dominated regime this memory persists for up to three cycles,
  whereas in the diffusion-dominated regime this memory persists for
  mainly one cycle. This indicates that solar cycle predictions based
  on these two different regimes would have to rely on fundamentally
  different inputs, which may be the cause of conflicting predictions. Our
  simulations also show that the observed solar cycle amplitude-period
  relationship arises more naturally in the diffusion-dominated regime,
  thereby supporting those dynamo models in which diffusive flux transport
  plays a dominant role in the solar convection zone.

Title: The Stars as Suns Project: Recent Results from Solar and
    Stellar Dynamo Modeling
Authors: Munoz, Andres; Nandy, D.; Martens, P. C.
Bibcode: 2007AAS...210.9209M
Altcode: 2007BAAS...39..210M
  Solar variability controls our space environment and is also believed
  to play a role in shaping the global climate. The variability of the Sun
  can be traced back to the presence and modulation of magnetic fields --
  which has its origin in a dynamo mechanism working in the interior. The
  "Stars as Suns" project aims to determine the long-term variability of
  Sun, through a combination of stellar magnetic activity observations
  of Sun-like stars and theoretical dynamo modeling. Here we present
  recent results from solar and stellar dynamo studies that addresses
  the goals of this project. This research is funded by a NASA Living
  With a Star grant NNG05GE47G to Montana State University.

Title: Long-Term Evolution of Solar Magnetic Activity Derived From
    Stellar Proxies
Authors: Nandy, D.; Martens, P. C.
Bibcode: 2007AGUSMSH54B..04N
Altcode:
  The variability of the Sun over stellar and planetary evolutionary
  timescales may have important consequences for planetary atmospheres
  such as the Earth's, including the forcing of global climate and
  evolution of life. This solar variability is in part due to the
  changing magnetism of the Sun, which has origins in the solar dynamo
  mechanism. A novel approach towards determining solar variability
  over such long timescales - stretching to billions of years - is
  to use Sun-like stars in various evolutionary phases as proxies of
  solar activity. In this talk, I will review efforts to derive this
  long-term variability of the Sun through theoretical dynamo modelling
  and observational analysis of stellar magnetic activity. This work is
  funded by the NASA Living With a Star program through grant NNG05GE47G.

Title: Active Region Magnetic Field Line Twist and Source of Coronal
    Magnetic Helicity.
Authors: Belur, Ravindra; Longcope, D.; Barnes, G.; Nandy, D.
Bibcode: 2007AAS...210.2401B
Altcode: 2007BAAS...39..128B
  Magnetic helicity is an important quantity which measures how the
  magnetic field lines are twisted and sheared. Recently it has become
  possible to measure the flux of magnetic helicity in active regions
  using the observational data. These observed helicity fluxes may
  arise due to the twist in the emerging active region flux tubes or it
  may come from the photospheric shearing motion. Here, we decompose
  the helicity flux into two different contributions called spin
  and braiding. These components typically come from twist and writhe
  helicity of a sub-photospheric flux tube anchored to the regions. The
  spin helicity of a given region quantifies the mean rotation rate of
  motion internal to that region and braiding helicity is injected by the
  motions of whole <P />regions about one another. The injected helicity
  flux due to spin and braiding motion leads to the coronal magnetic
  field line twist. The twist determined from vector magnetograms can be
  used to estimate the total helicity content of the coronal field at one
  time. The rate of change of this helicity estimate can be compared to
  the total helicity flux as well as its spin and braiding component. We
  make such a comparison for several active regions.

Title: A New Technique For Measuring The Twist Of Photospheric
Active Regions Without Recourse To The Force-Free-Field Equation:
    Reconfirming The Hemispheric Helicity Trend
Authors: Nandy, Dibyendu; Calhoun, A.; Windschitl, J.; Canfield,
   R. C.; Linton, M. G.
Bibcode: 2007AAS...210.2402N
Altcode: 2007BAAS...39..128N
  The twist component of magnetic helicity in solar active regions is
  known to be an important indicator of sub-photospheric flux tube
  dynamics and solar eruptive activity. Traditionally, estimates
  of the parameter alpha -- appearing in the force-free-field
  equation -- has been used to infer the twist of photospheric active
  regions. However, the photosphere is not force-free and this has
  lead to recent concerns on the validity of using the alpha parameter
  for determining photospheric active region twist. We have devised a
  new flux-tube-fitting technique for determining the twist of active
  regions without recourse to the force-free-field equation. This method
  assumes that the underlying active region flux system is cylindrically
  symmetric and uniformly twisted. By using this new technique, on a
  statistically compelling number of photospheric active region vector
  magnetograms, we re-confirm the hemispheric helicity rule independent
  of the force-free-field assumption. This research has been supported
  in parts by a NASA Living With a Star grant NNG05GE47G. A.C. and
  J.W. were supported by a NSF Research Experience for Undergraduates
  grant ATM-0243923 to Montana State University. M.G.L. acknowledges
  support from NASA and the Office of Naval Research.

Title: Space Climate and the Solar Stellar connection: What can we
    learn from the stars about long-term solar variability?
Authors: Nandy, Dibyendu; Martens, P. C. H.
Bibcode: 2007AdSpR..40..891N
Altcode:
  While it is well-known that solar variability influences the
  near-Earth Space environment at short timescales of days - an effect
  collectively termed as Space Weather, a more subtle influence of solar
  variability at longer timescales is also present and just beginning to
  be appreciated. Long-term solar forcing and its consequences - which has
  come to be known as Space Climate - has important consequences for the
  formation and evolution of planetary atmospheres, the evolution of life
  and global climate on Earth. Understanding the Sun's variability and
  its heliospheric influence at such scales, stretching from decennia
  to stellar and planetary evolutionary timescales, is therefore of
  fundamental importance. However, our knowledge of this variability,
  which is in part due to the evolution of the solar magnetic dynamo,
  is limited by direct solar observations which exist only from early
  17th Century onwards. In this review we introduce a novel concept -
  how the Solar-Stellar connection can be exploited to understand the
  long-term variability of the Sun and its influence on Space Climate. We
  present some preliminary studies, in which, through theoretical dynamo
  modeling and analysis of magnetic activity observations of solar-like
  stars at various evolutionary phases relative to the Sun, we show how
  the above concept is implemented in practice.

Title: Magnetic helicity and flux tube dynamics in the solar
convection zone: Comparisons between observation and theory
Authors: Nandy, Dibyendu
Bibcode: 2006JGRA..11112S01N
Altcode:
  Magnetic helicity, a conserved topological parameter in ideal MHD
  systems, conditions close to which are realized in the solar plasma, is
  intimately connected to the creation and subsequent dynamics of magnetic
  flux tubes in the solar interior. It can therefore be used as a tool to
  probe such dynamics. In this paper we show how photospheric observations
  of magnetic helicity of isolated magnetic flux tubes, manifested
  as the twist and writhe of solar active regions, can constrain the
  creation and dynamics of flux tubes in the solar convection zone and
  the nature of convective turbulence itself. We analyze the observed
  latitudinal distribution of twists in photospheric active regions,
  derived from solar vector magnetograms, in the largest such sample
  studied till-date. We confirm and put additional constraints on the
  hemispheric twist helicity trend and find that the dispersion in the
  active region twist distribution is latitude-independent, implying that
  the amplitude of turbulent fluctuations does not vary with latitude
  in the convection zone. Our data set also shows that the amplitude
  and dispersion of twist decreases with increasing magnetic size of
  active regions, supporting the conclusion that larger flux tubes are
  less affected by turbulence. Among the various theoretical models
  that have been proposed till-date to explain the origin of twist, our
  observations best match the Σ effect model, which invokes helical
  turbulent buffeting of rising flux tubes as the mechanism for twist
  creation. Finally, we complement our analysis of twists with past
  observations of tilts in solar active regions and tie them in with
  theoretical modeling studies, to build up a comprehensive picture
  of the dynamics of twisted magnetic flux tubes throughout the solar
  convection zone. This general framework, binding together theory and
  observations, suggests that flux tubes have a wide range of twists in
  the solar convection zone, with some as high as to make them susceptible
  to the kink instability mechanism that results in the formation of δ
  spot or non-Hale active regions.

Title: A Time Delay Model for Solar and Stellar Dynamos
Authors: Wilmot-Smith, A. L.; Nandy, D.; Hornig, G.; Martens, P. C. H.
Bibcode: 2006ApJ...652..696W
Altcode:
  Magnetohydrodynamic dynamos operating in stellar interiors produce
  the diverse range of magnetic activity observed in solar-like
  stars. Sophisticated dynamo models including realistic physics of
  convection zone flows and flux tube dynamics have been built for
  the Sun, for which appropriate observations exist to constrain such
  models. Nevertheless, significant differences exist in the physics that
  the models invoke, the most important being the nature and location
  of the dynamo α-effect and whether it is spatially segregated from
  the location of the Ω-effect. Spatial segregation of these source
  layers necessitates a physical mechanism for communication between
  them, involving unavoidable time delays. We construct a physically
  motivated reduced dynamo model in which, through the use of time delays,
  we mimic the generation of field components in spatially segregated
  layers and the communication between them. The model can be adapted
  to examine the underlying structures of more complicated and spatially
  extended numerical dynamo models with diverse α-effect mechanisms. A
  variety of dynamic behaviors arise as a direct consequence of the
  introduction of time delays in the system. Various parameter regimes
  give rise to periodic and aperiodic oscillations. Amplitude modulation
  leads to episodes of reduced activity, such as that observed during
  the Maunder minima, the length and duration of which depend on the
  dynamo number. Regular activity is more easily excited in the flux
  transport-dominated regime (when the time delay is smaller than the
  dissipative timescale), whereas irregular activity characterizes
  solutions in the diffusion-dominated regime (when the time delay is
  larger than the dissipative timescale).

Title: Generation And Dynamics Of Magnetic Fields In The Solar
    Convection Zone
Authors: Nandy, D.
Bibcode: 2006IAUJD...3E..14N
Altcode:
  Magnetic fields constituting solar active regions are generated by
  a dynamo mechanism in the deep interior of the Sun - from where they
  buoyantly rise to erupt through the photosphere as sunspots. During
  this buoyant rise through the convection zone, the magnetic flux
  tubes interact with a variety of physical processes, notable amongst
  them being the Coriolis force and helical turbulent convection. This
  interaction results in the flux tubes acquiring specific structural
  properties such as tilt and twist. In this talk I will briefly review
  the process of magnetic field generation and discuss how observations
  of twist and writhe - components of magnetic helicity - in solar
  active regions, can constrain the dynamics of magnetic flux tubes in
  the solar convection zone.

Title: Unravelling Long-Term Solar Variability: The Stars As Suns
    Project
Authors: Nandy, D.; Martens, P. C. H.
Bibcode: 2006IAUJD...8E..13N
Altcode:
  It is well known that solar variability influences the near-Earth
  Space environment at short timescales of days - an effect collectively
  termed as Space Weather. A lesser known and more subtle influence of
  solar variability at longer timescales, is however, just beginning
  to be appreciated. This long-term solar forcing, which is sometimes
  referred to as Space Climate, has important consequences for the
  formation and evolution of planetary atmospheres, evolution of life
  and global climate on Earth. Understanding the Sun's variability and
  its heliospheric influence at such scales stretching from millennia
  to stellar evolutionary timescales is therefore of fundamental
  importance. However, our understanding of this variability, which is
  partly due to the evolution of the solar magnetic dynamo, is limited by
  solar observations which exist only from early 17^th Century onwards. In
  this talk I will review the "Stars as Suns" project - in which we
  take a novel approach to unravelling long-term solar variability
  through theoretical modelling and magnetic activity observations of
  Sun-like stars, which are at various evolutionary phases relative to
  the Sun. The "Stars as Suns" project is funded by the NASA Living With
  a Star program through grant NNG05GE47G.

Title: Implementation of an Exponential Propagation Method to
    Numerically Solving the 2.5 D Stellar Dynamo Equations
Authors: Munoz, Andres; Martens, P. C.; Nandy, D.
Bibcode: 2006SPD....37.1202M
Altcode: 2006BAAS...38Q.240M
  Magnetic fields in stars such as the Sun originate via a MHD dynamo
  mechanism working in their interior. A complete understanding of the
  dynamo mechanism, which involves complex and non-linear interactions
  between plasma flows and magnetic fields, remains an elusive and
  outstanding problem in Astrophysics. As an integral step in a study of
  stellar dynamos, part of a new MSU project entitled "Stars as Suns:
  Unraveling Long-term Solar Variability by Stellar Dynamo Modeling",
  a numerical solution for the 2D dynamo equations is being developed
  that uses an exponential propagation method, in which, the exponential
  is approximated using a projection into a Krylov subspace. As has
  been found in other work, this kind of numerical scheme presents a
  promising alternative to explicit schemes, since it is not subject
  to the CFL condition, and to implicit methods, since an iteration
  using the projection onto a Krylov subspace converges faster than an
  equivalent solution of the implicit formulation. Here, we outline our
  preliminary efforts towards developing this new numerical scheme for
  addressing the stellar dynamo problem.This research is supported by
  NASA Grant NNGO5GE47G to MSU.

Title: Unraveling long-term solar variability and its impact on
space climate: The stars as suns project
Authors: Nandy, D.; Martens, P. C. H.
Bibcode: 2006ilws.conf..158N
Altcode:
  It is well-known that solar variability influences the near-Earth
  Space environment at short timescales of days - an effect collectively
  termed as Space Weather. A lesser known and more subtle influence of
  solar variability at longer timescales is however just beginning to be
  appreciated. This long-term solar forcing, which is sometimes referred
  to as Space Climate, has important consequences for the formation
  and evolution of planetary atmospheres and the evolution of life and
  global climate on Earth. Understanding the Sun's variability and its
  heliospheric influence at such scales stretching from millennia to
  stellar evolutionary timescales is therefore of fundamental importance
  and a very promising area of future research. However, our understanding
  of this variability, which is in part connected to the evolution of the
  solar magnetic dynamo, is limited by continuous sunspot observations,
  which exist only from the early 17th Century onwards. In this paper we
  review the "Stars as Suns" project - in which we take a radically new
  approach to unraveling long-term solar variability through theoretical
  modeling and magnetic activity observations of Sun-like stars, which
  are at various evolutionary phases relative to the Sun.

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: The Magnetic Activity of Solar-like Stars at Different
    Main-Sequence Ages
Authors: Lakatos, S. L.; Nandy, D.; Martens, P.
Bibcode: 2005AAS...20711104L
Altcode: 2005BAAS...37.1342L
  We report on a study of modeling stellar magnetic activity inferred
  through CaII H+K and ROSAT X-ray emission. The purpose of this project
  is to create a subset of stars with similar properties to the Sun,
  but with a wide range of ages (0.6 - 10 Gyrs); to study the CaII H+K
  emission data and decipher how the stars' emission changes with age;
  and to compare the X-ray activity to the CaII H+K activity. The ultimate
  goal of this project is to determine and use the relationships between
  the stellar parameters to understand the evolution of the magnetic
  dynamo from an younger Sun to an older Sun. This research is supported
  by a NSF Research Experience for Undergraduates grant ATM-0243923 and
  a NASA Living With a Star grant NNG05GE47G to Montana State University.

Title: Low-order stellar dynamo models
Authors: Wilmot-Smith, A. L.; Martens, P. C. H.; Nandy, D.; Priest,
   E. R.; Tobias, S. M.
Bibcode: 2005MNRAS.363.1167W
Altcode: 2005MNRAS.tmp..855W
  Stellar magnetic activity - which has been observed in a diverse set
  of stars including the Sun - originates via a magnetohydrodynamic
  dynamo mechanism working in stellar interiors. The full set of
  magnetohydrodynamic equations governing stellar dynamos is highly
  complex, and so direct numerical simulation is currently out of
  reach computationally. An understanding of the bifurcation structure,
  likely to be found in the partial differential equations governing such
  dynamos, is vital if we are to understand the activity of solar-like
  stars and its evolution with varying stellar parameters such as rotation
  rate. Low-order models are an important aid to this understanding,
  and can be derived either as approximations of the governing equations
  themselves or by using bifurcation theory to obtain systems with the
  desired structure. We use normal-form theory to derive a third-order
  model with robust behaviour. The model is able to reproduce many of the
  basic types of behaviour found in observations of solar-type stars. In
  the appropriate parameter regime, a chaotic modulation of the basic
  cycle is present, together with varying periods of low activity such
  as that observed during the solar Maunder minima.

Title: Spatial Relationship between Twist in Active Region Magnetic
    Fields and Solar Flares
Authors: Hahn, Michael; Gaard, Stacy; Jibben, Patricia; Canfield,
   Richard C.; Nandy, Dibyendu
Bibcode: 2005ApJ...629.1135H
Altcode:
  Twisted magnetic field lines in solar active regions constitute stressed
  flux systems, the reconnection of which can release the stored (excess)
  magnetic energy in the form of solar flares. Using co-registered
  photospheric vector magnetograms and chromospheric Hα images for 29
  flares, we explore the spatial relationship between these flares and
  the magnetic topology of the active regions in which they occur. We
  find two dominant trends. First, flares are preferentially initiated in
  subregions that have a high gradient in twist. Second, flare initiation
  occurs close to chirality inversion lines (which separate regions with
  twist of opposite handedness). Our results demonstrate that magnetic
  helicity, as manifested in the twist parameter, plays an important
  role in magnetic reconnection and solar flaring activity.

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: The Relationship Between Active Region Twist &amp; Solar
    Flaring Activity
Authors: Nandy, D.; Hahn, M.; Gaard, S.; Jibben, P.; Canfield, R. C.
Bibcode: 2005AGUSMSP23B..06N
Altcode:
  Twisted magnetic field lines in solar active regions constitute stressed
  flux systems -- the reconnection of which can release the stored
  (excess) energy in the form of solar flares. The explosive release of
  energy through such flares, beyond contributing to the heating of the
  solar corona, can sometimes affect the near-Earth Space environment and
  trigger geomagnetic storms. Here we explore the relationship between
  solar flares and the pre-flare magnetic topology (characterized by the
  twist α in the magnetic fields lines) of the active regions in which
  the flares originate. We have discovered that flares are preferentially
  initiated in sub-regions that have an high gradient in twist and
  lie close to chirality inversion lines (which separate regions with
  twist of opposite handedness). Our results imply that the topology of
  magnetic field lines -- as characterized by the twist parameter α --
  plays an important role in magnetic reconnection and flaring events.

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: Meridional Circulation and the Solar Magnetic Cycle
Authors: Nandy, D.
Bibcode: 2004ESASP.559..241N
Altcode: 2004soho...14..241N
  No abstract at ADS

Title: Exploring Magnetic Activity from The Sun to the Stars
Authors: Nandy, Dibyendu
Bibcode: 2004SoPh..224..161N
Altcode: 2005SoPh..224..161N
  Sun-like stars are known to display a wide variety of magnetic
  activity which is likely to be the signature of a hydromagnetic dynamo
  mechanism working in stellar interiors. This dynamo mechanism has been
  studied extensively in the context of the Sun. Here we take ideas and
  experiences gained from solar dynamo modeling and build upon it to
  study the inferred scaling laws, involving stellar parameters, from
  observations of stellar magnetic activity. We also discuss how such a
  synthesis of theoretical dynamo modeling of Sun-like stars and stellar
  cycle observations may help us reconstruct the long-term variability
  of the Sun - an important ingredient for understanding the effects of
  solar forcing on space and global climate.

Title: On the Tilt and Twist of Solar Active Regions
Authors: Holder, Zachary A.; Canfield, Richard C.; McMullen, Rebecca
   A.; Nandy, Dibyendu; Howard, Robert F.; Pevtsov, Alexei A.
Bibcode: 2004ApJ...611.1149H
Altcode:
  Tilt and twist are two measurable characteristics of solar active
  regions that can give us information about subsurface physical
  processes associated with the creation and subsequent evolution of
  magnetic flux tubes inside the Sun. Using Mees Solar Observatory active
  region vector magnetograms and Mount Wilson Observatory full-disk
  longitudinal magnetograms, we measure the magnetic twist and tilt
  angles of 368 bipolar active regions. In addition to two well-known
  phenomena, Joy's law and the hemispheric helicity rule, this data set
  also shows a lesser known twist-tilt relationship, which is the focus
  of this study. We find that those regions that closely follow Joy's
  law do not show any twist-tilt dependence. The dispersion in tilt
  angles and the dispersion in twist are also found to be uncorrelated
  with each other. Both of these results are predicted consequences of
  convective buffeting of initially untwisted and unwrithed flux tubes
  through the Σ-effect. However, we find that regions that strongly
  depart from Joy's law show significantly larger than average twist
  and very strong twist-tilt dependence-suggesting that the twist-tilt
  relationship in these regions is due to the kinking of flux tubes that
  are initially highly twisted, but not strongly writhed. This implies
  that some mechanism other than the Σ-effect (e.g., the solar dynamo
  itself or the process of buoyancy instability and flux tube formation)
  is responsible for imparting the initial twist (at the base of the
  solar convection zone) to the flux tubes that subsequently become
  kink-unstable.

Title: Erratum: ``Evidence that a Deep Meridional Flow Sets the
    Sunspot Cycle Period'' (<A href="/abs/2003ApJ...589..665H">ApJ, 589,
    665 [2003]</A>)
Authors: Hathaway, David H.; Nandy, Dibyendu; Wilson, Robert M.;
   Reichmann, Edwin J.
Bibcode: 2004ApJ...602..543H
Altcode:
  An error was made in entering the data used in Figure 6. This changes
  the results concerning the length of the time lag between the variations
  in the meridional flow speed and those in the cycle amplitude. The
  final paragraph on page 667 should read: <P />``Finally, we study
  the relationship between the drift velocities and the amplitudes
  of the hemisphere/cycles. In Figure 5 we compare the drift velocity
  at the maximum of the cycle to the amplitude of that cycle for that
  hemisphere. There is a positive (0.5) and significant (95%) correlation
  between the two. However, an even stronger relationship is found between
  the drift velocity and the amplitude of the N+2 cycle. The correlation
  is stronger (0.7) and more significant (99%), as shown in Figure 6. This
  relationship is suggestive of a ``memory'' in the solar cycle, again
  a property of dynamo models that use meridional circulation. Indeed,
  the two-cycle lag is precisely the relationship found by Charbonneau
  &amp; Dikpati (<A href="/abs/2003ApJ...589..665H">ApJ, 589, 665
  [2003]</A>). This behavior is, however, more difficult to interpret,
  and we elaborate on this in the next section. In either case, these
  correlations only explain part of the variance in cycle amplitude (25%
  for the current cycle and 50% for the N+2 cycle). Obviously, other
  mechanisms, such as variations in the gradient in the rotation rate,
  also contribute to the cycle amplitude variations. Our investigation of
  possible connections between drift rates and the amplitudes of the N+1
  and N+3 cycles gives no significant correlations at these alternative
  time lags.'' <P />The revised Figure 6 and its caption are given below

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: Detection of a Taylor-like Plasma Relaxation Process in the
    Sun and its Implication for Coronal Heating
Authors: Nandy, Dibyendu; Hahn, Michael; Canfield, Richard C.;
   Longcope, Dana W.
Bibcode: 2004IAUS..223..473N
Altcode: 2005IAUS..223..473N
  The relaxation dynamics of a magnetized plasma system is a subject
  of fundamental importance in MHD - with applications ranging from
  laboratory plasma devices like the Toroidal Field Pinch and Spheromaks
  to astrophysical plasmas, stellar flaring activity and coronal
  heating. Taylor in 1974 proposed that the magnetic field in a plasma
  (of small but finite resistivity) relaxes to a minimum energy state,
  subject to the constraint that its total magnetic helicity is conserved
  (Woltjer 1958), such that the final magnetic field configuration is a
  constant alpha (linear) force-free field - where alpha is a quantity
  describing the twist in magnetic field lines. However, a clear signature
  of this mechanism in astrophysical plasmas remained undetected. Here
  we report observational detection of a relaxation process, similar
  to what Taylor (1974, 1986) envisaged, in the magnetic fields of
  flare-productive solar active regions. The implications of this result
  for magnetic reconnection and the coronal heating problem are discussed.

Title: Detection of a Taylor-like Plasma Relaxation Process in the Sun
Authors: Nandy, Dibyendu; Hahn, Michael; Canfield, Richard C.;
   Longcope, Dana W.
Bibcode: 2003ApJ...597L..73N
Altcode:
  The relaxation dynamics of a magnetized plasma system is a subject of
  fundamental importance in magnetohydrodynamics-with applications ranging
  from laboratory plasma devices such as the toroidal-field pinch and
  spheromaks to astrophysical plasmas, stellar flaring activity, and
  coronal heating. Taylor in 1974 proposed that the magnetic field in
  a plasma, subject to certain constraints, relaxes to a minimum energy
  state such that the final magnetic field configuration is a constant α
  (linear) force-free field-where α is a quantity describing the twist in
  magnetic field lines. While Taylor's theory was remarkably successful in
  explaining some intriguing results from laboratory plasma experiments,
  a clear signature of this mechanism in astrophysical plasmas remained
  undetected. Here we report observational detection of a relaxation
  process, similar to what Taylor envisaged, in the magnetic fields of
  flare-productive solar active regions. The implications of this result
  for magnetic reconnection and the coronal heating problem are discussed.

Title: Evidence That a Deep Meridional Flow Sets the Sunspot Cycle
    Period
Authors: Hathaway, David H.; Nandy, Dibyendu; Wilson, Robert M.;
   Reichmann, Edwin J.
Bibcode: 2003ApJ...589..665H
Altcode:
  Sunspots appear on the Sun in two bands on either side of the equator
  that drift toward lower latitudes as each sunspot cycle progresses. We
  examine the drift of the centroid of the sunspot area toward the
  equator in each hemisphere from 1874 to 2002 and find that the drift
  rate slows as the centroid approaches the equator. We compare the
  drift rate at sunspot cycle maximum with the period of each cycle
  for each hemisphere and find a highly significant anticorrelation:
  hemispheres with faster drift rates have shorter periods. These
  observations are consistent with a meridional counterflow deep within
  the Sun as the primary driver of the migration toward the equator and
  the period associated with the sunspot cycle. We also find that the
  drift rate at maximum is significantly correlated with the amplitude of
  the following cycle, a prediction of dynamo models that employ a deep
  meridional flow toward the equator. Our results indicate an amplitude
  of about 1.2 m s<SUP>-1</SUP> for the meridional flow velocity at the
  base of the solar convection zone.

Title: Delving Deeper into the Solar Dynamo Mechanism: Alpha Effect,
    Parity Selection and Large Scale Flows.
Authors: Nandy, D.
Bibcode: 2003SPD....34.1906N
Altcode: 2003BAAS...35..843N
  Visible manifestations of the 22 year solar magnetic cycle have been
  the subject of study spanning centuries starting with the telescopic
  observations of sunspots by Johann Fabricius, Christoph Scheiner and
  Galileo Galilei in the early 1600s. Coupled with these observations
  of magnetic features on the solar surface, the advent of the field
  of helioseismology in recent years has made it possible to map large
  scale flows in the solar interior - believed to play a crucial role in
  sustaining the solar cycle. However, a complete understanding of the
  hydromagnetic dynamo mechanism that powers this solar cycle remains
  elusive. Here we report studies of the solar dynamo addressing some of
  the important unresolved questions regarding the nature and location
  of the alpha effect, solar magnetic parity selection and the role of
  large scale flows and their variation, with a goal to understand the
  exact means by which the Sun generates its magnetic cycle. This study
  was supported by NASA through SR&amp;T grant NAG5-6110.

Title: Evidence that a Deep Meridional Flow Sets the Sunspot Cycle
    Period
Authors: Hathaway, D. H.; Nandy, D.; Wilson, R. M.; Reichmann, E. J.
Bibcode: 2003SPD....34.2604H
Altcode: 2003BAAS...35..854H
  Sunspots appear on the Sun in two bands on either side of the equator
  that drift toward lower latitudes as each sunspot cycle progresses. We
  examine the equatorward drift of the centroid of the sunspot area in
  each hemisphere from 1874 to 2002 and find that the drift rate slows
  as the centroid approaches the equator. We compare the drift rate at
  sunspot cycle maximum to the cycle-period for each hemisphere and find
  a highly significant anti-correlation: hemispheres with faster drift
  rates have shorter periods. These observations are consistent with
  an equatorward meridional counterflow, deep within the Sun, as the
  primary driver of the equatorward migration and the period associated
  with the sunspot cycle. We also find that the drift rate at maximum is
  significantly correlated with the amplitude of the following cycle, a
  prediction of dynamo models that employ a deep equatorward meridional
  flow. Our results indicate an amplitude of about 1.2 m/s for the
  meridional flow velocity at the base of the solar convection zone.

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: Reviewing solar magnetic field generation in the light of
    helioseismology
Authors: Nandy, Dibyendu
Bibcode: 2003ESASP.517..123N
Altcode: 2003soho...12..123N
  Helioseismic observations, in recent years, have vastly improved our
  understanding of solar interior dynamics. This has challenged theorists
  to come up with up-to-date models for the origin and evolution of
  magnetic fields in the Sun - which on the one hand has to be consistent
  with these new results from the solar interior and on the other hand,
  has to correctly reproduce magnetic features that have been observed
  on the Sun since the advent of telescopes and magnetograms. We report
  here results from such dynamo simulations and address the following
  questions: Where in the Sun are the strong "sunspot-forming" toroidal
  magnetic field and the much weaker poloidal field generated? What is
  the role and nature of the unobserved meridional counterflow? We also
  discuss possible future helioseismic inputs that may usefully constrain
  the exact nature of the solar dynamo.

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: Constraints on the Solar Internal Magnetic Field from a
    Buoyancy Driven Solar Dynamo
Authors: Nandy, Dibyendu
Bibcode: 2002Ap&SS.282..209N
Altcode:
  We report here results from a dynamo model developed on the lines
  of the Babcock-Leighton idea that the poloidal field is generated at
  the surface of the Sun from the decay of active regions. In this model
  magnetic buoyancy is handled with a realistic recipe - wherein toroidal
  flux is made to erupt from the overshoot layer wherever it exceeds a
  specified critical field B<SUB>c</SUB> (10<SUP>5</SUP> G). The erupted
  toroidal field is then acted upon by the α-effect near the surface
  to give rise to the poloidal field. In this paper we study the effect
  of buoyancy on the dynamo generated magnetic fields. Specifically,
  we show that the mechanism of buoyant eruption and the subsequent
  depletion of the toroidal field inside the overshoot layer, is capable
  of constraining the magnitude and distribution of the magnetic field
  there. We also believe that a critical study of this mechanism may
  give us new information regarding the solar interior and end with an
  example, where we propose a method for estimating an upper limit of
  the difusivity within the overshoot layer.

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: Can theoretical solar dynamo models predict future solar
    activity?
Authors: Nandy, D.
Bibcode: 2002cosp...34E..53N
Altcode: 2002cosp.meetE..53N
  The origin of many solar eruptive events and activity can be traced
  back to the presence of magnetic fields in the Sun. It is believed
  that these strong magnetic fields are generated by a dynamo mechanism
  in the solar interior. This dynamo mechanism involves a recycling of
  magnetic flux, between the weak poloidal field observed on the solar
  surface and the much stronger toroidal field that is generated in the
  solar tachocline beneath the convection zone. Thus, in principle, a
  theoretical basis exists for predicting the magnitude of the strong
  toroidal field (that buoyantly rise to form active regions) of the
  next cycle, taking as input the observed poloidal flux of the present
  cycle. We explore this scenario here to find out, whether such a dynamo
  prediction of future solar activity is indeed possible.

Title: Insights of the physical processes in the Sun s interior from
    solar dynamo studies.
Authors: Nandy, D.
Bibcode: 2002cosp...34E..52N
Altcode: 2002cosp.meetE..52N
  With the coming of age of helioseismology, our knowledge of the dynamics
  of the solar convection zone has improved vastly over the last few
  years. This has challenged theorists to come up with up-to-date models
  for the origin and evolution of magnetic fields in the Sun, which
  on the one hand has to incorporate these new results from the solar
  interior and on the other hand, has to correctly reproduce features
  that have been observed on the Sun since the advent of telescopes and
  magnetograms. We have developed a hybrid model of the solar dynamo
  that includes the helioseismically deduced solar rotation profile
  and is consistent with the recent results from flux tube dynamics. We
  present here, a study of this new class of dynamo model and show how
  this synthesis of theoretical ideas and observational results, can
  lead to important advances in our understanding of flows and other
  physical processes in the solar interior.

Title: Characteristics Of A Magnetic Buoyancy Driven Solar Dynamo
    Model
Authors: Nandy, Dibyendu
Bibcode: 2001astro.ph..7564N
Altcode:
  We have developed a hybrid model of the solar dynamo on the lines
  of the Babcock-Leighton idea that the poloidal field is generated
  at the surface of the Sun from the decay of active regions. In this
  model magnetic buoyancy is handled with a realistic recipe - wherein
  toroidal flux is made to erupt from the overshoot layer wherever it
  exceeds a specified critical field ($10^5$ G). The erupted toroidal
  field is then acted upon by the alpha effect near the surface to give
  rise to the poloidal field. In the first half of this paper we present
  a parameter space study of this model, to bring out similarities and
  differences between it and other well studied models of the past. In
  the second half of this paper we show that the mechanism of buoyant
  eruptions and the subsequent depletion of the toroidal field inside
  the overshoot layer, is capable of constraining the magnitude of the
  dynamo generated magnetic field there, although a global quenching
  mechanism is still required to ensure that the magnetic fields do not
  blow up. We also believe that a critical study of this mechanism may
  give us new information regarding the solar interior and end with an
  example, where we propose a new method for estimating an upper limit
  of the diffusivity within the overshoot layer.

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, &amp;
  Dikpati in 1995 did not incorporate buoyancy. We extend this model by
  two contrasting methods. In one method, we incorporate the generation
  of the poloidal field near the solar surface by Durney's procedure
  of double-ring eruption. In the second method, the poloidal field
  generation is treated by a positive α-effect concentrated near the
  solar surface coupled with an algorithm for handling buoyancy. The
  two methods are found to give qualitatively similar results.

Title: The 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&amp;A 303, L29 Nandi D., Choudhuri A. R., 2000,
  submitted to ApJ

Title: Vacuum-field solutions of Ross and Sen-Dunn theories of
    gravitation
Authors: Krori, K. D.; Nandy, D.
Bibcode: 1978JPhA...11.1943K
Altcode:
  Vacuum-field solutions of Ross and Sen-Dunn theories of gravitation
  have been obtained with the aid of a Friedmann-type metric. Non-static
  solutions are found showing that the Birkhoff theorem holds for neither
  theory. It has been observed that the two theories have a limited scope
  for vacuum solution as against the Brans-Dicke theory. Mach's principle,
  however, holds for both the theories.

