Author name code: chatterjee ADS astronomy entries on 2022-09-14 author:"Chatterjee, Piyali" ------------------------------------------------------------------------ Title: The Solar Activity Monitor Network - SAMNet Authors: Erdélyi, Robertus; Korsós, Marianna B.; Huang, Xin; Yang, Yong; Pizzey, Danielle; Wrathmall, Steven A.; Hughes, Ifan G.; Dyer, Martin J.; Dhillon, Vikram S.; Belucz, Bernadett; Brajša, Roman; Chatterjee, Piyali; Cheng, Xuewu; Deng, Yuanyong; Domínguez, Santiago Vargas; Joya, Raúl; Gömöry, Peter; Gyenge, Norbert G.; Hanslmeier, Arnold; Kucera, Ales; Kuridze, David; Li, Faquan; Liu, Zhong; Xu, Long; Mathioudakis, Mihalis; Matthews, Sarah; McAteer, James R. T.; Pevtsov, Alexei A.; Pötzi, Werner; Romano, Paolo; Shen, Jinhua; Temesváry, János; Tlatov, Andrey G.; Triana, Charles; Utz, Dominik; Veronig, Astrid M.; Wang, Yuming; Yan, Yihua; Zaqarashvili, Teimuraz; Zuccarello, Francesca Bibcode: 2022JSWSC..12....2E Altcode: The Solar Activity Magnetic Monitor (SAMM) Network (SAMNet) is a future UK-led international network of ground-based solar telescope stations. SAMNet, at its full capacity, will continuously monitor the Sun's intensity, magnetic, and Doppler velocity fields at multiple heights in the solar atmosphere (from photosphere to upper chromosphere). Each SAMM sentinel will be equipped with a cluster of identical telescopes each with a different magneto-optical filter (MOFs) to take observations in K I, Na D, and Ca I spectral bands. A subset of SAMM stations will have white-light coronagraphs and emission line coronal spectropolarimeters. The objectives of SAMNet are to provide observational data for space weather research and forecast. The goal is to achieve an operationally sufficient lead time of e.g., flare warning of 2-8 h and provide many sought-after continuous synoptic maps (e.g., LoS magnetic and velocity fields, intensity) of the lower solar atmosphere with a spatial resolution limited only by seeing or diffraction limit, and with a cadence of 10 min. The individual SAMM sentinels will be connected to their master HQ hub where data received from all the slave stations will be automatically processed and flare warning issued up to 26 h in advance. Title: Polymeric jets throw light on the origin and nature of the forest of solar spicules Authors: Dey, Sahel; Chatterjee, Piyali; Murthy, O. V. S. N.; Korsós, Marianna B.; Liu, Jiajia; Nelson, Christopher J.; Erdélyi, Robertus Bibcode: 2022NatPh..18..595D Altcode: Spicules are plasma jets that are observed in the dynamic interface region between the visible solar surface and the hot corona. At any given time, it is estimated that about 3 million spicules are present on the Sun. We find an intriguing parallel between the simulated spicular forest in a solar-like atmosphere and the numerous jets of polymeric fluids when both are subjected to harmonic forcing. In a radiative magnetohydrodynamic numerical simulation with sub-surface convection, solar global surface oscillations are excited similarly to those harmonic vibrations. The jets thus produced match remarkably well with the forests of spicules detected in observations of the Sun. Taken together, the numerical simulations of the Sun and the laboratory fluid dynamics experiments provide insights into the mechanism underlying the ubiquity of jets. The non-linear focusing of quasi-periodic waves in anisotropic media of magnetized plasma as well as polymeric fluids under gravity is sufficient to generate a forest of jets. Title: Configuration files for simulations of the solar spicule forest Authors: Chatterjee, Piyali; Dey, Sahel Bibcode: 2022zndo...5807020C Altcode: The open source radiative MHD code, the pencil code and the manual, can be downloaded from https://github.com/pencil-code/pencil-code. Recommended pencil code version to check-out: 51b321b888ebb5adb60eaff205cee8e64d5cf689 (dated 5-Jan-2022) Executing >> git describe 51b321b888ebb5adb60eaff205cee8e64d5cf689 stable-4234-g51b321b Important note/Additional instruction: For the stratification.dat file, kindly replicated the first 3 rows at first and last 3 rows at last so that # of rows is 3078 (and not 3072). The beginning and end of this file should looks like -------------------------------------------------------------- -5.0351391 7.3070121 10.421405 -5.0351391 7.3070121 10.421405 -5.0351391 7.3070121 10.421405 -5.0351391 7.3070121 10.421405 ......rows in between ......rows in between 44.044453 -25.910588 13.815511 44.060440 -25.910931 13.815511 44.060440 -25.910931 13.815511 44.060440 -25.910931 13.815511 44.060440 -25.910931 13.815511 --------------------------------------------------------------- Tar file setup_fig2b contains the run directory used to produce Fig 2 b and c in Dey et al (2022, Nature Physics). Title: The Pencil Code, a modular MPI code for partial differential equations and particles: multipurpose and multiuser-maintained Authors: Pencil Code Collaboration; Brandenburg, Axel; Johansen, Anders; Bourdin, Philippe; Dobler, Wolfgang; Lyra, Wladimir; Rheinhardt, Matthias; Bingert, Sven; Haugen, Nils; Mee, Antony; Gent, Frederick; Babkovskaia, Natalia; Yang, Chao-Chin; Heinemann, Tobias; Dintrans, Boris; Mitra, Dhrubaditya; Candelaresi, Simon; Warnecke, Jörn; Käpylä, Petri; Schreiber, Andreas; Chatterjee, Piyali; Käpylä, Maarit; Li, Xiang-Yu; Krüger, Jonas; Aarnes, Jørgen; Sarson, Graeme; Oishi, Jeffrey; Schober, Jennifer; Plasson, Raphaël; Sandin, Christer; Karchniwy, Ewa; Rodrigues, Luiz; Hubbard, Alexander; Guerrero, Gustavo; Snodin, Andrew; Losada, Illa; Pekkilä, Johannes; Qian, Chengeng Bibcode: 2021JOSS....6.2807P Altcode: 2021JOSS....6.2807C; 2020arXiv200908231B The Pencil Code is a highly modular physics-oriented simulation code that can be adapted to a wide range of applications. It is primarily designed to solve partial differential equations (PDEs) of compressible hydrodynamics and has lots of add-ons ranging from astrophysical magnetohydrodynamics (MHD) to meteorological cloud microphysics and engineering applications in combustion. Nevertheless, the framework is general and can also be applied to situations not related to hydrodynamics or even PDEs, for example when just the message passing interface or input/output strategies of the code are to be used. The code can also evolve Lagrangian (inertial and noninertial) particles, their coagulation and condensation, as well as their interaction with the fluid. Title: Modeling the solar spicule forest and the coronal swirls Authors: Chatterjee, Piyali Bibcode: 2021cosp...43E1797C Altcode: Solar spicules, which can be observed when imaging the Sun using the Ca II H (396.9 nm) line, Mg II k(279.6 nm) line or the H Alpha (656.28 nm) spectral line filters, are thin cylindrical structures comprising of cold and dense plasma from the chromosphere making incursions -all the time- into the much hotter solar coronal plasma. Spicules seen in Ca II H filter are loosely classified by solar astronomers into two classes -- type-I and type-II, the latter being more energetic than the former. Our numerical magneto-hydrodynamic (MHD) experiments in two dimensions establishes that periodic forcing due to formation and destruction of convective granules on the visible solar surface is sufficient to form the forest of spicules with physical characteristics similar to the observed spicules. In our unified model of the spicule forest, the more energetic spicules are formed when this periodic forcing is further aided by magnetic reconnection. Therefore our simulations further contribute to unveiling the subtleties of spicule formation. We also study the characteristics of spicules formed in coronal holes versus in quiet sun or the active region. It is to be noted that spicules are not dependent on the 2-dimensionality of our model. We obtain spicules of similar characteristics in a 3-dimensional version of the same model. The behavior only gets richer because now the spicules can also have a torsional mode apart from the transverse kink and sausage modes also seen in the 2-dimensional model. New interesting features like short lived coronal swirls also appear in the 3-dimensional model alongside spicules but with a very small spatial overlap even though there exists striking temporal coincidence. We explore the presence of Kelvin-Helmholtz instability and its effect on the multi-stranded structure of spicules. Finally, we analyze different oscillation modes of the synthetic spicules using time-distance diagrams and estimate the energy in the modes. Title: Testing Alfvén wave propagation in a "realistic" set-up of the solar atmosphere Authors: Chatterjee, Piyali Bibcode: 2020GApFD.114..213C Altcode: We present a radiative magneto-hydrodynamic simulation set-up using the pencil code to study the generation, propagation and dissipation of Alfvén waves in the solar atmosphere which includes a convective layer, photosphere below and chromosphere, transition region and the corona above. We prepare a set-up of steady-state solar convection where the imposed external magnetic field also has reached the final value gradually starting from a very small value. From that state, we start several simulations by varying the magnetic Prandtl number and the forcing strengths. We find the propagation characteristics of waves excited in this simulation run depend strongly on the magnetic Prandtl number and the wave number of the forcing. For magnetic Prandtl number of unity, we obtain localised heating in the corona due to shock dissipation. Title: Dimerization Effect on HF Elimination from the Photoionized Fluorophenols Authors: Chatterjee, Piyali; Biswas, Souvick; Chakraborty, Tapas Bibcode: 2019isms.confEFC06C Altcode: A time of flight mass spectrometry study for multi-photon ionization dissociation of monomers and dimers of 2- and 3-fluorophenols (2FP and 3FP) by a pulsed UV laser light of wavelength 266 nm will be presented. For these molecules, HF elimination from the excited and ionic states is a vital reaction channel. Our measurements reveal that the reaction does not occur from the monomer of 3FP, but it does occur with a measurable yield from the monomer cation of 2FP. On the other hand, upon formation of hydrogen bonded dimers, this reaction is triggered in the cation of 3FP, but for 2FP dimer cation the reaction is so facile that no intact dimer cation survives and only the HF eliminated dimer ion shows up in the mass spectrum. Electronic structure theory predicts that in the D0 state of 2FP dimer cation, HF elimination is exothermic, but the process encounters a large barrier, 2.75 eV. However, in S1 state of the dimer the reaction is predicted to be barrierless. Thus, we propose that for this dimer, HF elimination takes place in the intermediate S1 state, and the remaining fragment that has relatively lower ionization energy is ionized effectively by an overall two-photon (1+1) process. For the reaction to occur from 3FP dimer cation, a rearrangement of the dimer geometry and formation of an intermediate adduct has been suggested, and it is argued that the latter could be produced by nucleophilic attack of the neutral moiety at the ortho site of the cationic counterpart, and the whole process requires 3-photon (2+1) absorption. Title: Testing Alfvén wave propagation in a "realistic" set-up of the solar atmosphere Authors: Chatterjee, Piyali Bibcode: 2018arXiv180608166C Altcode: We present a radiative magneto-hydrodynamic simulation set-up using the PENCIL CODE to study the generation, propagation and dissipation of Alfvén waves in the solar atmosphere which includes a convective layer, photosphere below and chromosphere, transition region and the corona above. We prepare a setup of steady-state solar convection where the imposed external magnetic field also has reached the final value gradually starting from a very small value. From that state, we start several simulations by varying the magnetic Prandtl number and the forcing strengths. We find the propagation characteristics of waves excited in this simulation run depend strongly on the magnetic Prandtl number and the wave number of the forcing. For magnetic Prandtl number of unity, we obtain localized heating in the corona due to shock dissipation. Title: Applying the Weighted Horizontal Magnetic Gradient Method to a Simulated Flaring Active Region Authors: Korsós, M. B.; Chatterjee, P.; Erdélyi, R. Bibcode: 2018ApJ...857..103K Altcode: 2018arXiv180410351K Here, we test the weighted horizontal magnetic gradient (WG M ) as a flare precursor, introduced by Korsós et al., by applying it to a magnetohydrodynamic (MHD) simulation of solar-like flares. The preflare evolution of the WG M and the behavior of the distance parameter between the area-weighted barycenters of opposite-polarity sunspots at various heights is investigated in the simulated δ-type sunspot. Four flares emanated from this sunspot. We found the optimum heights above the photosphere where the flare precursors of the WG M method are identifiable prior to each flare. These optimum heights agree reasonably well with the heights of the occurrence of flares identified from the analysis of their thermal and ohmic heating signatures in the simulation. We also estimated the expected time of the flare onsets from the duration of the approaching-receding motion of the barycenters of opposite polarities before each single flare. The estimated onset time and the actual time of occurrence of each flare are in good agreement at the corresponding optimum heights. This numerical experiment further supports the use of flare precursors based on the WG M method. Title: Strong nonlocality variations in a spherical mean‑field dynamo Authors: Brandenburg, Axel; Chatterjee, Piyali Bibcode: 2018AN....339..118B Altcode: 2018arXiv180204231B To explain the large-scale magnetic field of the Sun and other bodies, mean-field dynamo theory is commonly applied where one solves the averaged equations for the mean magnetic field. However, the standard approach breaks down when the scale of the turbulent eddies becomes comparable to the scale of the variations of the mean magnetic field. Models showing sharp magnetic field structures have therefore been regarded as unreliable. Our aim is to look for new effects that occur when we relax the restrictions of the standard approach, which becomes particularly important at the bottom of the convection zone where the size of the turbulent eddies is comparable to the depth of the convection zone itself. We approximate the underlying integro-differential equation by a partial differential equation corresponding to a reaction-diffusion type equation for the mean electromotive force, making an approach that is nonlocal in space and time feasible under conditions where spherical geometry and nonlinearity are included. In agreement with earlier findings, spatio-temporal nonlocality lowers the excitation conditions of the dynamo. Sharp structures are now found to be absent. However, in the surface layers the field remains similar to before. Title: Modeling Repeatedly Flaring δ Sunspots Authors: Chatterjee, Piyali; Hansteen, Viggo; Carlsson, Mats Bibcode: 2016PhRvL.116j1101C Altcode: 2016arXiv160100749C Active regions (ARs) appearing on the surface of the Sun are classified into α , β , γ , and δ by the rules of the Mount Wilson Observatory, California on the basis of their topological complexity. Amongst these, the δ sunspots are known to be superactive and produce the most x-ray flares. Here, we present results from a simulation of the Sun by mimicking the upper layers and the corona, but starting at a more primitive stage than any earlier treatment. We find that this initial state consisting of only a thin subphotospheric magnetic sheet breaks into multiple flux tubes which evolve into a colliding-merging system of spots of opposite polarity upon surface emergence, similar to those often seen on the Sun. The simulation goes on to produce many exotic δ sunspot associated phenomena: repeated flaring in the range of typical solar flare energy release and ejective helical flux ropes with embedded cool-dense plasma filaments resembling solar coronal mass ejections. Title: The Sun's Interior Structure and Dynamics, and the Solar Cycle Authors: Broomhall, A. -M.; Chatterjee, P.; Howe, R.; Norton, A. A.; Thompson, M. J. Bibcode: 2015sac..book..191B Altcode: No abstract at ADS Title: The Sun's Interior Structure and Dynamics, and the Solar Cycle Authors: Broomhall, A. -M.; Chatterjee, P.; Howe, R.; Norton, A. A.; Thompson, M. J. Bibcode: 2014SSRv..186..191B Altcode: 2014arXiv1411.5941B The Sun's internal structure and dynamics can be studied with helioseismology, which uses the Sun's natural acoustic oscillations to build up a profile of the solar interior. We discuss how solar acoustic oscillations are affected by the Sun's magnetic field. Careful observations of these effects can be inverted to determine the variations in the structure and dynamics of the Sun's interior as the solar cycle progresses. Observed variations in the structure and dynamics can then be used to inform models of the solar dynamo, which are crucial to our understanding of how the Sun's magnetic field is generated and maintained. Title: Erratum: "Simulation of Homologous and Cannibalistic Coronal Mass Ejections Produced by the Emergence of a Twisted Flux Rope into the Solar Corona" (2013, ApJL, 778, L8) Authors: Chatterjee, Piyali; Fan, Yuhong Bibcode: 2014ApJ...792L..24C Altcode: No abstract at ADS Title: Bifurcations of ion acoustic solitary and periodic waves in an electron-positron-ion plasma through non-perturbative approach Authors: Saha, Asit; Chatterjee, Prasanta; Chatterjee Bibcode: 2014JPlPh..80..553S Altcode: Ion acoustic solitary waves and periodic waves in an unmagnetized plasma with superthermal (kappa-distributed) electrons and positrons are investigated through a non-perturbative approach. Model equations are transformed to a planar dynamical system. Then by using the bifurcations of phase portraits of this planar dynamical system, we have established that our model has solitary wave and periodic wave solutions. We have obtained two analytical solutions for these solitary and periodic waves depending on the parameters. From these solitary wave and periodic wave solutions, we have shown the combined effects of temperature ratio (σ) of electrons and positrons, spectral index (κ), speed of the traveling wave (v), and density ratio (p) of positrons and electrons on the characteristics of ion acoustic solitary and periodic waves. The spectral index, density ratio, speed of the traveling wave, and temperature ratio significantly affect the characteristics of ion acoustic solitary and periodic structures. The present study might be helpful to understand the salient features of nonlinear ion acoustic solitary and periodic structures in the interstellar medium. Title: Head-on collisions of two types of dust-acoustic solitons in a magnetized quantum plasma Authors: Ghorui, M. K.; Samanta, U. K.; Maji, T. K.; Chatterjee, P. Bibcode: 2014Ap&SS.352..159G Altcode: The head-on collisions of dust-acoustic waves (DAWs) in a dense magnetized quantum dusty plasma are investigated by using the extended version of the Poincaré-Lighthill-Kuo (PLK) method. We discuss the theoretical predictions as regards the existence of compressive and rarefactive DAWs in the model. We also observe that, in the generic case, collisions are possible among the same polarity solitons, whereas in the special case, collisions are possible among the same or opposite polarity solitons. It is also observed that the phase shifts are significantly affected by the quantum diffraction parameter and the dust cyclotron frequency. The interesting observations of this manuscript are that the waves reach a maximum amplitude which is a superposition of the initial amplitude and they suffer a time delay during their collision. Our results may be useful in space and laboratory plasmas as well as in plasma applications. Title: MHD simulations of homologous and cannibalistic coronal mass ejections Authors: Fan, Yuhong; Chatterjee, Piyali Bibcode: 2014AAS...22421203F Altcode: We present magneto-hydrodynamic simulations of the development of a homologous sequence of coronal mass ejections (CMEs) and demonstrate their so-called cannibalistic behavior. These CMEs originate from the repeated formations and partial eruptions of kink unstable flux ropes as a result of the continued emergence of a twisted flux rope across the lower boundary into a pre-existing coronal potential arcade field. The simulations show that a CME erupting into the open magnetic field created by a preceding CME has a higher speed, and therefore tends to be cannibalistic, catching up and merging with the preceding one into a single fast CME. All the CMEs attained speeds of about 1000 km/s as they exit the domain. The reformation of a twisted flux rope after each CME eruption during the sustained flux emergence can naturally explain the X-ray observations of repeated reformations of sigmoids and “sigmoid-under-cusp” configurations at a low-coronal source of homologous CMEs. Title: MHD Simulations of the Initiation of Coronal Mass Ejections Authors: Fan, Yuhong; Chatterjee, Piyali Bibcode: 2014cosp...40E.833F Altcode: Using three-dimensional MHD simulations, we model the quasi-static evolution and the onset of eruption of twisted magnetic flux ropes in the solar corona. We present simulations where the eruption is triggered by either the onset of the torus instability or the helical kink instability of the line-tied coronal flux rope. The simulations show that S (or inverse S) shaped current sheets develop along topological structures identified as Quasi Separatrix Layers (QSLs), during the quasi-static phase before the eruption. Reconnections in the current sheets effectively add twisted flux to the flux rope and thus allow it to rise quasi-statically to the critical height for the onset of the torus instability. We examine the thermal features produced by the current sheet formation and the associated reconnections and found that they can explain some of the observed features in coronal prominence cavities as well as in pre-eruption active regions. We also present simulations of the development of a homologous sequence of CMEs caused by the repeated formation and partial eruption of kink unstable flux ropes as a result of continued flux emergence. It is found that such homologous CMEs tend to be cannibalistic, leading to the formation of more energetic, highly twisted ejecta. Title: Homologous and cannibalistic coronal mass ejections from twisted magnetic flux rope simulations Authors: Chatterjee, Piyali; Fan, Yuhong Bibcode: 2014cosp...40E.487C Altcode: We present results from magnetohydrodynamic simulations of the development of homologous sequence of coronal mass ejections (CMEs) and demonstrate their so-called cannibalistic behavior. These CMEs originate from the repeated formations and partial eruptions of kink unstable flux ropes as a result of continued emergence of a twisted flux rope across the lower boundary into a pre-existing coronal potential arcade field. Our simulation shows that a CME erupting into the open magnetic field created by a preceding CME has a higher speed. The second of the three successive CMEs in one of the simulations is cannibalistic, catching up and merging with the first into a single fast CME before exiting the domain. All the CMEs including the leading merged CME, attained speeds of about 1000 km s-1 as they exit the domain. The reformation of a twisted flux rope after each CME eruption during the sustained flux emergence can naturally explain the X-ray observations of repeated reformations of sigmoids and "sigmoid-under-cusp" configurations at a low-coronal source of homologous CMEs. We also investigate the initiation mechanism and ejecta topology of these energetic CMEs as a function of the twist parameter of the flux rope. Title: Simulation of Homologous and Cannibalistic Coronal Mass Ejections produced by the Emergence of a Twisted Flux Rope into the Solar Corona Authors: Chatterjee, Piyali; Fan, Yuhong Bibcode: 2013ApJ...778L...8C Altcode: 2013arXiv1309.4785C We report the first results of a magnetohydrodynamic simulation of the development of a homologous sequence of three coronal mass ejections (CMEs) and demonstrate their so-called cannibalistic behavior. These CMEs originate from the repeated formations and partial eruptions of kink unstable flux ropes as a result of continued emergence of a twisted flux rope across the lower boundary into a pre-existing coronal potential arcade field. The simulation shows that a CME erupting into the open magnetic field created by a preceding CME has a higher speed. The second of the three successive CMEs is cannibalistic, catching up and merging with the first into a single fast CME before exiting the domain. All the CMEs including the leading merged CME, attained speeds of about 1000 km s-1 as they exit the domain. The reformation of a twisted flux rope after each CME eruption during the sustained flux emergence can naturally explain the X-ray observations of repeated reformations of sigmoids and "sigmoid-under-cusp" configurations at a low-coronal source of homologous CMEs. Title: Effect of non-extensivity during the collision between inward and outward ion acoustic solitary waves in cylindrical and spherical geometry Authors: Ghosh, Uday Narayan; Chatterjee, Prasanta; Chatterjee Bibcode: 2013JPlPh..79..789G Altcode: The head-on collision between two cylindrical/spherical ion acoustic solitary waves (IASWs) in un-magnetized plasmas comprising inertial ions and q-non-extensive electrons and positrons is investigated using the extended version of the Poincaré-Lighthill-Kuo perturbation method. How the interactions are taking place in cylindrical and spherical geometry are studied, and the collision is shown at different times. The non-planar geometry can modify analytical phase shifts following the head-on collision are derived. The effects of q-non-extensive electrons and positrons on the phase shift are studied. It is shown that the properties of the interaction of IASWs in cylindrical and spherical geometry are very different. Title: Formation of hot channels in pre-CME coronal flux ropes and their role in the onset of eruptions Authors: Fan, Yuhong; Chatterjee, P. Bibcode: 2013SPD....4410302F Altcode: Using 3D magneto-hydrodynamic simulations of the eruption of coronal flux ropes, we examine the thermal features produced by current sheet formation and the associated “tether-cutting” reconnections. We find that current sheets form along topological structures identified as quasi-separatrix layers (QSLs) during the pre-eruption stage. Tether-cutting reconnections in the current sheets produce a hot channel containing reconnected, twisted flux threading under the axis of the flux rope. This accumulation of twisted flux allows the flux rope to rise quasi-statically to the critical height for the onset of the torus instability, which leads to the dynamic eruption of the flux rope. The current sheet morphology and the hot channel that forms above it may explain the observed prominence “horns” enclosing a central cavity seen in AIA observations of coronal cavities. They may also be the cause of the X-ray emitting cores observed in some coronal cavities. We present a sequence of simulations to examine how the temperature and density of the hot channel depend on the properties of the coronal flux rope, and compare the results with multi-wavelength coronal observations of CMEs. Title: Modelling of delta-spots with a focus on multiple eruptions Authors: Chatterjee, Piyali Bibcode: 2013SPD....44...08C Altcode: It is a generally accepted idea from observations that delta-spot regions are harbingers of 'super activity' in the Solar atmosphere. In this computational study we model a typical delta spot region by emerging two independent twisted flux ropes into the Solar corona. We support the above idea by invoking existing statistical studies of delta-spot regions and the associated flares. It has been found from observations that component sunspots locked into a delta-spot region often have different sizes, fluxes and evolutionary history. Also supporting the above idea is the fact that the connectivity between the two component spots is found to be stronger after a flare than before it. The two colliding bipoles can give rise to steep magnetic gradients and shear at the neutral line. Such collisions have the potential to increase the magnetic free energy available for not just one but multiple eruptions. In our isothermal compressible MHD simulations we vary the relative magnetic flux as well as the twist of the two emerging flux ropes. For colliding bipoles we do find signatures of multiple eruptions as well as intense current sheets. Further, we also find evidence of new magnetic connections between the components of the delta-spot after eruption. Title: Non-planar ion acoustic Gardner solitons in electron-positron-ion plasma with superthermal electrons and positrons Authors: Ghosh, Deb Kumar; Ghosh, Uday Narayan; Chatterjee, Prasanta; Chatterjee Bibcode: 2013JPlPh..79...37G Altcode: The properties of non-planar (cylindrical and spherical) ion acoustic solitary waves (IASWs) in an unmagnetized collisionless electron-positron-ion (e-p-i) plasma, whose constituents are inertial ions and superthermal/non-Maxwellian electrons and positrons (represented by the kappa (κ) distribution), are investigated by deriving the modified Gardner (MG) equation. The well-known reductive perturbation method is employed to derive the MG equation. The basic features of non-planar IA Gardner solitons (GSs) are discussed. It is seen that the properties of non-planar IAGSs (positive and negative) differ significantly as the value of spectral index kappa changes. Title: Alpha effect due to buoyancy instability of a magnetic layer Authors: Chatterjee, P.; Mitra, D.; Rheinhardt, M.; Brandenburg, A. Bibcode: 2011A&A...534A..46C Altcode: 2010arXiv1011.1218C Context. A strong toroidal field can exist in form of a magnetic layer in the overshoot region below the solar convection zone. This motivates a more detailed study of the magnetic buoyancy instability with rotation.
Aims: We calculate the α effect due to helical motions caused by an unstable magnetic layer in a rotating density-stratified system with angular velocity Ω making an angle θ with the vertical. We also study the dependence of the α effect on θ and the strength of the initial magnetic field.
Methods: We carry out three-dimensional hydromagnetic simulations in Cartesian geometry. A turbulent electromotive force (EMF) due to the correlations of the small scale velocity and magnetic field is generated. We use the test-field method to calculate the transport coefficients of the inhomogeneous turbulence produced by the layer.
Results: We show that the growth rate of the instability and the twist of the magnetic field vary monotonically with the ratio of thermal conductivity to magnetic diffusivity. The resulting α effect is non-uniform and increases with the strength of the initial magnetic field. It is thus an example of an "anti-quenched" α effect. The α effect is also nonlocal, i.e. scale dependent, requiring around 8-16 Fourier modes to reconstruct the actual mean EMF based on the actual mean field. Title: A theoretical model of torsional oscillations from a flux transport dynamo model Authors: Chatterjee, Piyali; Chakraborty, Sagar; Choudhuri, Arnab Rai Bibcode: 2011IAUS..273..366C Altcode: 2010arXiv1008.2161C Assuming that the torsional oscillation is driven by the Lorentz force of the magnetic field associated with the sunspot cycle, we use a flux transport dynamo to model it and explain its initiation at a high latitude before the beginning of the sunspot cycle. Title: Spontaneous chiral symmetry breaking by hydromagnetic buoyancy Authors: Chatterjee, Piyali; Mitra, Dhrubaditya; Brandenburg, Axel; Rheinhardt, Matthias Bibcode: 2011PhRvE..84b5403C Altcode: 2010arXiv1011.1251C Evidence for the parity-breaking nature of the magnetic buoyancy instability in a stably stratified gas is reported. In the absence of rotation, no helicity is produced, but the nonhelical state is found to be unstable to small helical perturbations during the development of the instability. The parity-breaking nature of this magnetohydrodynamic instability appears to be the first of its kind and has properties similar to those in chiral symmetry breaking in biochemistry. Applications to the production of mean fields in galaxy clusters are discussed. Title: Reynolds stress and heat flux in spherical shell convection Authors: Käpylä, P. J.; Mantere, M. J.; Guerrero, G.; Brandenburg, A.; Chatterjee, P. Bibcode: 2011A&A...531A.162K Altcode: 2010arXiv1010.1250K Context. Turbulent fluxes of angular momentum and enthalpy or heat due to rotationally affected convection play a key role in determining differential rotation of stars. Their dependence on latitude and depth has been determined in the past from convection simulations in Cartesian or spherical simulations. Here we perform a systematic comparison between the two geometries as a function of the rotation rate.
Aims: Here we want to extend the earlier studies by using spherical wedges to obtain turbulent angular momentum and heat transport as functions of the rotation rate from stratified convection. We compare results from spherical and Cartesian models in the same parameter regime in order to study whether restricted geometry introduces artefacts into the results. In particular, we want to clarify whether the sharp equatorial profile of the horizontal Reynolds stress found in earlier Cartesian models is also reproduced in spherical geometry.
Methods: We employ direct numerical simulations of turbulent convection in spherical and Cartesian geometries. In order to alleviate the computational cost in the spherical runs, and to reach as high spatial resolution as possible, we model only parts of the latitude and longitude. The rotational influence, measured by the Coriolis number or inverse Rossby number, is varied from zero to roughly seven, which is the regime that is likely to be realised in the solar convection zone. Cartesian simulations are performed in overlapping parameter regimes.
Results: For slow rotation we find that the radial and latitudinal turbulent angular momentum fluxes are directed inward and equatorward, respectively. In the rapid rotation regime the radial flux changes sign in accordance with earlier numerical results, but in contradiction with theory. The latitudinal flux remains mostly equatorward and develops a maximum close to the equator. In Cartesian simulations this peak can be explained by the strong "banana cells". Their effect in the spherical case does not appear to be as large. The latitudinal heat flux is mostly equatorward for slow rotation but changes sign for rapid rotation. Longitudinal heat flux is always in the retrograde direction. The rotation profiles vary from anti-solar (slow equator) for slow and intermediate rotation to solar-like (fast equator) for rapid rotation. The solar-like profiles are dominated by the Taylor-Proudman balance.

Movies and Appendix A are available in electronic form at http://www.aanda.org Title: Alpha effect due to magnetic buoyancy instability of a horizontal magnetic layer Authors: Chatterjee, Piyali Bibcode: 2011ASInC...2..137C Altcode: In this paper we study the hydromagnetic instability of a toroidal magnetic layer such as that thought to be located in the solar tachocline. The magnetic layer is located in a convectively stable layer and is subject to what is known as the magnetic buoyancy instability (MBI) and under suitable conditions breaks up into twisted and arching magnetic flux tubes. The MBI gives rise to an anti-quenched α effect which can be measured by using the sophisticated quasi-kinematic test field method. This paper aims at summarizing the main results of a much longer paper by Chatterjee et al. 2011, A&A (in press). Title: Magnetic helicity fluxes in interface and flux transport dynamos Authors: Chatterjee, P.; Guerrero, G.; Brandenburg, A. Bibcode: 2011A&A...525A...5C Altcode: 2010arXiv1005.5335C Context. Dynamos in the Sun and other bodies tend to produce magnetic fields that possess magnetic helicity of opposite sign at large and small scales, respectively. The build-up of magnetic helicity at small scales provides an important saturation mechanism.
Aims: In order to understand the nature of the solar dynamo we need to understand the details of the saturation mechanism in spherical geometry. In particular, we aim to understand the effects of magnetic helicity fluxes from turbulence and meridional circulation.
Methods: We consider a model with only radial shear confined to a thin layer (tachocline) at the bottom of the convection zone. The kinetic α owing to helical turbulence is assumed to be localized in a region above the convection zone. The dynamical quenching formalism is used to describe the build-up of mean magnetic helicity in the model, which results in a magnetic α effect that feeds back on the kinetic α effect. In some cases we compare these results with those obtained from a model with a simple algebraic α quenching formula.
Results: In agreement with earlier findings, the magnetic α effect has the opposite sign compared with the kinetic α effect and leads to a catastrophic decrease of the saturation field strength proportional to the inverse magnetic Reynolds number. At high latitudes this quenching effect can lead to secondary dynamo waves that propagate poleward because of the opposite sign of α. These secondary dynamo waves are driven by small-scale magnetic helicity instead of the small-scale kinetic helicity. Magnetic helicity fluxes both from turbulent mixing and from meridional circulation alleviate catastrophic quenching. Interestingly, supercritical diffusive helicity fluxes also give rise to secondary dynamo waves and grand minima-like episodes. Title: What do global p-modes tell us about banana cells? Authors: Chatterjee, Piyali Bibcode: 2011JPhCS.271a2066C Altcode: 2010arXiv1010.4204C We have calculated the effects of giant convection cells also know as sectoral rolls or banana cells, on p-mode splitting coefficients. We use the technique of quasi-degenerate perturbation theory formulated by Lavely & Ritzwoller in order to estimate the frequency shifts. A possible way of detecting giant cells is to look for even splitting coefficients of 'nearly degenerate' modes in the observational data since these modes have the largest shifts. We find that banana cells having an azimuthal wave number of 16 and maximum vertical velocity of 180 m s-1 cannot be ruled out from GONG data for even splitting coefficients. Title: Turbulent transport in hydromagnetic flows Authors: Brandenburg, A.; Chatterjee, P.; Del Sordo, F.; Hubbard, A.; Käpylä, P. J.; Rheinhardt, M. Bibcode: 2010PhST..142a4028B Altcode: 2010arXiv1004.5380B The predictive power of mean-field theory is emphasized by comparing theory with simulations under controlled conditions. The recently developed test-field method is used to extract turbulent transport coefficients both in the kinematic and the nonlinear or quasi-kinematic cases. A striking example of the quasi-kinematic method is provided by magnetic buoyancy-driven flows that produce an α effect and turbulent diffusion. Title: Shear-driven and diffusive helicity fluxes in αΩ dynamos Authors: Guerrero, G.; Chatterjee, P.; Brandenburg, A. Bibcode: 2010MNRAS.409.1619G Altcode: 2010MNRAS.tmp.1434G; 2010arXiv1005.4818G; 2010MNRAS.tmp.1451G We present non-linear mean-field αΩ dynamo simulations in spherical geometry with simplified profiles of kinetic α effect and shear. We take magnetic helicity evolution into account by solving a dynamical equation for the magnetic α effect. This gives a consistent description of the quenching mechanism in mean-field dynamo models. The main goal of this work is to explore the effects of this quenching mechanism in solar-like geometry, and in particular to investigate the role of magnetic helicity fluxes, specifically diffusive and Vishniac-Cho (VC) fluxes, at large magnetic Reynolds numbers (Rm). For models with negative radial shear or positive latitudinal shear, the magnetic α effect has predominantly negative (positive) sign in the Northern (Southern) hemisphere. In the absence of fluxes, we find that the magnetic energy follows an R-1m dependence, as found in previous works. This catastrophic quenching is alleviated in models with diffusive magnetic helicity fluxes resulting in magnetic fields comparable to the equipartition value even for Rm= 107. On the other hand, models with a shear-driven Vishniac-Cho flux show an increase in the amplitude of the magnetic field with respect to models without fluxes, but only for Rm < 104. This is partly a consequence of assuming a vacuum outside the Sun which cannot support a significant VC flux across the boundary. However, in contrast to the diffusive flux, the VC flux modifies the distribution of the magnetic field. In addition, if an ill-determined scaling factor in the expression for the VC flux is large enough, subcritical dynamo action is possible that is driven by the action of shear and the divergence of magnetic helicity flux. Title: Can catastrophic quenching be alleviated by separating shear and α effect? Authors: Chatterjee, Piyali; Brandenburg, Axel; Guerrero, Gustavo Bibcode: 2010GApFD.104..591C Altcode: 2010arXiv1005.5708C The small-scale magnetic helicity produced as a by-product of the large-scale dynamo is believed to play a major role in dynamo saturation. In a mean-field model the generation of small-scale magnetic helicity can be modelled by using the dynamical quenching formalism. Catastrophic quenching refers to a decrease of the saturation field strength with increasing Reynolds number. It has been suggested that catastrophic quenching only affects the region of non-zero helical turbulence (i.e. where the kinematic α operates) and that it is possible to alleviate catastrophic quenching by separating the region of strong shear from the α layer. We perform a systematic study of a simple axisymmetric two-layer αΩ dynamo in a spherical shell for Reynolds numbers in the range 1 ≤ R m ≤ 105. In the framework of dynamical quenching we show that this may not be the case, suggesting that magnetic helicity fluxes would be necessary. Title: Equatorial magnetic helicity flux in simulations with different gauges Authors: Mitra, D.; Candelaresi, S.; Chatterjee, P.; Tavakol, R.; Brandenburg, A. Bibcode: 2010AN....331..130M Altcode: 2009arXiv0911.0969M We use direct numerical simulations of forced MHD turbulence with a forcing function that produces two different signs of kinetic helicity in the upper and lower parts of the domain. We show that the mean flux of magnetic helicity from the small-scale field between the two parts of the domain can be described by a Fickian diffusion law with a diffusion coefficient that is approximately independent of the magnetic Reynolds number and about one third of the estimated turbulent magnetic diffusivity. The data suggest that the turbulent diffusive magnetic helicity flux can only be expected to alleviate catastrophic quenching at Reynolds numbers of more than several thousands. We further calculate the magnetic helicity density and its flux in the domain for three different gauges. We consider the Weyl gauge, in which the electrostatic potential vanishes, the pseudo-Lorenz gauge, where the speed of light is replaced by the sound speed, and the `resistive gauge' in which the Laplacian of the magnetic vector potential acts as a resistive term. We find that, in the statistically steady state, the time-averaged magnetic helicity density and the magnetic helicity flux are the same in all three gauges. Title: Why Does the Torsional Oscillation Precede the Sunspot Cycle? Authors: Chatterjee, P.; Chakraborty, S.; Choudhuri, A. R. Bibcode: 2010ASSP...19..500C Altcode: 2010mcia.conf..500C The Sun's rotation shows a periodic variation with the sunspot cycle, called torsional oscillations, the nature of which inside the solar convection zone has been determined from helioseismology. Several authors developed theoretical models of torsional oscillations by assuming that they are driven by the Lorentz force of the Sun's cyclically varying magnetic field. If this is true, then one would expect the torsional oscillations to follow the sunspot cycles. However, the torsional oscillations of a cycle begin a couple of years before the sunspots of that cycle appear and at a latitude higher than where the first sunspots are subsequently seen. Our aim in this paper is to provide an explanation for this seemingly causality defying phenomenon. Title: Solar Flows and Their Effect on Frequencies of Acoustic Modes Authors: Chatterjee, Piyali; Antia, H. M. Bibcode: 2009ApJ...707..208C Altcode: 2009arXiv0910.4137C We have calculated the effects of large-scale solar flows, such as the meridional circulation, giant convection cells, and solar rotation on the helioseismic splitting coefficients using quasi-degenerate perturbation theory (QDPT). Our investigation reveals that the effect of poloidal flows like the large-scale meridional circulation are difficult to detect in observational data of the global acoustic modes since the frequency shifts are much less than the errors. However, signatures of large-scale convective flows may be detected if their amplitude is sufficiently large by looking for frequency shifts due to nearly degenerate modes coupled by convection. In this comprehensive study, we attempt to put limits on the magnitude of flow velocities in giant cells by comparing the splitting coefficients obtained from the QDPT treatment with observational data. Title: Small-scale magnetic helicity losses from a mean-field dynamo Authors: Brandenburg, Axel; Candelaresi, Simon; Chatterjee, Piyali Bibcode: 2009MNRAS.398.1414B Altcode: 2009MNRAS.tmp..979B; 2009arXiv0905.0242B Using mean-field models with a dynamical quenching formalism, we show that in finite domains magnetic helicity fluxes associated with small-scale magnetic fields are able to alleviate catastrophic quenching. We consider fluxes that result from advection by a mean flow, the turbulent mixing down the gradient of mean small-scale magnetic helicity density or the explicit removal which may be associated with the effects of coronal mass ejections in the Sun. In the absence of shear, all the small-scale magnetic helicity fluxes are found to be equally strong for both large- and small-scale fields. In the presence of shear, there is also an additional magnetic helicity flux associated with the mean field, but this flux does not alleviate catastrophic quenching. Outside the dynamo-active region, there are neither sources nor sinks of magnetic helicity, so in a steady state this flux must be constant. It is shown that unphysical behaviour emerges if the small-scale magnetic helicity flux is forced to vanish within the computational domain. Title: Erratum: Why Does the Sun's Torsional Oscillation Begin before the Sunspot Cycle? [Phys. Rev. Lett. 102, 041102 (2009)] Authors: Chakraborty, Sagar; Choudhuri, Arnab Rai; Chatterjee, Piyali Bibcode: 2009PhRvL.103i9902C Altcode: No abstract at ADS Title: Why Does the Sun's Torsional Oscillation Begin before the Sunspot Cycle? Authors: Chakraborty, Sagar; Choudhuri, Arnab Rai; Chatterjee, Piyali Bibcode: 2009PhRvL.102d1102C Altcode: 2009arXiv0907.4842C Although the Sun’s torsional oscillation is believed to be driven by the Lorentz force associated with the sunspot cycle, this oscillation begins 2 3 yr before the sunspot cycle. We provide a theoretical explanation of this with the help of a solar dynamo model having a meridional circulation penetrating slightly below the bottom of the convection zone, because only in such dynamo models does the strong toroidal field form a few years before the sunspot cycle and at a higher latitude. Title: How Do f-Mode Frequencies Change with Solar Radius? Authors: Chatterjee, Piyali; Antia, H. M. Bibcode: 2008ApJ...688L.123C Altcode: 2008arXiv0810.4213C We test the relation between relative f-mode frequency variation (δ ν/ν) and Lagrangian perturbation in the solar radius (δ r/r) obtained by Dziembowski and Goode using several pairs of solar models and show that it does not hold true for any of the model pairs we have used. We attempt to derive a better approximation for the kernel linking the relative frequency changes and the solar radius variation in the subsurface layers. 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: Solar activity forecast with a dynamo model Authors: Jiang, Jie; Chatterjee, Piyali; Choudhuri, Arnab Rai Bibcode: 2007MNRAS.381.1527J Altcode: 2007arXiv0707.2258J; 2007MNRAS.tmp..899J Although systematic measurements of the Sun's polar magnetic field exist only from mid-1970s, other proxies can be used to infer the polar field at earlier times. The observational data indicate a strong correlation between the polar field at a sunspot minimum and the strength of the next cycle, although the strength of the cycle is not correlated well with the polar field produced at its end. This suggests that the Babcock-Leighton mechanism of poloidal field generation from decaying sunspots involves randomness, whereas the other aspects of the dynamo process must be reasonably ordered and deterministic. Only if the magnetic diffusivity within the convection zone is assumed to be high (of order 1012cm2s-1), we can explain the correlation between the polar field at a minimum and the next cycle. We give several independent arguments that the diffusivity must be of this order. In a dynamo model with diffusivity like this, the poloidal field generated at the mid-latitudes is advected toward the poles by the meridional circulation and simultaneously diffuses towards the tachocline, where the toroidal field for the next cycle is produced. To model actual solar cycles with a dynamo model having such high diffusivity, we have to feed the observational data of the poloidal field at the minimum into the theoretical model. We develop a method of doing this in a systematic way. Our model predicts that cycle 24 will be a very weak cycle. Hemispheric asymmetry of solar activity is also calculated with our model and compared with observational data. Title: Predicting Solar Cycle 24 With a Solar Dynamo Model Authors: Choudhuri, Arnab Rai; Chatterjee, Piyali; Jiang, Jie Bibcode: 2007PhRvL..98m1103C Altcode: 2007astro.ph..1527C Whether or not the upcoming cycle 24 of solar activity will be strong is being hotly debated. The solar cycle is produced by a complex dynamo mechanism. We model the last few solar cycles by “feeding” observational data of the Sun’s polar magnetic field into our solar dynamo model. Our results fit the observed sunspot numbers of cycles 21 23 reasonably well and predict that cycle 24 will be about 35% weaker than cycle 23. Title: On Magnetic Coupling Between the Two Hemispheres in Solar Dynamo Models Authors: Chatterjee, Piyali; Choudhuri, Arnab Rai Bibcode: 2006SoPh..239...29C Altcode: 2006SoPh..tmp...77C By introducing an asymmetry between the two hemispheres, we study whether the solar dynamo solutions in the two hemispheres remain coupled with each other. Our calculations are based on the solar dynamo code SURYA, which incorporates the helioseismically-determined solar-rotation profile, a Babcock-Leighton α effect concentrated near the surface, and a meridional circulation. When the magnetic coupling between the hemispheres is enhanced by either increasing the diffusion or introducing an α effect distributed throughout the convection zone, we find that the solutions in the two hemispheres evolve together with a single period even when we make the meridional circulation or the α effect different in the two hemispheres. On the other hand, when the hemispheric coupling is weaker for other values of parameters, an asymmetry between the hemispheres can make solutions in the two hemispheres evolve independently with different periods. Title: Forecasting Cycle 24 with a Solar Dynamo Model Authors: Jiang, Jie; Chatterjee, P.; Choudhuri, A. R. Bibcode: 2006ihy..workE..28J Altcode: A challenge before solar physicists right now is to forecast the strength of the next solar cycle (Cycle 24). Several contrary forecasts have already been made. Most of the forecasts are based on various precursor methods. Only one forecast is based on a dynamo model (Dikpati and Gilman 2006). Since we find some aspects of this work questionable, it is desirable to have another independent forecast based on a dynamo model. We are carrying out an analysis based on our dynamo model, using a methodology different from what was used by Dikpati and Gilman (2006). We shall present the methodology of our approach and, most probably, we shall also have some results by the time of the IHY meeting. Title: Helicity of Solar Active Regions from a Dynamo Model Authors: Chatterjee, Piyali Bibcode: 2006JApA...27...87C 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. We use our solar dynamo model based on the Babcock-Leighton α-effect to study how helicity varies with latitude and time. Title: On the Origin of Current Helicity in Active Regions Authors: Petrovay, K.; Chatterjee, P.; Choudhuri, A. Bibcode: 2006ESASP.617E..67P Altcode: 2006soho...17E..67P No abstract at ADS Title: Development of twist in an emerging magnetic flux tube by poloidal field accretion Authors: Chatterjee, P.; Choudhuri, A. R.; Petrovay, K. Bibcode: 2006A&A...449..781C Altcode: 2005astro.ph.12472C Aims.Following an earlier proposal for the origin of twist in the magnetic fields of solar active regions, we model the penetration of a wrapped up background poloidal field into a toroidal magnetic flux tube rising through the solar convective zone. Methods.The rise of the straight, cylindrical flux tube is followed by numerically solving the induction equation in a comoving Lagrangian frame, while an external poloidal magnetic field is assumed to be radially advected onto the tube with a speed corresponding to the rise velocity. Results.One prediction of our model is the existence of a ring of reverse current helicity on the periphery of active regions. On the other hand, the amplitude of the resulting twist depends sensitively on the assumed structure (diffuse vs. concentrated/intermittent) of the active region magnetic field right before its emergence, and on the assumed vertical profile of the poloidal field. Nevertheless, in the model with the most plausible choice of assumptions a mean twist comparable to the observations results. Conclusions.Our results indicate that the contribution of this mechanism to the twist can be quite significant, and under favourable circumstances it can potentially account for most of the current helicity observed in active regions. Title: Theoretical model for calculation of helicity in solar active regions Authors: Chatterjee, P. Bibcode: 2006ilws.conf...30C Altcode: We (Choudhuri, Chatterjee and Nandy, 2005) calculate helicities of solar active regions based on the idea of Choudhuri (2003) 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 favourably 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. Next we (Chatterjee, Choudhuri and Petrovay 2006) use the above idea along with the sunspot decay model of Petrovay and Moreno-Insertis, (1997) to estimate the distribution of helicity inside a flux tube as it keeps collecting more azimuthal flux during its rise through the convection zone and as turbulent diffusion keeps acting on it. By varying parameters over reasonable ranges in our simple 1-d model, we find that the azimuthal flux penetrates the flux tube to some extent instead of being confined to a narrow sheath outside. Title: A theoretical model for the magnetic helicity of solar active regions Authors: Choudhuri, A. R.; Chatterjee, P.; Petrovay, K.; Nandy, D. Bibcode: 2006cosp...36..714C Altcode: 2006cosp.meet..714C Active regions on the solar surface are known to possess magnetic helicity which is predominantly negative in the northern hemisphere and positive in the southern hemisphere Choudhuri 2003 Sol Phys 123 217 proposed that the magnetic helicity arises due to the wrapping up of the poloidal field of the convection zone around rising flux tubes which form active regions Choudhuri Chatterjee and Nandy 2004 ApJ 615 L57 used this idea to calculate magnetic helicity from their solar dynamo model and found broad agreements with observational data Chatterjee Choudhuri and Petrovay 2006 A A in press have studied the penetration of the wrapped poloidal field into the rising flux tube and concluded that more detailed observational data will throw light on the physical conditions of flux tubes just before their emergence to the photosphere Title: Reply to the Comments of Dikpati et al. Authors: Choudhuri, A. R.; Nandy, D.; Chatterjee, P. Bibcode: 2005A&A...437..703C Altcode: 2005astro.ph..5232C We respond to Dikpati et al.'s criticism of our recent solar dynamo model. A different treatment of the magnetic buoyancy is the most probable reason for their different results. Title: Full-sphere simulations of a circulation-dominated solar dynamo: Exploring the parity issue Authors: Chatterjee, P.; Nandy, D.; Choudhuri, A. R. Bibcode: 2004A&A...427.1019C Altcode: 2004astro.ph..5027C We explore a two-dimensional kinematic solar dynamo model in a full sphere, based on the helioseismically determined solar rotation profile and with an α effect concentrated near the solar surface, which captures the Babcock-Leighton idea that the poloidal field is created from the decay of tilted bipolar active regions. The meridional circulation, assumed to penetrate slightly below the tachocline, plays an important role. Some doubts have recently been raised regarding the ability of such a model to reproduce solar-like dipolar parity. We specifically address the parity issue and show that the dipolar mode is preferred when certain reasonable conditions are satisfied, the most important condition being the requirement that the poloidal field should diffuse efficiently to get coupled across the equator. Our model is shown to reproduce various aspects of observational data, including the phase relation between sunspots and the weak, diffuse field. Title: Helicity of Solar Active Regions from a Dynamo Model Authors: Choudhuri, Arnab Rai; Chatterjee, Piyali; Nandy, Dibyendu Bibcode: 2004ApJ...615L..57C Altcode: We calculate helicities of solar active regions based on the idea that poloidal flux lines get wrapped around a toroidal flux tube rising through the convection zone, thereby giving rise to the helicity. Rough estimates based on this idea compare favorably with the observed magnitude of helicity. We use our solar dynamo model based on the Babcock-Leighton α-effect to study how helicity varies with latitude and time. At the time of solar maximum, our theoretical model gives negative helicity in the northern hemisphere and positive helicity in the south, in accordance with observed hemispheric trends. However, we find that during a short interval at the beginning of a cycle, helicities tend to be opposite of the preferred hemispheric trends. Title: Full Sphere Axisymmetric Simulations of the Solar Dynamo Authors: Nandy, Dibyendu; Chatterjee, Piyali; Choudhuri, Arnab Rai Bibcode: 2004IAUS..223..133N Altcode: 2005IAUS..223..133N We explore a full sphere (2D axisymmetric) kinematic solar dynamo model based on the Babcock-Leighton idea that the poloidal field is generated in the surface layers from the decay of tilted bipolar solar active regions. This model incorporates the helioseismically deduced solar rotation profile and an algorithm for buoyancy motivated from simulations of flux tube dynamics. A prescribed deep meridional circulation plays an important role in the advection of magnetic flux. We specifically address the parity issue and show that - contrary to some recent claims - the Babcock-Leighton dynamo can reproduce solar-like dipolar parity if certain reasonable conditions are satisfied in the solar interior, the most important requirement being that the poloidal field of the two hemispheres be efficiently coupled across the equator. Title: The Origin of Helicity in Solar Active Regions Authors: Choudhuri, Arnab Rai; Chatterjee, Piyali; Nandy, Dibyendu Bibcode: 2004IAUS..223...45C Altcode: 2005IAUS..223...45C; 2004astro.ph..6598C We present calculations of helicity based on our solar dynamo model and show that the results are consistent with observational data. Title: On Solar Radius Variation with Magnetic Field Authors: Choudhuri, Arnab Rai; Chatterjee, Piyali Bibcode: 2003astro.ph.11028C Altcode: In response to the claim by Dziembowski et al. (2001) that the solar radius decreases with magnetic activity at the rate of 1.5 km/yr, we consider the theoretical question whether a radius variation is expected with the solar cycle. If the radius variation is caused by the magnetic pressure of toroidal flux tubes at the bottom of the convection zone, then the dynamo model of Nandy and Choudhuri (2002) would suggest a radius decrease with magnetic activity, in contrast to other dynamo models which would suggest a radius increase. However, the radius decrease is estimated to be only of the order of hundreds of metres. Title: Space-Time Torsion, Broken Lorentz Symmetry and Inflation in the Early Universe Authors: Chatterjee, P.; Bhattacharya, B. Bibcode: 1993MPLA....8.2249C Altcode: We have shown here, a possible scheme of inflationary scenario at an early stage of the universe during the radiation dominated era by introducing a local Lorentz symmetry violating term in the space-time torsion of Einstein-Cartan action in E(4,1) space. This 'additional' mechanism for producing inflation may also suggest the answer to the question of the local relative abundance of particles over antiparticles in the observed universe.