Simulations Of Astrophysical Jets Research Articles

Resistive relativistic MHD simulations of astrophysical jets

Aims. The main goal of the present paper is to provide the first systematic numerical study of the propagation of astrophysical relativistic jets, in the context of high-resolution, shock-capturing Resistive Relativistic MagnetoHydroDynamic (RRMHD) simulations. We aim to investigate different values and models for the plasma resistivity coefficient, and to assess their impact on the level of turbulence, the formation of current sheets and reconnection plasmoids, the electromagnetic energy content, and the dissipated power. Methods. We used the PLUTO code for simulations and we assumed an axisymmetric setup for the jets, endowed with both poloidal and toroidal magnetic fields, and propagating in a uniform magnetized medium. The gas was assumed to be characterized by a realistic, Synge-like equation of state (the Taub equation), appropriate for such astrophysical jets. The Taub equation was combined here for the first time with the implicit-explicit Runge-Kutta time-stepping procedure, as required in RRMHD simulations. Results. The main result is that turbulence is clearly suppressed for the highest values of resistivity (low Lundquist numbers), current sheets are broader, and plasmoids are barely present, while for low values of resistivity the results are very similar to ideal runs, in which dissipation is purely numerical. We find that recipes employing a variable resistivity based on the advection of a jet tracer or on the assumption of a uniform Lundquist number improve on the use of a constant coefficient and are probably more realistic possible sites for the acceleration of the nonthermal particles that produce the observed high-energy emission, preserving as they do the development of turbulence and of sharp current sheets.

Internal Structure of the Jets from Young Stars Simulated at Plasma Focus Facilities

The laboratory simulations of jets from young stars that have been carried out for many years at plasma focus facilities allow the internal structure of the active regions emerging during the interaction of the jet with the surrounding plasma to be studied in detail. We have found a new wide class of solutions for the equations of ideal magnetohydrodynamics describing the closed axisymmetric stationary flows that are apparently realized in the active regions. Such flows are shown to well reproduce the internal structure of the plasma structures observed in laboratory simulations of astrophysical jets.

Numerical Simulations of Magnetized Astrophysical Jets and Comparison with Laboratory Laser Experiments

The results of MHD numerical simulations of the formation and development of magnetized jets are presented. Similarity criteria for comparisons of the results of laboratory laser experiments and numerical simulations of astrophysical jets are discussed. The results of laboratory simulations of jets generated in experiments at the Neodim laser installation are presented.

Laboratory simulations of astrophysical jets: results from experiments at the PF-3, PF-1000U, and KPF-4 facilities

Results are presented from laboratory simulations of plasma jets emitted by young stellar objects carried out at the plasma focus facilities. The experiments were performed at three facilities: the PF-3, PF-1000U and KPF-4. The operation modes were realized enabling the formation of narrow plasma jets which can propagate over long distances. The main parameters of plasma jets and background plasma were determined. In order to control the ratio of a jet density to that of background plasma, some special operation modes with pulsed injection of the working gas were used.

Localization of the magnetic field in a plasma flow in laboratory simulations of astrophysical jets at the KPF-4-PHOENIX installation

The results of experiments aimed at investigating axial plasma flows forming during the compression of a current–plasma sheath are presented. These experiments were carried out at the KPF-4-PHOENIX plasma-focus installation, as part of a program of laboratory simulations of astrophysical jets. The plasma flows were generated in a discharge when the chamber was filled with the working gas (argon) at initial pressures of 0.5–2 Torr. Experimental data obtained using a magnetic probe and optical diagnostics are compared. The data obtained can be used to determine the location of trapped magnetic field relative to regions of intense optical glow in the plasma flow.

Properties of the distribution of azimuthal magnetic field in a plasma flow during laboratory simulations of astrophysical jets in a plasma-focus installation

The results of laboratory simulations of astrophysical jets are presented. Plasma flows generated in the PF-3 plasma-focus installation of the NRC “Kurchatov Institute” and propagating to distances substantially exceeding their transverse dimensions are studied. It is shown usingmagnetic probes that the plasma flow propagates with a frozen-in magnetic field. The resulting radial distribution of the azimuthal magnetic field corresponds well to the distribution created by a longitudinal current of ~10 kA flowing in a region with a radius of 1–2 cm near the axis. Structures associated with return currents are observed at the periphery of the flow. The magnetic field decays rapidly as the flow propagates along the axis. Nevertheless, the leading lobe of the plasma flow is preserved to substantial distances in a neon discharge, possibly due to radiative cooling of the plasma.

Laboratory simulations of astrophysical jets

AbstractCollimated outflows (jets) are ubiquitous in the universe, appearing around sources as diverse as protostars and extragalactic supermassive black holes. Jets are thought to be magnetically collimated, and launched from a magnetized accretion disk surrounding a compact gravitating object. We have developed the first laboratory experiment to address time-dependent, episodic phenomena relevant to the poorly understood jet acceleration and collimation region (Ciardi et al., 2009). The experiments were performed on the MAGPIE pulsed power facility (1.5 MA, 250 ns) at Imperial College. The experimental results show the periodic ejections of magnetic bubbles naturally evolving into a heterogeneous jet propagating inside a channel made of self-collimated magnetic cavities. The results provide a unique view of the possible transition from a relatively steady-state jet launching to the observed highly structured outflows.

Long-Time Simulations of Astrophysical Jets: Energy Structure and Quasi-Periodic Ejection

Abstract We have performed self-consistent 2.5-dimensional nonsteady MHD numerical simulations of jet formation as long as possible, including the dynamics of accretion disks. Previous simulations showed that, in the case where the calculation time of the simulations is very short as compared with the time scale of observed jets, there is no significant difference between the characteristics of the nonsteady and steady MHD simulations. Thus, we have investigated long-time evolutions of the mass-accretion rate, mass-outflow rate, jet velocity, and various energy fluxes. We found that the ejection of a jet is quasi-periodic. The period of the ejection, $T_{\rm ejection}$, is related to the time needed for the initial magnetic field, $B_0$, to be twisted to generate a toroidal filed, $T_{\rm ejection} \propto {V_{\rm A}}^{-1} \propto {B_0}^{-1} \propto {E_{\rm mg}}^{-1/2}$, where $V_{\rm A}$ is the Alfvén velocity and $E_{\rm mg}$ the initial magnetic energy. We compared our results with both the steady-state theory and the previous 2.5-dimensional nonsteady MHD simulations. We found that the time-averaged velocity of the jet, $V_{{\rm jet}, \mathrm{avg}}$, is $\sim 0.1V_{\mathrm{K}}$ and $\sim 0.1V_{{\rm jet}, \mathrm{max}}$, where $V_{\mathrm{K}}$ is the Keplerian velocity at $(r,{z})=(1,0)$ and $V_{{\rm jet},\mathrm{max}}$ the maximum velocity of the jet. Nevertheless, the characteristics of our simulations are consistent with those of the steady solution and previous short-time simulations. We found that the dependences of the time-averaged velocity and the mass-outflow rate, $\dot{M}_{{\rm w},\mathrm{avg}}$, on the initial magnetic field are approximately $V_{{\rm jet},\mathrm{avg}} \propto {B_0}^{0.3}$ and $\dot{M}_{{\rm w},\mathrm{avg}} \propto {B_0}^{0.32}$, respectively.

Relativistic Hydrodynamic Flows Using Spatial and Temporal Adaptive Structured Mesh Refinement

Astrophysical relativistic flow problems require high-resolution three-dimensional (3D) numerical simulations. In this paper, we describe a new parallel 3D code for simulations of special relativistic hydrodynamics (SRHD) using both spatially and temporally structured adaptive mesh refinement (AMR). We used the method of lines to discretize the SRHD equations spatially and a total variation diminishing (TVD) Runge-Kutta scheme for time integration. For spatial reconstruction, we have implemented piecewise linear method (PLM), piecewise parabolic method (PPM), third-order convex essentially nonoscillatory (CENO) and third- and fifth-order weighted essentially nonoscillatory (WENO) schemes. Flux is computed using either direct flux reconstruction or approximate Riemann solvers including HLL, modified Marquina flux, local Lax-Friedrichs flux formulas, and HLLC. The AMR part of the code is built on top of the cosmological Eulerian AMR code enzo. We discuss the coupling of the AMR framework with the relativistic solvers. Via various test problems, we emphasize the importance of resolution studies in relativistic flow simulations because extremely high resolution is required especially when shear flows are present in the problem. We also present the results of two 3D simulations of astrophysical jets: AGN jets and GRB jets. Resolution study of those two cases further highlights the need of high resolutions to calculate accurately relativistic flow problems.

Dynamic and Stagnating Plasma Flow Leading to Magnetic-Flux-Tube Collimation

Highly collimated, plasma-filled magnetic-flux tubes are frequently observed on galactic, stellar, and laboratory scales. We propose that a single, universal magnetohydrodynamic pumping process explains why such collimated, plasma-filled magnetic-flux tubes are ubiquitous. Experimental evidence from carefully diagnosed laboratory simulations of astrophysical jets confirms this assertion and is reported here. The magnetohydrodynamic process pumps plasma into a magnetic-flux tube and the stagnation of the resulting flow causes this flux tube to become collimated.

Long Term Simulations of Astrophysical Jets

We have performed self-consistent 2.5-dimensional nonsteady MHD numerical simulations of jet formation, including the dynamics of accretion disks. Although the previous nonsteady MHD simulations for astrophysical jets revealed that the characteristics of nonsteady jets are very similar to those of steady jets, the calculation time of these simulations is very short compared with the time scale of observed jets. Thus we have investigated long term evolutions of the mass accretion rate (M a ), mass outflow rate (M w ), and energy of the toroidal magnetic field (E m g t ). We found that the jet is ejected intermittently with a period of around 2π which is similar to the growth time of the magnetorotational instability (MRI).

Numerical Simulations of Astrophysical Jets from Keplerian Disks with Periodic Ejection

We present 2.5-dimensional time-dependent simulations of nonrelativistic and nonradiative outflows from sinusoidally perturbed Keplerian accretion disks. A sinusoidal perturbation is introduced in the velocity of the gas ejected from the surface of the disk into a cold corona. In the simulations, the disk is a fixed boundary from which the gas is ejected with a pulsed velocity. The maximum value of this velocity is taken to be a thousandth of the local Keplerian disk velocity. It was found that for large periods, the structures in the jet tend to fragment into smaller substructures. For small values of the period, the structures tend to dissipate, while for medium values of the period, they tend to persist.

Numerical simulations of astrophysical jets from randomly perturbed Keplerian disks

We study the outflows from randomly perturbed Keplerian accretion disks. In the velocity field of the gas at the surface of the disk, we introduce a random perturbation proportional to ra, where r is the radial distance and a is a fixed parameter in the simulation (with values of , -1, , 0, or 1). In the simulations, the disk is a fixed boundary from which the gas is ejected with continuous and random velocities added together. The continuous ejection velocity is taken to be a thousandth of that of the local Keplerian disk; the maximum value of the random velocity is 10 times the continuous velocity. We observe the formation of periodic structures along the outflow axis. The distance of the axial separation between the structures is found to be about 11 times the innermost radius of the accretion disk. For a negative radial slope a, the structures are well defined and situated near the jet axis, whereas for a positive slope, decollimation of the jet occurs.

Initial magnetic field configurations for 3-dimensional simulations of astrophysical jets

We solve, and provide analytical expressions, for current-free magnetic configurations in the context of initial setups of 3-dimensional simulations of astrophysical jets involving an accretion disk corona in hydrostatic balance around a central object. These configurations which thread through the accretion disk and its corona preserve the initial hydrostatic state. This work sets stage for future 3-dimensional jet simulations (including disk rotation and mass-load) where launching, acceleration and collimation mechanisms can be investigated.

Numerical simulations of astrophysical jets from Keplerian discs -- III. The effects of mass loading

We present 2.5D time-dependent simulations of the non-linear evolution of non-relativistic outflows from the surface of Keplerian accretion discs. The gas is accelerated from the surface of the disc (which is a fixed platform in these simulations) into a cold corona in stable hydrostatic equilibrium. We explore the dependence of the resulting jet characteristics upon the mass loading of the winds. Two initial configurations of the threading disc magnetic field are studied: a potential field and a uniform vertical field configuration. We show that the nature of the resulting highly collimated, jet-like outflows (steady or episodic) is determined by the mass load of the disc wind. The mass load controls the interplay between the collimating effects of the toroidal field and the kinetic energy density in the outflow. In this regard, we demonstrate that the onset of episodic behaviour of jets appears to be determined by the quantity which compares the speed for a toroidal Alfven wave to cross the diameter of the jet, with the flow speed vp along the jet. This quantity decreases with increasing load. For sufficiently large N (small mass loads), disturbances appear to grow leading to instabilities and shocks. Knots are then generated and the outflow becomes episodic. These effects are qualitatively independent of the initial magnetic configuration that we employed and are probably generic to a wide variety of magnetized accretion disc models.

Instabilities in Three‐dimensional Simulations of Astrophysical Jets Crossing Tilted Interfaces

Three-dimensional numerical simulations of light supersonic hydrodynamic jets have been performed to quantify the crucial roles of the interstellar medium (ISM) and intracluster medium (ICM) in defining the gross morphologies of powerful radio galaxies. Such a jet emerges through a power-law atmosphere (ISM) of its host galaxy and then crosses into a hotter, but less dense, ICM. Our eight medium-resolution simulations are followed to lengths of 45 initial jet radii. Simulations with different jet velocities, jet densities, extensions of the ISM (along the jet's axis), and inclination angles of the ISM/ICM interface are compared. The shear layer between the jet and the shock-processed gas is affected by nonlinear hydrodynamical instabilities. Complex patterns of asymmetric vortex rings and superposed streamwise vortex tubes arise in the cocoon, while internal shocks form in the jet. The low-Mach number jets have higher growth rates of instabilities, in comparison with higher Mach number jets, and the Mach disks or working surfaces at the heads of the jets break down. The growth of Rayleigh-Taylor instabilities along the contact discontinuity between the shocked ambient plasma and the shocked jet's plasma appears to trigger the entrainment of heavier external gas into the jet. Greater tilts of the interface induce more wiggling and ribboning of the jets, but none of these simulations evinces substantial jet bending.

Numerical Simulations of Astrophysical Jets from Keplerian Disks. I. Stationary Models

We present 2.5-dimensional time-dependent simulations of the evolution of nonrelativistic outflows from Keplerian accretion disks orbiting low-mass protostars or black holes accreting at sub-Eddington rates. The gas is injected at a very small speed (vinj = 10-3vK) from the surface of the disk (a fixed boundary in our simulations) into a cold corona. The corona is in stable equilibrium and is supported by Alfvénic turbulent pressure. The initial magnetic field configuration in the corona is poloidal and is given by a potential field (J = 0). This configuration is extended smoothly into the disk where the toroidal magnetic field is taken to scale inversely with the disk radius. We present the analytical and the numerical approaches to our problem, as well as many results for a steady state simulation. We find that the gas is centrifugally accelerated through the Alfvén and the fast magnetosonic (FM) surfaces and collimated into cylinders parallel to the disk's axis. The collimation of the outflow is due to the pinch force exerted by the dominant toroidal magnetic field generated by the outflow itself. Beyond the FM surface, we found that a "Hubble" flow is present; vz ∝ z. The velocities achieved in our simulations are of the order of 180 km s-1 for our standard young stellar object (a 0.5 M☉ protostar) and of the order or 105 km s-1 for our standard active galactic nuclei (a 108 M☉ black hole). Our jet solutions, dominated mainly by the poloidal kinetic energy ρv2p, are very efficient in magnetically extracting angular momentum and energy from the disk. We find the ratio of the disk accretion rate to the wind mass flux rate to be of the order of a/w ≃ 6.0. We find that our stationary outflows have many similarities to steady state models of MHD disk winds.

Magnetically Driven Jets from Accretion Disks: Comparison Between 2.5D Nonsteady Simulations and 1.5D Nonsteady/Steady Solutions

We have performed 1D(1.5D) and 2D(2.5D) nonsteady MHD numerical simulations of astrophysical jets which are magnetically driven from Keplerian disks, in order to clarify the origin and structure of jets ejected from protostars and active galactic nuclei. The initial and boundary conditions are similar to those of 2D(2.5D) nonsteady MHD simulations of Shibata and Uchida (1986) and Matsumoto et al. (1996); there is initially a Keplerian disk with a nonrotating corona outside, both of which are penetrated by vertical magnetic fields. The subsequent interaction between the disk/corona and the vertical fields are studied as an initial value problem. Against the current belief that this kind of simulations show simply a transient jet caused by nonsteady interaction between the disk/corona and the magnetic field, we have found that the jets ejected from the disk in this way have the same properties of the steady magnetically driven jets that were investigated by using ID steady wind solution (Kudoh & Shibata 1995), even if the jets are not exactly in steady state.

Contour Simulations of Astrophysical Jets

We apply the theory of the Kelvin-Helmholtz instabilities to trace the jet's pattern. A certain level of noise is introduced.