Papers by Ramit Bhattacharyya
Self-organization in continuous systems is associated with dissipative processes. In particular, ... more Self-organization in continuous systems is associated with dissipative processes. In particular, for magnetized plasmas, it is known as magnetic relaxation, where the magnetic energy is converted into heat and kinetic energy of flow through the magnetic reconnection process. An example of such a system is the solar corona, where reconnection manifests as solar transients like flares and jets. Consequently, toward investigation of plasma relaxation in solar transients, we utilize the novel approach of data-constrained MHD simulation for an observed solar flare. The selected active region NOAA 12253 hosts a GOES M1.3 class flare. The investigation of extrapolated coronal magnetic field in conjunction with the spatiotemporal evolution of flare reveals a hyperbolic flux tube (HFT), overlying the observed brightenings. MHD simulation is carried out with the EULAG-MHD numerical model to explore reconnection dynamics. Three distinct sub-volumes are chosen and are subjected to analysis of m...
Frontiers in Astronomy and Space Sciences
Magnetohydrodynamics simulation of active region NOAA 11515 is performed to examine the initiatio... more Magnetohydrodynamics simulation of active region NOAA 11515 is performed to examine the initiation of the M5.6 flaring event that starts around 10:43 UT on 2 July 2012. The simulation is conducted using an extrapolated non-force-free magnetic field generated from the photospheric vector magnetogram of the active region as the initial magnetic field. The magnetic field shows the presence of a three-dimensional (3D) magnetic null with the corresponding dome overlying a filament and a low-lying magnetic flux rope, observed in 304 Å and 131 Å respectively. The simulated dynamics, triggered by the initial Lorentz force, lead to the bifurcations of the flux rope, which is similar to the observed bifurcation in the 131 Å brightenings. Additionally, the rope exhibits a rise and reconnects at the 3D null. These reconnections convert field lines of the rope into the anchored outer spine of the 3D null—explaining the occurrence of a nearby confined C-class flare. Further, the results show that...
2019 IEEE MTT-S International Microwave and RF Conference (IMARC)
This paper presents a detailed description of various subsystems of CALLISTO solar radio spectrog... more This paper presents a detailed description of various subsystems of CALLISTO solar radio spectrograph installed at the USO/PRL. In the front-end system, a log periodic dipole antenna (LPDA) is designed for the frequency range of 40-900 MHz. In this paper LPDA design, its modifications, and simulation results are presented. We also present some initial observations taken by CALLISTO at Udaipur.
The Astrophysical Journal, 2022
This work analyzes the Hall magnetohydrodynamics (HMHD) and magnetohydrodynamics (MHD) numerical ... more This work analyzes the Hall magnetohydrodynamics (HMHD) and magnetohydrodynamics (MHD) numerical simulations of a flaring solar active region as a test bed while idealizing the coronal Alfvén speed to be less by two orders of magnitude. HMHD supports faster magnetic reconnection and shows richer complexity in magnetic field line evolution compared to the MHD. The magnetic reconnections triggering the flare are explored by numerical simulations augmented with relevant multiwavelength observations. The initial coronal magnetic field is constructed by non-force-free extrapolation of photospheric vector magnetic field. Magnetic structure involved in the flare is identified to be a flux rope, with its overlying magnetic field lines constituting the quasi-separatrix layers (QSLs) along with a three-dimensional null point and a null line. Compared to the MHD simulation, the HMHD simulation shows a higher and faster ascent of the rope together with the overlying field lines, which further r...
Solar Physics, 2021
Three-dimensional (3D) magnetohydrodynamic simulations are carried out to explore magnetic reconn... more Three-dimensional (3D) magnetohydrodynamic simulations are carried out to explore magnetic reconnections in the presence of 3D magnetic nulls and quasi-separatrix layers (QSLs). The initial magnetic fields are created by superposing uniform vertical magnetic fields of two different magnitudes on a linear force-free field. The interior of the numerical box contains two 3D nulls with separatrix domes separated by a quasi-separator (or hyperbolic flux tube) with QSLs. In the first simulation, the uniform vertical field is so large that the nulls are located at low heights and the domes are separate. Initially unbalanced Lorentz forces drive rotational flows that form strong electric currents and strong torsional fan reconnection at the 3D nulls and weak QSL reconnection at the hyperbolic flux tube. Flipping or slipping of field lines is observed in both cases. In the second simulation, with a weaker vertical field and larger domes, the separatrix surfaces meet at the central quasi-separator and their rotation drives stronger QSL reconnection than before.
Physics of Plasmas, 2015
In this work, evolution of an incompressible, thermally homogeneous, infinitely conducting, visco... more In this work, evolution of an incompressible, thermally homogeneous, infinitely conducting, viscous magnetofluid is numerically explored as the fluid undergoes repeated events of magnetic reconnection. The initial magnetic field is constructed by a superposition of two linear force-free fields and has similar morphology as the magnetic loops observed in the solar corona. The results are presented for computations with three distinct sets of footpoint geometries. To onset reconnection we rely on numerical model magnetic diffusivity, in the spirit of Implicit Large Eddy Simulation (ILES). It is generally expected that in a high Lundquist number fluid, repeated magnetic reconnections are ubiquitous and hence can lead to a host of magnetic structures with considerable observational importance. In particular, the simulations presented here illustrate formations of magnetic islands, rotating magnetic helices and rising flux ropes-depending on the initial footpoint geometry but through the common process of repeated magnetic reconnections. Further, we observe development of extended current sheets in two case studies, where the footpoint reconnections generate favorable dynamics.
Physical Review E, 2002
The principle of minimum dissipation of energy is utilized to obtain the spheromak configuration ... more The principle of minimum dissipation of energy is utilized to obtain the spheromak configuration as a relaxed state. The Euler-Lagrange equation for the minimum dissipative relaxed state is solved in terms of Chandrasekhar-Kendall eigenfunctions analytically generalized in the complex domain. This state is non-forcefree and further shows the nonconstancy of the ratio of parallel current to the magnetic field.
The solar corona is the tenuous atmosphere of the Sun characterized by a temperature of the order... more The solar corona is the tenuous atmosphere of the Sun characterized by a temperature of the order of million degrees Kelvin, an ambient magnetic field of 10 to 15 Gauss and a very high magnetic Reynolds number because of which it qualifies as a near-ideal magnetofluid system. It is well known that for such a system, the magnetic flux across every fluid surface remains effectively constant to a good approximation. Under this so called ``frozen-in'' condition then, it is possible to partition this magnetofluid into contiguous magnetic subvolumes each entrapping its own subsystem of magnetic flux. Thin magnetic flux tubes are an elementary example of such magnetic subvolumes evolving in time with no exchange of fluid among them. The internal twists and interweaving of these flux tubes, collectively referred as the magnetic topology, remains conserved under the frozen-in condition. Because of the dynamical evolution of the magnetofluid, two such subvolumes can come into direct c...
The dynamics of spontaneous current sheet formation is demonstrated in a viscous, perfectly condu... more The dynamics of spontaneous current sheet formation is demonstrated in a viscous, perfectly conducting, incompressible magnetofluid through numerical simulations. The magnetic field is represented in terms of evolving flux surfaces which are the possible sites of current sheet formation. The computation follows global magnetic flux surfaces of simple initial geometry as they deform into more complex forms creating current sheets in the process. Ultimately, the flux surfaces break their initial topology, as the spatial scale of surface folds decreases below the model resolution. This breaking is used to identify the sites of the current sheets formation.
The Astrophysical Journal, 2011
Solar Physics, 2007
Arcade-type magnetic field structures originating from the photosphere are relevant to the unders... more Arcade-type magnetic field structures originating from the photosphere are relevant to the understanding of different types of solar prominences, coronal loops and coronal heating. In most of the existing literature, these loop-like magnetic structures are modeled as force-free fields (FFF) without any plasma flow. The system is assumed to be isolated and non-dissipative. In reality the photospheric plasma hardly qualifies to be isolated in nature and the existence of an external drive in the form of a dynamo field is always possible. Solar prominences are not ideal either since dissipative effects are believed to play a major role in coronal heating. The above observations indicate that a force-free model based on a non-dissipative plasma may not be a suitable candidate to replicate the arcade structures and further investigations are required. In our work, we have proposed the arcade structures as minimum dissipative relaxed states (including both the viscous and resistive channels) pertaining to a two-fluid description of the plasma. The obtained relaxed state is non force-free in nature and appropriate to an open system with external drives. The Euler-Lagrange equations are solved in Cartesian coordinates subject to the existing photospheric boundary conditions. The solutions are seen to support flow-containing arcade like magnetic field configurations with inherent dissipative properties that may play an important role in the coronal heating. An interesting feature observed is the generation of different types of arcades with the variation of a single parameter characterizing the relaxed state. Also, recent observations with the LASCO coronagraph on board the SOHO spacecraft suggest that the helmet streamers originating from the sun may have an internal triple-arcade structure. The two-fluid relaxed state obtained here is also seen to support such structures.
Pramana, 2000
ABSTRACT Relaxation of toroidal discharges is described by the principle of minimum energy dissip... more ABSTRACT Relaxation of toroidal discharges is described by the principle of minimum energy dissipation together with the constraint of conserved global helicity. The resulting Euler-Lagrange equation is solved in toroidal coordinates for an axisymmetric torus by expressing the solutions in terms of Chandrasekhar-Kendall (C-K) eigenfunctions analytically continued in the complex domain. The C-K eigenfunctions are obtained as hypergeometric functions that are solutions of scalar Helmholtz equation in toroidal coordinates in the large aspect-ratio approximation. Equilibria are constructed by assuming the current to vanish at the edge of plasma. For the m=0, n=0 (m and n are the poloidal and toroidal mode numbers respectively) relaxed states, the magnetic field, current, q (safety factor) and pressure profiles are calculated for a given value of aspect-ratio of the torus and for different values of the eigenvalue λ r 0. The new feature of the present model is that solutions allow for both tokamak as well as RFP-like behaviour with increase in the values of λ r 0, which is related directly to volt-sec in the experiment.
Physics of Plasmas, 2003
ABSTRACT Electron-magnetohydrodynamic equilibrium is studied through a variational approach. The ... more ABSTRACT Electron-magnetohydrodynamic equilibrium is studied through a variational approach. The equilibrium state is described by the Beltrami equation. Through a constrained minimization of the pseudo-resistive dissipation in the system, a variational problem is formulated to mimic the equilibrium state. It is found that a subclass of the Euler–Lagrange equations has strong morphological resemblance with the equilibrium topology of the system. © 2003 American Institute of Physics.
Physics of Plasmas, 2013
In ideal magnetohydrodynamics characterized by an infinite electrical conductivity, the magnetic ... more In ideal magnetohydrodynamics characterized by an infinite electrical conductivity, the magnetic flux across an arbitrary fluid surface is conserved in time. The magnetofluid then can be partitioned into contiguous subvolumes of fluid, each of which entraps its own subsystem of magnetic flux. During dynamical evolution of the magnetofluid these subvolumes press into each other, and in the process two such subvolumes may come into direct contact while ejecting a third interstitial subvolume. Depending on the orientations of magnetic fields of the two interacting subvolumes the magnetic field at the common surface of interaction may become discontinuous and a current sheet is formed there. This process of current sheet formation and their subsequent decay is believed to be a plausible mechanism for coronal heating and may also be responsible for various eruptive phenomena at the solar corona. In this work, we explore this theoretical concept through numerical simulations of a viscous, incompressible magnetofluid characterized by infinite electrical conductivity. In particular, we show that if the initial magnetic field is prescribed by superposition of two linear force-free fields with different torsion coefficients, then formation of current sheets are numerically realizable in the neighborhood of magnetic nulls.
Physics of Plasmas, 2010
ABSTRACT This is a study of the spontaneous formation of electric current sheets in an incompress... more ABSTRACT This is a study of the spontaneous formation of electric current sheets in an incompressible viscous fluid with perfect electrical conductivity, governed by the magnetohydrodynamic Navier–Stokes equations. Numerical solutions to two initial value problems are presented for a three-dimensional, periodic, untwisted magnetic field evolving, with no change in magnetic topology under the frozen-in condition and at characteristic fluid Reynolds numbers of the order of 500, from a nonequilibrium initial state with the fluid at rest. The evolution converts magnetic free energy into kinetic energy to be all dissipated away by viscosity so that the field settles into a minimum-energy, static equilibrium. The solutions demonstrate that, as a consequence of the frozen-in condition, current sheets must form during the evolution despite the geometric simplicity of the prescribed initial fields. In addition to the current sheets associated with magnetic neutral points and field reversal layers, other sheets not associated with such magnetic features are also in evidence. These current sheets form on magnetic flux surfaces. This property is used to achieve a high degree of the frozen-in condition in the simulations, by describing the magnetic field entirely in terms of the advection of its flux surfaces and integrating the resulting governing equations with a customized version of a general-purpose high-resolution (viz., nonoscillatory) hydrodynamical simulation code EULAG [ J. M. Prusa et al., Comput. Fluids 37, 1193 (2008) ]. Incompressibility imposes the additional global constraint that the flux surfaces must evolve with no change in the spatial volumes they enclose. In this approach, current sheet formation is demonstrated graphically by the progressive pressing together of suitably selected flux surfaces until their separation has diminished below the minimal resolved distance on a fixed grid. The frozen-in condition then fails in the simulation as the field reconnects through an effecting numerical resistivity. The principal results are related to the Parker theory of current-sheet formation and dissipation in the solar corona.
Physics of Plasmas, 2011
In this work, the well established two-fluid relaxation model based on the minimum energy princip... more In this work, the well established two-fluid relaxation model based on the minimum energy principle is extended to include open systems like the solar corona. The Euler-Lagrange equations obtained are of double curl in nature and support non-zero plasma-b along with mass flow of the magnetofluid. These equations are solved in Cartesian coordinates utilizing a geometry relevant to the solar atmosphere, and a basic comparative study of the non force-free, force-free, and potential magnetic field obtained as solutions of the same Euler-Lagrange equations is presented.
Physics Letters A, 2003
For a non-relativistic electron-positron plasma the possibility of obtaining a relaxed state is e... more For a non-relativistic electron-positron plasma the possibility of obtaining a relaxed state is explored. The Euler-Lagrange equations are obtained by minimizing the hyper-resistivity with generalized helicities and magnetofluid energy as the constraints. The relaxed state is shown to be morphologically similar to the steady-state equilibria of the electron-positron plasma system.
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Papers by Ramit Bhattacharyya