arXiv subjects

Michael Rowan

Publications and source records attributed to Michael Rowan.

3 recordsLinked to original sources

arXiv2024

Application of mesh refinement to relativistic magnetic reconnection

During relativistic magnetic reconnection, antiparallel magnetic fields undergo a rapid change in topology, releasing a large amount of energy in the form of non-thermal particle acceleration. This work explores the application of mesh refinement to 2D reconnection simulations to efficiently model the ineherent disparity in length-scales. We have systematically investigated the effects of mesh refinement and determined necessary modifications to the algorithm required to mitigate non-physical artifacts at the coarse-fine interface. We have used the ultrahigh-order Pseudo-Spectral Analytical Time-Domain (PSATD) Maxwell solver to analyze how its use can mitigate the numerical dispersion that occurs with the finite-difference time-domain (FDTD) (or ``Yee'') method. Absorbing layers are introduced at the coarse-fine interface to eliminate spurious effects that occur with mesh refinement. We also study how damping the electromagnetic fields and current density in the absorbing layer can help prevent the non-physical accumulation of charge and current density at the coarse-fine interface. Using a mesh refinement ratio of 8 for two-dimensional magnetic reconnection simulations, we obtained good agreement with the high resolution baseline simulation, using only 36% of the macroparticles and 71% of the node-hours needed for the baseline. The methods presented here are especially applicable to 3D systems where higher memory savings are expected than in 2D, enabling comprehensive, computationally efficient 3D reconnection studies in the future.

physics.comp-ph↗

arXiv2022

The Kelvin-Helmholtz instability at the boundary of relativistic magnetized jets

We study the linear stability of a planar interface separating two fluids in relative motion, focusing on conditions appropriate for the boundaries of relativistic jets. The jet is magnetically dominated, whereas the ambient wind is gas-pressure dominated. We derive the most general form of the dispersion relation and provide an analytical approximation of its solution for an ambient sound speed much smaller than the jet Alfv\'en speed $v_A$, as appropriate for realistic systems. The stability properties are chiefly determined by the angle $\psi$ between the wavevector and the jet magnetic field. For $\psi=\pi/2$, magnetic tension plays no role, and our solution resembles the one of a gas-pressure dominated jet. Here, only sub-Alfv\'enic jets are unstable ($0<M_e\equiv(v/v_A)\cos\theta<1$, where $v$ is the shear velocity and $\theta$ the angle between the velocity and the wavevector). For $\psi=0$, the free energy in the velocity shear needs to overcome the magnetic tension, and only super-Alfv\'enic jets are unstable ($1<M_e<\sqrt{(1+\Gamma_w^2)/[1+(v_A/c)^2\Gamma_w^2]}$, with $\Gamma_w$ the wind adiabatic index). Our results have important implications for the propagation and emission of relativistic magnetized jets.

astro-ph.HE↗

arXiv2016

Revisiting the Dark Matter - Comet Shower Connection

We revisit the question of whether the observed periodicity of comet impacts on Earth is consistent with Solar oscillation about the Galactic midplane and spiral arm crossings, here in the context of dissipative dark matter models. Consider whether a hypothetical thin dark disk, a signature of these models, is necessary to give the right periodicity, and whether such a dark disk is allowed given kinematic and other observational constraints on the Galaxy's gravitational potential, taking into account recent updates of these limits based on the vertical epicyclic oscillations of the tracer populations. Our analysis contains updated parameters for the Galactic disk, a self-consistent gravitational potential under the Poisson-Jeans equations, and includes prior probabilities from local stellar kinematics and the distribution of Milky Way interstellar gas. Moreover, our analysis also includes radial oscillations and Galactic spiral arm crossings. We find a dark disk explanation for the comet periodicity to be 10 times more likely than a constant average rate model. Moreover, we find that spiral arm crossing is necessary to correctly predict the date of the Chicxulub crater dated to 66 My ago.

astro-ph.EP↗