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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
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.
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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
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.
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Title: Adaptive hyperspectral imaging using structured illumination
in a spatial light modulator-based interferometer
Authors: Chandra, Amar Deo; Karmakar, Mintu; Nandy, Dibyendu;
Banerjee, Ayan
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.
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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
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.
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Title: Simulating Magnetic Switchback Structures in the near-Sun
Solar Wind Observed by Parker Probe
Authors: Roddanavar, Arpita; Nandy, Dibyendu
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.
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Title: The Roles of Intrinsic and Imposed Magnetospheres in Shielding
Planetary Atmospheres
Authors: Basak, Arnab; Nandy, Dibyendu
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 & Nandy, MNRAS 502, 3569 (2021)]
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Title: Understanding the Global Coronal Magnetic Fields using
Data-constrained Magnetohydrodynamic Model
Authors: Dash, Soumyaranjan; Nandy, Dibyendu; Vaidya, Bhargav
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.
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Title: Subsurface Plasma Flows and the Flare Productivity of Solar
Active Regions
Authors: Biji, Lekshmi; Jain, Kiran; Komm, Rudolf; Nandy, Dibyendu
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.
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Title: Estimating magnetospheric currents and geoeffectiveness of
interplanetary CMEs with magnetohydrodynamic simulations
Authors: ROY, Souvik; Nandy, Dibyendu
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.
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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
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.
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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
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.
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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
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.
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Title: Stellar mid-life crisis: subcritical magnetic dynamos of
solar-like stars and the breakdown of gyrochronology
Authors: Tripathi, Bindesh; Nandy, Dibyendu; Banerjee, Soumitro
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.
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Title: Modelling the Impact of Magnetic Storms on Planetary
Environments
Authors: Roy, Souvik; Nandy, Dibyendu
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.
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Title: Modelling the imposed magnetospheres of Mars-like exoplanets:
star-planet interactions and atmospheric losses
Authors: Basak, Arnab; Nandy, Dibyendu
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.
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Title: Modelling the solar wind forced Martian environment
Authors: Basak, Arnab; Nandy, Dibyendu
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.
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Title: Progress in Solar Cycle Predictions: Sunspot Cycles 24-25
in Perspective
Authors: Nandy, Dibyendu
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.
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Title: Magnetohydrodynamical Understanding of the Interactions
Between Coronal Mass Ejections and Earth's Magnetosphere.
Authors: ROY, S.; Nandy, D.
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.
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Title: Flux Erosion of Magnetic Clouds by Reconnection With the
Sun's Open Flux
Authors: Pal, S.; Dash, S.; Nandy, D.
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.
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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.
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.
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Title: A Physics-based Prediction of Solar Cycle 25 and Reconstruction
of Century-Scale Solar Activity
Authors: Nandy, D.; Bhowmik, P.
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.
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Title: Dynamics of the Sun's Polar Field: Possible Insights from
Out of the Ecliptic Observations
Authors: Dash, S.; Nandy, D.; Pal, S.
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.
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.
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.
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
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.
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
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
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
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
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
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
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.
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.
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.
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
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.
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
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
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
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.
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
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
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
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
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
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.
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.
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
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.
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
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
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
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
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.
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
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.
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
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.
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
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
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.
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
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.
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.
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
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.
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.
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.
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&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.
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.
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.
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
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.
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.
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.
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.
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
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.
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.
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.
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.
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
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.
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
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.
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| > 3, N_D < 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
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
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.
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
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
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.
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.
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| > 3,ND < 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.
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.
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.
2011SPD....42.1705M Altcode: 2011BAAS..43S.1705M
Nandy, Munoz, & 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 &
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 & 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 & Rightmire,
Basu & Antia, and the model assumptions of Nandy, Munoz, &
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.
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.
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.
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
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.
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
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.
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.
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.
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.
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.
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.
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.
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
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.
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.
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
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.
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.
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.
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.
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.
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.
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.
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.
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
2008AdSpR..41..893C Altcode:
Active regions on the solar surface are known to possess magnetic
helicity, which is predominantly negative in the northern hemisphere
and positive in the southern hemisphere. Choudhuri et al. [Choudhuri,
A.R. On the connection between mean field dynamo theory and flux
tubes. Solar Phys. 215, 31 55, 2003] proposed that the magnetic helicity
arises due to the wrapping up of the poloidal field of the convection
zone around rising flux tubes which form active regions. Choudhuri
[Choudhuri, A.R., Chatterjee, P., Nandy, D. Helicity of solar active
regions from a dynamo model. ApJ 615, L57 L60, 2004] used this idea to
calculate magnetic helicity from their solar dynamo model. Apart from
getting broad agreements with observational data, they also predict
that the hemispheric helicity rule may be violated at the beginning
of a solar cycle. Chatterjee et al. [Chatterjee, P., Choudhuri, A.R.,
Petrovay, K. Development of twist in an emerging magnetic flux tube
by poloidal field accretion. A&A 449, 781 789, 2006] study the
penetration of the wrapped poloidal field into the rising flux tube
due to turbulent diffusion using a simple 1-d model. They find that
the extent of penetration of the wrapped field will depend on how
weak the magnetic field inside the rising flux tube becomes before
its emergence. They conclude that more detailed observational data
will throw light on the physical conditions of flux tubes just before
their emergence to the photosphere.
---------------------------------------------------------
Title: 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.
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.
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.
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.
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.
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.
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
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.
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.
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.
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.
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.
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.
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.
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.
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
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.
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 & Solar
Flaring Activity
Authors: Nandy, D.; Hahn, M.; Gaard, S.; Jibben, P.; Canfield, R. C.
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.
2004A&A...427.1019C Altcode: 2004astro.ph..5027C
We explore a two-dimensional kinematic solar dynamo model in a full
sphere, based on the helioseismically determined solar rotation
profile and with an α effect concentrated near the solar surface,
which captures the Babcock-Leighton idea that the poloidal field is
created from the decay of tilted bipolar active regions. The meridional
circulation, assumed to penetrate slightly below the tachocline, plays
an important role. Some doubts have recently been raised regarding
the ability of such a model to reproduce solar-like dipolar parity. We
specifically address the parity issue and show that the dipolar mode
is preferred when certain reasonable conditions are satisfied, the
most important condition being the requirement that the poloidal field
should diffuse efficiently to get coupled across the equator. Our model
is shown to reproduce various aspects of observational data, including
the phase relation between sunspots and the weak, diffuse field.
---------------------------------------------------------
Title: Helicity of Solar Active Regions from a Dynamo Model
Authors: Choudhuri, Arnab Rai; Chatterjee, Piyali; Nandy, Dibyendu
2004ApJ...615L..57C Altcode:
We calculate helicities of solar active regions based on the idea that
poloidal flux lines get wrapped around a toroidal flux tube rising
through the convection zone, thereby giving rise to the helicity. Rough
estimates based on this idea compare favorably with the observed
magnitude of helicity. We use our solar dynamo model based on the
Babcock-Leighton α-effect to study how helicity varies with latitude
and time. At the time of solar maximum, our theoretical model gives
negative helicity in the northern hemisphere and positive helicity in
the south, in accordance with observed hemispheric trends. However,
we find that during a short interval at the beginning of a cycle,
helicities tend to be opposite of the preferred hemispheric trends.
---------------------------------------------------------
Title: Meridional Circulation and the Solar Magnetic Cycle
Authors: Nandy, D.
2004ESASP.559..241N Altcode: 2004soho...14..241N
No abstract at ADS
---------------------------------------------------------
Title: Exploring Magnetic Activity from The Sun to the Stars
Authors: Nandy, Dibyendu
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.
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.
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
& 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
2004IAUS..223..133N Altcode: 2005IAUS..223..133N
We explore a full sphere (2D axisymmetric) kinematic solar dynamo
model based on the Babcock-Leighton idea that the poloidal field is
generated in the surface layers from the decay of tilted bipolar solar
active regions. This model incorporates the helioseismically deduced
solar rotation profile and an algorithm for buoyancy motivated from
simulations of flux tube dynamics. A prescribed deep meridional
circulation plays an important role in the advection of magnetic
flux. We specifically address the parity issue and show that -
contrary to some recent claims - the Babcock-Leighton dynamo can
reproduce solar-like dipolar parity if certain reasonable conditions
are satisfied in the solar interior, the most important requirement
being that the poloidal field of the two hemispheres be efficiently
coupled across the equator.
---------------------------------------------------------
Title: The Origin of Helicity in Solar Active Regions
Authors: Choudhuri, Arnab Rai; Chatterjee, Piyali; Nandy, Dibyendu
2004IAUS..223...45C Altcode: 2005IAUS..223...45C; 2004astro.ph..6598C
We present calculations of helicity based on our solar dynamo model
and show that the results are consistent with observational data.
---------------------------------------------------------
Title: 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.
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.
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.
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.
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&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.
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.
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
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
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
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
2002Sci...296.1671N Altcode:
Sunspots, dark magnetic regions occurring at low latitudes on the Sun's
surface, are tracers of the magnetic field generated by the dynamo
mechanism. Recent solar dynamo models, which use the helioseismically
determined solar rotation, indicate that sunspots should form at high
latitudes, contrary to observations. We present a dynamo model with
the correct latitudinal distribution of sunspots and demonstrate that
this requires a meridional flow of material that penetrates deeper
than hitherto believed, into the stable layers below the convection
zone. Such a deep material flow may have important implications for
turbulent convection and elemental abundance in the Sun and similar
stars.
---------------------------------------------------------
Title: On the absence of sunspots at high solar latitudes and
associated constraints on the meridional flow in the solar interior
Authors: Nandy, D.; Choudhuri, A. R.
2002AAS...200.8901N Altcode: 2002BAAS...34..791N
Sunspots -- dark magnetic regions -- occur at low latitudes on the Sun's
surface and are believed to be tracers of the magnetic field generated
in the interior by the dynamo mechanism. An important ingredient
in recent models of this dynamo mechanism is the meridional flow of
material, which is observed to be poleward on the Sun's surface and must
be balanced by an equatorward counterflow in the interior. The nature
and exact location of this counterflow, however, is unknown. Recent
solar dynamo models, which use the helioseismically determined internal
rotation of the Sun and confine the counterflow within the convection
zone (as classical theories of solar convection would suggest),
indicate that sunspots should form at higher latitudes, contrary to
observations. Here we present a solar dynamo model with the correct
latitudinal distribution of sunspots and show that this requires a
counterflow that penetrates much deeper than hitherto believed - into
the stable layers below the convection zone. The existence of such
a deep counterflow of material may have important implications for
turbulent convection and elemental abundance in the Sun and related
stellar atmospheres.
---------------------------------------------------------
Title: Can theoretical solar dynamo models predict future solar
activity?
Authors: Nandy, D.
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.
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
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
2001ApJ...551..576N Altcode: 2001astro.ph..7466N
We develop a model of the solar dynamo in which, on the one hand, we
follow the Babcock-Leighton approach to include surface processes,
such as the production of poloidal field from the decay of active
regions, and, on the other hand, we attempt to develop a mean field
theory that can be studied in quantitative detail. One of the main
challenges in developing such models is to treat the buoyant rise
of the toroidal field and the production of poloidal field from it
near the surface. A previous paper by Choudhuri, Schüssler, &
Dikpati in 1995 did not incorporate buoyancy. We extend this model by
two contrasting methods. In one method, we incorporate the generation
of the poloidal field near the solar surface by Durney's procedure
of double-ring eruption. In the second method, the poloidal field
generation is treated by a positive α-effect concentrated near the
solar surface coupled with an algorithm for handling buoyancy. The
two methods are found to give qualitatively similar results.
---------------------------------------------------------
Title: The Role of Magnetic Buoyancy in a Babcock-Leighton Type
Solar Dynamo
Authors: Nandy, D.; Choudhuri, A. R.
2000JApA...21..381N Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Incorporating magnetic buoyancy in solar dynamo models:
New results, problems -- and their possible solutions.
Authors: Nandy, D.; Choudhuri, A. R.
2000SPD....31.0134N Altcode:
There have been traditionally two kinds of approaches
to understand the origin of the solar magnetic cycle: the
Parker--Steenbeck--Krause--Rädler approach and the Babcock--Leighton
approach. It seems at present that the most promising models of the
solar dynamo are those which incorporate the best features of both
these traditional approaches. One of the uncertainties in these hybrid
models (Choudhuri et. al. 1995) lies in the treatment of magnetic
buoyancy within a mean field framework, a subject which has rarely been
explored in the past (Durney 1997). We study this problem by exploring
possible ways of incorporating magnetic buoyancy in a dynamo code --
to simulate the formation and subsequent decay of sunspots and the
recycling of fields, with magnetic buoyancy as an important player in
the flux-transport process. The results as well as some problems faced
by such new generation of dynamo models, will be discussed. References:
Durney B. R., 1997, ApJ 486, 1065 Choudhuri A. R., Schussler M.,
Dikpati M., 1995, A&A 303, L29 Nandi D., Choudhuri A. R., 2000,
submitted to ApJ
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Title: Vacuum-field solutions of Ross and Sen-Dunn theories of
gravitation
Authors: Krori, K. D.; Nandy, D.
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.