Author name code: nandy ADS astronomy entries on 2022-09-14 author:"Nandy, Dibyendu" ------------------------------------------------------------------------ Title: A Comparative Analysis of Machine-learning Models for Solar Flare Forecasting: Identifying High-performing Active Region Flare Indicators Authors: Sinha, Suvadip; Gupta, Om; Singh, Vishal; Lekshmi, B.; Nandy, Dibyendu; Mitra, Dhrubaditya; Chatterjee, Saikat; Bhattacharya, Sourangshu; Chatterjee, Saptarshi; Srivastava, Nandita; Brandenburg, Axel; Pal, Sanchita Bibcode: 2022ApJ...935...45S Altcode: 2022arXiv220405910S Solar flares create adverse space weather impacting space- and Earth-based technologies. However, the difficulty of forecasting flares, and by extension severe space weather, is accentuated by the lack of any unique flare trigger or a single physical pathway. Studies indicate that multiple physical properties contribute to active region flare potential, compounding the challenge. Recent developments in machine learning (ML) have enabled analysis of higher-dimensional data leading to increasingly better flare forecasting techniques. However, consensus on high-performing flare predictors remains elusive. In the most comprehensive study to date, we conduct a comparative analysis of four popular ML techniques (k nearest neighbors, logistic regression, random forest classifier, and support vector machine) by training these on magnetic parameters obtained from the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory for the entirety of solar cycle 24. We demonstrate that the logistic regression and support vector machine algorithms perform extremely well in forecasting active region flaring potential. The logistic regression algorithm returns the highest true skill score of 0.967 ± 0.018, possibly the highest classification performance achieved with any strictly parametric study. From a comparative assessment, we establish that magnetic properties like total current helicity, total vertical current density, total unsigned flux, R_VALUE, and total absolute twist are the top-performing flare indicators. We also introduce and analyze two new performance metrics, namely, severe and clear space weather indicators. Our analysis constrains the most successful ML algorithms and identifies physical parameters that contribute most to active region flare productivity. Title: Long-term forcing of Sun's coronal field, open flux and cosmic ray modulation potential during grand minima, maxima and regular activity phases by the solar dynamo mechanism Authors: Dash, Soumyaranjan; Nandy, Dibyendu; Usoskin, Ilya Bibcode: 2022arXiv220812103D Altcode: Magnetic fields generated in the Sun's interior by the solar dynamo mechanism drive solar activity over a range of time-scales. While space-based observations of the Sun's corona exist only for few decades, direct sunspot observations exist for a few centuries, solar open flux and cosmic ray flux variations can be reconstructed through studies of cosmogenic isotopes over thousands of years. While such reconstructions indicate the presence of extreme solar activity fluctuations in the past, causal links between millennia scale dynamo activity, consequent coronal field, solar open flux and cosmic ray modulation remain elusive. By utilizing a stochastically forced solar dynamo model we perform long-term simulations to illuminate how the dynamo generated magnetic fields govern the structure of the solar corona and the state of the heliosphere -- as indicated by variations in the open flux and cosmic ray modulation potential. We establish differences in the nature of the large-scale structuring of the solar corona during grand maximum, minimum, and regular solar activity phases and simulate how the open flux and cosmic ray modulation potential varies over time scales encompassing these different phases of solar activity. We demonstrate that the power spectrum of simulated and reconstructed solar open flux are consistent with each other. Our study provides the theoretical basis for interpreting long-term solar cycle variability based on reconstructions relying on cosmogenic isotopes and connects solar internal variations to the forcing of the state of the heliosphere. Title: Adaptive hyperspectral imaging using structured illumination in a spatial light modulator-based interferometer Authors: Chandra, Amar Deo; Karmakar, Mintu; Nandy, Dibyendu; Banerjee, Ayan Bibcode: 2022OExpr..3019930C Altcode: 2022arXiv220410587D We develop a novel hyperspectral imaging system using structured illumination in an SLM-based Michelson interferometer. In our design, we use a reflective SLM as a mirror in one of the arms of a Michelson interferometer, and scan the interferometer by varying the phase across the SLM display. For achieving the latter, we apply a checkerboard phase mask on the SLM display where the gray value varies between 0-255, thereby imparting a dynamic phase of up to 262° to the incident light beam. We couple a supercontinuum source into the interferometer in order to mimic an astronomical object such as the Sun, and choose a central wavelength of 637.4 nm akin to the strong emission line of Fe X present in the solar spectrum. We use a bandwidth of 30 nm, and extract fringes corresponding to a spectral resolution of 3.8 nm which is limited by the reflectivity of the SLM. We also demonstrate a maximum wavelength tunability of ~8 nm by varying the phase over the phase mask with a spectral sampling of around 0.03 nm between intermediate fringes. The checkerboard phase mask can be adapted close to real time on time-scales of a few tens of milliseconds to obtain spectral information for other near-contiguous wavelengths. The compactness, potential low cost, low power requirements, real-time tunability and lack of moving mechanical parts in the setup implies that it can have very useful applications in settings which require near real-time, multi-wavelength spectroscopic applications, and is especially relevant in space astronomy. Title: A magnetic cloud prediction model for forecasting space weather relevant properties of Earth-directed coronal mass ejections Authors: Pal, Sanchita; Nandy, Dibyendu; Kilpua, Emilia K J Bibcode: 2022arXiv220305231P Altcode: Coronal Mass Ejections (CMEs) are energetic storms in the Sun that result in the ejection of large-scale magnetic clouds (MCs) in interplanetary space that contain enhanced magnetic fields with coherently changing field direction. The severity of geomagnetic perturbations depends on the direction and strength of the interplanetary magnetic field (IMF), as well as the speed and duration of passage of the storm. The coupling between the heliospheric environment and Earth's magnetosphere is the strongest when the IMF direction is persistently southward for a prolonged period. Predicting the magnetic profile of such Earth-directed CMEs is crucial for estimating their geomagnetic impact. We aim to build upon and integrate diverse techniques towards development of a comprehensive magnetic cloud prediction (MCP) model that can forecast the magnetic field vectors, Earth-impact time, speed and duration of passage of solar storms. A novelty of our scheme is the ability to predict the passage duration of the storm without recourse to computationally intensive, time-dependent dynamical equations. Our methodology is validated by comparing the MCP model output with observations of ten MCs at 1 AU. In our sample, we find that eight MCs show a root mean square deviation of less than 0.1 between predicted and observed magnetic profiles and the passage duration of seven MCs fall within the predicted range. Based on the success of this approach, we conclude that predicting the near-Earth properties of MCs based on analysis and modelling of near-Sun CME observations is a viable endeavor with potential benefits for space weather assessment. Title: Simulating Magnetic Switchback Structures in the near-Sun Solar Wind Observed by Parker Probe Authors: Roddanavar, Arpita; Nandy, Dibyendu Bibcode: 2021AGUFMSH15C2049R Altcode: The Suns atmosphere, the Corona drives solar plasma winds in to the interplanetary medium that consist of charged particles embedded with magnetic fields. The solar wind assumes the shape of a spiral moving radially outwards from the Sun which is known as the Parker Spiral. The Parker Solar Probe Mission has observed that the magnetic fields in the near-Sun solar wind undergo magnetic polarity inversions. These deviations are termed magnetic switchbacks. Magnetic switchbacks are also accompanied by variations in the solar wind plasma parameters. We present proof of concept simulations to test a hypothesis for the origin of magnetic switchback structures based on vortical turbulence in the solar wind and present our preliminary results here. Title: The Roles of Intrinsic and Imposed Magnetospheres in Shielding Planetary Atmospheres Authors: Basak, Arnab; Nandy, Dibyendu Bibcode: 2021AGUFMSM55C1792B Altcode: We present results of three-dimensional compressible magnetohydrodynamic (MHD) simulations of the interactions between stellar wind and planets with different magnetospheric strengths, using the Star-Planet Interaction Module (CESSI-SPIM) developed at CESSI, IISER Kolkata. A gravitationally stratified planetary atmosphere is considered which is in hydrostatic equilibrium with the ambient medium, while the stellar wind follows magnetized shock conditions. The module is benchmarked using the observations from specific spacecraft orbits of Mars Global Surveyor (MGS) and Mars Atmosphere and Volatile EvolutioN (MAVEN) missions. The magnetopause stand-off distance is found to be the greatest for a strong intrinsic magnetosphere and the least for an imposed magnetosphere, implying that the stellar wind penetration is the maximum for a non-magnetized planet, resulting in higher atmospheric loss. This indicates that although the imposed magnetosphere protects the planetary atmosphere from the stellar wind, its level of shielding is not as effective as that of intrinsic magnetosphere. The study is important from the perspective of planetary habitability in solar and exoplanetary systems. [Basak & Nandy, MNRAS 502, 3569 (2021)] Title: Understanding the Global Coronal Magnetic Fields using Data-constrained Magnetohydrodynamic Model Authors: Dash, Soumyaranjan; Nandy, Dibyendu; Vaidya, Bhargav Bibcode: 2021AGUFMSH15G2085D Altcode: Coronal magnetic field evolution modulates our space environment via coronal mass ejections, flares, and changes in solar wind conditions. However, routine observations of magnetic field in the optically thin solar corona are not yet well-developed. Hence understanding the coronal magnetic field distribution using data-constrained global magnetohydrodynamic (MHD) models is of paramount importance nowadays. There are several factors which can drive the evolution e.g. flux emergence, photospheric flows, stratification in thermodynamic variables, solar wind conditions. Simulations using MHD models will help us generate the global magnetic field distribution, which can be constrained using total solar eclipse observations. Apart from this, such models have the potential to generate polarization characteristics utilizing the model output, which will help in better interpretation of data from future solar missions like PUNCH. We discuss the magnetic field distribution and polarization characteristics for past solar eclipses based on MHD simulations for global solar corona using a variety of approaches based on data-driven surface flux transport models, potential field source surface models and full MHD models. Title: Subsurface Plasma Flows and the Flare Productivity of Solar Active Regions Authors: Biji, Lekshmi; Jain, Kiran; Komm, Rudolf; Nandy, Dibyendu Bibcode: 2021AGUFMSH54A..07B Altcode: Highly energetic solar events such as solar flares and Coronal Mass Ejections (CMEs) can lead to extreme space weather. Hence, it is essential to understand their physical drivers and explore what governs their occurrence and intensity. By using the near-surface velocities derived by the ring-diagram analysis of active region patches using Global Oscillation Network Group (GONG) Doppler velocity measurements, we seek to explore the connection between subsurface flow properties and solar flares. The temporal evolution of vorticity and kinetic helicity of flaring and non-flaring active regions is investigated. The integrated vorticity, kinetic and current helicities, and magnetic flux one day prior to the flare are observed to be correlated with the integrated flare intensity. We show that active regions with strong subsurface vorticity and kinetic helicity tend to generate high intensity flares. We hypothesize that this is achieved via energy injection into subsurface magnetic flux systems by helical plasmas flows. Title: Estimating magnetospheric currents and geoeffectiveness of interplanetary CMEs with magnetohydrodynamic simulations Authors: ROY, Souvik; Nandy, Dibyendu Bibcode: 2021AGUFMSM41A..08R Altcode: The high energetic plasma and the embedded magnetic field of coronal mass ejections interact with planetary magnetospheres giving rise to transient perturbations such as geomagnetic storms. Predicting the geomagnetic impact of such interplanetary coronal mass ejections (ICME) is of utmost importance for the protection of our technological infrastructure that is affected by space weather. We use 3D compressible magnetohydrodynamic simulation of a star-planet system to model and study an ICME-Earth interaction event of 20th November 2003. In the modelled interaction, we observe a change in magnetopause shape and stand-off distance on ICME impact, day and night side reconnections and induction of high currents in the magnetosphere. We also notice the formation of a ring of strong equatorial current around the Earth, leading to a reduction of the geomagnetic field. We calculate the simulated reduction in the magnetic field and compare that to the observed geomagnetic indices in order to establish a predictive approach for geomagnetic storms. These simulations are expected to illuminate the physical processes that result in space weather impacts of stellar magnetic storms in planetary and exoplanetary systems. Title: Impact of Anomalous Active Regions on the Large Scale Magnetic Fields of the Solar Cycle Authors: Pal, Shaonwita; Nandy, Dibyendu; Bhowmik, Prantika; Dash, Soumyaranjan; Mahajan, Sushant; Munoz-Jaramillo, Andres Bibcode: 2021AGUFMSH55D1878P Altcode: Emergence of anomalous bipolar magnetic regions (combinations of anti-hale and anti-joy regions) on the solar surface can influence cycle to cycle variability and irregularities. We perform a comprehensive analysis of the dipole moment and polar field build up due to the appearance of anomalous active regions on the solar surface using a solar surface flux transport model. Our aim is to study the differences and the similarities between these anomalous regions and their effect in global solar cycle dynamics. Although these regions appear in small numbers, if they carry significant flux, they are found to significantly impact the polar field strength and thereby, the amplitude of future cycles. Title: Solar evolution and extrema: current state of understanding of long-term solar variability and its planetary impacts Authors: Nandy, Dibyendu; Martens, Petrus C. H.; Obridko, Vladimir; Dash, Soumyaranjan; Georgieva, Katya Bibcode: 2021PEPS....8...40N Altcode: The activity of stars such as the Sun varies over timescales ranging from the very short to the very long—stellar and planetary evolutionary timescales. Experience from our solar system indicates that short-term, transient events such as stellar flares and coronal mass ejections create hazardous space environmental conditions that impact Earth-orbiting satellites and planetary atmospheres. Extreme events such as stellar superflares may play a role in atmospheric mass loss and create conditions unsuitable for life. Slower, long-term evolutions of the activity of Sun-like stars over millennia to billions of years result in variations in stellar wind properties, radiation flux, cosmic ray flux, and frequency of magnetic storms. This coupled evolution of star-planet systems eventually determines planetary and exoplanetary habitability. The Solar Evolution and Extrema (SEE) initiative of the Variability of the Sun and Its Terrestrial Impact (VarSITI) program of the Scientific Committee on Solar-Terrestrial Physics (SCOSTEP) aimed to facilitate and build capacity in this interdisciplinary subject of broad interest in astronomy and astrophysics. In this review, we highlight progress in the major themes that were the focus of this interdisciplinary program, namely, reconstructing and understanding past solar activity including grand minima and maxima, facilitating physical dynamo-model-based predictions of future solar activity, understanding the evolution of solar activity over Earth's history including the faint young Sun paradox, and exploring solar-stellar connections with the goal of illuminating the extreme range of activity that our parent star—the Sun—may have displayed in the past, or may be capable of unleashing in the future. Title: Solar Cycle Evolution of Filaments over a Century: Investigations with the Meudon and McIntosh Hand-drawn Archives Authors: Mazumder, Rakesh; Chatterjee, Subhamoy; Nandy, Dibyendu; Banerjee, Dipankar Bibcode: 2021ApJ...919..125M Altcode: 2021arXiv210604320M Hand-drawn synoptic maps from the Meudon Observatory (1919 onwards) and the McIntosh archive (1967 onwards) are two important sources of long-term, manually recorded filament observations. In this study, we calibrate the Meudon maps and subsequently identify filaments through an automated method. We extract physical parameters from this filament database and perform a comparative study of their long-term evolution focusing on the cotemporal period of the McIntosh and Meudon observations. The spatiotemporal evolution of filaments manifests in the form of a filament butterfly diagram, further indicating that they are intimately related to the large-scale solar cycle. Physical descriptors such as the number and length of filaments, which are tracers of the solar surface magnetic field, have cycles which are phase locked with the ~11 yr sunspot cycle. The tilt-angle distribution of filaments-both near to or distant from active region locations-indicates that their origin is due to either large-scale surface magnetic field or inter-active-region field evolution. This study paves the way for constructing a composite series of hand-drawn filament data with minimal gaps stretching over the time span of solar filament observations up to a century. On the one hand, this would serve as a useful constraint for models of magnetic field emergence and evolution on the Sun's surface over multiple solar cycles, and on the other hand, this filament database may be used to guide the reconstruction of filament/prominence associated eruptive events before the space age. Title: Stellar mid-life crisis: subcritical magnetic dynamos of solar-like stars and the breakdown of gyrochronology Authors: Tripathi, Bindesh; Nandy, Dibyendu; Banerjee, Soumitro Bibcode: 2021MNRAS.506L..50T Altcode: 2018arXiv181205533T Recent observations have revealed the surprising breakdown of stellar gyrochronology relations at about the age of the Sun hinting the middle-aged, solar-like stars transition to a magnetically inactive future. We provide a theoretical basis for these intriguing observations inspired by simulations with a mathematical-dynamo model that can explore long-term solar cycle fluctuations. We reproduce the observed bimodal distribution of sunspot numbers, but only for subcritical dynamos. Based on a bifurcation analysis, we argue that the ageing of solar-like stars makes the magnetically weak dynamo regime readily accessible. Weak magnetic field production in this regime compromises wind-driven angular momentum losses, thus disrupting the hegemony of magnetic braking on stellar rotational spin-down. This hypothesis of subcritical magnetic dynamos of solar-like stars provides a self-consistent, unifying physical basis for a diversity of solar-stellar phenomena such as why stars beyond their mid-life do not spin-down as fast as in their youth, the break-down of stellar gyrochronology relations, the observed bimodal distribution of long-term sunspot observations, and recent findings suggesting that the Sun may be transitioning to a magnetically inactive future. Title: Modelling the Impact of Magnetic Storms on Planetary Environments Authors: Roy, Souvik; Nandy, Dibyendu Bibcode: 2021EGUGA..23.8863R Altcode: Coronal mass ejections (CMEs), large scale transient eruptions observed in the Sun, are thought to also be spawned by other magnetically active stars. The magnetic flux ropes intrinsic to these storms, and associated high-speed plasma ejecta perturb planetary environments creating hazardous conditions. To understand the physics of CME impact and consequent perturbations in planetary environments, we use 3D compressible magnetohydrodynamic simulation of a star-planet module (CESSI-SPIM) developed at CESSI, IISER Kolkata based on the PLUTO code architecture. We explore magnetohydrodynamic processes such as the formation of a bow-shock, magnetopause, magnetotail, planet-bound current sheets and atmospheric mass loss as a consequence of magnetic-storm-planetary interactions. Specifically, we utilize a realistic, twisted flux rope model for our CME, which leads to interesting dynamics related to helicity injection into the magnetosphere. Such studies will help us understand how energetic magnetic storms from host stars impact magnetospheres and atmospheres with implications for planetary and exoplanetary habitability. Title: Modelling the imposed magnetospheres of Mars-like exoplanets: star-planet interactions and atmospheric losses Authors: Basak, Arnab; Nandy, Dibyendu Bibcode: 2021MNRAS.502.3569B Altcode: 2021MNRAS.tmp..252B Based on 3D compressible magnetohydrodynamic simulations, we explore the interactions between the magnetized wind from a solar-like star and a Mars-like planet - with a gravitionally stratified atmosphere - that is either non-magnetized or hosts a weak intrinsic dipolar field. The primary mechanism for the induction of a magnetosphere around a non-magnetized conducting planet is the pile-up of stellar magnetic fields in the day-side region. The magnetopause stand-off distance decreases as the strength of the planetary dipole field is lowered and saturates to a minimum value for the case of a planet with no magnetic field. Global features such as bow shock, magnetosheath, magnetotail, and strong current sheets are observed in the imposed magnetosphere. We explore variations in atmospheric mass loss rates for different stellar wind strengths to understand the impact of stellar magnetic activity and plasma winds - and their evolution - on (exo)planetary habitability. In order to simulate a case analogous to the present-day Mars, a planet without atmosphere is considered. Our simulations are found to be in good agreement with observational data from Mars Global Surveyor and Mars Atmosphere and Volatile EvolutioN missions and is expected to complement observations from the Emirates (Hope) Mars Mission, China's Tianwen-1 and NASA's Mars 2020 Perseverance mission. Title: Modelling the solar wind forced Martian environment Authors: Basak, Arnab; Nandy, Dibyendu Bibcode: 2021csss.confE...2B Altcode: Since the halting of the Martian dynamo, the constant bombardment of solar wind particles on the planet led to the erosion of a significant portion of its atmosphere due to the lack of proper magnetic shielding. Several missions devoted to the exploration of the "red planet", such as Phobos, Mars Global Surveyor, Mars Atmosphere and Volatile Evolution, InSight, etc., provide vital information about the Martian environment which is important from the aspect of planetary habitability. We present results of three dimensional compressible magnetohydrodynamic simulations of solar wind and Mars interaction, using the Star Planet Interaction Module (CESSI-SPIM) developed at CESSI, IISER Kolkata. We elucidate mechanisms that lead to the formation of an imposed magnetosphere around Mars and provide a theoretical viewpoint on the interaction of stellar winds with non-magnetized planets with/without atmospheres. The results are found to be in agreement with observational data from various missions. The above study is not only relevant for planets and moons in our solar system but also provides important insight for the exploration of habitable planets in extrasolar systems. Title: Progress in Solar Cycle Predictions: Sunspot Cycles 24-25 in Perspective Authors: Nandy, Dibyendu Bibcode: 2021SoPh..296...54N Altcode: 2020arXiv200901908N The dynamic activity of the Sun—sustained by a magnetohydrodynamic dynamo mechanism working in its interior—modulates the electromagnetic, particulate, and radiative environment in space. While solar activity variations on short timescale create space weather, slow long-term modulation forms the basis of space climate. Space weather impacts diverse space-reliant technologies while space climate influences planetary atmospheres and climate. Having prior knowledge of the Sun's activity is important in these contexts. However, forecasting solar-stellar magnetic activity has remained an outstanding challenge. In this review, predictions for Sunspot Cycle 24 and the upcoming Solar Cycle 25 are summarized, and critically assessed. The analysis demonstrates that while predictions based on diverse techniques disagree across Solar Cycles 24-25, physics-based predictions for Solar Cycle 25 have converged and indicates a weak to moderate-weak sunspot cycle. I argue that this convergence in physics-based predictions is indicative of progress in the fundamental understanding of solar cycle predictability. Based on this understanding, resolutions to several outstanding questions related to solar cycle predictions are discussed; these questions include: is it possible to predict the solar cycle, what is the best proxy for predictions, how early can we predict the solar cycle and how many cycles into the future can we predict relying on our current understanding? Based on our analysis, we also suggest a rigorous pathway towards generating and disseminating a "consensus forecast" by any solar cycle prediction panels tasked with such a challenge. Title: Magnetohydrodynamical Understanding of the Interactions Between Coronal Mass Ejections and Earth's Magnetosphere. Authors: ROY, S.; Nandy, D. Bibcode: 2020AGUFMSM0510004R Altcode: Coronal mass ejections (CMEs), the large scale transient eruptions from the Sun, interact with the Earth's magnetosphere while travelling into the heliosphere. The energetic interplanetary CME (ICME) at 1AU not only creates geomagnetic storms and disrupts the magnetic field structure around the Earth but also impacts the plasma environment, causes strong aurorae, and disturbs the radio and electrical transmission massively. We use 3D compressible magnetohydrodynamic simulation of a star-planet system and study the interesting magnetohydrodynamic processes like bow-shock, magnetopause, magnetotail, planet-bound current sheets, magnetic reconnections, atmospheric mass loss as well as particle injection, etc., when an ICME flux rope crosses the Earth at 1 AU. We use the uniformly twisted force-free flux rope model proposed by Gold and Hoyle in 1960 to initiate the ICME and vary the flux rope properties using actual observational data. We observe a change in magnetopause's shape and the stand-off distance to the magnetopause. We notice twist helicity injection inside the magnetotail current system. We discover comparative increment in both the rates of atmospheric mass out-flow and solar wind in-flow in the vicinity of Earth during the geo-storm. Such studies will help us understand how energetic magnetic storms from a host star impact planetary magnetospheres and atmospheres with implications for planetary and exoplanetary habitability. Title: Flux Erosion of Magnetic Clouds by Reconnection With the Sun's Open Flux Authors: Pal, S.; Dash, S.; Nandy, D. Bibcode: 2020AGUFMSH041..07P Altcode: Magnetic clouds (MCs) are flux rope magnetic structures forming a subset of solar coronal mass ejections, which have significant space weather impacts. The geoeffectiveness of MCs depends on their properties, which evolve during their interplanetary passage. Based on an analysis of observations spanning two solar cycles, we establish that MCs interacting with the ambient solar wind magnetic field (i.e., heliospheric open flux) lose a substantial amount of their initial magnetic flux via magnetic reconnection, which, in some cases, reduce their geoeffectiveness. We find a linear correlation between the eroded flux of MCs and solar open flux which is consistent with the scenario that MC erosion is mediated via the local heliospheric magnetic field draping around an MC during its interplanetary propagation. The solar open flux is governed by the sunspot cycle. This work therefore uncovers a hitherto unknown pathway for solar cycle modulation of the properties of MCs. Title: COSPAR International Space Weather Action Teams: Addressing Challenges Across the Field of Space Weather. Authors: Kuznetsova, M. M.; Belehaki, A.; Bisi, M. M.; Bruinsma, S.; Fung, S. F.; Glover, A.; Grande, M.; Guo, J.; Jun, I.; Linker, J.; Mann, I. R.; Masson, A.; Mendoza, A. M. M.; Murray, S. A.; Nandy, D.; Opgenoorth, H. J.; Pevtsov, A. A.; Plainaki, C.; Reiss, M.; Sutton, E. K.; Temmer, M.; Usoskin, I. G.; Yao, Z.; Yardley, S.; Zheng, Y. Bibcode: 2020AGUFMSH0030022K Altcode: Advanced predictions of space weather impacts require improved understanding and modeling capabilities of coupled chains of space environment processes. It is necessary to assemble parts of the source-to-impact puzzle by identifying, addressing and solving problems focused on specific physical domains, and then to connect all validated solutions from space weather origins on the sun to impacts on coupled geospace system, humans and technologies. To address the need for multi-disciplinary international space weather research community connecting experts in space weather phenomena across all domains and experts in space environment impact, the COSPAR Panel on Space Weather facilitated establishment of a network of International Space Weather Action Teams (ISWAT, https://www.iswat-cospar.org, @IswatCosparOrg). ISWAT serves as a global hub for community coordinated topical collaborations focused on different aspects of space weather including advancing understanding, assessment and improvement of modeling capabilities, transitioning advances in research to operations, optimized utilization of available observations, and generating inputs to future instrumentation deployment. Action teams are building blocks of ISWAT initiative. ISWAT action teams are organized into domain-based ISWAT clusters. Action teams are working in coordinated effort across physical domain and across borders. The primary ISWAT goal is to advance space weather predictive capabilities based on best science available. The ISWAT currently includes more than 250 active participants and more than 50 action teams. The presentation will overview the outcome from the COSPAR ISWAT Inaugural Working Meeting in February 2020, highlight recent progress in advancing physics-based predictive capabilities and discuss plans for transforming COSPAR space weather Roadmap into a living document maintained by the community. Title: A Physics-based Prediction of Solar Cycle 25 and Reconstruction of Century-Scale Solar Activity Authors: Nandy, D.; Bhowmik, P. Bibcode: 2020AGUFMSH053..06N Altcode: The activity of the Sun generates space weather and forces the atmosphere and climate of planets. The basis of this dynamic solar activity is its magnetic cycle, which is manifest in variations of sunspots observed on the solar surface. These sunspots are generated by complex interactions of plasma flows and magnetic fields in the solar interior via the magnetohydrodynamic dynamo mechanism. While predictions of the future state of the solar cycle, including its strength and timing are critical to space weather awareness and mitigation strategies, making accurate physical model based predictions remain a challenging task. In this talk, I shall discuss the advances made in understanding the physics of solar cycle predictions in the last decade. I shall also discuss a prediction of the strength and timing of solar cycle 25 which is based on a methodology of coupling a surface flux transport model to a solar dynamo model; this methodology explains well the last century of solar activity and provides new insights in to the inner workings of the solar dynamo mechanism. Title: Dynamics of the Sun's Polar Field: Possible Insights from Out of the Ecliptic Observations Authors: Dash, S.; Nandy, D.; Pal, S. Bibcode: 2020AGUFMSH014..06D Altcode: The solar polar fields play a crucial role in sustaining the solar dynamo mechanism. Observations and computational modelling indicates that the surface dynamics of active regions mediated via plasma flows, which leads to the polar field build-up, is a fundamental aspect of the solar dynamo mechanism; in fact, this surface mediated flux dynamics also known as the Babcock-Leighton mechanism, is now implicated to be the leading contributor to the Sun's polar field, and thus also provides a window to the future solar cycle. This emergent understanding is one of the major drivers of novel mission concepts aiming to image the solar high latitudes from out of the ecliptic locations. Based on numerical simulations of the near-surface magnetic field dynamics and the Sun's polar landscape, here we discuss what new insights may be provided by out-of-the ecliptic solar physics missions. Title: Metis - Solar Orbiter Topical Team on "Modelling of CME propagation/evolution in corona and solar wind in connection with Space Weather" Authors: Bemporad, A.; Banerjee, D.; Berlicki, A.; Biondo, R.; Boe, B.; Calchetti, D.; Capuano, G.; De Leo, Y.; Del Moro, D.; Feng, L.; Foldes, R.; Frassati, F.; Frazin, R. A.; Giovannelli, L.; Giunta, A. S.; Heinzel, P.; Ippolito, A.; Janvier, M.; Jerse, G.; Kilpua, K. E. J.; Laurenza, M.; Lloveras, D.; Magdalenic, J.; Mancuso, S.; Messerotti, M.; Mierla, M.; Nandy, D.; Napoletano, G.; Nuevo, F.; Pagano, P.; Pinto, R.; Plainaki, C.; Reale, F.; Romoli, M.; Rodriguez, L.; Slemer, A.; Spadaro, D.; Susino, R.; Stangalini, M.; Vainio, R. O.; Valori, G.; Vásquez, A. M.; West, M. J. Bibcode: 2020AGUFMSH0360027B Altcode: Despite the current availability of multi-spacecraft observations of Coronal Mass Ejections (CMEs) and their interplanetary counterpart (ICMEs), at present we still don't understand which physical phenomena are driving their expansion and propagation phases. This also limits our understanding on how CMEs (observed with remote sensing data) become ICMEs (observed in situ), how they interact with the background solar wind, and how their final geo-effectiveness can be modified during their interplanetary evolution. Such problems match some of the scientific objectives of the Solar Orbiter Science Activity Plan and of the Metis coronagraph. Thanks to its multi-channel capability, Metis (acquiring images in the visible light and at the same time in the UV HI Lyman-alpha emission) will really provide an unprecedented view of CMEs and in particular of their thermodynamic evolution. At closest approaches to the Sun (in the nominal mission), Metis will acquire high spatial resolution and/or temporal cadence multi-channel images of CMEs. Farther from the Sun, Metis will shed light on the early Interplanetary propagation of CMEs. Later on (in the extended mission) Metis will observe for the first time the CME/ICME propagation out-of-ecliptic. These novelties will be combined with the unique vantage point that will be offered by the Solar Orbiter spacecraft, and supported with valuable data acquired by other on-board remote sensing (e.g. SPICE, EUI, SoloHI) and in situ (e.g. EPD, MAG, SWA, RPW) instruments. In this contribution we present the ongoing activities of the Metis Topical Team on "CME/ICME propagation", (http://metis.oato.inaf.it/topical_teams.html), an international working group recently established and gathering scientists from different countries, experts of both in-situ and remote sensing observations, as well as numerical simulations, and we summarize the main science objectives discussed during the last months. Title: Solar Cycle Variation of Meridional Flow Authors: Biji, L.; Nandy, D. Bibcode: 2020AGUFMSH0020002B Altcode: The solar meridional flow plays a crucial role in driving the solar dynamo. We study the near-surface hemispherical asymmetries in the meridional flow over 18 years using the velocities obtained from the Global Oscillation Network Group (GONG) and Helioseismic and Magnetic Imager (HMI) ring-diagram pipelines. Our analysis shows that the hemispherical asymmetry in the flow is strongly related to the solar cycle asymmetry. We observe a time-delayed anti-correlation with the asymmetry in meridional flow preceding the asymmetry in the sunspot cycle. We investigate the relationship between sunspot tilt angles and plasma inflows, and uncover the first observational evidence of the hypothesized non-linear quenching in the Babcock Leighton poloidal field generation mechanism mediated by surface plasma flows. Our observations illuminate feedback processes between magnetic fields and plasma flows and set new constraints on magneto-hydrodynamic dynamo models of the solar cycle. Title: Magnetohydrodynamic Simulations of the Solar Wind Interaction with Mars Authors: Basak, A.; Nandy, D. Bibcode: 2020AGUFMSM0530004B Altcode: Present day Mars is known to be a planet devoid of any intrinsic global dipolar field due to the absence of a "once active" dynamo process that existed billions of years ago. The lack of proper magnetic shielding led to the erosion of the Martian atmosphere mediated by solar wind interactions, thereby impacting the habitability of the planet. Since the dawn of space age, there have been numerous missions devoted to the exploration of the Martian environment to understand its evolution and current conditions. In the present study, we carry out three dimensional compressible magnetohydrodynamic simulations of the interactions between solar wind and a Mars-like planet using the Star Planet Interaction Module (CESSI-SPIM) developed at CESSI, IISER Kolkata. We ignore the presence of remnant crustal fields and take precautions for not allowing any undesirable numerical outflows from the planet by using gravitational stratification. The mechanism of formation of an imposed magnetosphere around the planet is presented in detail with emphasis on magnetic reconnections that lead to the same. The pile up of stellar magnetic field on the day side of the planet, which is responsible for the induction of magnetosphere, may be inhibited if the planet hosts even a weak dipolar field. The structures of the bow shock, magnetotail and magnetic pile up region are found to be in good agreement with observational data from missions such as Phobos, Mars Global Surveyor (MGS) and Mars Atmosphere and Volatile Evolution (MAVEN). The results presented are not only applicable to planets and moons in our solar system but also provide important insight for the exploration of habitable planets in far out exoplanetary systems. Title: Does the mean-field α effect have any impact on the memory of the solar cycle? Authors: Hazra, Soumitra; Brun, Allan Sacha; Nandy, Dibyendu Bibcode: 2020A&A...642A..51H Altcode: 2020arXiv200302776H Context. Predictions of solar cycle 24 obtained from advection-dominated and diffusion-dominated kinematic dynamo models are different if the Babcock-Leighton mechanism is the only source of the poloidal field. Some previous studies argue that the discrepancy arises due to different memories of the solar dynamo for advection- and diffusion-dominated solar convection zones.
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.
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.
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.
Conclusions: Our results imply that inclusion of a mean-field α effect in the framework of a flux transport Babcock-Leighton dynamo model leads to additional complexities that may impact memory and predictability of predictive dynamo models of the solar cycle. Title: Prediction of Sunspot Cycle 25: Based on a Century-scale Data-driven Magnetic Field Simulations Authors: Bhowmik, P.; Nandy, D. Bibcode: 2020SPD....5120902B Altcode: Solar variability governs the electromagnetic, radiative, and particulate environment in the heliosphere creating hazardous weather in space through eruptive events such as solar flares, coronal mass ejections. Moreover, modulation in solar output in terms of solar irradiance defines space climate. Both the short and long-term solar variabilities are closely associated with and mostly dominated by the sunspot cycle. Thus, in the context of space weather studies, predicting the sunspot cycle has gained a significant impetus in recent times. However, scientific studies have shown that the intrinsic stochastic nature of the solar convection zone limits the range of predictability to half a solar cycle, and, the dipolar field during the cycle minimum is one of the best precursors for sunspot cycle prediction. In comparison, we have devised a methodology by combining an observational data-driven surface flux transport model and an interior dynamo model to extend the prediction time window to decadal scale. This new methodology has been validated by performing a century-scale data-driven simulation which reproduced the past observation quite successfully — the first of its kind. Subsequently, we employ this technique for predicting sunspot cycle 25 while considering various possible uncertainties. Our ensemble forecast indicates cycle 25 would be similar or slightly stronger than the current cycle and peak around 2024. Sunspot cycle 25 may thus reverse the persistent weakening trend in solar activity which has led to speculation of an imminent Maunder-like grand minimum and cooling of global climate. Title: A model-free, data-based forecast for sunspot cycle 25 Authors: Espuña-Fontcuberta, Aleix; Chatterjee, Saikat; Mitra, Dhrubaditya; Nandy, Dibyendu Bibcode: 2020arXiv200512166E Altcode: The dynamic activity of the Sun, governed by its cycle of sunspots -- strongly magnetized regions that are observed on its surface -- modulate our solar system space environment creating space weather. Severe space weather leads to disruptions in satellite operations, telecommunications, electric power grids and air-traffic on polar routes. Forecasting the cycle of sunspots, however, has remained a challenging problem. We use reservoir computing -- a model-free, neural--network based machine-learning technique -- to forecast the upcoming solar cycle, sunspot cycle 25. The standard algorithm forecasts that solar cycle 25 is going to last about ten years, the maxima is going to appear in the year 2024 and the maximum number of sunspots is going to be 113 ($\pm15$). We also develop a novel variation of the standard algorithm whose forecasts for duration and peak timing matches that of the standard algorithm, but whose peak amplitude forecast is 124 ($\pm2$) -- within the upper bound of the standard reservoir computing algorithm. We conclude that sunspot cycle 25 is likely to be a weak, lower than average solar cycle, somewhat similar in strength to sunspot cycle 24. Title: Flux Erosion of Magnetic Clouds by Reconnection With the Sun's Open Flux Authors: Pal, Sanchita; Dash, Soumyaranjan; Nandy, Dibyendu Bibcode: 2020GeoRL..4786372P Altcode: 2021arXiv210305990P Magnetic clouds (MCs) are flux rope magnetic structures forming a subset of solar coronal mass ejections, which have significant space weather impacts. The geoeffectiveness of MCs depends on their properties, which evolve during their interplanetary passage. Based on an analysis of observations spanning two solar cycles, we establish that MCs interacting with the ambient solar wind magnetic field (i.e., heliospheric open flux) lose a substantial amount of their initial magnetic flux via magnetic reconnection, which, in some cases, reduce their geoeffectiveness. We find a linear correlation between the eroded flux of MCs and solar open flux which is consistent with the scenario that MC erosion is mediated via the local heliospheric magnetic field draping around an MC during its interplanetary propagation. The solar open flux is governed by the sunspot cycle. This work therefore uncovers a hitherto unknown pathway for solar cycle modulation of the properties of MCs. Title: Sunspot Cycle 25 is Brewing: Early Signs Herald its Onset Authors: Nandy, Dibyendu; Bhatnagar, Aditi; Pal, Sanchita Bibcode: 2020RNAAS...4...30N Altcode: No abstract at ADS Title: Prediction of the Sun's Coronal Magnetic Field and Forward-modeled Polarization Characteristics for the 2019 July 2 Total Solar Eclipse Authors: Dash, Soumyaranjan; Bhowmik, Prantika; Athira, B. S.; Ghosh, Nirmalya; Nandy, Dibyendu Bibcode: 2020ApJ...890...37D Altcode: 2019arXiv190610201D On 2019 July 2 a total solar eclipse—visible across parts of the Southern Pacific Ocean, Chile, and Argentina—enabled observations of the Sun's corona. The structure and emission characteristics of the corona are determined by underlying magnetic fields, which also govern coronal heating and solar eruptive events. However, coronal magnetic field measurements remain an outstanding challenge. Coronal magnetic field models serve an important purpose in this context. Earlier work has demonstrated that the large-scale coronal structure is governed by surface flux evolution and memory buildup, which allows for its prediction on solar rotational timescales. Utilizing this idea and based upon a 51 day forward run of a predictive solar surface flux transport model and a potential field source surface model, we predict the coronal structure of the 2019 July 2 solar eclipse. We also forward model the polarization characteristics of the coronal emission. Our prediction of two large-scale streamer structures and their locations on the east and west limbs of the Sun match eclipse observations reasonably well. We demonstrate that the Sun's polar fields strongly influence the modeled corona, concluding that accurate polar field observations are critical. This study is relevant for coronal magnetometry initiatives envisaged with the Daniel K. Inouye Solar Telescope, Coronal Multichannel Polarimeter and upcoming space-based instruments such as Solar Orbiter, Solar Ultraviolet Imaging Telescope and the Variable Emission Line Coronagraph on board the Indian Space Research Organisation's Aditya-L1 space mission. Title: Subcritical Magnetic Dynamos of Middle-aged Sun-like Stars Reconcile Solar-Stellar Activity Observation Authors: Tripathi, Bindesh; Nandy, Dibyendu; Banerjee, Soumitro Bibcode: 2020APS..DPPZ07009T Altcode: Long-term solar magnetic activity reconstructions indicate the solar dynamo operates in two distinct - grand minimum and regular activity - modes. By employing bifurcation analysis of a physically-motivated time delay dynamo model, we establish this to be a direct consequence of dynamo hysteresis. We reproduce the observed bimodal distribution of sunspots, but only for subcritical dynamos. We also demonstrate how the Sun can enter into the grand minima episodes and recover from it. A theoretical framework consistent with these findings explain confounding observations of an abrupt midlife transition in stellar activity, characterized by reduced angular momentum loss rates and breakdown of gyrochronology relations. Our study indicates that an evolving dynamo bridges a diversity of phenomena in Sun-like stars across their lifetime.

Supported by the Center of Excellence in Space Sciences India, MHRD, India; St. Xavier's College, Nepal. Title: Single-shot measurement of the space-varying polarization state of light through interferometric quantification of the geometric phase Authors: B S, Athira; Pal, Mandira; Mukherjee, Sounak; Mishra, Jatadhari; Nandy, Dibyendu; Ghosh, Nirmalya Bibcode: 2020PhRvA.101a3836B Altcode: A light beam carrying a spatially varying state of polarization generates a space-varying Pancharatnam-Berry geometric phase while propagating through a homogeneous anisotropic medium. We show that determination of such a space-varying geometric phase provides a unique way to quantify the space-varying polarization state of light using a single-shot interferometric measurement. We demonstrate this concept in a Mach-Zehnder interferometric arrangement using a linearly polarized reference light beam, where full information on the spatially varying polarization state is successfully recovered by quantifying the space-varying geometric phase and the contrast of interference. The proposed method enables single-shot measurement of any space-varying polarization state of light from the measured interference pattern with a polarized reference beam. This approach shows considerable potential for instantaneous mapping of complex space-varying polarization of light in diverse applications, such as astronomy, biomedical imaging, and nanophotonics, where high precision and near real-time measurement of spatial polarization patterns are desirable. Title: International Scientific Coordination on Space Weather: A COSPAR Panel on Space Weather Perspective Authors: Bisi, M. M.; Kuznetsova, M. M.; Temmer, M.; Opgenoorth, H. J.; Belehaki, A.; Bruinsma, S.; Glover, A.; Heynderickx, D.; Linker, J.; Mann, I. R.; Murray, S. A.; Nandy, D. Bibcode: 2019AGUFMSM31C3543B Altcode: The understanding and prediction of space-weather phenomena and their respective impact(s) on society have been widely-acknowledged as an international challenge and something that requires a global coordination and focus. In order to address this need to form more-formal worldwide collaboration and coordination, and to maximise return on such efforts (particularly scientifically), the Committee on Space Research (COSPAR) Panel on Space Weather (PSW) has created a network of International Space Weather Action Teams (ISWATs).

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.

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.

This presentation will re-introduce the ISWAT initiative, review its current status and plans for community-wide campaigns, highlight the overarching current plans for PSW, and place a focus on two key space-weather areas: the ambient heliosphere/background solar wind (designated as ISWAT theme H1) and CME structure, evolution and propagation through heliosphere (designated as ISWAT theme H2). Title: Showcasing the just released ISWAT website (http://www.iswat-cospar.org) built with a content management platform to serve as an online presence for the ISWAT (International Space Weather Action Teams) - community driven effort hosted by the COSPAR Panel on Space Weather. Authors: Mendoza, A. M. M.; Kuznetsova, M.; Opgenoorth, H. J.; Belehaki, A.; Bisi, M. M.; Bruinsma, S.; Heynderickx, D.; Linker, J.; Mann, I. R.; Murray, S. A.; Nandy, D.; Temmer, M. Bibcode: 2019AGUFMSM31C3181M Altcode: We will showcase the just released ISWAT website (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.

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.

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)

shown in the image is links to dedicated webpages that contain information about cluster goals and links to entry pages of registered action teams.

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

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

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

registered ISWAT team will be added in the near future.

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.

The website will be eventually maintained and facilitated by the COSPAR Panel on Space Weather Chairs/Vice-chairs, ISWAT cluster moderators, and ISWAT team

representatives. Title: International Scientific Coordination on Space Weather: A COSPAR Panel on Space Weather Perspective Authors: Kuznetsova, M.; Bisi, M. M.; Kusano, K.; Fuller-Rowell, T. J.; Mann, I.; Belehaki, A.; Minow, J. I.; Munoz-Jaramillo, A.; Masson, A.; Bruinsma, S.; Bisi, M. M.; Kuznetsova, M. M.; Temmer, M.; Opgenoorth, H. J.; Belehaki, A.; Bruinsma, S.; Glover, A.; Heynderickx, D.; Linker, J.; Mann, I. R.; Murray, S. A.; Nandy, D. Bibcode: 2019AGUFMSM31C3543K Altcode: The understanding and prediction of space-weather phenomena and their respective impact(s) on society have been widely-acknowledged as an international challenge and something that requires a global coordination and focus. In order to address this need to form more-formal worldwide collaboration and coordination, and to maximise return on such efforts (particularly scientifically), the Committee on Space Research (COSPAR) Panel on Space Weather (PSW) has created a network of International Space Weather Action Teams (ISWATs).

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.

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.

This presentation will re-introduce the ISWAT initiative, review its current status and plans for community-wide campaigns, highlight the overarching current plans for PSW, and place a focus on two key space-weather areas: the ambient heliosphere/background solar wind (designated as ISWAT theme H1) and CME structure, evolution and propagation through heliosphere (designated as ISWAT theme H2). Title: The origin of parity changes in the solar cycle Authors: Hazra, Soumitra; Nandy, Dibyendu Bibcode: 2019MNRAS.489.4329H Altcode: 2019MNRAS.tmp.2131H; 2019arXiv190606780H Although sunspots have been systematically observed on the Sun's surface over the last four centuries, their magnetic properties have been revealed and documented only since the early 1900s. Sunspots typically appear in pairs of opposite magnetic polarities which have a systematic orientation. This polarity orientation is opposite across the equator - a trend that has persisted over the last century. Taken together with the configuration of the global poloidal field of the Sun - this phenomena is consistent with the dipolar parity state of an underlying magnetohydrodynamic dynamo. Although transient hemispheric asymmetry in sunspot emergence is observed, a global parity shift has never been observed. We simulate hemispheric asymmetry through introduction of random fluctuations in a computational dynamo model of the solar cycle and demonstrate that changes in parity are indeed possible in long-term simulations covering thousands of years. Quadrupolar modes are found to exist over significant fraction of the simulated time. In particular, we find that a parity shift in the underlying nature of the sunspot cycle is more likely to occur when sunspot activity dominates in any one hemisphere for a time which is significantly longer than the cycle period. We establish causal pathways connecting hemispheric asymmetry to parity flips mediated via a decoupling of the dynamo cycle period across the two solar hemispheres. Our findings indicate that the solar cycle may have resided in quadrupolar parity states in the past, and provides a possible pathway for predicting parity flips in the future. Title: Hemispheric asymmetry in meridional flow and the sunspot cycle Authors: Lekshmi, B.; Nandy, Dibyendu; Antia, H. M. Bibcode: 2019MNRAS.489..714L Altcode: Magnetohydrodynamic dynamo modelling shows that the large-scale solar meridional plasma flow plays an important role in governing the dynamics of the sunspot cycle. Observations indicate that meridional flow velocities at each solar latitude and depth vary over time and are asymmetric across the equator. Here, using helioseismic observations we explore the temporal variation in the hemispherical asymmetry of near-surface residual (time-varying) component of the Sun's meridional flow velocity. The meridional flow velocities obtained from Global Oscillation Network Group (GONG) and Helioseismic and Magnetic Imager (HMI) onboard Solar Dynamics Observatory (SDO) ring-diagram pipelines are used in this work. Our data set covers the declining phase of cycle 23 and cycle 24 (from July 2001 till December 2018) and the flow velocities are poleward for the observed depth range. We observe a time delayed anticorrelation between the hemispherical asymmetry in near-surface meridional flow velocities and the sunspot cycle quantified in terms of magnetic flux and sunspot number. Interestingly, asymmetry in meridional flow velocity precedes the asymmetry in sunspot cycle by 3.1-3.5 yr. We propose that meridional flow asymmetry is a precursor of asymmetry in hemispherical cycle strength. The symmetric component of meridional flow is observed to be positively correlated with the corresponding symmetric components of the magnetic cycle, also with a time delay. Our analysis sets important constraints on theories for the origin of meridional plasma flow asymmetries and its temporal variations and is relevant for understanding the role of plasma flux transport processes in determining hemispheric asymmetry in the sunspot cycle. Title: Solar Filament Eruptions as Precursors to Flare-CME Events: Establishing the Temporal Connection Authors: Sinha, Suvadip; Srivastava, Nandita; Nandy, Dibyendu Bibcode: 2019ApJ...880...84S Altcode: Elongated structures on the Sun’s surface known as filaments are known to have a connection with energetic events of space weather consequence (flares and coronal mass ejections (CMEs)). In this work, we explore the connection between the eruptive dynamics of filaments and the initiation of solar flares and CMEs. We estimate the filament eruption start time by tracking the filament throughout its eruption phase. We define the filament eruption start time as the time from which the filament area starts to decrease as observed in Hα images. A total of 33 eruptive filament events are reported in this study, out of which 73% are CME associated and 76% are related to solar flares. We find a good correlation between area decay rate of the quiescent filaments and the speed of the associated CMEs with a correlation coefficient of 0.75. By analyzing the time delay of the extreme ultraviolet brightening of solar flares relative to the start time of associated filament eruption, we show that in 83% of cases, filament eruption precedes the flare brightening, which indicates that eruptive filaments can be considered as one of the precursors for the occurrence of a solar flare. Finally, we study the time delay of the CME onset from the time of initiation of the filament eruption process and show that for most of the cases, CMEs occur within 2 hr from the start time of the filament eruptions. This study would be useful for space weather assessment and characterization based on automated trackers of solar filament dynamics. Title: Modeling Star-Planet Interactions in Far-out Planetary and Exoplanetary Systems Authors: Bharati Das, Srijan; Basak, Arnab; Nandy, Dibyendu; Vaidya, Bhargav Bibcode: 2019ApJ...877...80B Altcode: 2019ApJ...877...80D; 2018arXiv181207767B The magnetized wind from a host star plays a vital role in shaping the magnetospheric configuration of the planets it harbors. We carry out three-dimensional (3D) compressible magnetohydrodynamic simulations of the interactions between magnetized stellar winds and planetary magnetospheres corresponding to a far-out star-planet system, with and without planetary dipole obliquity. We identify the pathways that lead to the formation of a dynamical steady-state magnetosphere and find that magnetic reconnection plays a fundamental role in the process. The magnetic energy density is found to be greater on the nightside than on the dayside, and the magnetotail is comparatively more dynamic. It is found that stellar wind plasma injection into the inner magnetosphere is possible through the magnetotail. We further study magnetospheres with extreme tilt angles, keeping in perspective the examples of Uranus and Neptune. High dipole obliquities may also manifest due to polarity excursions during planetary field reversals. We find that global magnetospheric reconnection sites change for large planetary dipole obliquity, and more complex current sheet structures are generated. We discuss the implications of these findings for atmospheric erosion, the introduction of stellar and interplanetary species that modify the composition of the atmosphere, auroral activity, and magnetospheric radio emission. This study is relevant for exploring star-planet interactions and its consequence on atmospheric dynamics and habitability in solar system planets and exoplanets. Title: Prediction of the Sun’s Corona for the Total Solar Eclipse on 2019 July 2 Authors: Dash, Soumyaranjan; Bhowmik, Prantika; Nandy, Dibyendu Bibcode: 2019RNAAS...3...86D Altcode: 2019RNAAS...3f..86D No abstract at ADS Title: Work Done by Lorentz Force Drives Solar-Stellar Magnetic Cycles Authors: Mahajan, Sushant Sushil; Nandy, Dibyendu; Martens, Petrus C. Bibcode: 2019shin.confE.199M Altcode: Our theoretical analysis of the equations of magnetohydrodynamics applied to the solar dynamo suggests that the work done by Lorentz force is the source of magnetic energy inside the solar convection zone. The action of Lorentz Force on poloidal field inside the convection zone is expected to leave behind signs of magnetic tension which manifest in the form of reduced latitudinal shear in differential rotation. We show that these expected signs of magnetic tension are consistent with the torsional oscillation profile of the Sun measured by three different instruments and hence can be used to locate regions inside the Sun where the magnetic field in sunspots originates. Title: A 3D kinematic Babcock Leighton solar dynamo model sustained by dynamic magnetic buoyancy and flux transport processes Authors: Kumar, Rohit; Jouve, Laurène; Nandy, Dibyendu Bibcode: 2019A&A...623A..54K Altcode: 2019arXiv190104251K Context. Magnetohydrodynamic interactions between plasma flows and magnetic fields is fundamental to the origin and sustenance of the 11-year sunspot cycle. These processes are intrinsically three-dimensional (3D) in nature.
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.
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.
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.
Conclusions: This study elucidates the relative roles of different flux-transport processes in the Sun's convection zone in determining the properties and physics of the sunspot cycle and could potentially lead to realistic, data-driven 3D dynamo models for solar-activity predictions and exploration of stellar magnetism and starspot formation in other stars. Title: Prediction of the strength and timing of sunspot cycle 25 reveal decadal-scale space environmental conditions Authors: Bhowmik, Prantika; Nandy, Dibyendu Bibcode: 2018NatCo...9.5209B Altcode: 2019arXiv190904537B The Sun's activity cycle governs the radiation, particle and magnetic flux in the heliosphere creating hazardous space weather. Decadal-scale variations define space climate and force the Earth's atmosphere. However, predicting the solar cycle is challenging. Current understanding indicates a short window for prediction best achieved at previous cycle minima. Utilizing magnetic field evolution models for the Sun's surface and interior we perform the first century-scale, data-driven simulations of solar activity and present a scheme for extending the prediction window to a decade. Our ensemble forecast indicates cycle 25 would be similar or slightly stronger than the current cycle and peak around 2024. Sunspot cycle 25 may thus reverse the substantial weakening trend in solar activity which has led to speculation of an imminent Maunder-like grand minimum and cooling global climate. Our simulations demonstrate fluctuation in the tilt angle distribution of sunspots is the dominant mechanism responsible for solar cycle variability. Title: The Association of Filaments, Polarity Inversion Lines, and Coronal Hole Properties with the Sunspot Cycle: An Analysis of the McIntosh Database Authors: Mazumder, Rakesh; Bhowmik, Prantika; Nandy, Dibyendu Bibcode: 2018ApJ...868...52M Altcode: 2018arXiv181002133M Filaments and coronal holes, two principal features observed in the solar corona, are sources of space weather variations. Filament formation is closely associated with polarity inversion lines (PILs) on the solar photosphere which separate positive and negative polarities of the surface magnetic field. The origin of coronal holes is governed by large-scale unipolar magnetic patches on the photosphere from where open magnetic field lines extend to the heliosphere. We study the properties of filaments, PILs, and coronal holes in solar cycles 20, 21, 22, and 23 utilizing the McIntosh archive. We detect a prominent cyclic behavior of filament length, PIL length, and coronal hole area with significant correspondence with the solar magnetic cycle. The spatio-temporal evolution of the geometric centers of filaments shows a butterfly-like structure and distinguishable poleward migration of long filaments during cycle maxima. We identify this rush to the poles of filaments to be co-temporal with the initiation of polar field reversal as gleaned from Mount Wilson and Wilcox Solar Observatory polar field observations, and quantitatively establish their temporal correspondence. We analyze the filament tilt angle distribution to constrain their possible origins. The majority of the filaments exhibit negative and positive tilt angles in the northern and the southern hemispheres, respectively, strongly suggesting that their formation is governed by the overall large-scale magnetic field distribution on the solar photosphere and not by the small-scale intra-active region magnetic field configurations. We also investigate the hemispheric asymmetry in filaments, PILs, and coronal holes. We find that the hemispheric asymmetry in filaments and PILs is positively correlated with sunspot area asymmetry, whereas coronal hole asymmetry is uncorrelated. Title: The Extended Solar Cycle: Muddying the Waters of Solar/Stellar Dynamo Modeling Or Providing Crucial Observational Constraints? Authors: Srivastava, Abhishek K.; McIntosh, Scott W.; Arge, N.; Banerjee, Dipankar; Dikpati, Mausumi; Dwivedi, Bhola N.; Guhathakurta, Madhulika; Karak, B. B.; Leamon, Robert J.; Matthew, Shibu K.; Munoz-Jaramillo, Andres; Nandy, D.; Norton, Aimee; Upton, L.; Chatterjee, S.; Mazumder, Rakesh; Rao, Yamini K.; Yadav, Rahul Bibcode: 2018FrASS...5...38S Altcode: 2018arXiv180707601S In 1844 Schwabe discovered that the number of sunspots increased and decreased over a period of about 11 years, that variation became known as the sunspot cycle. Almost eighty years later, Hale described the nature of the Sun's magnetic field, identifying that it takes about 22 years for the Sun's magnetic polarity to cycle. It was also identified that the latitudinal distribution of sunspots resembles the wings of a butterfly showing migration of sunspots in each hemisphere that abruptly start at mid-latitudes (about ±35(o) ) towards the Sun's equator over the next 11 years. These sunspot patterns were shown to be asymmetric across the equator. In intervening years, it was deduced that the Sun (and sun-like stars) possess magnetic activity cycles that are assumed to be the physical manifestation of a dynamo process that results from complex circulatory transport processes in the star's interior. Understanding the Sun's magnetism, its origin and its variation, has become a fundamental scientific objective the distribution of magnetism, and its interaction with convective processes, drives various plasma processes in the outer atmosphere that generate particulate, radiative, eruptive phenomena and shape the heliosphere. In the past few decades, a range of diagnostic techniques have been employed to systematically study finer scale magnetized objects, and associated phenomena. The patterns discerned became known as the ``Extended Solar Cycle'' (ESC). The patterns of the ESC appeared to extend the wings of the activity butterfly back in time, nearly a decade before the formation of the sunspot pattern, and to much higher solar latitudes. In this short review, we describe their observational patterns of the ESC and discuss possible connections to the solar dynamo as we depart on a multi-national collaboration to investigate the origins of solar magnetism through a blend of archived and contemporary data analysis with the goal of improving solar dynamo understanding and modeling. Title: Dependence of Coronal Mass Ejection Properties on Their Solar Source Active Region Characteristics and Associated Flare Reconnection Flux Authors: Pal, Sanchita; Nandy, Dibyendu; Srivastava, Nandita; Gopalswamy, Nat; Panda, Suman Bibcode: 2018ApJ...865....4P Altcode: 2018arXiv180804144P The near-Sun kinematics of coronal mass ejections (CMEs) determine the severity and arrival time of associated geomagnetic storms. We investigate the relationship between the deprojected speed and kinetic energy of CMEs and magnetic measures of their solar sources, reconnection flux of associated eruptive events, and intrinsic flux-rope characteristics. Our data covers the period 2010-2014 in solar cycle 24. Using vector magnetograms of source active regions, we estimate the size and nonpotentiality. We compute the total magnetic reconnection flux at the source regions of CMEs using the post-eruption arcade method. By forward modeling the CMEs, we find their deprojected geometric parameters and constrain their kinematics and magnetic properties. Based on an analysis of this database, we report that the correlation between CME speed and their source active region size and global nonpotentiality is weak, but not negligible. We find the near-Sun velocity and kinetic energy of CMEs to be well correlated with the associated magnetic reconnection flux. We establish a statistically significant empirical relationship between the CME speed and reconnection flux that may be utilized for prediction purposes. Furthermore, we find CME kinematics to be related with the axial magnetic field intensity and relative magnetic helicity of their intrinsic flux ropes. The amount of coronal magnetic helicity shed by CMEs is found to be well correlated with their near-Sun speeds. The kinetic energy of CMEs is well correlated with their intrinsic magnetic energy density. Our results constrain processes related to the origin and propagation of CMEs and may lead to better empirical forecasting of their arrival and geoeffectiveness. Title: The solar dynamo as an interplay of rotational shear and magnetic field Authors: Mahajan, Sushant Sushil; Nandy, Dibyendu; Martens, Petrus C. Bibcode: 2018shin.confE.154M Altcode: No abstract at ADS Title: Asymmetry in Solar Torsional Oscillation and the Sunspot Cycle Authors: Lekshmi, B.; Nandy, Dibyendu; Antia, H. M. Bibcode: 2018ApJ...861..121L Altcode: 2018arXiv180703588B; 2018ApJ...861..121B; 2018arXiv180703588L Solar torsional oscillations are migrating bands of slower- and faster-than-average rotation, which are strongly related to the Sun’s magnetic cycle. We perform a long-term study (16 yr) of hemispherical asymmetry in solar torsional oscillation velocity using helioseismic data for the first time. We study the north-south asymmetry in the velocity using the zonal flow velocities obtained by ring diagram analysis of the Global Oscillation Network Group (GONG) Doppler images. We find significant hemispherical asymmetry in the torsional oscillation velocity and explore its variation with respect to depth, time, and latitude. We also calculate the hemispherical asymmetry in the surface velocity measurements from the Mount Wilson Observatory and the zonal flow velocities obtained from the Helioseismic and Magnetic Imager ring diagram pipeline. These asymmetries are found to be consistent with the asymmetry obtained from GONG observations. We show that the asymmetry in near-surface torsional oscillation velocity is correlated with the asymmetry in magnetic flux and sunspot number at the solar surface, with the velocity asymmetry preceding the flux and sunspot number asymmetries. We speculate that the asymmetry in torsional oscillation velocity may help in predicting the hemispherical asymmetry in sunspot cycles. Title: Origin and Recovery from Grand Solar Minima in a Time Delay Dynamo Model with Magnetic Noise as an Additional Poloidal Source Authors: Tripathi, Bindesh; Nandy, Dibyendu; Banerjee, Soumitro Bibcode: 2018arXiv180411350T Altcode: We explore a reduced Babcock-Leighton (BL) dynamo model based on delay differential equations using numerical bifurcation analysis. This model reveals hysteresis, seen in the recent mean-field dynamo model and the direct numerical simulations of turbulent dynamos. The BL model with 'magnetic noise' as an additional weak-source of the poloidal field recovers the solar cycle every time from grand minima, which BL source alone cannot do. The noise-incorporated model exhibits a bimodal distribution of toroidal field energy confirming two modes of solar activity. It also shows intermittency and reproduces phase space collapse, an experimental signature of the Maunder Minimum. The occurrence statistics of grand minima in our model agree reasonably well with the observed statistics in the reconstructed sunspot number. Finally, we demonstrate that the level of magnetic noise controls the duration of grand minima and even has a handle over its waiting period, suggesting a triggering effect of grand minima by the noise and thus shutting down the global dynamo. Therefore, we conclude that the 'magnetic noise' due to small-scale turbulent dynamo action (or other sources) plays a vital role even in Babcock-Leighton dynamo models. Title: Torsional Oscillations in the Suns rotation contribute to the Waldmeier-effect in Solar Cycles Authors: Mahajan, Sushant S.; Nandy, Dibyendu; Antia, H. M.; Dwivedi, B. N. Bibcode: 2018arXiv180307758M Altcode: Temporal variations in the Suns internal velocity field with a periodicity of about 11 years have been observed over the last four decades. The period of these torsional oscillations and their latitudinal propagation roughly coincides with the period and equatorward propagation of sunspots which originate from a magnetohydrodynamic dynamo mechanism operating in the Suns interior. While the solar differential rotation plays an important role in this dynamo mechanism by inducting the toroidal component of magnetic field, the impact of torsional oscillations on the dynamo mechanism and hence the solar cycle is not well understood. Here, we include the observed torsional oscillations into a flux transport dynamo model of the solar cycle to investigate their effect. We find that the overall amplitude of the solar cycle does not change significantly on inclusion of torsional oscillations. However, all the characteristics of the Waldmeier effect in the sunspot cycle are qualitatively reproduced by varying only the amplitude of torsional oscillations. The Waldmeier effect, first noted in 1935, includes the important characteristic that the amplitude of sunspot cycles is anti-correlated to their rise time; cycles with high initial rise rate tend to be stronger. This has implications for solar cycle predictions. Our results suggest that the Waldmeier effect could be a plausible outcome of cycle to cycle modulation of torsional oscillations and provides a physical basis for sunspot cycle forecasts based on torsional oscillation observations. We also provide a theoretical explanation based on the magnetic induction equation thereby connecting two apparently disparate phenomena. Title: A Magnetofrictional model for the solar corona Authors: Dash, Soumyaranjan; Nandy, Dibyendu Bibcode: 2018IAUS..340...87D Altcode: Regular reconstruction of global solar corona constrained by observational data is required to monitor the space weather variations. We develop a model for simulating the global coronal magnetic field using magnetofrictional approach. Here we perform simulations to study the evolution of the magnetic field associated with a bipolar active region in response to photospheric flows. Title: The activity evolution of Solar-like stars with age and its planetary impact Authors: Das, Srijan Bharati; Basak, Arnab; Nandy, Dibyendu Bibcode: 2018IAUS..340..240D Altcode: The age-dependent activity of a star dictates the extent of its planetary impact. We study the interaction of the stellar wind produced by Solar-like stars with the magnetosphere of Earth-like planets using three dimensional (3D) magnetohydrodynamic (MHD) simulations. The numerical simulations reveal important features of star-planet interaction e.g. bow-shock, magnetopause, magnetotail, etc. Interesting phenomena such as particle injection into the planetary atmosphere as well as atmospheric mass loss are also observed which are instrumental in determining the atmospheric retention by the planet. Title: Study of starspots in fully convective stars using three dimensional MHD simulations Authors: Basak, Arnab; Nandy, Dibyendu Bibcode: 2018IAUS..340..303B Altcode: Concentrated magnetic structures such as sunspots and starspots play a fundamental role in driving solar and stellar activity. However, as opposed to the sun, observations as well as numerical simulations have shown that stellar spots are usually formed as high-latitude patches extended over wide areas. Using a fully spectral magnetohydrodynamic (MHD) code, we simulate polar starspots produced by self-consistent dynamo action in rapidly rotating convective shells. We carry out high resolution simulations and investigate various properties related to stellar dynamics which lead to starspot formation. Title: Properties of Coronal Holes in Solar Cycle 21-23 using McIntosh archive Authors: Mazumder, Rakesh; Bhowmik, Prantika; Nandy, Dibyendu Bibcode: 2018IAUS..340..187M Altcode: We study the properties of coronal holes during solar cycle 21-23 from the McIntosh archive. In the spatial distribution of coronal hole area we find that there is a sharp increase in coronal hole area at high latitude in agreement with expected open flux configuration there. In overall spatiotemporal distribution of coronal hole centroids, we find the dominance of high latitude coronal holes except for the maximum of the solar cycle, when coronal holes mostly appear in low latitudes. This is in agreement with the expected solar cycle evolution of surface magnetic flux. Title: Asymmetry in Solar Torsional Oscillation Authors: Lekshmi, B.; Nandy, Dibyendu; Antia, H. M. Bibcode: 2018IAUS..340...11L Altcode: Solar torsional oscillations are migrating bands of slower and faster than average rotation, which are thought to be related to the Sun's magnetic cycle. We perform the first long-term study (16 years) of hemispherical asymmetry in solar torsional oscillation velocity using helioseismic data. We explore the spatial and temporal variation of North-South asymmetry using zonal flow velocities obtained from ring diagram analysis of the Global Oscillation Network Group (GONG) Doppler images. We find a strong correlation between the asymmetries of near-surface torsional oscillation with magnetic flux and sunspot number, with the velocity asymmetry preceding in both the cases. We speculate that the asymmetry in torsional oscillation velocity may help in predicting the hemispherical asymmetry in the sunspot cycle. Title: The Large-scale Coronal Structure of the 2017 August 21 Great American Eclipse: An Assessment of Solar Surface Flux Transport Model Enabled Predictions and Observations Authors: Nandy, Dibyendu; Bhowmik, Prantika; Yeates, Anthony R.; Panda, Suman; Tarafder, Rajashik; Dash, Soumyaranjan Bibcode: 2018ApJ...853...72N Altcode: On 2017 August 21, a total solar eclipse swept across the contiguous United States, providing excellent opportunities for diagnostics of the Sun’s corona. The Sun’s coronal structure is notoriously difficult to observe except during solar eclipses; thus, theoretical models must be relied upon for inferring the underlying magnetic structure of the Sun’s outer atmosphere. These models are necessary for understanding the role of magnetic fields in the heating of the corona to a million degrees and the generation of severe space weather. Here we present a methodology for predicting the structure of the coronal field based on model forward runs of a solar surface flux transport model, whose predicted surface field is utilized to extrapolate future coronal magnetic field structures. This prescription was applied to the 2017 August 21 solar eclipse. A post-eclipse analysis shows good agreement between model simulated and observed coronal structures and their locations on the limb. We demonstrate that slow changes in the Sun’s surface magnetic field distribution driven by long-term flux emergence and its evolution governs large-scale coronal structures with a (plausibly cycle-phase dependent) dynamical memory timescale on the order of a few solar rotations, opening up the possibility for large-scale, global corona predictions at least a month in advance. Title: An Early Prediction of Sunspot Cycle 25 Authors: Nandy, D.; Bhowmik, P. Bibcode: 2017AGUFMSH21A2639N Altcode: The Sun's magnetic activity governs our space environment, creates space weather and impacts our technologies and climate. With increasing reliance on space- and ground-based technologies that are subject to space weather, the need to be able to forecast the future activity of the Sun has assumed increasing importance. However, such long-range, decadal-scale space weather prediction has remained a great challenge as evident in the diverging forecasts for solar cycle 24. Based on recently acquired understanding of the physics of solar cycle predictability, we have devised a scheme to extend the forecasting window of solar cycles. Utilizing this we present an early forecast for sunspot cycle 25 which would be of use for space mission planning, satellite life-time estimates, and assessment of the long-term impacts of space weather on technological assets and planetary atmospheres. Title: A Sun-to-Earth Analysis of Magnetic Helicity of the 2013 March 17-18 Interplanetary Coronal Mass Ejection Authors: Pal, Sanchita; Gopalswamy, Nat; Nandy, Dibyendu; Akiyama, Sachiko; Yashiro, Seiji; Makela, Pertti; Xie, Hong Bibcode: 2017ApJ...851..123P Altcode: 2017arXiv171201114P We compare the magnetic helicity in the 2013 March 17-18 interplanetary coronal mass ejection (ICME) flux rope at 1 au and in its solar counterpart. The progenitor coronal mass ejection (CME) erupted on 2013 March 15 from NOAA active region 11692 and is associated with an M1.1 flare. We derive the source region reconnection flux using the post-eruption arcade (PEA) method that uses the photospheric magnetogram and the area under the PEA. The geometrical properties of the near-Sun flux rope is obtained by forward-modeling of white-light CME observations. Combining the geometrical properties and the reconnection flux, we extract the magnetic properties of the CME flux rope. We derive the magnetic helicity of the flux rope using its magnetic and geometric properties obtained near the Sun and at 1 au. We use a constant-α force-free cylindrical flux rope model fit to the in situ observations in order to derive the magnetic and geometric information of the 1 au ICME. We find a good correspondence in both amplitude and sign of the helicity between the ICME and the CME, assuming a semi-circular (half torus) ICME flux rope with a length of π au. We find that about 83% of the total flux rope helicity at 1 au is injected by the magnetic reconnection in the low corona. We discuss the effect of assuming flux rope length in the derived value of the magnetic helicity. This study connecting the helicity of magnetic flux ropes through the Sun-Earth system has important implications for the origin of helicity in the interplanetary medium and the topology of ICME flux ropes at 1 au and hence their space weather consequences. Title: Living Around Active Stars Authors: Nandy, D.; Valio, A.; Petit, P. Bibcode: 2017IAUS..328.....N Altcode: No abstract at ADS Title: A Data Driven, Zero-Dimensional Time Delay Model with Radiative Forcing for Simulating Global Climate Authors: Tarafder, Rajashik; Nandy, Dibyendu Bibcode: 2017arXiv170908860T Altcode: Several complicated non-linear models exist which simulate the physical processes leading to fluctuations in global climate. Some of these more advanced models use observations to constrain various parameters involved. However, they tend to be very computationally expensive. Also, the exact physical processes that affect the climate variations have not been completely comprehended. Therefore, to obtain an insight into global climate, we have developed a physically motivated reduced climate model. The model utilizes a novel mathematical formulation involving a non-linear delay differential equation to study temperature fluctuations when subjected to imposed radiative forcing. We have further incorporated simplified equations to test the effect of speculated mechanisms of climate forcing and evaluated the extent of their influence. The findings are significant in our efforts to predict climate change and help in policy framing necessary to tackle it. Title: Strong Hemispheric Asymmetry can Trigger Parity Changes in the Sunspot Cycle Authors: Hazra, Soumitra; Nandy, Dibyendu Bibcode: 2017SPD....4830604H Altcode: Although sunspots have been systematically observed on the Sun’s surface over the last four centuries, their magnetic properties have been revealed and documented only since the early 1900s. Sunspots typically appear in pairs of opposite magnetic polarity which have a systematic orientation. This polarity orientation is opposite across the equator - a trend that has persisted over the last century since observations of sunspot magnetic fields exist. Taken together with the configurationof the global poloidal field of the Sun - that governs the heliospheric open flux and cosmic ray flux at Earth - this phenomena is consistent with the dipolar parity state of an underlying magnetohydrodynamic dynamo mechanism. Although, hemispheric asymmetry in the emergence of sunspots is observed in the Sun, a parity shift has never been observed. We simulate hemispheric asymmetry through introduction of random fluctuations in a computational dynamo model of the solar cycle and demonstrate that changes in parity are indeed possible over long time-scales. In particular, we find that a parity shift in the underlying nature of the sunspot cycle is more likely to occur when sunspot activity dominates in any one hemisphere for a time which is significantly longer compared to the sunspot cycle period. Our simulations suggest that the sunspot cycle may have resided in quadrupolar parity states in the distant past, and provides a possible pathway for predicting parity flips in the future. Title: The Solar Ultraviolet Imaging Telescope on-board Aditya-L1 Authors: Tripathi, Durgesh; Ramaprakash, A. N.; Khan, Aafaque; Ghosh, Avyarthana; Chatterjee, Subhamoy; Banerjee, Dipankar; Chordia, Pravin; Gandorfer, Achim; Krivova, Natalie; Nandy, Dibyendu; Rajarshi, Chaitanya; Solanki, Sami K. Bibcode: 2017CSci..113..616T Altcode: 2022arXiv220407732T The Solar Ultraviolet Imaging Telescope (SUIT) is an instrument onboard the Aditya-L1 mission of ISRO that will measure and monitor the solar radiation emitted in the near-ultraviolet wavelength range (200-400 nm). SUIT will simultaneously map the photosphere and the chromosphere of the Sun using 11 filters sensitive to different wavelengths and covering different heights in the solar atmosphere and help us understand the processes involved in the transfer of mass and energy from one layer to the other. SUIT will also allow us to measure and monitor spatially resolved solar spectral irradiance that governs the chemistry of oxygen and ozone in the stratosphere of Earth's atmosphere. This is central to our understanding of the Sun climate relationship. Title: Solar Surface Magnetic Field Simulation Enabled Prediction of the Large-Scale Coronal Structure of the 21 August 2017 Great American Eclipse: An Assessment of Model Predictions and Observations Authors: Nandy, Dibyendu; Bhowmik, Prantika; Yeates, Anthony R.; Panda, Suman; Tarafder, Rajashik; Dash, Soumyaranjan Bibcode: 2017arXiv170805996N Altcode: On 21 August 2017 a total solar eclipse swept across the contiguous United States providing excellent opportunities for diagnostics of the Sun's corona. The Sun's coronal structure is notoriously difficult to observe except during solar eclipses; thus theoretical models must be relied upon for inferring the underlying magnetic structure of the Sun's outer atmosphere. These models are necessary for understanding the role of magnetic fields in the heating of the corona to a million degrees and generation of severe space weather. Here we present a methodology for predicting the structure of the coronal field based on long-term surface flux transport simulations whose output is utilized to extrapolate the coronal magnetic field structures. This prescription was applied to the 21 August 2017 solar eclipse. Post-eclipse analysis shows good agreement between model simulated and observed coronal structures and their locations on the limb. We demonstrate that slow changes in the Sun's surface magnetic field distribution driven by long-term flux emergence and evolution govern large-scale coronal structures with a (plausibly cycle-phase dependent) dynamical memory timescale on the order of few solar rotations -- opening up the possibility of large-scale, global corona predictions at least a month in advance. Title: The Solar-Stellar Connection Authors: Brun, A. S.; García, R. A.; Houdek, G.; Nandy, D.; Pinsonneault, M. Bibcode: 2017hdsi.book..309B Altcode: No abstract at ADS Title: Grand Minima in the Light of Kinematic Flux Transport Solar Dynamo Model Authors: Hazra, S.; Nandy, D. Bibcode: 2016AGUFMSH43D2590H Altcode: Fluctuations in solar magnetic output,including episodes of grand minima such as maunder minima are studied using kinematic flux transport dynamo model. We show that cycle to cycle variation and maunder like grand minima is naturally produced by introduction of stochastic fluctuation in the poloidal field generation mechanism. Our anyalisis shows that Babcock-Leighton alpha effect alone can not restart the solar cycle once it gets into a grand minimum. An additional alpha effect capable of working on weak toroidal magnetic fields - such as that driven by helical turbulence - is necessary to recover the solar cycle. Our result demonstrates the importance of small scale alpha effect and shows how self consistent entry and exit from grand minima like episodes is possible. We also investigate the condition which triggers the grand minima like episode and find that if the amplitude of the poloidal field at the minima of the previous solar cycle drops near about 60 % from the mean value of peak poloidal fields, then solar cycle entering into grand minima like episode. Finally we address the question whether it is possible to predict the onset of grand minima like episodes. Title: A Proposed Paradigm for Solar Cycle Dynamics Mediated via Turbulent Pumping of Magnetic Flux in Babcock-Leighton-type Solar Dynamos Authors: Hazra, Soumitra; Nandy, Dibyendu Bibcode: 2016ApJ...832....9H Altcode: 2016arXiv160808167H At present, the Babcock-Leighton flux transport solar dynamo models appear to be the most promising models for explaining diverse observational aspects of the sunspot cycle. The success of these flux transport dynamo models is largely dependent upon a single-cell meridional circulation with a deep equatorward component at the base of the Sun’s convection zone. However, recent observations suggest that the meridional flow may in fact be very shallow (confined to the top 10% of the Sun) and more complex than previously thought. Taken together, these observations raise serious concerns on the validity of the flux transport paradigm. By accounting for the turbulent pumping of magnetic flux, as evidenced in magnetohydrodynamic simulations of solar convection, we demonstrate that flux transport dynamo models can generate solar-like magnetic cycles even if the meridional flow is shallow. Solar-like periodic reversals are recovered even when meridional circulation is altogether absent. However, in this case, the solar surface magnetic field dynamics does not extend all the way to the polar regions. Very importantly, our results demonstrate that the Parker-Yoshimura sign rule for dynamo wave propagation can be circumvented in Babcock-Leighton dynamo models by the latitudinal component of turbulent pumping, which can generate equatorward propagating sunspot belts in the absence of a deep, equatorward meridional flow. We also show that variations in turbulent pumping coefficients can modulate the solar cycle amplitude and periodicity. Our results suggest the viability of an alternate magnetic flux transport paradigm—mediated via turbulent pumping—for sustaining solar-stellar dynamo action. Title: The Solar Ultraviolet Imaging Telescope onboard Aditya-L1 Authors: Ghosh, Avyarthana; Chatterjee, Subhamoy; Khan, Aafaque R.; Tripathi, Durgesh; Ramaprakash, A. N.; Banerjee, Dipankar; Chordia, Pravin; Gandorfer, Achim M.; Krivova, Natalie; Nandy, Dibyendu; Rajarshi, Chaitanya; Solanki, Sami K.; Sriram, S. Bibcode: 2016SPIE.9905E..03G Altcode: The Solar Ultraviolet Imaging Telescope (SUIT) is an instrument onboard the Aditya-L1 spacecraft, the first dedicated solar mission of the Indian Space Research Organization (ISRO), which will be put in a halo orbit at the Sun-Earth Langrage point (L1). SUIT has an off-axis Ritchey-Chrétien configuration with a combination of 11 narrow and broad bandpass filters which will be used for full-disk solar imaging in the Ultravoilet (UV) wavelength range 200-400 nm. It will provide near simultaneous observations of lower and middle layers of the solar atmosphere, namely the Photosphere and Chromosphere. These observations will help to improve our understanding of coupling and dynamics of various layers of the solar atmosphere, mechanisms responsible for stability, dynamics and eruption of solar prominences and Coronal Mass ejections, and possible causes of solar irradiance variability in the Near and Middle UV regions, which is of central interest for assessing the Sun's influence on climate. Title: The Impact Of Torsional Oscillations On The Solar Cycle: The Waldmeier-effect As An Outcome Authors: Mahajan, Sushant S.; Nandy, Dibyendu; Dwivedi, Bhola N.; Antia, H. M. Bibcode: 2016SPD....47.0718M Altcode: Temporal variations in the Sun’s internal velocity field with a periodicity of about 11 years have been observed in the last three decades. The period of these torsional oscillations and their latitudinal propagation roughly coincide with the period and equatorward propagation of sunspots which originate from a magnetohydrodynamic dynamo mechanism operating in the Sun’s interior. While the solar differential rotation plays an important role in this dynamo mechanism by inducting the toroidal component of magnetic field, the impact of torsional oscillations on the dynamo mechanism - and hence the solar cycle - is not well understood. Here, we include the observed torsional oscillations into a flux transport dynamo model of the solar cycle to inves- tigate their effect. Although the overall amplitude of the solar cycle does not change significantly on inclusion of torsional oscillations we find that all the characteristics of the Waldmeier effect inthe sunspot cycle are qualitatively reproduced by varying only the amplitude of torsional oscillations. The Waldmeier effect, first noted in 1935, includes the important characteristic that the amplitude of sunspot cycles is anti-correlated to their rise time; cycles with high initial rise rate tend to be stronger. This has implications for solar cycle predictions. Our result suggests that the Waldmeier effect is a plausible outcome of cycle-to-cycle modulation of torsional oscillations and provides a physical basis for sunspot cycle forecasts based on torsional oscillation observations. Title: The Solar-Stellar Connection Authors: Brun, A. S.; García, R. A.; Houdek, G.; Nandy, D.; Pinsonneault, M. Bibcode: 2015SSRv..196..303B Altcode: 2014SSRv..tmp...54B; 2015arXiv150306742B We discuss how recent advances in observations, theory and numerical simulations have allowed the stellar community to progress in its understanding of stellar convection, rotation and magnetism and to assess the degree to which the Sun and other stars share similar dynamical properties. Ensemble asteroseismology has become a reality with the advent of large time domain studies, especially from space missions. This new capability has provided improved constraints on stellar rotation and activity, over and above that obtained via traditional techniques such as spectropolarimetry or CaII H&K observations. New data and surveys covering large mass and age ranges have provided a wide parameter space to confront theories of stellar magnetism. These new empirical databases are complemented by theoretical advances and improved multi-D simulations of stellar dynamos. We trace these pathways through which a lucid and more detailed picture of magnetohydrodynamics of solar-like stars is beginning to emerge and discuss future prospects. Title: Erratum: Erratum to: The Solar-Stellar Connection Authors: Brun, A. S.; García, R. A.; Houdek, G.; Nandy, D.; Pinsonneault, M. Bibcode: 2015SSRv..196..357B Altcode: 2015SSRv..tmp...30B No abstract at ADS Title: a Roadmap to Advance Understanding of the Science of Space Weather Authors: Schrijver, K.; Kauristie, K.; Aylward, A.; De Nardin, C. M.; Gibson, S. E.; Glover, A.; Gopalswamy, N.; Grande, M.; Hapgood, M. A.; Heynderickx, D.; Jakowski, N.; Kalegaev, V. V.; Lapenta, G.; Linker, J.; Liu, S.; Mandrini, C. H.; Mann, I. R.; Nagatsuma, T.; Nandy, D.; Obara, T.; O'Brien, T. P., III; Onsager, T. G.; Opgenoorth, H. J.; Terkildsen, M. B.; Valladares, C. E.; Vilmer, N. Bibcode: 2015AGUFMSH12A..01S Altcode: There is a growing appreciation that the environmental conditions that we call space weather impact the technological infrastructure that powers the coupled economies around the world. With that comes the need to better shield society against space weather by improving forecasts, environmental specifications, and infrastructure design. A COSPAR/ILWS team recently completed a roadmap that identifies the scientific focus areas and research infrastructure that are needed to significantly advance our understanding of space weather of all intensities and of its implications and costs for society. This presentation provides a summary of the highest-priority recommendations from that roadmap. Title: Radiative properties of few F- and Cl- like alkali and alkaline-earth metal ions Authors: Nandy, D. K.; Singh, Sukhjit; Sahoo, B. K. Bibcode: 2015MNRAS.452.2546N Altcode: We present high-accuracy calculations of radiative properties such as oscillator strengths and transition probabilities, of the allowed ns 2S1/2 → np 2P1/2, 3/2 transitions and of the forbidden np 2P1/2 → np 2P3/2 transitions in the F- and Cl-like alkali and alkaline-earth ions with the ground state principal quantum number n of the respective ion. For this purpose, we have employed the Dirac-Fock, relativistic second-order many-body perturbation theory and an all-order perturbative relativistic method in the coupled-cluster (CC) theory framework. To test the validity of these methods for giving accurate results, we first evaluated the ionization potentials in the creation processes of these ions and compare them with their experimental values listed in the National Institute of Science and Technology data base. Moreover, both the allowed and forbidden transition amplitudes are estimated using the above three methods and a comparative analysis is made to follow-up the electron correlation trends in order to demonstrate the need of using a sophisticated method like the CC theory for their precise determination. For astrophysical use, we provide the most precise values of the transition properties by combining the experimental energies, which suppresses uncertainties from the calculated energies, using the transition amplitudes from the CC method. These data will be useful in the abundance analysis of the considered ions in the astronomical objects and for the diagnostic processes of astrophysical plasmas. Title: Understanding space weather to shield society: A global road map for 2015-2025 commissioned by COSPAR and ILWS Authors: Schrijver, Carolus J.; Kauristie, Kirsti; Aylward, Alan D.; Denardini, Clezio M.; Gibson, Sarah E.; Glover, Alexi; Gopalswamy, Nat; Grande, Manuel; Hapgood, Mike; Heynderickx, Daniel; Jakowski, Norbert; Kalegaev, Vladimir V.; Lapenta, Giovanni; Linker, Jon A.; Liu, Siqing; Mandrini, Cristina H.; Mann, Ian R.; Nagatsuma, Tsutomu; Nandy, Dibyendu; Obara, Takahiro; Paul O'Brien, T.; Onsager, Terrance; Opgenoorth, Hermann J.; Terkildsen, Michael; Valladares, Cesar E.; Vilmer, Nicole Bibcode: 2015AdSpR..55.2745S Altcode: 2015arXiv150306135S There is a growing appreciation that the environmental conditions that we call space weather impact the technological infrastructure that powers the coupled economies around the world. With that comes the need to better shield society against space weather by improving forecasts, environmental specifications, and infrastructure design. We recognize that much progress has been made and continues to be made with a powerful suite of research observatories on the ground and in space, forming the basis of a Sun-Earth system observatory. But the domain of space weather is vast - extending from deep within the Sun to far outside the planetary orbits - and the physics complex - including couplings between various types of physical processes that link scales and domains from the microscopic to large parts of the solar system. Consequently, advanced understanding of space weather requires a coordinated international approach to effectively provide awareness of the processes within the Sun-Earth system through observation-driven models. This roadmap prioritizes the scientific focus areas and research infrastructure that are needed to significantly advance our understanding of space weather of all intensities and of its implications for society. Advancement of the existing system observatory through the addition of small to moderate state-of-the-art capabilities designed to fill observational gaps will enable significant advances. Such a strategy requires urgent action: key instrumentation needs to be sustained, and action needs to be taken before core capabilities are lost in the aging ensemble. We recommend advances through priority focus (1) on observation-based modeling throughout the Sun-Earth system, (2) on forecasts more than 12 h ahead of the magnetic structure of incoming coronal mass ejections, (3) on understanding the geospace response to variable solar-wind stresses that lead to intense geomagnetically-induced currents and ionospheric and radiation storms, and (4) on developing a comprehensive specification of space climate, including the characterization of extreme space storms to guide resilient and robust engineering of technological infrastructures. The roadmap clusters its implementation recommendations by formulating three action pathways, and outlines needed instrumentation and research programs and infrastructure for each of these. An executive summary provides an overview of all recommendations. Title: Forbidden transition properties in the ground-state configurations of singly ionized noble gas atoms for stellar and interstellar media Authors: Nandy, D. K.; Sahoo, B. K. Bibcode: 2015MNRAS.450.1012N Altcode: High-accuracy calculations of the forbidden transition amplitudes for the np 2P1/2 → np 2P3/2 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 2P1/2 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 2P1/2 states. Title: The Relationship Between Solar Coronal X-Ray Brightness and Active Region Magnetic Fields: A Study Using High-Resolution Hinode Observations Authors: Hazra, Soumitra; Nandy, Dibyendu; Ravindra, B. Bibcode: 2015SoPh..290..771H Altcode: 2015SoPh..tmp...19H; 2014arXiv1406.1683H By using high-resolution observations of nearly co-temporal and co-spatial Solar Optical Telescope spectropolarimeter and X-Ray Telescope coronal X-ray data onboard Hinode, we revisit the problematic relationship between global magnetic quantities and coronal X-ray brightness. Co-aligned vector magnetogram and X-ray data were used for this study. The total X-ray brightness over active regions is well correlated with integrated magnetic quantities such as the total unsigned magnetic flux, the total unsigned vertical current, and the area-integrated square of the vertical and horizontal magnetic fields. On accounting for the inter-dependence of the magnetic quantities, we inferred that the total magnetic flux is the primary determinant of the observed integrated X-ray brightness. Our observations indicate that a stronger coronal X-ray flux is not related to a higher non-potentiality of active-region magnetic fields. The data even suggest a slightly negative correlation between X-ray brightness and a proxy of active-region non-potentiality. Although there are small numerical differences in the established correlations, the main conclusions are qualitatively consistent over two different X-ray filters, the Al-poly and Ti-poly filters, which confirms the strength of our conclusions and validate and extend earlier studies that used low-resolution data. We discuss the implications of our results and the constraints they set on theories of solar coronal heating. Title: Relativistic calculations of radiative properties and fine structure constant varying sensitivity coefficients in the astrophysically relevant Zn II, Si IV and Ti IV ions Authors: Nandy, D. K.; Sahoo, B. K. Bibcode: 2015MNRAS.447.3812N Altcode: We have carried out calculations of the relativistic sensitivity coefficients, oscillator strengths, transition probabilities, lifetimes and magnetic dipole hyperfine structure constants for a number of low-lying states in the Zn II, Si IV and Ti IV ions which are abundant in the distant quasars and various stellar plasmas. These spectroscopic data will be very useful for probing temporal variation of the fine structure constant (αe) 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 VN-1 and VN+1 potentials and then the electron-electron correlation effects are taken into account by constructing all possible singly and doubly excited configurations, involving both the core and valence electrons, from the respective reference states. We have also determined one electron affinities and ionization potentials of many excited states in these Zn II, Si IV and Ti IV ions. Except for a few states we have attained accuracies within 1 per cent for the energies compared with their experimental values. Our calculated sensitivity coefficients are estimated to have similar accuracies as of the calculated energies. Furthermore, combining our calculated transition matrix elements with the experimental wavelengths we evaluate transition probabilities, oscillator strengths and lifetimes of some of the excited states in these ions. These results are compared with the available data in a few cases and found to be in very good agreement among themselves. Using our reported hyperfine structure constants due to the dominant magnetic dipole interaction, it is possible to determine hyperfine splittings approximately in the above considered ions. Title: Long-term Activity Evolution of the Sun-as-a-Star Authors: Nandy, D. Bibcode: 2014spih.confE..16N Altcode: No abstract at ADS Title: A Stochastically Forced Time Delay Solar Dynamo Model: Self-consistent Recovery from a Maunder-like Grand Minimum Necessitates a Mean-field Alpha Effect Authors: Hazra, Soumitra; Passos, Dário; Nandy, Dibyendu Bibcode: 2014ApJ...789....5H Altcode: 2013arXiv1307.5751H Fluctuations in the Sun's magnetic activity, including episodes of grand minima such as the Maunder minimum have important consequences for space and planetary environments. However, the underlying dynamics of such extreme fluctuations remain ill-understood. Here, we use a novel mathematical model based on stochastically forced, non-linear delay differential equations to study solar cycle fluctuations in which time delays capture the physics of magnetic flux transport between spatially segregated dynamo source regions in the solar interior. Using this model, we explicitly demonstrate that the Babcock-Leighton poloidal field source based on dispersal of tilted bipolar sunspot flux, alone, cannot recover the sunspot cycle from a grand minimum. We find that an additional poloidal field source effective on weak fields—e.g., the mean-field α effect driven by helical turbulence—is necessary for self-consistent recovery of the sunspot cycle from grand minima episodes. Title: Implementation and application of the relativistic equation-of-motion coupled-cluster method for the excited states of closed-shell atomic systems Authors: Nandy, D. K.; Singh, Yashpal; Sahoo, B. K. Bibcode: 2014PhRvA..89f2509N Altcode: 2014arXiv1405.1936N We report the implementation of the equation-of-motion coupled-cluster (EOMCC) method in the four-component relativistic framework with a spherical atomic potential to generate the excited states from a closed-shell atomic configuration. This theoretical development will be very useful in carrying out high-precision calculations of various atomic properties in many atomic systems. We employ this method to calculate the excitation energies of many low-lying states in a few Ne-like highly charged ions, such as Cr xv, Fe xvii, Co xviii, and Ni xix ions, and compare them against their corresponding experimental values to demonstrate the accomplishment of the EOMCC implementation. The ions considered are appropriate to substantiate accurate inclusion of the relativistic effects in the evaluation of atomic properties and are also interesting for astrophysical studies. Investigation of the temporal variation of the fine-structure constant (α) obtained from astrophysical observations is a modern research problem for which we also estimate the α sensitivity coefficients in the above ions. Title: A solar dynamo model driven by mean-field alpha and Babcock-Leighton sources: fluctuations, grand-minima-maxima, and hemispheric asymmetry in sunspot cycles Authors: Passos, D.; Nandy, D.; Hazra, S.; Lopes, I. Bibcode: 2014A&A...563A..18P Altcode: 2013arXiv1309.2186P Context. Extreme solar activity fluctuations and the occurrence of solar grand minima and maxima episodes, such as the Maunder minimum and Medieval maximum are well-established, observed features of the solar cycle. Nevertheless, such extreme activity fluctuations and the dynamics of the solar cycle during Maunder minima-like episodes remain ill understood.
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.
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.
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.
Conclusions: We conclude that stochastic fluctuations in two interacting poloidal field sources working with distinct operating thresholds is a viable candidate for triggering episodes of extreme solar activity and that the mean-field α effect capable of working on weak, sub-equipartition fields is critical to the recovery of the solar cycle following an extended solar minimum. Based on our results, we also postulate that solar activity can exhibit significant parity shifts and hemispheric asymmetry, including phases when only one hemisphere is completely quiescent while the other remains active, to, successful grand minima like conditions in both hemispheres. Title: Spectral properties of a few F-like ions Authors: Nandy, D. K.; Sahoo, B. K. Bibcode: 2014A&A...563A..25N Altcode:
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 (αe) by observing their transition lines from the distant astronomical objects.
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.
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 (αe) through the precise astrophysical observation of the spectral lines of the transitions involving the above low-lying states. The accuracies of these sensitivity coefficients are appraised from the comparison of our calculated energies with the experimental values. We also present the line strengths by calculating the reduced matrix elements of the allowed transitions and of the M1 and E2 forbidden transitions. By combining our calculated line strengths with the observed wavelengths, we determine the oscillator strengths, transition probabilities, and lifetimes of the first two excited states of the above ions and compare them with the other theoretical calculations. The hyperfine splittings of the states are also presented by calculating their magnetic dipole and electric quadrupole hyperfine structure constants. Title: Helioseismic Perspective of the Solar Dynamo Authors: Muñoz-Jaramillo, A.; Martens, P. C. H.; Nandy, D. Bibcode: 2013ASPC..478..271M Altcode: Helioseismology has been, without a doubt, one of the greatest contributors to our understanding of the solar cycle. In particular, its results have been critical in the development of solar dynamo models, by providing modelers with detailed information about the internal, large scale flows of solar plasma.

This review will give a historical overview of the evolution of our understanding of the solar cycle, placing special emphasis on advances driven by helioseismic results. We will discuss some of the outstanding modeling issues, and discuss how Helioseismology can help push our understanding forward during the next decade. Title: Forecasting the solar activity cycle: new insights Authors: Nandy, Dibyendu; Karak, Bidya Binay Bibcode: 2013IAUS..294..439N Altcode: 2013arXiv1312.7613N Having advance knowledge of solar activity is important because the Sun's magnetic output governs space weather and impacts technologies reliant on space. However, the irregular nature of the solar cycle makes solar activity predictions a challenging task. This is best achieved through appropriately constrained solar dynamo simulations and as such the first step towards predictions is to understand the underlying physics of the solar dynamo mechanism. In Babcock-Leighton type dynamo models, the poloidal field is generated near the solar surface whereas the toroidal field is generated in the solar interior. Therefore a finite time is necessary for the coupling of the spatially segregated source layers of the dynamo. This time delay introduces a memory in the dynamo mechanism which allows forecasting of future solar activity. Here we discuss how this forecasting ability of the solar cycle is affected by downward turbulent pumping of magnetic flux. With significant turbulent pumping the memory of the dynamo is severely degraded and thus long term prediction of the solar cycle is not possible; only a short term prediction of the next cycle peak may be possible based on observational data assimilation at the previous cycle minimum. Title: EXPLORING SOLAR GRAND MINIMA THROUGH A TIME DELAY DYNAMO MODEL Authors: Hazra, Soumitra; Nandy, D. Bibcode: 2013SPD....4440305H Altcode: Fluctuations in solar magnetic output, including episodes of grand minima such as the Maunder minimum are studied using a mathematical model based on time delay differential equations. Time delays physically capture the effect of the finite time required for magnetic flux transport between two spatially segregated source regions for toroidal and poloidal field creation. We show that cycle to cycle variations and Maunder-like grand minima are naturally produced by introduction of stochastic fluctuation in the poloidal field generation mechanism. Our analysis shows that the Babcock-Leighton mechanism, alone, cannot restart the sunspot cycle once it settles into a grand minimum. An additional poloidal field source capable of working on weak toroidal magnetic fields - such as that driven by helical turbulence (mean-field alpha-effect) - is necessary to recover the solar cycle. Our result demonstrates the importance of a small-scale alpha-effect in the context of the large-scale solar cycle dynamics and shows, how self consistent entry and exit from grand minima like episodes are possible. Title: Long-term solar activity and its implications to the heliosphere, geomagnetic activity, and the Earth's climate. Preface to the Special Issue on Space Climate Authors: Mursula, Kalevi; Manoharan, Periasamy; Nandy, Dibyendu; Tanskanen, Eija; Verronen, Pekka Bibcode: 2013JSWSC...3A..21M Altcode: The Sun's long-term magnetic variability is the primary driver of space climate. This variability is manifested not only in the long-observed and dramatic change of magnetic fields on the solar surface, but also in the changing solar radiative output across all wavelengths. The Sun's magnetic variability also modulates the particulate and magnetic fluxes in the heliosphere, which determine the interplanetary conditions and impose significant electromagnetic forces and effects upon planetary atmospheres. All these effects due to the changing solar magnetic fields are also relevant for planetary climates, including the climate of the Earth. The ultimate cause of solar variability, at time scales much shorter than stellar evolutionary time scales, i.e., at decadal to centennial and, maybe, even millennial or longer scales, has its origin in the solar dynamo mechanism. Therefore, in order to better understand the origin of space climate, one must analyze different proxies of solar magnetic variability and develop models of the solar dynamo mechanism that correctly produce the observed properties of the magnetic fields. This Preface summarizes the most important findings of the papers of this Special Issue, most of which were presented in the Space Climate-4 Symposium organized in 2011 in Goa, India. Title: Use of a time delay dynamo model to obtain solar-like sunspot cycles Authors: Amouzou, E.; Nandy, D.; Muñoz-Jaramillo, A.; Martens, P. Bibcode: 2013ASInC..10...83A Altcode: Using a delay-differential equation model, we simulate the solar dynamo. We find that solar-like dynamo solutions exist in certain parameter regimes for which the dynamo number is less than or about equal to -3 (|N_D| > 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 Bibcode: 2013ASInC..10..115H Altcode: 2013arXiv1302.3133H Recent results indicate that the Babcock-Leighton mechanism for poloidal field creation plays an important role in the solar cycle. However, modelling this mechanism has not always correctly captured the underlying physics. In particular, it has been demonstrated that using a spatially distributed near-surface alpha-effect to parametrize the Babcock-Leighton mechanism generates results which do not agree with observations. Motivated by this, we are developing a physically more consistent model of the solar cycle in which we model poloidal field creation by the emergence and flux dispersal of double-ring structures. Here, we present preliminary results from this new dynamo model. Title: Turbulent Pumping of Magnetic Flux Reduces Solar Cycle Memory and thus Impacts Predictability of the Sun's Activity Authors: Karak, Bidya Binay; Nandy, Dibyendu Bibcode: 2012ApJ...761L..13K Altcode: 2012arXiv1206.2106K Prediction of the Sun's magnetic activity is important because of its effect on space environment and climate. However, recent efforts to predict the amplitude of the solar cycle have resulted in diverging forecasts with no consensus. Yeates et al. have shown that the dynamical memory of the solar dynamo mechanism governs predictability, and this memory is different for advection- and diffusion-dominated solar convection zones. By utilizing stochastically forced, kinematic dynamo simulations, we demonstrate that the inclusion of downward turbulent pumping of magnetic flux reduces the memory of both advection- and diffusion-dominated solar dynamos to only one cycle; stronger pumping degrades this memory further. Thus, our results reconcile the diverging dynamo-model-based forecasts for the amplitude of solar cycle 24. We conclude that reliable predictions for the maximum of solar activity can be made only at the preceding minimum—allowing about five years of advance planning for space weather. For more accurate predictions, sequential data assimilation would be necessary in forecasting models to account for the Sun's short memory. Title: All Quiet on the Solar Front: Origin and Heliospheric Consequences of the Unusual Minimum of Solar Cycle 23 Authors: Nandy, D.; Muñoz-Jaramillo, A.; Martens, P. C. H. Bibcode: 2012SunGe...7...17N Altcode: The magnetic activity of the Sun shapes the heliospheric space environment through modulation of the solar wind, interplanetary magnetic field, cosmic ray flux and solar irradiance. Sunspots - strongly magnetized regions on the solar surface - also spawns solar storms such as flares and coronal mass ejections which generate severe space weather affecting space-based technologies. The Sun's magnetic output varies in a cyclic manner going through phases of maximum and minimum activity. Following solar cycle 23 the Sun entered a prolonged and unusually long minimum with a large number of days without sunspots that was unprecedented in the space age. This long phase of very low solar activity resulted in record high cosmic ray flux at Earth, weak solar wind speeds and low interplanetary magnetic field. We provide an overview of this peculiar solar minimum, critically explore theories for its origin and argue that the unusual conditions in the heliosphere that we experienced during this minimum eventually originated in solar internal dynamics. Title: Modeling the solar cycle: what the future holds Authors: Nandy, Dibyendu Bibcode: 2012IAUS..286...54N Altcode: 2011arXiv1111.5352N Stellar magnetic fields are produced by a magnetohydrodynamic dynamo mechanism working in their interior - which relies on the interaction between plasma flows and magnetic fields. The Sun, being a well-observed star, offers an unique opportunity to test theoretical ideas and models of stellar magnetic field generation. Solar magnetic fields produce sunspots, whose number increases and decreases with a 11 year periodicity - giving rise to what is known as the solar cycle. Dynamo models of the solar cycle seek to understand its origin, variation and evolution with time. In this review, I summarize observations of the solar cycle and describe theoretical ideas and kinematic dynamo modeling efforts to address its origin. I end with a discussion on the future of solar cycle modeling - emphasizing the importance of a close synergy between observational data assimilation, kinematic dynamo models and full magnetohydrodynamic models of the solar interior. Title: The Solar Cycle: From Understanding to Forecasting Authors: Nandy, Dibyendu Bibcode: 2012AAS...22030001N Altcode: Solar and stellar magnetic cycles are born out of a magnetohydrodynamic dynamo mechanism involving interactions between internal plasma flows and magnetic fields. The Sun offers a unique opportunity of exploring this dynamo mechanism in detail. The solar cycle is manifested as a periodic variation in the number of sunspots. This magnetic activity spawns severe space weather which can adversely affect technologies exposed to environmental conditions in space. It is also thought that slower, long-term variations in the Sun's magnetic activity influence planetary climates such as that of the Earth. Understanding the physical processes that generate the solar cycle is therefore of fundamental importance. While it is expected that this understanding should also lead to reliable predictive capabilities, unfortunately, forecasts for the amplitude of the (ongoing) solar cycle 24 have not converged. In this talk, after providing an introduction to solar dynamo theory, I will review our current state of understanding and critically discuss the underlying physics of solar cycle predictability. Title: The Sun Has A Short Memory: Turbulent Pumping Of Magnetic Flux Reduces Solar Cycle Memory And Precludes Long-term Predictions Authors: Nandy, Dibyendu; Karak, B. B. Bibcode: 2012AAS...22052118N Altcode: Predicting the activity of the Sun is important because of its effect on space environmental conditions and climate. However, recent efforts to predict the amplitude of the solar cycle have resulted in diverging forecasts with no consensus. It is understood that the dynamical memory of the solar dynamo mechanism governs predictability and this memory is different for advection- and diffusion-dominated solar convection zones. By utilizing stochastically forced, kinematic dynamo simulations, we demonstrate that the inclusion of downward turbulent pumping of magnetic flux reduces the memory of both advection- and diffusion-dominated solar dynamos to only one cycle; stronger pumping degrades this memory further. We conclude that the dynamical memory of the solar cycle is short; reliable predictions for the maximum of solar activity can be made only at the preceding minimum which explains why early forecasts for the maximum of solar cycle 24 have widely diverged. Our analysis suggests that for more accurate predictions, sequential data assimilation would be necessary in forecasting models to account for the Sun's short memory. Title: Use of a Time Delay Dynamo Model to Obtain Sun-Like Sunspot Cycles Authors: Amouzou, Ernest C.; Nandy, D.; Munoz-Jaramillo, A.; Martens, P. C. H. Bibcode: 2012AAS...22020611A Altcode: Using a time delay-based, simplified dynamo model, we attempted to produce results characteristic of the Sun when the parameters are set to solar values. We found that dynamo solutions exist for dynamo numbers less than or about equal to -3 (|ND| > 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. Bibcode: 2011JPhB...44v5701N Altcode: 2011arXiv1107.5453N Transition properties such as oscillator strengths, transition rates, branching ratios and lifetimes of many low-lying states in the doubly ionized scandium (Sc III) are reported. A relativistic method in the coupled-cluster framework has been employed to incorporate the electron correlation effects due to the Coulomb interaction to all orders by considering all possible singly and doubly excited electronic configurations conjointly with the leading order triply excited configurations in a perturbative approach. Present results are compared with the previously reported results for the transition lines of astrophysical interest. In addition, some of the transition properties and lifetimes of a few low-lying states are given for the first time. The role of the correlation effects in the evaluation of the transition strengths are described concisely. Title: Recent Improvements of Kinematic Models of the Solar Magnetic Cycle Authors: Muñoz-Jaramillo, Andrés; Nandy, Dibyendu; Martens, Petrus C. H. Bibcode: 2011shin.confE...3M Altcode: One of the best tools we have for understanding the origin of solar magnetic variability are kinematic dynamo models. During the last decade, this type of models has seen a continuous evolution and has become increasingly successful at reproducing solar cycle characteristics. Unfortunately, most of ingredients that make up a kinematic dynamo model remain poorly constrained allowing one to obtain solar-like solutions by 'tuning' the input parameters' leading to controversy regarding which parameter set is more appropriate. In this poster we will revisit two of those ingredients and show how to constrain them better by using observational data and theoretical considerations.

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.

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.

This work is funded by NASA Living With a Star Grant NNX08AW53G to Montana State University/Harvard-Smithsonian Center for Astrophysics and the Government of India's Ramanujan Fellowship. Title: Meridional Surface Flows and the Recent Extended Solar Minimum Authors: Martens, Petrus C.; Nandy, D.; Munoz-Jaramillo, A. Bibcode: 2011SPD....42.1705M Altcode: 2011BAAS..43S.1705M Nandy, Munoz, & 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.

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.

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. Bibcode: 2011SPD....42.1743M Altcode: 2011BAAS..43S.1743M The minimum of solar cycle 23 was characterized by very weak polar field strength and a large number of sunspot-less days that was unprecedented in the space age. This has had significant consequences in the heliospheric space environment in terms of record-high cosmic-ray flux and low levels of solar irradiance - which is the primary natural driver of the climate system. During this un-anticipated phase, there was some speculation as to whether the solar minimum could lead to a Maunder-like grand minimum which coincided with the Little Ice Age. Here we present the first consistent explanation of the defining characteristics of this unusual minimum based on variations in the solar meridional plasma flows, and discuss how our results compare with observations.

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

This work is funded by NASA Living With a Star Grant NNX08AW53G to Montana State University/Harvard-Smithsonian Center for Astrophysics and the Government of India's Ramanujan Fellowship. Title: The unusual minimum of sunspot cycle 23 caused by meridional plasma flow variations Authors: Nandy, Dibyendu; Muñoz-Jaramillo, Andrés; Martens, Petrus C. H. Bibcode: 2011Natur.471...80N Altcode: 2013arXiv1303.0349N Direct observations over the past four centuries show that the number of sunspots observed on the Sun's surface varies periodically, going through successive maxima and minima. Following sunspot cycle 23, the Sun went into a prolonged minimum characterized by a very weak polar magnetic field and an unusually large number of days without sunspots. Sunspots are strongly magnetized regions generated by a dynamo mechanism that recreates the solar polar field mediated through plasma flows. Here we report results from kinematic dynamo simulations which demonstrate that a fast meridional flow in the first half of a cycle, followed by a slower flow in the second half, reproduces both characteristics of the minimum of sunspot cycle 23. Our model predicts that, in general, very deep minima are associated with weak polar fields. Sunspots govern the solar radiative energy and radio flux, and, in conjunction with the polar field, modulate the solar wind, the heliospheric open flux and, consequently, the cosmic ray flux at Earth. Title: A review of Space Climate and an introduction to the papers of the JASTP special issue on Space Climate Authors: Mursula, Kalevi; Marsh, Dan; Nandy, Dibyendu; Usoskin, Ilya Bibcode: 2011JASTP..73..179M Altcode: No abstract at ADS Title: Magnetic Quenching of Turbulent Diffusivity: Reconciling Mixing-length Theory Estimates with Kinematic Dynamo Models of the Solar Cycle Authors: Muñoz-Jaramillo, Andrés; Nandy, Dibyendu; Martens, Petrus C. H. Bibcode: 2011ApJ...727L..23M Altcode: 2010arXiv1007.1262M The turbulent magnetic diffusivity in the solar convection zone is one of the most poorly constrained ingredients of mean-field dynamo models. This lack of constraint has previously led to controversy regarding the most appropriate set of parameters, as different assumptions on the value of turbulent diffusivity lead to radically different solar cycle predictions. Typically, the dynamo community uses double-step diffusivity profiles characterized by low values of diffusivity in the bulk of the convection zone. However, these low diffusivity values are not consistent with theoretical estimates based on mixing-length theory, which suggest much higher values for turbulent diffusivity. To make matters worse, kinematic dynamo simulations cannot yield sustainable magnetic cycles using these theoretical estimates. In this work, we show that magnetic cycles become viable if we combine the theoretically estimated diffusivity profile with magnetic quenching of the diffusivity. Furthermore, we find that the main features of this solution can be reproduced by a dynamo simulation using a prescribed (kinematic) diffusivity profile that is based on the spatiotemporal geometric average of the dynamically quenched diffusivity. This bridges the gap between dynamically quenched and kinematic dynamo models, supporting their usage as viable tools for understanding the solar magnetic cycle. Title: Dynamo models of the solar cycle: current trends and future prospects Authors: Nandy, Dibyendu Bibcode: 2011ASInC...2...91N Altcode: 2011arXiv1110.5725N The magnetic cycle of the Sun, as manifested in the cyclic appearance of sunspots, significantly influences our space environment and space-based technologies by generating what is now termed as space weather. Long-term variation in the Sun's magnetic output also influences planetary atmospheres and climate through modulation of solar irradiance. Here, I summarize the current state of understanding of this magnetic cycle, highlighting important observational constraints, detailing the kinematic dynamo modeling approach and commenting on future prospects. Title: A Double-ring Algorithm for Modeling Solar Active Regions: Unifying Kinematic Dynamo Models and Surface Flux-transport Simulations Authors: Muñoz-Jaramillo, Andrés; Nandy, Dibyendu; Martens, Petrus C. H.; Yeates, Anthony R. Bibcode: 2010ApJ...720L..20M Altcode: 2010arXiv1006.4346M The emergence of tilted bipolar active regions (ARs) and the dispersal of their flux, mediated via processes such as diffusion, differential rotation, and meridional circulation, is believed to be responsible for the reversal of the Sun's polar field. This process (commonly known as the Babcock-Leighton mechanism) is usually modeled as a near-surface, spatially distributed α-effect in kinematic mean-field dynamo models. However, this formulation leads to a relationship between polar field strength and meridional flow speed which is opposite to that suggested by physical insight and predicted by surface flux-transport simulations. With this in mind, we present an improved double-ring algorithm for modeling the Babcock-Leighton mechanism based on AR eruption, within the framework of an axisymmetric dynamo model. Using surface flux-transport simulations, we first show that an axisymmetric formulation—which is usually invoked in kinematic dynamo models—can reasonably approximate the surface flux dynamics. Finally, we demonstrate that our treatment of the Babcock-Leighton mechanism through double-ring eruption leads to an inverse relationship between polar field strength and meridional flow speed as expected, reconciling the discrepancy between surface flux-transport simulations and kinematic dynamo models. Title: Empirical Modeling of Radiative versus Magnetic Flux for the Sun-as-a-Star Authors: Preminger, Dora; Nandy, Dibyendu; Chapman, Gary; Martens, Petrus C. H. Bibcode: 2010SoPh..264...13P Altcode: 2010arXiv1006.4354P; 2010SoPh..tmp...92P We study the relationship between full-disk solar radiative flux at different wavelengths and average solar photospheric magnetic-flux density, using daily measurements from the Kitt Peak magnetograph and other instruments extending over one or more solar cycles. We use two different statistical methods to determine the underlying nature of these flux - flux relationships. First, we use statistical correlation and regression analysis and show that the relationships are not monotonic for total solar irradiance and for continuum radiation from the photosphere, but are approximately linear for chromospheric and coronal radiation. Second, we use signal theory to examine the flux - flux relationships for a temporal component. We find that a well-defined temporal component exists and accounts for some of the variance in the data. This temporal component arises because active regions with high magnetic-field strength evolve, breaking up into small-scale magnetic elements with low field strength, and radiative and magnetic fluxes are sensitive to different active-region components. We generate empirical models that relate radiative flux to magnetic flux, allowing us to predict spectral-irradiance variations from observations of disk-averaged magnetic-flux density. In most cases, the model reconstructions can account for 85 - 90% of the variability of the radiative flux from the chromosphere and corona. Our results are important for understanding the relationship between magnetic and radiative measures of solar and stellar variability. Title: Towards better Constrained Kinematic Dynamo Models: Turbulent Diffusivity and Diffusivity Quenching Authors: Munoz-Jaramillo, Andres; Nandy, D.; Martens, P. C. H. Bibcode: 2010AAS...21640116M Altcode: The turbulent magnetic diffusivity in the Solar Convection Zone (SCZ) is one of the most poorly constrained ingredients of mean-field dynamo models. This lack of constrain has previously led to controversy regarding which set of parameters is more appropriate (yielding better solar like solutions) and the generation of radically different cycle predictions. Furthermore, due to the relative freedom in the different parameters associated with it, more often than not it is used to finely tune the dynamo solutions. As of now, the dynamo community seems to have settled on double step diffusivity profiles characterized by low values of diffusivity inside most of the convection zone; notwithstanding that these values of diffusivity are not consistent with theoretical considerations based on mixing-length theory, which suggest much higher values of turbulent diffusivity. To make matters worse, standard kinematic dynamo simulations cannot yield sustainable magnetic cycles using theoretical estimates. Here we study how magnetic diffusivity quenching can provide a physically meaningful way out of this discrepancy and whether standard diffusivity profiles are truly a representation of a physical process. This work is funded by NASA Living With a Star grant NNG05GE47G. Title: Are Active Regions as Relevant for the Solar Cycle as we Think? Authors: Munoz-Jaramillo, Andres; Nandy, D.; Martens, P. C. H. Bibcode: 2010AAS...21640108M Altcode: 2010BAAS...41R.858M The long and short term variability of the Sun is strongly determined by the evolution of the solar magnetic cycle, which is sustained through the action of a magneto-hydrodynamic dynamo. In our current understanding of the dynamo, the poloidal field (which acts as a starting point for the cycle) is recreated through the emergence and decay of active regions subjected to the collective effect of meridional circulation and turbulent diffusion; a process commonly referred to as the Babcock-Leighton mechanism. Dynamo models based on this mechanism have been quite successful in reproducing the different properties of the solar cycle and have also been used to make predictions of cycle 24. However, the question of whether the BL mechanism is enough to sustain the solar cycle has not yet been addressed quantitatively. By including real active region data in our state of the art kinematic dynamo model we are able to take the first steps into answering this question.

This work is funded by NASA Living With a Star grant NNG05GE47G. Title: The Unusual Minimum of Solar Cycle 23 Explained Authors: Nandy, Dibyendu; Munoz-Jaramillo, A.; Martens, P. C. H. Bibcode: 2010AAS...21631703N Altcode: 2010BAAS...41..898N The minimum in activity between solar cycle 23 and 24 has been the deepest in the space age, with an unusually large number of days without sunspots and weak solar dipolar field strength. This has had consequences for the heliosphere and planetary atmospheres - given the weak solar wind, low solar irradiance and radio flux and historically high values of cosmic ray flux that has characterized this minimum epoch. The origin of this peculiar minimum has not yet been clearly understood. Here we present the first theoretical explanation of this deep minimum based on simulations of the solar dynamo mechanism - which seeks to explain the origin and variability of solar magnetic fields. Our simulations have uncovered a somewhat surprising explanation, which however, provides a consistent solution to both of the unusual features of this minimum; namely, the long period when sunspots were missing and the very weak solar polar field strength. Title: Comparison of a Global Magnetic Evolution Model with Observations of Coronal Mass Ejections Authors: Yeates, A. R.; Attrill, G. D. R.; Nandy, Dibyendu; Mackay, D. H.; Martens, P. C. H.; van Ballegooijen, A. A. Bibcode: 2010ApJ...709.1238Y Altcode: 2009arXiv0912.3347Y The relative importance of different initiation mechanisms for coronal mass ejections (CMEs) on the Sun is uncertain. One possible mechanism is the loss of equilibrium of coronal magnetic flux ropes formed gradually by large-scale surface motions. In this paper, the locations of flux rope ejections in a recently developed quasi-static global evolution model are compared with observed CME source locations over a 4.5 month period in 1999. Using extreme ultraviolet data, the low-coronal source locations are determined unambiguously for 98 out of 330 CMEs. An alternative method of determining the source locations using recorded Hα events was found to be too inaccurate. Despite the incomplete observations, positive correlation (with coefficient up to 0.49) is found between the distributions of observed and simulated ejections, but only when binned into periods of 1 month or longer. This binning timescale corresponds to the time interval at which magnetogram data are assimilated into the coronal simulations, and the correlation arises primarily from the large-scale surface magnetic field distribution; only a weak dependence is found on the magnetic helicity imparted to the emerging active regions. The simulations are limited in two main ways: they produce fewer ejections, and they do not reproduce the strong clustering of observed CME sources into active regions. Due to this clustering, the horizontal gradient of radial photospheric magnetic field is better correlated with the observed CME source distribution (coefficient 0.67). Our results suggest that while the gradual formation of magnetic flux ropes over weeks can account for many observed CMEs, especially at higher latitudes, there exists a second class of CMEs (at least half) for which dynamic active region flux emergence on shorter timescales must be the dominant factor. Improving our understanding of CME initiation in future will require both more comprehensive observations of CME source regions and more detailed magnetic field simulations. Title: Outstanding Issues in Solar Dynamo Theory Authors: Nandy, D. Bibcode: 2010ASSP...19...86N Altcode: 2009arXiv0906.4748N; 2010mcia.conf...86N The magnetic activity of the Sun, as manifested in the sunspot cycle, originates deep within its convection zone through a dynamo mechanism, which involves nontrivial interactions between the plasma and the magnetic field in the solar interior. Recent advances in magnetohydrodynamic dynamo theory have led us closer towards a better understanding of the physics of the solar magnetic cycle. In conjunction, helioseismic observations of large-scale flows in the solar interior has nowmade it possible to constrain some of the parameters used in models of the solar cycle. In the first part of this review, I briefly describe this current state of understanding of the solar cycle. In the second part, I highlight some of the outstanding issues in solar dynamo theory related to the nature of the dynamo α-effect, magnetic buoyancy, and the origin of Maunder-like minima in activity. I also discuss how poor constraints on key physical processes such as turbulent diffusion, meridional circulation, and turbulent flux pumping confuse the relative roles of these vis-a-vis magnetic flux transport. I argue that unless some of these issues are addressed, no model of the solar cycle can claim to be "the standard model," nor can any predictions from such models be trusted; in other words, we are still not there yet. Title: Dynamo Processes Authors: Nandy, Dibyendu Bibcode: 2010ASSP...18...35N Altcode: 2010hepr.book...35N Magnetic fields play an important role in defining and modulating the space environment within the heliosphere. How these magnetic fields originate and evolve in stars such as the Sun or planets such as the Earth, is in itself a compelling question - the answer to which is of universal importance to astrophysics and space science. It is thought that magnetic fields are created through a dynamo process which involves complex, non-linear interactions within the magnetized plasma that is often encountered in astrophysical systems. In this chapter, after briefly discussing the importance of magnetic fields in the heliosphere, I provide a gentle introduction to concepts in magnetohydrodynamics, describe the mathematical foundation of mean-field dynamo theory, and explain the basic physical processes that constitute the dynamo mechanism. In doing this, I focus mainly on our star - the Sun - which is the primary source of heliospheric magnetic fields and their variability. Title: What do Solar Kinematic Models Tell us About the Current Minimum? Authors: Muñoz-Jaramillo, A.; Nandy, D.; Martens, P. C. Bibcode: 2009AGUFMSH11A1505M Altcode: In the last three years the sun has reached the most unusual minimum in the space age. Although minima as long as this one have happened several times in the past, this one has come as a surprise in contrast with the previous four who where fairly regular. However, such an event is a perfect opportunity to learn more about the solar cycle and the processes that drive it. In order to understand this event we turn to kinematic dynamo models, which are the best tool we currently have for understanding the solar cycle. Although modelers have been aware of the role of the different components into setting the period of the solar cycle, little work has been done in understanding the nature of solar minima. Can kinematic models reproduce such an event with all it's signatures? In this study we attempt to address this question using our state of the art kinematic dynamo model. Title: ERRATUM: "Helioseismic Data Inclusion in Solar Dynamo Models" (2009, ApJ, 698, 461) Authors: Muñoz-Jaramillo, Andrés; Nandy, Dibyendu; Martens, Petrus C. H. Bibcode: 2009ApJ...707.1852M Altcode: No abstract at ADS Title: Solar Cycle Variations of Coronal Null Points: Implications for the Magnetic Breakout Model of Coronal Mass Ejections Authors: Cook, G. R.; Mackay, D. H.; Nandy, Dibyendu Bibcode: 2009ApJ...704.1021C Altcode: In this paper, we investigate the solar cycle variation of coronal null points and magnetic breakout configurations in spherical geometry, using a combination of magnetic flux transport and potential field source surface models. Within the simulations, a total of 2843 coronal null points and breakout configurations are found over two solar cycles. It is found that the number of coronal nulls present at any time varies cyclically throughout the solar cycle, in phase with the flux emergence rate. At cycle maximum, peak values of 15-17 coronal nulls per day are found. No significant variation in the number of nulls is found from the rising to the declining phase. This indicates that the magnetic breakout model is applicable throughout both phases of the solar cycle. In addition, it is shown that when the simulations are used to construct synoptic data sets, such as those produced by Kitt Peak, the number of coronal nulls drops by a factor of 1/6. The vast majority of the coronal nulls are found to lie above the active latitudes and are the result of the complex nature of the underlying active region fields. Only 8% of the coronal nulls are found to be connected to the global dipole. Another interesting feature is that 18% of coronal nulls are found to lie above the equator due to cross-equatorial interactions between bipoles lying in the northern and southern hemispheres. As the majority of coronal nulls form above active latitudes, their average radial extent is found to be in the low corona below 1.25 R sun (175, 000 km above the photosphere). Through considering the underlying photospheric flux, it is found that 71% of coronal nulls are produced though quadrupolar flux distributions resulting from bipoles in the same hemisphere interacting. When the number of coronal nulls present in each rotation is compared to the number of bipoles emerging, a wide scatter is found. The ratio of coronal nulls to emerging bipoles is found to be approximately 1/3. Overall, the spatio-temporal evolution of coronal nulls is found to follow the typical solar butterfly diagram and is in qualitative agreement with the observed time dependence of coronal mass ejection source-region locations. Title: Helioseismic Data Inclusion in Solar Dynamo Models Authors: Muñoz-Jaramillo, Andrés; Nandy, Dibyendu; Martens, Petrus C. H. Bibcode: 2009ApJ...698..461M Altcode: 2008arXiv0811.3441M An essential ingredient in kinematic dynamo models of the solar cycle is the internal velocity field within the simulation domain—the solar convection zone (SCZ). In the last decade or so, the field of helioseismology has revolutionized our understanding of this velocity field. In particular, the internal differential rotation of the Sun is now fairly well constrained by helioseismic observations almost throughout the SCZ. Helioseismology also gives us some information about the depth dependence of the meridional circulation in the near-surface layers of the Sun. The typical velocity inputs used in solar dynamo models, however, continue to be an analytic fit to the observed differential rotation profile and a theoretically constructed meridional circulation profile that is made to match the flow speed only at the solar surface. Here, we take the first steps toward the use of more accurate velocity fields in solar dynamo models by presenting methodologies for constructing differential rotation and meridional circulation profiles that more closely conform to the best observational constraints currently available. We also present kinematic dynamo simulations driven by direct helioseismic measurements for the rotation and four plausible profiles for the internal meridional circulation—all of which are made to match the helioseismically inferred near-surface depth dependence, but whose magnitudes are made to vary. We discuss how the results from these dynamo simulations compare with those that are driven by purely analytic fits to the velocity field. Our results and analysis indicate that the latitudinal shear in the rotation in the bulk of the SCZ plays a more important role, than either the tachocline or surface radial shear, in the induction of the toroidal field. We also find that it is the speed of the equatorward counterflow in the meridional circulation right at the base of the SCZ, and not how far into the radiative interior it penetrates, that primarily determines the dynamo cycle period. Improved helioseismic constraints are expected to be available from future space missions such as the Solar Dynamics Observatory and through analysis of more long-term continuous data sets from ground-based instruments such as the Global Oscillation Network Group. Our analysis lays the basis for the assimilation of these helioseismic data within dynamo models to make future solar cycle simulations more realistic. Title: The Unusual Minimum of Cycle 23: Observations and Interpretation Authors: Martens, Petrus C.; Nandy, D.; Munoz-Jaramillo, A. Bibcode: 2009SPD....40.2403M Altcode: The current minimum of cycle 23 is unusual in its long duration, the very low level to which Total Solar Irradiance (TSI) has fallen, and the small flux of the open polar fields. The deep minimum of TSI seems to be related to an unprecedented dearth of polar faculae, and hence to the small amount of open flux. Based upon surface flux transport models it has been suggested that the causes of these phenomena may be an unusually vigorous meridional flow, or even a deviation from Joy's law resulting in smaller Joy angles than usual for emerging flux in cycle 23. There is also the possibility of a connection with the recently inferred emergence in polar regions of bipoles that systematically defy Hale's law.

Much speculation has been going on as to the consequences of this exceptional minimum: are we entering another global minimum, is this the end of the 80 year period of exceptionally high solar activity, or is this just a statistical hiccup? Dynamo simulations are underway that may help answer this question. As an aside it must be mentioned that the current minimum of TSI puts an upper limit in the TSI input for global climate simulations during the Maunder minimum, and that a possible decrease in future solar activity will result in a very small but not insignificant reduction in the pace of global warming. Title: Towards Better Constrained Solar Dynamo Models: The Velocity Field And Turbulent Diffusivity Profiles Authors: Munoz-Jaramillo, Andres; Nandy, D.; Martens, P. C. H. Bibcode: 2009SPD....40.0405M Altcode: The best tool we have for understanding the origin of solar magnetic variability is the kinematic dynamo model. During the last decade this type of models have seen a continuous evolution and have become increasingly successful at reproducing solar cycle characteristics. However, some of the key ingredients used in dynamo models remain poorly constrained which allows one to obtain solar-like solutions by "tuning" the input parameters. Here we present out efforts to better constrain two of the most important ingredients of solar dynamo models:: The internal velocity field (meridional flow and differential rotation) and the turbulent diffusivity. To accomplish this goal, we formulate techniques to assimilate the latest results from helioseismology to constrain the velocity fields. We also apply mixing length theory to the Solar Model S, in conjunction with magnetic quenching of the turbulent diffusivity, to generate more realistic effective turbulent diffusivity profiles for kinematic dynamo models. In essence therefore, we try to address some of these outstanding issues in a first-principle physics based approach, rather than an ad-hoc manner. Title: Effect of the Magnetic Quenching of the Turbulent Diffusivity in a Mean-Field Kinematic Solar Dynamo Authors: Muñoz-Jaramillo, A.; Nandy, D.; Martens, P. C. Bibcode: 2008AGUSMSP41A..09M Altcode: The fundamental model used to study the solar dynamo mechanism is based on the electromagnetic induction equation coupled with Ohm's law. Apart from mean-field or other phenomenological source terms (such as a Babcock-Leighton alpha-effect), the resultant dynamo equation is composed of two terms: An advection and a diffusion term. Depending on the relative importance of these two terms, the dynamo can operate either in an advection-dominated or a diffusion dominated regime. One of the parameters that determine which of these regimes the dynamo operates in is the effective magnetic diffusivity, this parameter is expected to be enhanced by convective turbulence in stellar convection zones. The diffusivity values can range from 104 cm2/s in the radiative zone (where there is no turbulence) to 1012-14 cm2/s in the upper convection zone. The depth dependence of this effective diffusivity is not particularly well-constrained and most commonly used profiles involve a relatively low diffusivity in the convection zone (1010-11 cm2/s) - which makes the dynamo operate in the advection-dominated regime. The underlying problem here is that these values of diffusivity are not consistent with theoretical considerations based on mixing-length theory, which suggest much higher values of turbulent diffusivity; this would make the dynamo operate in a diffusion-dominated regime. However, a possible solution to this inconsistency may be in the quenching effect that strong magnetic fields have on turbulence. We have recently developed a kinematic solar dynamo based on a novel numerical technique called the exponential- propagation method. Using this model, we study magnetic diffusivity quenching and discuss how its effect may reconcile the theoretically suggested turbulent diffusivity values with the effective diffusivity profiles most commonly used in this type of models. Title: Kinematic Dynamo Models Of The Solar Cycle Authors: Nandy, D. Bibcode: 2008AGUSMSP33A..01N Altcode: In the kinematic approach to solar dynamo modeling, one solves for the mean magnetic field with given velocity fields such as differential rotation and meridional circulation; in addition, key physical processes such as turbulent diffusion, magnetic buoyancy and the dynamo α-effect have to be parameterized appropriately. Kinematic dynamo models perhaps do not capture the full range of complexities of stellar convection zones and approximates complex processes through simple parameterizations. Nevertheless, the underlying physics of this class of models is relatively more transparent, they are computationally less demanding and they successfully explain many observed features of the solar cycle. In this lecture, I will trace the historical development of solar kinematic dynamo models, describe the basic physical ingredients and observational inputs necessary to build one, present our current state of understanding, and highlight outstanding problems. Title: Twisted solar active region magnetic fields as drivers of space weather: Observational and theoretical investigations Authors: Nandy, Dibyendu; Mackay, Duncan H.; Canfield, Richard C.; Martens, P. C. H. Bibcode: 2008JASTP..70..605N Altcode: The properties and dynamics of magnetic fields on the Sun's photosphere and outer layers--notably those within solar active regions--govern the eruptive activity of the Sun. These photospheric magnetic fields also act as the evolving lower boundary of the Sun-Earth coupled system. Quantifying the physical attributes of these magnetic fields and exploring the mechanisms underlying their influence on the near-Earth space environment are of vital importance for forecasting and mitigating adverse space weather effects. In this context, we discuss here a novel technique for measuring twist in the magnetic field lines of solar active regions that does not invoke the force-free field assumption. Twist in solar active regions can play an important role in flaring activity and the initiation of CMEs via the kink instability mechanism; we outline a procedure for determining this solar active region eruptive potential. We also discuss how twist in active region magnetic fields can be used as inputs in simulations of the coronal and heliospheric fields; specifically, we explore through simulations, the formation, evolution and ejection of magnetic flux ropes that originate in twisted magnetic structures. The results and ideas presented here are relevant for exploring the role of twisted solar active region magnetic fields and flux ropes as drivers of space weather in the Sun-Earth system. Title: Magnetic Helicity, Coronal Heating and Solar Flaring Activity: A Review of the Role of Active Region Twist Authors: Nandy, D. Bibcode: 2008ASPC..383..201N Altcode: Magnetic helicity of solar active regions is quantified by the twist and writhe of their underlying flux tubes. The twist component, in particular, is believed to be an important determinant of the energetics and dynamics of active regions that ultimately lead to coronal heating and solar eruptive activity. Here, I review the role of active region twist in the context of solar activity --- laying emphasis on constraints that are set by observations. The results, taken cumulatively, suggest a new paradigm. In this paradigm, the distribution and evolution of twist in active region sub-structures play a more predominant role in driving solar dynamic activity than the magnitude of the global active region twist itself. I discuss the implications of these observational constraints for theories of stellar coronal heating and solar eruptive activity. Title: Kinematic properties of solar coronal mass ejections: Correction for projection effects in spacecraft coronagraph measurements Authors: Howard, T. A.; Nandy, D.; Koepke, A. C. Bibcode: 2008JGRA..113.1104H Altcode: One of the main sources of uncertainty in quantifying the kinematic properties of coronal mass ejections (CMEs) using coronagraphs is the fact that coronagraph images are projected into the sky plane, resulting in measurements which can differ significantly from their actual values. By identifying solar surface source regions of CMEs using X-ray and Hα flare and disappearing filament data, and through considerations of CME trajectories in three-dimensional (3-D) geometry, we have devised a methodology to correct for the projection effect. We outline this method here. The methodology was automated and applied to over 10,000 CMEs in the Coordinated Data Analysis Workshop (CDAW) (SOHO Large Angle Spectroscopic Coronagraph) catalog spanning 1996-2005, in which we could associate 1961 CMEs with an appropriate surface event. In the latter subset, deprojected speeds, accelerations, and launch angles were determined to study CME kinematics. Our analysis of this subset of events reconfirms some important trends, notably that previously uncovered solar cycle variation of CME properties are preserved, CMEs with greater width have higher speeds, and slower CMEs tend to accelerate while faster CMEs tend to decelerate. This points out that statistical trends in CME properties, recovered from plane-of-sky measurements, may be preserved even in the face of more sophisticated 3-D measurements from spacecrafts such as STEREO, if CME trajectories are predominantly radial. However, our results also show that the magnitude of corrected measurements can differ significantly from the projected plane-of-sky measurements on a case-by-case basis and that acceleration is more sensitive to the deprojection process than speed. Average corrected speed and acceleration tend to be a factor of 1.7 and 4.4 higher than their projected values, with mean corrected speed and acceleration magnitudes being on the order of 1000 km/s and 50 m/s2, respectively. We conclude that while using the plane-of-sky measurements may be suitable for studies of general trends in a large sample of events, correcting for projection effects is mandatory for those investigations which rely on a numerically precise determination of the properties of individual CMEs. Title: A theoretical model for the magnetic helicity of solar active regions Authors: Chatterjee, Piyali; Choudhuri, Arnab Rai; Petrovay, Kristof; Nandy, Dibyendu Bibcode: 2008AdSpR..41..893C Altcode: Active regions on the solar surface are known to possess magnetic helicity, which is predominantly negative in the northern hemisphere and positive in the southern hemisphere. Choudhuri et al. [Choudhuri, A.R. On the connection between mean field dynamo theory and flux tubes. Solar Phys. 215, 31 55, 2003] proposed that the magnetic helicity arises due to the wrapping up of the poloidal field of the convection zone around rising flux tubes which form active regions. Choudhuri [Choudhuri, A.R., Chatterjee, P., Nandy, D. Helicity of solar active regions from a dynamo model. ApJ 615, L57 L60, 2004] used this idea to calculate magnetic helicity from their solar dynamo model. Apart from getting broad agreements with observational data, they also predict that the hemispheric helicity rule may be violated at the beginning of a solar cycle. Chatterjee et al. [Chatterjee, P., Choudhuri, A.R., Petrovay, K. Development of twist in an emerging magnetic flux tube by poloidal field accretion. A&A 449, 781 789, 2006] study the penetration of the wrapped poloidal field into the rising flux tube due to turbulent diffusion using a simple 1-d model. They find that the extent of penetration of the wrapped field will depend on how weak the magnetic field inside the rising flux tube becomes before its emergence. They conclude that more detailed observational data will throw light on the physical conditions of flux tubes just before their emergence to the photosphere. Title: Exploring the Physical Basis of Solar Cycle Predictions: Flux Transport Dynamics and Persistence of Memory in Advection- versus Diffusion-dominated Solar Convection Zones Authors: Yeates, Anthony R.; Nandy, Dibyendu; Mackay, Duncan H. Bibcode: 2008ApJ...673..544Y Altcode: 2007arXiv0709.1046Y The predictability, or lack thereof, of the solar cycle is governed by numerous separate physical processes that act in unison in the interior of the Sun. Magnetic flux transport and the finite time delay that it introduces, specifically in the so-called Babcock-Leighton models of the solar cycle with spatially segregated source regions for the α- and Ω-effects, play a crucial rule in this predictability. Through dynamo simulations with such a model, we study the physical basis of solar cycle predictions by examining two contrasting regimes, one dominated by diffusive magnetic flux transport in the solar convection zone, the other dominated by advective flux transport by meridional circulation. Our analysis shows that diffusion plays an important role in flux transport, even when the solar cycle period is governed by the meridional flow speed. We further examine the persistence of memory of past cycles in the advection- and diffusion-dominated regimes through stochastically forced dynamo simulations. We find that in the advection-dominated regime this memory persists for up to three cycles, whereas in the diffusion-dominated regime this memory persists for mainly one cycle. This indicates that solar cycle predictions based on these two different regimes would have to rely on fundamentally different inputs, which may be the cause of conflicting predictions. Our simulations also show that the observed solar cycle amplitude-period relationship arises more naturally in the diffusion-dominated regime, thereby supporting those dynamo models in which diffusive flux transport plays a dominant role in the solar convection zone. Title: The Stars as Suns Project: Recent Results from Solar and Stellar Dynamo Modeling Authors: Munoz, Andres; Nandy, D.; Martens, P. C. Bibcode: 2007AAS...210.9209M Altcode: 2007BAAS...39..210M Solar variability controls our space environment and is also believed to play a role in shaping the global climate. The variability of the Sun can be traced back to the presence and modulation of magnetic fields -- which has its origin in a dynamo mechanism working in the interior. The "Stars as Suns" project aims to determine the long-term variability of Sun, through a combination of stellar magnetic activity observations of Sun-like stars and theoretical dynamo modeling. Here we present recent results from solar and stellar dynamo studies that addresses the goals of this project. This research is funded by a NASA Living With a Star grant NNG05GE47G to Montana State University. Title: Long-Term Evolution of Solar Magnetic Activity Derived From Stellar Proxies Authors: Nandy, D.; Martens, P. C. Bibcode: 2007AGUSMSH54B..04N Altcode: The variability of the Sun over stellar and planetary evolutionary timescales may have important consequences for planetary atmospheres such as the Earth's, including the forcing of global climate and evolution of life. This solar variability is in part due to the changing magnetism of the Sun, which has origins in the solar dynamo mechanism. A novel approach towards determining solar variability over such long timescales - stretching to billions of years - is to use Sun-like stars in various evolutionary phases as proxies of solar activity. In this talk, I will review efforts to derive this long-term variability of the Sun through theoretical dynamo modelling and observational analysis of stellar magnetic activity. This work is funded by the NASA Living With a Star program through grant NNG05GE47G. Title: Active Region Magnetic Field Line Twist and Source of Coronal Magnetic Helicity. Authors: Belur, Ravindra; Longcope, D.; Barnes, G.; Nandy, D. Bibcode: 2007AAS...210.2401B Altcode: 2007BAAS...39..128B Magnetic helicity is an important quantity which measures how the magnetic field lines are twisted and sheared. Recently it has become possible to measure the flux of magnetic helicity in active regions using the observational data. These observed helicity fluxes may arise due to the twist in the emerging active region flux tubes or it may come from the photospheric shearing motion. Here, we decompose the helicity flux into two different contributions called spin and braiding. These components typically come from twist and writhe helicity of a sub-photospheric flux tube anchored to the regions. The spin helicity of a given region quantifies the mean rotation rate of motion internal to that region and braiding helicity is injected by the motions of whole

regions about one another. The injected helicity flux due to spin and braiding motion leads to the coronal magnetic field line twist. The twist determined from vector magnetograms can be used to estimate the total helicity content of the coronal field at one time. The rate of change of this helicity estimate can be compared to the total helicity flux as well as its spin and braiding component. We make such a comparison for several active regions. Title: A New Technique For Measuring The Twist Of Photospheric Active Regions Without Recourse To The Force-Free-Field Equation: Reconfirming The Hemispheric Helicity Trend Authors: Nandy, Dibyendu; Calhoun, A.; Windschitl, J.; Canfield, R. C.; Linton, M. G. Bibcode: 2007AAS...210.2402N Altcode: 2007BAAS...39..128N The twist component of magnetic helicity in solar active regions is known to be an important indicator of sub-photospheric flux tube dynamics and solar eruptive activity. Traditionally, estimates of the parameter alpha -- appearing in the force-free-field equation -- has been used to infer the twist of photospheric active regions. However, the photosphere is not force-free and this has lead to recent concerns on the validity of using the alpha parameter for determining photospheric active region twist. We have devised a new flux-tube-fitting technique for determining the twist of active regions without recourse to the force-free-field equation. This method assumes that the underlying active region flux system is cylindrically symmetric and uniformly twisted. By using this new technique, on a statistically compelling number of photospheric active region vector magnetograms, we re-confirm the hemispheric helicity rule independent of the force-free-field assumption. This research has been supported in parts by a NASA Living With a Star grant NNG05GE47G. A.C. and J.W. were supported by a NSF Research Experience for Undergraduates grant ATM-0243923 to Montana State University. M.G.L. acknowledges support from NASA and the Office of Naval Research. Title: Space Climate and the Solar Stellar connection: What can we learn from the stars about long-term solar variability? Authors: Nandy, Dibyendu; Martens, P. C. H. Bibcode: 2007AdSpR..40..891N Altcode: While it is well-known that solar variability influences the near-Earth Space environment at short timescales of days - an effect collectively termed as Space Weather, a more subtle influence of solar variability at longer timescales is also present and just beginning to be appreciated. Long-term solar forcing and its consequences - which has come to be known as Space Climate - has important consequences for the formation and evolution of planetary atmospheres, the evolution of life and global climate on Earth. Understanding the Sun's variability and its heliospheric influence at such scales, stretching from decennia to stellar and planetary evolutionary timescales, is therefore of fundamental importance. However, our knowledge of this variability, which is in part due to the evolution of the solar magnetic dynamo, is limited by direct solar observations which exist only from early 17th Century onwards. In this review we introduce a novel concept - how the Solar-Stellar connection can be exploited to understand the long-term variability of the Sun and its influence on Space Climate. We present some preliminary studies, in which, through theoretical dynamo modeling and analysis of magnetic activity observations of solar-like stars at various evolutionary phases relative to the Sun, we show how the above concept is implemented in practice. Title: Magnetic helicity and flux tube dynamics in the solar convection zone: Comparisons between observation and theory Authors: Nandy, Dibyendu Bibcode: 2006JGRA..11112S01N Altcode: Magnetic helicity, a conserved topological parameter in ideal MHD systems, conditions close to which are realized in the solar plasma, is intimately connected to the creation and subsequent dynamics of magnetic flux tubes in the solar interior. It can therefore be used as a tool to probe such dynamics. In this paper we show how photospheric observations of magnetic helicity of isolated magnetic flux tubes, manifested as the twist and writhe of solar active regions, can constrain the creation and dynamics of flux tubes in the solar convection zone and the nature of convective turbulence itself. We analyze the observed latitudinal distribution of twists in photospheric active regions, derived from solar vector magnetograms, in the largest such sample studied till-date. We confirm and put additional constraints on the hemispheric twist helicity trend and find that the dispersion in the active region twist distribution is latitude-independent, implying that the amplitude of turbulent fluctuations does not vary with latitude in the convection zone. Our data set also shows that the amplitude and dispersion of twist decreases with increasing magnetic size of active regions, supporting the conclusion that larger flux tubes are less affected by turbulence. Among the various theoretical models that have been proposed till-date to explain the origin of twist, our observations best match the Σ effect model, which invokes helical turbulent buffeting of rising flux tubes as the mechanism for twist creation. Finally, we complement our analysis of twists with past observations of tilts in solar active regions and tie them in with theoretical modeling studies, to build up a comprehensive picture of the dynamics of twisted magnetic flux tubes throughout the solar convection zone. This general framework, binding together theory and observations, suggests that flux tubes have a wide range of twists in the solar convection zone, with some as high as to make them susceptible to the kink instability mechanism that results in the formation of δ spot or non-Hale active regions. Title: A Time Delay Model for Solar and Stellar Dynamos Authors: Wilmot-Smith, A. L.; Nandy, D.; Hornig, G.; Martens, P. C. H. Bibcode: 2006ApJ...652..696W Altcode: Magnetohydrodynamic dynamos operating in stellar interiors produce the diverse range of magnetic activity observed in solar-like stars. Sophisticated dynamo models including realistic physics of convection zone flows and flux tube dynamics have been built for the Sun, for which appropriate observations exist to constrain such models. Nevertheless, significant differences exist in the physics that the models invoke, the most important being the nature and location of the dynamo α-effect and whether it is spatially segregated from the location of the Ω-effect. Spatial segregation of these source layers necessitates a physical mechanism for communication between them, involving unavoidable time delays. We construct a physically motivated reduced dynamo model in which, through the use of time delays, we mimic the generation of field components in spatially segregated layers and the communication between them. The model can be adapted to examine the underlying structures of more complicated and spatially extended numerical dynamo models with diverse α-effect mechanisms. A variety of dynamic behaviors arise as a direct consequence of the introduction of time delays in the system. Various parameter regimes give rise to periodic and aperiodic oscillations. Amplitude modulation leads to episodes of reduced activity, such as that observed during the Maunder minima, the length and duration of which depend on the dynamo number. Regular activity is more easily excited in the flux transport-dominated regime (when the time delay is smaller than the dissipative timescale), whereas irregular activity characterizes solutions in the diffusion-dominated regime (when the time delay is larger than the dissipative timescale). Title: Generation And Dynamics Of Magnetic Fields In The Solar Convection Zone Authors: Nandy, D. Bibcode: 2006IAUJD...3E..14N Altcode: Magnetic fields constituting solar active regions are generated by a dynamo mechanism in the deep interior of the Sun - from where they buoyantly rise to erupt through the photosphere as sunspots. During this buoyant rise through the convection zone, the magnetic flux tubes interact with a variety of physical processes, notable amongst them being the Coriolis force and helical turbulent convection. This interaction results in the flux tubes acquiring specific structural properties such as tilt and twist. In this talk I will briefly review the process of magnetic field generation and discuss how observations of twist and writhe - components of magnetic helicity - in solar active regions, can constrain the dynamics of magnetic flux tubes in the solar convection zone. Title: Unravelling Long-Term Solar Variability: The Stars As Suns Project Authors: Nandy, D.; Martens, P. C. H. Bibcode: 2006IAUJD...8E..13N Altcode: It is well known that solar variability influences the near-Earth Space environment at short timescales of days - an effect collectively termed as Space Weather. A lesser known and more subtle influence of solar variability at longer timescales, is however, just beginning to be appreciated. This long-term solar forcing, which is sometimes referred to as Space Climate, has important consequences for the formation and evolution of planetary atmospheres, evolution of life and global climate on Earth. Understanding the Sun's variability and its heliospheric influence at such scales stretching from millennia to stellar evolutionary timescales is therefore of fundamental importance. However, our understanding of this variability, which is partly due to the evolution of the solar magnetic dynamo, is limited by solar observations which exist only from early 17^th Century onwards. In this talk I will review the "Stars as Suns" project - in which we take a novel approach to unravelling long-term solar variability through theoretical modelling and magnetic activity observations of Sun-like stars, which are at various evolutionary phases relative to the Sun. The "Stars as Suns" project is funded by the NASA Living With a Star program through grant NNG05GE47G. Title: Implementation of an Exponential Propagation Method to Numerically Solving the 2.5 D Stellar Dynamo Equations Authors: Munoz, Andres; Martens, P. C.; Nandy, D. Bibcode: 2006SPD....37.1202M Altcode: 2006BAAS...38Q.240M Magnetic fields in stars such as the Sun originate via a MHD dynamo mechanism working in their interior. A complete understanding of the dynamo mechanism, which involves complex and non-linear interactions between plasma flows and magnetic fields, remains an elusive and outstanding problem in Astrophysics. As an integral step in a study of stellar dynamos, part of a new MSU project entitled "Stars as Suns: Unraveling Long-term Solar Variability by Stellar Dynamo Modeling", a numerical solution for the 2D dynamo equations is being developed that uses an exponential propagation method, in which, the exponential is approximated using a projection into a Krylov subspace. As has been found in other work, this kind of numerical scheme presents a promising alternative to explicit schemes, since it is not subject to the CFL condition, and to implicit methods, since an iteration using the projection onto a Krylov subspace converges faster than an equivalent solution of the implicit formulation. Here, we outline our preliminary efforts towards developing this new numerical scheme for addressing the stellar dynamo problem.This research is supported by NASA Grant NNGO5GE47G to MSU. Title: Unraveling long-term solar variability and its impact on space climate: The stars as suns project Authors: Nandy, D.; Martens, P. C. H. Bibcode: 2006ilws.conf..158N Altcode: It is well-known that solar variability influences the near-Earth Space environment at short timescales of days - an effect collectively termed as Space Weather. A lesser known and more subtle influence of solar variability at longer timescales is however just beginning to be appreciated. This long-term solar forcing, which is sometimes referred to as Space Climate, has important consequences for the formation and evolution of planetary atmospheres and the evolution of life and global climate on Earth. Understanding the Sun's variability and its heliospheric influence at such scales stretching from millennia to stellar evolutionary timescales is therefore of fundamental importance and a very promising area of future research. However, our understanding of this variability, which is in part connected to the evolution of the solar magnetic dynamo, is limited by continuous sunspot observations, which exist only from the early 17th Century onwards. In this paper we review the "Stars as Suns" project - in which we take a radically new approach to unraveling long-term solar variability through theoretical modeling and magnetic activity observations of Sun-like stars, which are at various evolutionary phases relative to the Sun. Title: A theoretical model for the magnetic helicity of solar active regions Authors: Choudhuri, A. R.; Chatterjee, P.; Petrovay, K.; Nandy, D. Bibcode: 2006cosp...36..714C Altcode: 2006cosp.meet..714C Active regions on the solar surface are known to possess magnetic helicity which is predominantly negative in the northern hemisphere and positive in the southern hemisphere Choudhuri 2003 Sol Phys 123 217 proposed that the magnetic helicity arises due to the wrapping up of the poloidal field of the convection zone around rising flux tubes which form active regions Choudhuri Chatterjee and Nandy 2004 ApJ 615 L57 used this idea to calculate magnetic helicity from their solar dynamo model and found broad agreements with observational data Chatterjee Choudhuri and Petrovay 2006 A A in press have studied the penetration of the wrapped poloidal field into the rising flux tube and concluded that more detailed observational data will throw light on the physical conditions of flux tubes just before their emergence to the photosphere Title: The Magnetic Activity of Solar-like Stars at Different Main-Sequence Ages Authors: Lakatos, S. L.; Nandy, D.; Martens, P. Bibcode: 2005AAS...20711104L Altcode: 2005BAAS...37.1342L We report on a study of modeling stellar magnetic activity inferred through CaII H+K and ROSAT X-ray emission. The purpose of this project is to create a subset of stars with similar properties to the Sun, but with a wide range of ages (0.6 - 10 Gyrs); to study the CaII H+K emission data and decipher how the stars' emission changes with age; and to compare the X-ray activity to the CaII H+K activity. The ultimate goal of this project is to determine and use the relationships between the stellar parameters to understand the evolution of the magnetic dynamo from an younger Sun to an older Sun. This research is supported by a NSF Research Experience for Undergraduates grant ATM-0243923 and a NASA Living With a Star grant NNG05GE47G to Montana State University. Title: Low-order stellar dynamo models Authors: Wilmot-Smith, A. L.; Martens, P. C. H.; Nandy, D.; Priest, E. R.; Tobias, S. M. Bibcode: 2005MNRAS.363.1167W Altcode: 2005MNRAS.tmp..855W Stellar magnetic activity - which has been observed in a diverse set of stars including the Sun - originates via a magnetohydrodynamic dynamo mechanism working in stellar interiors. The full set of magnetohydrodynamic equations governing stellar dynamos is highly complex, and so direct numerical simulation is currently out of reach computationally. An understanding of the bifurcation structure, likely to be found in the partial differential equations governing such dynamos, is vital if we are to understand the activity of solar-like stars and its evolution with varying stellar parameters such as rotation rate. Low-order models are an important aid to this understanding, and can be derived either as approximations of the governing equations themselves or by using bifurcation theory to obtain systems with the desired structure. We use normal-form theory to derive a third-order model with robust behaviour. The model is able to reproduce many of the basic types of behaviour found in observations of solar-type stars. In the appropriate parameter regime, a chaotic modulation of the basic cycle is present, together with varying periods of low activity such as that observed during the solar Maunder minima. Title: Spatial Relationship between Twist in Active Region Magnetic Fields and Solar Flares Authors: Hahn, Michael; Gaard, Stacy; Jibben, Patricia; Canfield, Richard C.; Nandy, Dibyendu Bibcode: 2005ApJ...629.1135H Altcode: Twisted magnetic field lines in solar active regions constitute stressed flux systems, the reconnection of which can release the stored (excess) magnetic energy in the form of solar flares. Using co-registered photospheric vector magnetograms and chromospheric Hα images for 29 flares, we explore the spatial relationship between these flares and the magnetic topology of the active regions in which they occur. We find two dominant trends. First, flares are preferentially initiated in subregions that have a high gradient in twist. Second, flare initiation occurs close to chirality inversion lines (which separate regions with twist of opposite handedness). Our results demonstrate that magnetic helicity, as manifested in the twist parameter, plays an important role in magnetic reconnection and solar flaring activity. Title: Reply to the Comments of Dikpati et al. Authors: Choudhuri, A. R.; Nandy, D.; Chatterjee, P. Bibcode: 2005A&A...437..703C Altcode: 2005astro.ph..5232C We respond to Dikpati et al.'s criticism of our recent solar dynamo model. A different treatment of the magnetic buoyancy is the most probable reason for their different results. Title: The Relationship Between Active Region Twist & Solar Flaring Activity Authors: Nandy, D.; Hahn, M.; Gaard, S.; Jibben, P.; Canfield, R. C. Bibcode: 2005AGUSMSP23B..06N Altcode: Twisted magnetic field lines in solar active regions constitute stressed flux systems -- the reconnection of which can release the stored (excess) energy in the form of solar flares. The explosive release of energy through such flares, beyond contributing to the heating of the solar corona, can sometimes affect the near-Earth Space environment and trigger geomagnetic storms. Here we explore the relationship between solar flares and the pre-flare magnetic topology (characterized by the twist α in the magnetic fields lines) of the active regions in which the flares originate. We have discovered that flares are preferentially initiated in sub-regions that have an high gradient in twist and lie close to chirality inversion lines (which separate regions with twist of opposite handedness). Our results imply that the topology of magnetic field lines -- as characterized by the twist parameter α -- plays an important role in magnetic reconnection and flaring events. Title: Full-sphere simulations of a circulation-dominated solar dynamo: Exploring the parity issue Authors: Chatterjee, P.; Nandy, D.; Choudhuri, A. R. Bibcode: 2004A&A...427.1019C Altcode: 2004astro.ph..5027C We explore a two-dimensional kinematic solar dynamo model in a full sphere, based on the helioseismically determined solar rotation profile and with an α effect concentrated near the solar surface, which captures the Babcock-Leighton idea that the poloidal field is created from the decay of tilted bipolar active regions. The meridional circulation, assumed to penetrate slightly below the tachocline, plays an important role. Some doubts have recently been raised regarding the ability of such a model to reproduce solar-like dipolar parity. We specifically address the parity issue and show that the dipolar mode is preferred when certain reasonable conditions are satisfied, the most important condition being the requirement that the poloidal field should diffuse efficiently to get coupled across the equator. Our model is shown to reproduce various aspects of observational data, including the phase relation between sunspots and the weak, diffuse field. Title: Helicity of Solar Active Regions from a Dynamo Model Authors: Choudhuri, Arnab Rai; Chatterjee, Piyali; Nandy, Dibyendu Bibcode: 2004ApJ...615L..57C Altcode: We calculate helicities of solar active regions based on the idea that poloidal flux lines get wrapped around a toroidal flux tube rising through the convection zone, thereby giving rise to the helicity. Rough estimates based on this idea compare favorably with the observed magnitude of helicity. We use our solar dynamo model based on the Babcock-Leighton α-effect to study how helicity varies with latitude and time. At the time of solar maximum, our theoretical model gives negative helicity in the northern hemisphere and positive helicity in the south, in accordance with observed hemispheric trends. However, we find that during a short interval at the beginning of a cycle, helicities tend to be opposite of the preferred hemispheric trends. Title: Meridional Circulation and the Solar Magnetic Cycle Authors: Nandy, D. Bibcode: 2004ESASP.559..241N Altcode: 2004soho...14..241N No abstract at ADS Title: Exploring Magnetic Activity from The Sun to the Stars Authors: Nandy, Dibyendu Bibcode: 2004SoPh..224..161N Altcode: 2005SoPh..224..161N Sun-like stars are known to display a wide variety of magnetic activity which is likely to be the signature of a hydromagnetic dynamo mechanism working in stellar interiors. This dynamo mechanism has been studied extensively in the context of the Sun. Here we take ideas and experiences gained from solar dynamo modeling and build upon it to study the inferred scaling laws, involving stellar parameters, from observations of stellar magnetic activity. We also discuss how such a synthesis of theoretical dynamo modeling of Sun-like stars and stellar cycle observations may help us reconstruct the long-term variability of the Sun - an important ingredient for understanding the effects of solar forcing on space and global climate. Title: On the Tilt and Twist of Solar Active Regions Authors: Holder, Zachary A.; Canfield, Richard C.; McMullen, Rebecca A.; Nandy, Dibyendu; Howard, Robert F.; Pevtsov, Alexei A. Bibcode: 2004ApJ...611.1149H Altcode: Tilt and twist are two measurable characteristics of solar active regions that can give us information about subsurface physical processes associated with the creation and subsequent evolution of magnetic flux tubes inside the Sun. Using Mees Solar Observatory active region vector magnetograms and Mount Wilson Observatory full-disk longitudinal magnetograms, we measure the magnetic twist and tilt angles of 368 bipolar active regions. In addition to two well-known phenomena, Joy's law and the hemispheric helicity rule, this data set also shows a lesser known twist-tilt relationship, which is the focus of this study. We find that those regions that closely follow Joy's law do not show any twist-tilt dependence. The dispersion in tilt angles and the dispersion in twist are also found to be uncorrelated with each other. Both of these results are predicted consequences of convective buffeting of initially untwisted and unwrithed flux tubes through the Σ-effect. However, we find that regions that strongly depart from Joy's law show significantly larger than average twist and very strong twist-tilt dependence-suggesting that the twist-tilt relationship in these regions is due to the kinking of flux tubes that are initially highly twisted, but not strongly writhed. This implies that some mechanism other than the Σ-effect (e.g., the solar dynamo itself or the process of buoyancy instability and flux tube formation) is responsible for imparting the initial twist (at the base of the solar convection zone) to the flux tubes that subsequently become kink-unstable. Title: Erratum: ``Evidence that a Deep Meridional Flow Sets the Sunspot Cycle Period'' (ApJ, 589, 665 [2003]) Authors: Hathaway, David H.; Nandy, Dibyendu; Wilson, Robert M.; Reichmann, Edwin J. Bibcode: 2004ApJ...602..543H Altcode: An error was made in entering the data used in Figure 6. This changes the results concerning the length of the time lag between the variations in the meridional flow speed and those in the cycle amplitude. The final paragraph on page 667 should read:

``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 (ApJ, 589, 665 [2003]). 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.''

The revised Figure 6 and its caption are given below Title: Full Sphere Axisymmetric Simulations of the Solar Dynamo Authors: Nandy, Dibyendu; Chatterjee, Piyali; Choudhuri, Arnab Rai Bibcode: 2004IAUS..223..133N Altcode: 2005IAUS..223..133N We explore a full sphere (2D axisymmetric) kinematic solar dynamo model based on the Babcock-Leighton idea that the poloidal field is generated in the surface layers from the decay of tilted bipolar solar active regions. This model incorporates the helioseismically deduced solar rotation profile and an algorithm for buoyancy motivated from simulations of flux tube dynamics. A prescribed deep meridional circulation plays an important role in the advection of magnetic flux. We specifically address the parity issue and show that - contrary to some recent claims - the Babcock-Leighton dynamo can reproduce solar-like dipolar parity if certain reasonable conditions are satisfied in the solar interior, the most important requirement being that the poloidal field of the two hemispheres be efficiently coupled across the equator. Title: The Origin of Helicity in Solar Active Regions Authors: Choudhuri, Arnab Rai; Chatterjee, Piyali; Nandy, Dibyendu Bibcode: 2004IAUS..223...45C Altcode: 2005IAUS..223...45C; 2004astro.ph..6598C We present calculations of helicity based on our solar dynamo model and show that the results are consistent with observational data. Title: Detection of a Taylor-like Plasma Relaxation Process in the Sun and its Implication for Coronal Heating Authors: Nandy, Dibyendu; Hahn, Michael; Canfield, Richard C.; Longcope, Dana W. Bibcode: 2004IAUS..223..473N Altcode: 2005IAUS..223..473N The relaxation dynamics of a magnetized plasma system is a subject of fundamental importance in MHD - with applications ranging from laboratory plasma devices like the Toroidal Field Pinch and Spheromaks to astrophysical plasmas, stellar flaring activity and coronal heating. Taylor in 1974 proposed that the magnetic field in a plasma (of small but finite resistivity) relaxes to a minimum energy state, subject to the constraint that its total magnetic helicity is conserved (Woltjer 1958), such that the final magnetic field configuration is a constant alpha (linear) force-free field - where alpha is a quantity describing the twist in magnetic field lines. However, a clear signature of this mechanism in astrophysical plasmas remained undetected. Here we report observational detection of a relaxation process, similar to what Taylor (1974, 1986) envisaged, in the magnetic fields of flare-productive solar active regions. The implications of this result for magnetic reconnection and the coronal heating problem are discussed. Title: Detection of a Taylor-like Plasma Relaxation Process in the Sun Authors: Nandy, Dibyendu; Hahn, Michael; Canfield, Richard C.; Longcope, Dana W. Bibcode: 2003ApJ...597L..73N Altcode: The relaxation dynamics of a magnetized plasma system is a subject of fundamental importance in magnetohydrodynamics-with applications ranging from laboratory plasma devices such as the toroidal-field pinch and spheromaks to astrophysical plasmas, stellar flaring activity, and coronal heating. Taylor in 1974 proposed that the magnetic field in a plasma, subject to certain constraints, relaxes to a minimum energy state such that the final magnetic field configuration is a constant α (linear) force-free field-where α is a quantity describing the twist in magnetic field lines. While Taylor's theory was remarkably successful in explaining some intriguing results from laboratory plasma experiments, a clear signature of this mechanism in astrophysical plasmas remained undetected. Here we report observational detection of a relaxation process, similar to what Taylor envisaged, in the magnetic fields of flare-productive solar active regions. The implications of this result for magnetic reconnection and the coronal heating problem are discussed. Title: Evidence That a Deep Meridional Flow Sets the Sunspot Cycle Period Authors: Hathaway, David H.; Nandy, Dibyendu; Wilson, Robert M.; Reichmann, Edwin J. Bibcode: 2003ApJ...589..665H Altcode: Sunspots appear on the Sun in two bands on either side of the equator that drift toward lower latitudes as each sunspot cycle progresses. We examine the drift of the centroid of the sunspot area toward the equator in each hemisphere from 1874 to 2002 and find that the drift rate slows as the centroid approaches the equator. We compare the drift rate at sunspot cycle maximum with the period of each cycle for each hemisphere and find a highly significant anticorrelation: hemispheres with faster drift rates have shorter periods. These observations are consistent with a meridional counterflow deep within the Sun as the primary driver of the migration toward the equator and the period associated with the sunspot cycle. We also find that the drift rate at maximum is significantly correlated with the amplitude of the following cycle, a prediction of dynamo models that employ a deep meridional flow toward the equator. Our results indicate an amplitude of about 1.2 m s-1 for the meridional flow velocity at the base of the solar convection zone. Title: Delving Deeper into the Solar Dynamo Mechanism: Alpha Effect, Parity Selection and Large Scale Flows. Authors: Nandy, D. Bibcode: 2003SPD....34.1906N Altcode: 2003BAAS...35..843N Visible manifestations of the 22 year solar magnetic cycle have been the subject of study spanning centuries starting with the telescopic observations of sunspots by Johann Fabricius, Christoph Scheiner and Galileo Galilei in the early 1600s. Coupled with these observations of magnetic features on the solar surface, the advent of the field of helioseismology in recent years has made it possible to map large scale flows in the solar interior - believed to play a crucial role in sustaining the solar cycle. However, a complete understanding of the hydromagnetic dynamo mechanism that powers this solar cycle remains elusive. Here we report studies of the solar dynamo addressing some of the important unresolved questions regarding the nature and location of the alpha effect, solar magnetic parity selection and the role of large scale flows and their variation, with a goal to understand the exact means by which the Sun generates its magnetic cycle. This study was supported by NASA through SR&T grant NAG5-6110. Title: Evidence that a Deep Meridional Flow Sets the Sunspot Cycle Period Authors: Hathaway, D. H.; Nandy, D.; Wilson, R. M.; Reichmann, E. J. Bibcode: 2003SPD....34.2604H Altcode: 2003BAAS...35..854H Sunspots appear on the Sun in two bands on either side of the equator that drift toward lower latitudes as each sunspot cycle progresses. We examine the equatorward drift of the centroid of the sunspot area in each hemisphere from 1874 to 2002 and find that the drift rate slows as the centroid approaches the equator. We compare the drift rate at sunspot cycle maximum to the cycle-period for each hemisphere and find a highly significant anti-correlation: hemispheres with faster drift rates have shorter periods. These observations are consistent with an equatorward meridional counterflow, deep within the Sun, as the primary driver of the equatorward migration and the period associated with the sunspot cycle. We also find that the drift rate at maximum is significantly correlated with the amplitude of the following cycle, a prediction of dynamo models that employ a deep equatorward meridional flow. Our results indicate an amplitude of about 1.2 m/s for the meridional flow velocity at the base of the solar convection zone. Title: Insights on Turbulent Flows Authors: Nandy, D.; Choudhuri, A. R. Bibcode: 2003PADEU..13...21N Altcode: Turbulent flows in the interior of the Sun, both at small and large scales, are believed to feed and sustain the solar hydromagnetic dynamo that generates the solar cycle. The solar cycle itself strikingly manifests in a 11-year periodic variation in the number of sunspots seen on the solar surface. Sunspots are regions of concentrated magnetic fields, occurring at low latitudes on the solar surface and are believed to be tracers of the underlying dynamo mechanism. An important ingredient in recent models of the dynamo mechanism is the meridional flow of material, which is believed to originate from turbulent stresses in the solar convection zone. This meridional circulation is observed to be poleward in the outer 15% of the Sun and must be balanced by an equatorward counterflow in the interior. The nature and exact location of this counterflow, however, is unknown. We discuss here results from a dynamo model that reproduces the correct latitudinal distribution of sunspots and show that this requires a meridional counterflow of material that penetrates much deeper than hitherto believed -- into the radiative layers below the convection zone. We comment on the viability of such a deep counterflow of material and discuss its implications for turbulent convection and elemental abundance in the Sun and related stellar atmospheres. Title: Reviewing solar magnetic field generation in the light of helioseismology Authors: Nandy, Dibyendu Bibcode: 2003ESASP.517..123N Altcode: 2003soho...12..123N Helioseismic observations, in recent years, have vastly improved our understanding of solar interior dynamics. This has challenged theorists to come up with up-to-date models for the origin and evolution of magnetic fields in the Sun - which on the one hand has to be consistent with these new results from the solar interior and on the other hand, has to correctly reproduce magnetic features that have been observed on the Sun since the advent of telescopes and magnetograms. We report here results from such dynamo simulations and address the following questions: Where in the Sun are the strong "sunspot-forming" toroidal magnetic field and the much weaker poloidal field generated? What is the role and nature of the unobserved meridional counterflow? We also discuss possible future helioseismic inputs that may usefully constrain the exact nature of the solar dynamo. Title: Solar dynamo models with realistic internal rotation Authors: Choudhuri, Arnab Rai; Nandy, Dibyendu Bibcode: 2002ESASP.505...91C Altcode: 2002IAUCo.188...91C; 2002solm.conf...91C Solar dynamo models based on differetial rotation inferred from helioseismology tend to produce rather strong magnetic activity at high solar latitudes, in contrast to the observed fact that sunspots appear at low latitudes. We show that a meridional circulation penetrating below the tachocline can solve this problem. Title: Constraints on the Solar Internal Magnetic Field from a Buoyancy Driven Solar Dynamo Authors: Nandy, Dibyendu Bibcode: 2002Ap&SS.282..209N Altcode: We report here results from a dynamo model developed on the lines of the Babcock-Leighton idea that the poloidal field is generated at the surface of the Sun from the decay of active regions. In this model magnetic buoyancy is handled with a realistic recipe - wherein toroidal flux is made to erupt from the overshoot layer wherever it exceeds a specified critical field Bc (105 G). The erupted toroidal field is then acted upon by the α-effect near the surface to give rise to the poloidal field. In this paper we study the effect of buoyancy on the dynamo generated magnetic fields. Specifically, we show that the mechanism of buoyant eruption and the subsequent depletion of the toroidal field inside the overshoot layer, is capable of constraining the magnitude and distribution of the magnetic field there. We also believe that a critical study of this mechanism may give us new information regarding the solar interior and end with an example, where we propose a method for estimating an upper limit of the difusivity within the overshoot layer. Title: Explaining the Latitudinal Distribution of Sunspots with Deep Meridional Flow Authors: Nandy, Dibyendu; Choudhuri, Arnab Rai Bibcode: 2002Sci...296.1671N Altcode: Sunspots, dark magnetic regions occurring at low latitudes on the Sun's surface, are tracers of the magnetic field generated by the dynamo mechanism. Recent solar dynamo models, which use the helioseismically determined solar rotation, indicate that sunspots should form at high latitudes, contrary to observations. We present a dynamo model with the correct latitudinal distribution of sunspots and demonstrate that this requires a meridional flow of material that penetrates deeper than hitherto believed, into the stable layers below the convection zone. Such a deep material flow may have important implications for turbulent convection and elemental abundance in the Sun and similar stars. Title: On the absence of sunspots at high solar latitudes and associated constraints on the meridional flow in the solar interior Authors: Nandy, D.; Choudhuri, A. R. Bibcode: 2002AAS...200.8901N Altcode: 2002BAAS...34..791N Sunspots -- dark magnetic regions -- occur at low latitudes on the Sun's surface and are believed to be tracers of the magnetic field generated in the interior by the dynamo mechanism. An important ingredient in recent models of this dynamo mechanism is the meridional flow of material, which is observed to be poleward on the Sun's surface and must be balanced by an equatorward counterflow in the interior. The nature and exact location of this counterflow, however, is unknown. Recent solar dynamo models, which use the helioseismically determined internal rotation of the Sun and confine the counterflow within the convection zone (as classical theories of solar convection would suggest), indicate that sunspots should form at higher latitudes, contrary to observations. Here we present a solar dynamo model with the correct latitudinal distribution of sunspots and show that this requires a counterflow that penetrates much deeper than hitherto believed - into the stable layers below the convection zone. The existence of such a deep counterflow of material may have important implications for turbulent convection and elemental abundance in the Sun and related stellar atmospheres. Title: Can theoretical solar dynamo models predict future solar activity? Authors: Nandy, D. Bibcode: 2002cosp...34E..53N Altcode: 2002cosp.meetE..53N The origin of many solar eruptive events and activity can be traced back to the presence of magnetic fields in the Sun. It is believed that these strong magnetic fields are generated by a dynamo mechanism in the solar interior. This dynamo mechanism involves a recycling of magnetic flux, between the weak poloidal field observed on the solar surface and the much stronger toroidal field that is generated in the solar tachocline beneath the convection zone. Thus, in principle, a theoretical basis exists for predicting the magnitude of the strong toroidal field (that buoyantly rise to form active regions) of the next cycle, taking as input the observed poloidal flux of the present cycle. We explore this scenario here to find out, whether such a dynamo prediction of future solar activity is indeed possible. Title: Insights of the physical processes in the Sun s interior from solar dynamo studies. Authors: Nandy, D. Bibcode: 2002cosp...34E..52N Altcode: 2002cosp.meetE..52N With the coming of age of helioseismology, our knowledge of the dynamics of the solar convection zone has improved vastly over the last few years. This has challenged theorists to come up with up-to-date models for the origin and evolution of magnetic fields in the Sun, which on the one hand has to incorporate these new results from the solar interior and on the other hand, has to correctly reproduce features that have been observed on the Sun since the advent of telescopes and magnetograms. We have developed a hybrid model of the solar dynamo that includes the helioseismically deduced solar rotation profile and is consistent with the recent results from flux tube dynamics. We present here, a study of this new class of dynamo model and show how this synthesis of theoretical ideas and observational results, can lead to important advances in our understanding of flows and other physical processes in the solar interior. Title: Characteristics Of A Magnetic Buoyancy Driven Solar Dynamo Model Authors: Nandy, Dibyendu Bibcode: 2001astro.ph..7564N Altcode: We have developed a hybrid model of the solar dynamo on the lines of the Babcock-Leighton idea that the poloidal field is generated at the surface of the Sun from the decay of active regions. In this model magnetic buoyancy is handled with a realistic recipe - wherein toroidal flux is made to erupt from the overshoot layer wherever it exceeds a specified critical field ($10^5$ G). The erupted toroidal field is then acted upon by the alpha effect near the surface to give rise to the poloidal field. In the first half of this paper we present a parameter space study of this model, to bring out similarities and differences between it and other well studied models of the past. In the second half of this paper we show that the mechanism of buoyant eruptions and the subsequent depletion of the toroidal field inside the overshoot layer, is capable of constraining the magnitude of the dynamo generated magnetic field there, although a global quenching mechanism is still required to ensure that the magnetic fields do not blow up. We also believe that a critical study of this mechanism may give us new information regarding the solar interior and end with an example, where we propose a new method for estimating an upper limit of the diffusivity within the overshoot layer. Title: Toward a Mean Field Formulation of the Babcock-Leighton Type Solar Dynamo. I. α-Coefficient versus Durney's Double-Ring Approach Authors: Nandy, Dibyendu; Choudhuri, Arnab Rai Bibcode: 2001ApJ...551..576N Altcode: 2001astro.ph..7466N We develop a model of the solar dynamo in which, on the one hand, we follow the Babcock-Leighton approach to include surface processes, such as the production of poloidal field from the decay of active regions, and, on the other hand, we attempt to develop a mean field theory that can be studied in quantitative detail. One of the main challenges in developing such models is to treat the buoyant rise of the toroidal field and the production of poloidal field from it near the surface. A previous paper by Choudhuri, Schüssler, & Dikpati in 1995 did not incorporate buoyancy. We extend this model by two contrasting methods. In one method, we incorporate the generation of the poloidal field near the solar surface by Durney's procedure of double-ring eruption. In the second method, the poloidal field generation is treated by a positive α-effect concentrated near the solar surface coupled with an algorithm for handling buoyancy. The two methods are found to give qualitatively similar results. Title: The Role of Magnetic Buoyancy in a Babcock-Leighton Type Solar Dynamo Authors: Nandy, D.; Choudhuri, A. R. Bibcode: 2000JApA...21..381N Altcode: No abstract at ADS Title: Incorporating magnetic buoyancy in solar dynamo models: New results, problems -- and their possible solutions. Authors: Nandy, D.; Choudhuri, A. R. Bibcode: 2000SPD....31.0134N Altcode: There have been traditionally two kinds of approaches to understand the origin of the solar magnetic cycle: the Parker--Steenbeck--Krause--Rädler approach and the Babcock--Leighton approach. It seems at present that the most promising models of the solar dynamo are those which incorporate the best features of both these traditional approaches. One of the uncertainties in these hybrid models (Choudhuri et. al. 1995) lies in the treatment of magnetic buoyancy within a mean field framework, a subject which has rarely been explored in the past (Durney 1997). We study this problem by exploring possible ways of incorporating magnetic buoyancy in a dynamo code -- to simulate the formation and subsequent decay of sunspots and the recycling of fields, with magnetic buoyancy as an important player in the flux-transport process. The results as well as some problems faced by such new generation of dynamo models, will be discussed. References: Durney B. R., 1997, ApJ 486, 1065 Choudhuri A. R., Schussler M., Dikpati M., 1995, A&A 303, L29 Nandi D., Choudhuri A. R., 2000, submitted to ApJ Title: Vacuum-field solutions of Ross and Sen-Dunn theories of gravitation Authors: Krori, K. D.; Nandy, D. Bibcode: 1978JPhA...11.1943K Altcode: Vacuum-field solutions of Ross and Sen-Dunn theories of gravitation have been obtained with the aid of a Friedmann-type metric. Non-static solutions are found showing that the Birkhoff theorem holds for neither theory. It has been observed that the two theories have a limited scope for vacuum solution as against the Brans-Dicke theory. Mach's principle, however, holds for both the theories.