Unmagnetized Medium Research Articles

Runko: Modern multiphysics toolbox for plasma simulations

runko is a new open-source plasma simulation framework implemented in C++ and Python. It is designed to function as an easy-to-extend general toolbox for simulating astrophysical plasmas with different theoretical and numerical models. Computationally intensive low-level kernels are written in modern C++ taking advantage of polymorphic classes, multiple inheritance, and template metaprogramming. High-level functionality is operated with Python scripts. The hybrid program design ensures good code performance together with ease of use. The framework has a modular object-oriented design that allows the user to easily add new numerical algorithms to the system. The code can be run on various computing platforms ranging from laptops (shared-memory systems) to massively parallel supercomputer architectures (distributed-memory systems). The framework supports heterogeneous multiphysics simulations in which different physical solvers can be combined and run simultaneously. Here, we showcase the framework’s relativistic particle-in-cell (PIC) module by presenting (i) 1D simulations of relativistic Weibel instability, (ii) 2D simulations of relativistic kinetic turbulence in a suddenly stirred magnetically-dominated pair plasma, and (iii) 3D simulations of collisionless shocks in an unmagnetized medium.

Two-jet structure of the flow produced by magnetized hypersonic spherical source into the steady unmagnetized medium

In the present paper we consider the steady flow produced by the hypersonic magnetized spherical source into the steady unmagnetized medium. The magnetic field of the source is considered purely azimuthal. The basic dimensionless parameter of the problem is Alfvén number at the inflow boundary at the source equatorial plane, which we denote as ε. We first review the numerical results, that was already presented in our recent paper (Golikov et al, 2017). According to our numerical results there should be a tangential discontinuity dividing the flow and the ambient medium. In the present paper we analytically derive the asymptotic behaviour of the tangential discontinuity radius as epsilon approaches zero.

MAGNETOHYDRODYNAMIC EFFECTS IN RELATIVISTIC EJECTA

We study the problem of deceleration of an arbitrarily magnetized relativistic ejecta in a static unmagnetized medium and its connection to the physics of gamma-ray bursts (GRBs). By computing exact solutions of the Riemann problem describing this scenario, we find that with the same initial Lorentz factor, the reverse shock becomes progressively weaker with increasing magnetization parameter σ (the Poynting-to-kinetic flux ratio). The reverse shock becomes a rarefaction wave when σ exceeds a critical value defined by the balance between magnetic pressure in the ejecta and thermal pressure in the forward shock. In the rarefaction wave regime, the rarefied region is accelerated to a Lorentz factor that is significantly larger than the initial value due to the strong magnetic pressure in the ejecta. We discuss the implications for models of GRBs.

Large jets from small-scale magnetic fields

We consider the conditions under which a rotating magnetic object can produce a magnetically powered outflow in an initially unmagnetized medium stratified under gravity. 3D MHD simulations are presented in which the footpoints of localized, arcade-shaped magnetic fields are put into rotation. It is shown how the effectiveness in producing a collimated magnetically powered outflow depends on the rotation rate, the strength and the geometry of the field. The flows produced by uniformly rotating, non-axisymmetric fields are found to consist mainly of buoyant plumes heated by dissipation of rotational energy. Collimated magnetically powered flows are formed if the field and the rotating surface are arranged such that a toroidal magnetic field is produced. This requires a differential rotation of the arcades' footpoints. Such jets are well-collimated; we follow their propagation through the stratified atmosphere over 100 times the source size. The magnetic field is tightly wound and its propagation is dominated by the development of non-axisymmetric instabilities. We observe a Poynting flux conversion efficiency of over 75% in the longest simulations. Applications to the collapsar model and protostellar jets are discussed.

MAGNETOHYDRODYNAMIC EFFECTS IN PROPAGATING RELATIVISTIC JETS: REVERSE SHOCK AND MAGNETIC ACCELERATION

We solve the Riemann problem for the deceleration of an arbitrarily magnetized relativistic flow injected into a static unmagnetized medium in one dimension. We find that for the same initial Lorentz factor, the reverse shock becomes progressively weaker with increasing magnetization \sigma (the Poynting-to kinetic energy flux ratio), and the shock becomes a rarefaction wave when \sigma exceeds a critical value, \sigma_c, defined by the balance between the magnetic pressure in the flow and the thermal pressure in the forward shock. In the rarefaction wave regime, we find that the rarefied region is accelerated to a Lorentz factor that is significantly larger than the initial value. This acceleration mechanism is due to the strong magnetic pressure in the flow. We discuss the implications of these results for models of gamma-ray bursts and active galactic nuclei.

Gamma-Ray Burst Phenomenology, Shock Dynamo, and the First Magnetic Fields

A relativistic collisionless shock propagating into an unmagnetized medium leaves behind a strong large-scale magnetic field. This seems to follow from two assumptions: (1) Gamma-ray burst (GRB) afterglows are explained by synchrotron emission of a relativistic shock. (2) The magnetic field cannot exist on microscopic scales only; it would decay by phase-space mixing. Assumption 1 is generally accepted because of an apparent success of the shock synchrotron phenomenological model of GRB afterglows. Assumption 2 is confirmed in this work by a low-dimensional numerical simulation. One may hypothesize that relativistic shock velocities are not essential for the magnetic field generation and that all collisionless shocks propagating into an unmagnetized medium generate strong large-scale magnetic fields. If this hypothesis is true, the first cosmical magnetic fields could have been generated in shocks of the first virialized objects.

Neutrino-electron processes in a strongly magnetized thermal plasma

We present a new method of calculating the rate of neutrino-electron interactions in a strong magnetic field based on finite temperature field theory. Using this method, in which the effect of the magnetic field on the electron states is taken into account exactly, we calculate the rates of all of the lowest order neutrino-electron interactions in a plasma. As an example of the use of this technique, we explicitly calculate the rate at which neutrinos and antineutrinos annihilate in a highly magnetized plasma, and compare that to the rate in an unmagnetized plasma. The most important channel for energy deposition is the gyromagnetic absorption of a neutrino-antineutrino pair on an electron or positron in the plasma ($\nu\bar{\nu} e^\pm\leftrightarrow e^\pm$). Our results show that the rate of annihilation increases with the magnetic field strength once it reaches a certain critical value, which is dependent on the incident neutrino energies and the ambient temperature of the plasma. It is also shown that the annihilation rates are strongly dependent on the angle between the incident particles and the direction of the magnetic field. If sufficiently strong fields exist in the regions surrounding the core of a type II supernovae or in the central engines of gamma ray bursts, these processes will lead to more efficient plasma heating mechanism than in an unmagnetized medium, and moreover, one which is intrinsically anisotropic.

The Evolution and Fragmentation of Rising Magnetic Flux Tubes

view Abstract Citations (95) References (16) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Evolution and Fragmentation of Rising Magnetic Flux Tubes Longcope, D. W. ; Fisher, G. H. ; Arendt, S. Abstract From its source at the base of the convection zone magnetic flux is believed to rise buoyantly to the solar surface in the form of isolated tubes. As it rises, the cross section of such a tube will be distorted through its interaction with the surrounding unmagnetized medium. This distortion greatly affects its rate of rise and can ultimately lead to its fragmentation into two parallel tubes. We derive a set of Boussinesq fluid equations for studying the perpendicular dynamics of a rising flux tube. Integrating these numerically shows the tube distorting and then separating into two fragments with opposing senses of fluid circulation. The same behavior was observed in numerical simulations by Schüssler (1979). These counter-rotating elements move apart horizontally from each other and eventually stop rising. A simple picture of isolated buoyant material explains this result and confirms that the rise of the flux is prevented by its fragmentation. This has important consequences for theories of magnetic flux tube emergence. Publication: The Astrophysical Journal Pub Date: June 1996 DOI: 10.1086/177387 Bibcode: 1996ApJ...464..999L Keywords: MAGNETOHYDRODYNAMICS: MHD; SUN: INTERIOR; SUN: MAGNETIC FIELDS full text sources ADS |

Symmetry and selection rules for wave‐wave interactions

Selection rules for nonlinear three‐wave interactions are derived on the basis of the symmetry of the background medium and the waves themselves. The process described bypasses the formal derivation of the coupling coefficient itself. The primary concern is with the effect of the polarization of the waves on the selection rules. Although consideration is limited to three‐wave interactions, the method is easily generalized to more waves. Results are derived for these examples: three waves with parallel propagation vectors in an unmagnetized medium, and three waves propagating parallel, and then perpendicular, to a static magnetic field.