Test particles in relativistic resistive magnetohydrodynamics

The Black Hole Accretion Code (BHAC) has recently been extended with the ability to evolve charged test particles according to the Lorentz force within resistive relativistic magnetohydrodynamics simulations. We apply this method to evolve particles in a reconnecting current sheet that forms due to the coalescence of two magnetic flux tubes in 2D Minkowski spacetime. This is the first analysis of charged test particle evolution in resistive relativistic magnetohydrodynamics simulations. The energy distributions of an ensemble of 100.000 electrons are analyzed, as well as the acceleration of particles in the plasmoids that form in the reconnection layer. The effect of the Lundquist number, magnetization, and plasma-$\beta$ on the particle energy distribution is explored for a range of astrophysically relevant parameters. We find that electrons accelerate to non-thermal energies in the thin current sheets in all cases. We find two separate acceleration regimes: An exponential increase of the Lorentz factor during the island coalescence where the acceleration depends linearly on the resistivity and a nonlinear phase with high variability. These results are relevant for determining energy distributions and acceleration sites obtaining radiation maps in large-scale magnetohydrodynamics simulations of black hole accretion disks and jets.Comment: Matching accepted version in J. Phys.: Conf. Ser. Astronum 2018 Proceeding

Particle orbits at the magnetopause: Kelvin-Helmholtz induced trapping

The Kelvin-Helmholtz instability (KHI) is a known mechanism for penetration of solar wind matter into the magnetosphere. Using three-dimensional, resistive magnetohydrodynamic simulations, the double mid-latitude reconnection (DMLR) process was shown to efficiently exchange solar wind matter into the magnetosphere, through mixing and reconnection. Here, we compute test particle orbits through DMLR configurations. In the instantaneous electromagnetic fields, charged particle trajectories are integrated using the guiding centre approximation. The mechanisms involved in the electron particle orbits and their kinetic energy evolutions are studied in detail, to identify specific signatures of the DMLR through particle characteristics. The charged particle orbits are influenced mainly by magnetic curvature drifts. We identify complex, temporarily trapped, trajectories where the combined electric field and (reconnected) magnetic field variations realize local cavities where particles gain energy before escaping. By comparing the orbits in strongly deformed fields due to the KHI development, with the textbook mirror-drift orbits resulting from our initial configuration, we identify effects due to current sheets formed in the DMLR process. We do this in various representative stages during the DMLR development.Comment: Matching accepted version in AGU JGR: Space Physic

Relativistic resistive magnetohydrodynamic reconnection and plasmoid formation in merging flux tubes

We apply the general relativistic resistive magnetohydrodynamics code {\tt BHAC} to perform a 2D study of the formation and evolution of a reconnection layer in between two merging magnetic flux tubes in Minkowski spacetime. Small-scale effects in the regime of low resistivity most relevant for dilute astrophysical plasmas are resolved with very high accuracy due to the extreme resolutions obtained with adaptive mesh refinement. Numerical convergence in the highly nonlinear plasmoid-dominated regime is confirmed for a sweep of resolutions. We employ both uniform resistivity and non-uniform resistivity based on the local, instantaneous current density. For uniform resistivity we find Sweet-Parker reconnection, from $\eta = 10^{-2}$ down to $\eta = 10^{-4}$, for a reference case of magnetisation $\sigma = 3.33$ and plasma-$\beta = 0.1$. {For uniform resistivity $\eta=5\times10^{-5}$ the tearing mode is recovered, resulting in the formation of secondary plasmoids. The plasmoid instability enhances the reconnection rate to $v_{\rm rec} \sim 0.03c$ compared to $v_{\rm rec} \sim 0.01c$ for $\eta=10^{-4}$.} For non-uniform resistivity with a base level $\eta_0 = 10^{-4}$ and an enhanced current-dependent resistivity in the current sheet, we find an increased reconnection rate of $v_{\rm rec} \sim 0.1c$. The influence of the magnetisation $\sigma$ and the plasma-$\beta$ is analysed for cases with uniform resistivity $\eta=5\times10^{-5}$ and $\eta=10^{-4}$ in a range $0.5 \leq \sigma \leq 10$ and $0.01 \leq \beta \leq 1$ in regimes that are applicable for black hole accretion disks and jets. The plasmoid instability is triggered for Lundquist numbers larger than a critical value of $S_{\rm c} \approx 8000$.Comment: Matching accepted version in MNRA

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)

Radiative reconnection-powered TeV flares from the black hole magnetosphere in M87

Active Galactic Nuclei in general, and the supermassive black hole in M87 in particular, show bright and rapid gamma-ray flares up to energies of 100 GeV and above. For M87, the flares show multiwavelength components, and the variability timescale is comparable to the dynamical time of the event horizon, suggesting that the emission may come from a compact region nearby the nucleus. However, the emission mechanism for these flares is not well understood. Recent high-resolution general relativistic magnetohydrodynamics simulations show the occurrence of episodic magnetic reconnection events that can power flares nearby the black hole event horizon. In this work we analyze the radiative properties of the reconnecting current layer under the extreme plasma conditions applicable to the black hole in M87 from first principles. We show that abundant pair production is expected in the vicinity of the reconnection layer, to the extent that the produced secondary pair-plasma dominates the reconnection dynamics. Using analytic estimates backed by two-dimensional particle-in-cell simulations we demonstrate that even in the presence of strong synchrotron cooling, reconnection can still produce a hard power-law distribution of pair plasma imprinted in the outgoing synchrotron (up to few tens of MeV) and the inverse-Compton signal (up to TeV). We produce synthetic radiation spectra from our simulations, which can be directly compared with the results of future multiwavelength observations of M87* flares.Comment: 16 pages, 5 figures, 1 tabl

Particle acceleration by magnetic Rayleigh-Taylor instability: mechanism for flares in black-hole accretion flows

We study the magnetic Rayleigh-Taylor instability in relativistic collisionless plasma, as an astrophysical process for nonthermal particle acceleration. We consider dense plasma on top of a highly magnetized cavity with sheared magnetic field. Using particle-in-cell simulations, we show that small plumes grow and merge progressively to form a large-scale plume, which broadens to drive rapid magnetic reconnection in the cavity. We find that this leads to efficient particle acceleration capable of explaining flares from the inner accretion flow onto the black hole Sgr A*.Comment: 10 pages, 7 figures; accepted for publication in Physical Review Researc

Plasmoid Instability in the Multiphase Interstellar Medium

The processes controlling the complex clump structure, phase distribution, and magnetic field geometry that develops across a broad range of scales in the turbulent interstellar medium remains unclear. Using unprecedentedly high resolution three-dimensional magnetohydrodynamic simulations of thermally unstable turbulent systems, we show that large current sheets unstable to plasmoid-mediated reconnection form regularly throughout the volume. The plasmoids form in three distinct environments: (i) within cold clumps, (ii) at the asymmetric interface of the cold and warm phases, and (iii) within the warm, volume-filling phase. We then show that the complex magneto-thermal phase structure is characterized by a predominantly highly magnetized cold phase, but that regions of high magnetic curvature, which are the sites of reconnection, span a broad range in temperature. Furthermore, we show that thermal instabilities change the scale dependent anisotropy of the turbulent magnetic field, reducing the increase in eddy elongation at smaller scales. Finally, we show that most of the mass is contained in one contiguous cold structure surrounded by smaller clumps that follow a scale free mass distribution. These clumps tend to be highly elongated and exhibit a size versus internal velocity relation consistent with supersonic turbulence, and a relative clump distance-velocity scaling consistent with subsonic motion. We discuss the striking similarity of cold plasmoids to observed tiny scale atomic and ionized structures and HI fibers, and consider how the prevalence of plasmoids will modify the motion of charged particles thereby impacting cosmic ray transport and thermal conduction in the ISM and other similar systems.Comment: 19 pages, 10 figures. For associated movies, see https://dfielding14.github.io/movies