Research

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Research Interests

  • Computational Fluid Dynamics : Hydrodynamics, Magneto-hydrodynamics & Radiative transfer.
  • Theoretical star formation : Accretion disk physics, Jets and Outflow formation.
  • Particle Acceleration in AGN jets : Modeling Non-Thermal spectral signatures
  • Inter-Stellar Medium : Shock-Cloud Interaction, Collapse of molecular cores, Shock induced chemistry.
  • Astrophysical Code Development : MPI Parallel Programming, Visualization software tools, best practices of coding with C and Python.

List of Recent Selected Published Works

1. A Particle Module for the PLUTO Code II - Hybrid Framework for Modeling Non-thermal emission from Relativistic Magnetized flows

Cartoon figure showing the hybrid framework of
incorporating Lagrangian particles on fixed Eulerian grid.

We describe a new hybrid framework to model non-thermal spectral signatures from highly energetic particles embedded in a large-scale classical or relativistic MHD flow. Our method makes use of Lagrangian particles moving through an Eulerian grid where the (relativistic) MHD equations are solved concurrently. Lagrangian particles follow fluid streamlines and represent ensembles of (real) relativistic particles with a finite energy distribution. The spectral distribution of each particle is updated in time by solving the relativistic cosmic ray transport equation based on local fluid conditions. This enables us to account for a number of physical processes, such as adiabatic expansion, synchrotron and inverse Compton emission. An accurate semi-analytically numerical scheme that combines the method of characteristics with a Lagrangian discretization in the energy coordinate is described.In presence of (relativistic) magnetized shocks, a novel approach to consistently model particle energization due to diffusive shock acceleration has been presented.

Multi-wavelength emission and polarisation maps of oblique shocks in slab jets.

Our approach relies on a refined shock-detection algorithm and updates the particle energy distribution based on the shock compression ratio, magnetic field orientation and amount of (parameterized) turbulence.The evolved distribution from each Lagrangian particle is further used to produce observational signatures like emission maps and polarization signals accounting for proper relativistic corrections. We further demonstrate the validity of this hybrid framework using standard numerical benchmarks and evaluate the applicability of such a tool to study high energy emission from extra-galactic jets.

2. A Particle Module for the PLUTO Code. I. An Implementation of the MHD–PIC Equations

Density snapshots for the collisionless shock problem at four different times. Only a reduced portion of the domain, in proximity of the unperturbed shock position is shown.

We describe an implementation of a particle physics module available for the PLUTO code appropriate for the dynamical evolution of a plasma consisting of a thermal fluid and a nonthermal component represented by relativistic charged particles or cosmic rays (CRs). While the fluid is approached using standard numerical schemes for magnetohydrodynamics, CR particles are treated kinetically using conventional Particle-In-Cell (PIC) techniques. The module can be used either to describe test-particle motion in the fluid electromagnetic field or to solve the fully coupled magnetohydrodynamics (MHD)–PIC system of equations with particle backreaction on the fluid as originally introduced by Bai et al. Particle backreaction on the fluid is included in the form of momentum–energy feedback and by introducing the CR-induced Hall term in Ohm’s law. The hybrid MHD–PIC module can be employed to study CR kinetic effects on scales larger than the (ion) skin depth provided that the Larmor gyration scale is properly resolved. When applicable, this formulation avoids resolving microscopic scales, offering substantial computational savings with respect to PIC simulations.We present a fully conservative formulation that is second-order accurate in time and space, and extends to either the Runge–Kutta (RK) or the corner transport upwind time-stepping schemes (for the fluid), while a standard Boris integrator is employed for the particles. For highly energetic relativistic CRs and in order to overcome the time-step restriction, a novel subcycling strategy that retains second-order accuracy in time is presented. Numerical benchmarks and applications including Bell instability, diffusive shock acceleration, and test-particle acceleration in reconnecting layers are discussed.

3. Simulation of MHD modes in Active Region of Sun.

There is considerable observational evidence of implosion of magnetic loop systems inside solar coronal active regions following high-energy events like solar flares. In this work, we propose that such collapse can be modeled in three dimensions quite accurately within the framework of ideal magnetohydrodynamics. We furthermore argue that the dynamics of loop implosion is only sensitive to the transmitted disturbance of one or more of the system variables, e.g., velocity generated at the event site. This indicates that to understand loop implosion, it is sensible to leave the event site out of the simulated active region. Toward our goal, a velocity pulse is introduced to model the transmitted disturbance generated at the event site. Magnetic field lines inside our simulated active region are traced in real time, and it is demonstrated that the subsequent dynamics of the simulated loops closely resemble observed imploding loops. Our work highlights the role of plasma β in regards to the rigidity of the loop systems and how that might affect the imploding loops’ dynamics. Compressible magnetohydrodynamic modes such as kink and sausage are also shown to be generated during such processes, in accordance with observations.

Density evolution along a field line showing indicatication of the standing sausage wave oscillation.

4. Runge-Kutta Legendre Method for Parabolic PDEs in PLUTO code : Application to Thermal Conduction Problems.

Effect of Anisotropic Thermal conduction in the problem of 2D blast wave.

An important ingredient in numerical modelling of high temperature magnetized astrophysical plasmas is the anisotropic transport of heat along magnetic field lines from higher to lower temperatures. Magnetohydrodynamics typically involves solving the hyperbolic set of conservation equations along with the induction equation. Incorporating anisotropic thermal conduction requires to also treat parabolic terms arising from the diffusion operator. An explicit treatment of parabolic terms will considerably reduce the simulation time step due to its dependence on the square of the grid resolution ( x) for stability. Although an implicit scheme relaxes the constraint on stability, it is difficult to distribute efficiently on a parallel architecture.

Strong scaling test for the newly implemented Runge-Kutta Legendre Method in PLUTO code.

Treating parabolic terms with accelerated super-time- stepping (STS) methods has been discussed in literature, but these methods suffer from poor accuracy (first order in time) and also have difficult-to- choose tuneable stability parameters. In this work, we highlight a second-order (in time) Runge–Kutta–Legendre (RKL) scheme (first described by Meyer, Balsara & Aslam 2012) that is robust, fast and accurate in treating parabolic terms alongside the hyperbolic conversation laws. We demonstrate its superiority over the first-order STS schemes with standard tests and astrophysical applications. We also show that explicit conduction is particularly robust in handling saturated thermal conduction. Parallel scaling of explicit conduction using RKL scheme is demonstrated up to more than 104 processors.

5. Interaction of Hydrodynamic Shock with Self-gravitating cloud.

Time evolution of density (X-Y plane) of interaction of 3D self-gravitating cloud with weak shock

We describe the results of 3D simulations of the interaction of hydrodynamic shocks with Bonnor-Ebert spheres performed with an adaptive mesh refinement code. The calculations are isothermal and the clouds are embedded in a medium in which the sound speed is either 4 or 10 times that in the cloud. The strengths of the shocks are such that they induce gravitational collapse in some cases and not in others, and we derive a simple estimate for the shock strength required for this to occur. These results are relevant to dense cores and Bok globules in star-forming regions subjected to shocks produced by stellar feedback.

6. Interplay Magnetic reconnection and non-axisymmetric instabilities in Jets.

Formation of short wavelength pressure driven instabilities in density of jet column with toroidal magnetic field lines.

Magnetic reconnection is a plasma phenomenon where a topological rearrangement of magnetic field lines with opposite polarity results in dissipation of magnetic energy into heat, kinetic energy and particle acceleration. Such a phenomenon is considered as an efficient mechanism for energy release in laboratory and astrophysical plasmas. An important question is how to make the process fast enough to account for observed explosive energy releases. The classical model for steady state magnetic reconnection predicts reconnection times scaling as S 1/2 (where S is the Lundquist number) and yields time-scales several order of magnitude larger than the observed ones. Earlier two- dimensional MHD simulations showed that for large Lundquist number the reconnection time becomes independent of S (`fast reconnection' regime) due to the presence of the secondary tearing instability that takes place for S ≳ 1 × 10 4 . We report on our 3D MHD simulations of magnetic reconnection in a magnetically confined cylindrical plasma column under either a pressure balanced or a force-free equilibrium and compare the results with 2D simulations of a circular current sheet. We find that the 3D instabilities acting on these configurations result in a fragmentation of the initial current sheet in small filaments, leading to enhanced dissipation rate that becomes independent of the Lundquist number already at S ≃ 1 × 10 3 .

The dominant kink mode for current driven instability seen in the density along turbulent current sheets.