39 research outputs found
Synchrotron radiation of self-collimating relativistic MHD jets
The goal of this paper is to derive signatures of synchrotron radiation from
state-of-the-art simulation models of collimating relativistic
magnetohydrodynamic (MHD) jets featuring a large-scale helical magnetic field.
We perform axisymmetric special relativistic MHD simulations of the jet
acceleration region using the PLUTO code. The computational domain extends from
the slow magnetosonic launching surface of the disk up to 6000^2 Schwarzschild
radii allowing to reach highly relativistic Lorentz factors. The Poynting
dominated disk wind develops into a jet with Lorentz factors of 8 and is
collimated to 1 degree. In addition to the disk jet, we evolve a thermally
driven spine jet, emanating from a hypothetical black hole corona. Solving the
linearly polarized synchrotron radiation transport within the jet, we derive
VLBI radio and (sub-) mm diagnostics such as core shift, polarization
structure, intensity maps, spectra and Faraday rotation measure (RM), directly
from the Stokes parameters. We also investigate depolarization and the
detectability of a lambda^2-law RM depending on beam resolution and observing
frequency. We find non-monotonic intrinsic RM profiles which could be detected
at a resolution of 100 Schwarzschild radii. In our collimating jet geometry,
the strict bi-modality in polarization direction (as predicted by Pariev et
al.) can be circumvented. Due to relativistic aberration, asymmetries in the
polarization vectors across the jet can hint to the spin direction of the
central engine.Comment: Submitted to Ap
MHD simulations of three-dimensional Resistive Reconnection in a cylindrical plasma column
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 (where is the Lundquist number) and
yields times 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 ("fast reconnection" regime)
due to the presence of the secondary tearing instability that takes place for
. 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 .Comment: 11 pages, 11 figures, accepted for publication in MNRA
Scalable explicit implementation of anisotropic diffusion with Runge-Kutta-Legendre super-time-stepping
An important ingredient in numerical modelling of high temperature magnetised
astrophysical plasmas is the anisotropic transport of heat along magnetic field
lines from higher to lower temperatures.Magnetohydrodynamics (MHD) 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 () for
stability. Although an implicit scheme relaxes the constraint on stability, it
is difficult to distribute efficiently on a parallel architecture. 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 et. al. 2012) that is robust, fast and
accurate in treating parabolic terms alongside the hyperbolic conversation
laws. We demonstrate its superiority over the first order super time stepping
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 processors.Comment: 15 pages, 9 figures, incorporated comments from the referee. This
version is now accepted for publication in MNRA
A Particle Module for the PLUTO code: II - Hybrid Framework for Modeling Non-thermal emission from Relativistic Magnetized flows
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 \textit{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. 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 \textit{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.Comment: 23 pages, 14 figures, Accepted for publication in The Astrophysical
Journa
Theory of Disks and Outflows around Massive Young Stellar Objects
The inner most regions around massive young stellar objects (YSO) are associated with complex interactions between numerous physical processes. Since the inner few Astronomical Units (AU) are tough to resolve observationally, a theoretical approach is important to create a qualitative picture for these regions around young high-mass stars. This thesis investigates the interplay between important physical processes with respect to the dynamics of jets and inner accretion disks. This thesis provides a bridge between the physical structures of the inner and the outer disk, where the later is observationally easier to access. Above all, the importance of the radiative force in altering the dynamics of a magnetically launched jet is outlined in this thesis. A thin accretion disk model with proper gas and dust opacities is applied for a luminous young high-mass star. This study has furnished estimates of various physical quantities in the inner few AU of the accretion disk. In particular, I have found that the mid-plane temperature around 0.1 AU could be as high as 10^5 K for a 10 Msun star. Such high temperatures in the disk destroy most of the dust grains already at large radii from the central star. This in turn reduces the opacity of the accreted matter thereby overcoming the central radiation-pressure from the young massive star. In addition, such disks are stable to gravitational fragmentation inside of 100 AU from the central star. Thus they form an ideal launching base for long-lasting outflows. Outflows and jets are an ubiquitous phenomenon in young massive star forming regions. Obser- vational surveys have suggested that the outflows become wider as the star grows in luminosity (thus mass) with time. I have performed magneto-hydrodynamical simulations of jet launching in presence of radiative forces from the luminous star and the inner hot accretion disk. The major outcome of this work, is that the radiative force from the central star plays a dominating role in accelerating and de-collimating the magnetically launched jet, while the influence of the disk ra- diative force is rather small. In addition, conducting an extensive parameter study, I have found that the outflows become wider as the mass (or luminosity) of the central star increases. The degree of collimation is also affected by the magnetic field strength and optical thickness of the line. This interplay of radiative and magnetic forces provides a physical insight to the trend in degree of collimation suggested by observations. Finally, a fully three-dimensional simulation is conducted to understand the manner in which the inner accretion disk transports material onto the central massive star. The hydrodynamic flow in the disk is simulated in the presence of radiative transfer and/or self-gravity. The transport of angular momentum is solely due to gravitational torques. My first results indicate that a locally isothermal disk becomes gravitationally instable and fragments in the inner parts as it is fed with matter from the outer massive core with a steady accretion of 10^-3 Msun yr^-1. About 10% of the mass added onto the disk is accreted onto the central star in form of clumps. On the other hand, no fragmentation is seen in an adiabatic disk whose initial temperature profile is consistently derived from radiative transfer calculations. This investigation complements the above semi-analytical study of the inner disk to single out the physics of angular momentum transport in massive accretion disks
Acceleration of cosmic rays in presence of magnetohydrodynamic fluctuations at small scales
This work investigates the evolution of the distribution of charged particles
(cosmic rays) due to the mechanism of stochastic turbulent acceleration (STA)
in presence of small-scale turbulence with a mean magnetic field. STA is
usually modelled as a biased random walk process in the momentum space of the
non-thermal particles. This results in an advection-diffusion type transport
equation for the non-thermal particle distribution function. Under quasilinear
approximation, and by assuming a turbulent spectrum with single scale injection
at sub-gyroscale, we find that the Fokker-Planck diffusion coefficients
and scale with the Lorentz factor as:
and .
Furthermore, with the calculated transport coefficients, we numerically solve
the advection-diffusion type transport equation for the non-thermal particles.
We demonstrate the interplay of various mircophysical processes such as STA,
synchrotron loss and particle escape on the particle distribution by
systematically varying the parameters of the problem.Comment: 15 pages, 8 figures, Submitted, Comments are welcom
Global-MHD Simulations using MagPIE : Impact of Flux Transfer Events on the Ionosphere
This study presents a recently developed two-way coupled
magnetosphere-ionosphere model named MagPIE that enables the investigation of
the impact of flux transfer events (FTEs) on the ionosphere. Our findings
highlight the prominent role of cusp-FTE reconnection in influencing the
ionosphere. The typical morphology of an FTE signal, represented by
field-aligned currents (FACs) on the ionosphere, is shown to exhibit a distinct
pattern characterized by an I-shaped patch surrounded by a U-shaped patch.
Furthermore, we demonstrate that the effects of FACs resulting from FTEs may
extend well into the region of closed field lines on the ionosphere. These FACs
are seen to exhibit a remarkable resemblance to discrete dayside auroral arcs,
providing further evidence that FTEs can be considered as a probable cause of
such phenomena. Additionally, FTEs generate vortex-like patterns of ionospheric
flow, which can manifest as either twin vortices or a combination of multiple
vortices, depending on the characteristics of the FACs producing them.
Furthermore, we present compelling evidence of morphological similarity between
the simulated ionospheric signatures obtained from the MagPIE model and an
observation made by the SWARM satellites. The agreement between our model and
observational data further strengthens the credibility of our model and opens
up new avenues to theoretically explore the complex ionospheric effects caused
by FTEs.Comment: 37 pages, 9 manuscript figures, 2 manuscript tables, 1 appendix, 4
appendix figures. Accepted for publication in JGR: Space Physic
SWASTi-CME: A physics-based model to study CME evolution and its interaction with Solar Wind
Coronal mass ejections (CMEs) are primary drivers of space weather and
studying their evolution in the inner heliosphere is vital to prepare for a
timely response. Solar wind streams, acting as background, influence their
propagation in the heliosphere and associated geomagnetic storm activity. This
study introduces SWASTi-CME, a newly developed MHD-based CME model integrated
into the Space Weather Adaptive SimulaTion (SWASTi) framework. It incorporates
a non-magnetized elliptic cone and a magnetized flux rope CME model. To
validate the model's performance with in-situ observation at L1, two Carrington
rotations were chosen: one during solar maxima with multiple CMEs, and one
during solar minima with a single CME. The study also presents a quantitative
analysis of CME-solar wind interaction using this model. To account for ambient
solar wind effects, two scenarios of different complexity in solar wind
conditions were established. The results indicate that ambient conditions can
significantly impact some of the CME properties in the inner heliosphere. We
found that the drag force on the CME front exhibits a variable nature,
resulting in asymmetric deformation of the CME leading edge. Additionally, the
study reveals that the impact on the distribution of CME internal pressure
primarily occurs during the initial stage, while the CME density distribution
is affected throughout its propagation. Moreover, regardless of the ambient
conditions, it was observed that after a certain propagation time (t), the CME
volume follows a non-fractal power-law expansion () due
to the attainment of a balanced state with ambient.Comment: Accepted for publication in ApJ