211 research outputs found
Generalized, energy-conserving numerical simulations of particles in general relativity. II. Test particles in electromagnetic fields and GRMHD
Direct observations of compact objects, in the form of radiation spectra,
gravitational waves from VIRGO/LIGO, and forthcoming direct imaging, are
currently one of the primary source of information on the physics of plasmas in
extreme astrophysical environments. The modeling of such physical phenomena
requires numerical methods that allow for the simulation of microscopic plasma
dynamics in presence of both strong gravity and electromagnetic fields. In
Bacchini et al. (2018) we presented a detailed study on numerical techniques
for the integration of free geodesic motion. Here we extend the study by
introducing electromagnetic forces in the simulation of charged particles in
curved spacetimes. We extend the Hamiltonian energy-conserving method presented
in Bacchini et al. (2018) to include the Lorentz force and we test its
performance compared to that of standard explicit Runge-Kutta and implicit
midpoint rule schemes against analytic solutions. Then, we show the application
of the numerical schemes to the integration of test particle trajectories in
general relativistic magnetohydrodynamic (GRMHD) simulations, by modifying the
algorithms to handle grid-based electromagnetic fields. We test this approach
by simulating ensembles of charged particles in a static GRMHD configuration
obtained with the Black Hole Accretion Code (BHAC)
Magnetohydrodynamic-Particle-in-Cell Method for Coupling Cosmic Rays with a Thermal Plasma: Application to Non-relativistic Shocks
We formulate a magnetohydrodynamic-particle-in-cell (MHD-PIC) method for
describing the interaction between collisionless cosmic ray (CR) particles and
a thermal plasma. The thermal plasma is treated as a fluid, obeying equations
of ideal MHD, while CRs are treated as relativistic Lagrangian particles
subject to the Lorentz force. Backreaction from CRs to the gas is included in
the form of momentum and energy feedback. In addition, we include the
electromagnetic feedback due to CR-induced Hall effect that becomes important
when the electron-ion drift velocity of the background plasma induced by CRs
approaches the Alfv\'en velocity. Our method is applicable on scales much
larger than the ion inertial length, bypassing the microscopic scales that must
be resolved in conventional PIC methods, while retaining the full kinetic
nature of the CRs. We have implemented and tested this method in the Athena MHD
code, where the overall scheme is second-order accurate and fully conservative.
As a first application, we describe a numerical experiment to study particle
acceleration in non-relativistic shocks. Using a simplified prescription for
ion injection, we reproduce the shock structure and the CR energy spectra
obtained with more self-consistent hybrid-PIC simulations, but at substantially
reduced computational cost. We also show that the CR-induced Hall effect
reduces the growth rate of the Bell instability and affects the gas dynamics in
the vicinity of the shock front. As a step forward, we are able to capture the
transition of particle acceleration from non relativistic to relativistic
regimes, with momentum spectrum connecting smoothly through
the transition, as expected from the theory of Fermi acceleration.Comment: 24 pages, 15 figures, accepted for publication in Ap
Leapfrog methods for relativistic charged-particle dynamics
A basic leapfrog integrator and its energy-preserving and variational /
symplectic variants are proposed and studied for the numerical integration of
the equations of motion of relativistic charged particles in an electromagnetic
field. The methods are based on a four-dimensional formulation of the equations
of motion. Structure-preserving properties of the numerical methods are
analysed, in particular conservation and long-time near-conservation of energy
and mass shell as well as preservation of volume in phase space. In the
non-relativistic limit, the considered methods reduce to the Boris algorithm
for non-relativistic charged-particle dynamics and its energy-preserving and
variational / symplectic variants.Comment: 18 pages, 3 figure
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