Bulletin of the American Physical Society
61st Annual Meeting of the APS Division of Plasma Physics
Volume 64, Number 11
Monday–Friday, October 21–25, 2019; Fort Lauderdale, Florida
Session UM9: Mini-conference: Topics in Plasma Astrophysics |
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Chair: Stas Boldyrev, University of Wisconsin Room: Grand C/E |
Thursday, October 24, 2019 2:00PM - 2:20PM |
UM9.00001: Kinetic Simulations of Black-Hole Magnetospheres: Jets, Reconnection, and the Connection to Turbulence Kyle Parfrey Black holes launch powerful jets of plasma to relativistic velocities, using magnetic fields supplied by turbulent flows of accreting matter. Reconnection may be important for heating regions in or near the accretion flow to very high temperatures, producing a corona similar to that of the sun. I will describe the first general-relativistic simulations of collisionless black-hole magnetospheres, showing jet production directly tied to a reconnecting current sheet extending through the hole's event horizon. I will also show results of more idealized simulations which illustrate how the corona and jet can both be created by turbulence-generated magnetic structures. [Preview Abstract] |
Thursday, October 24, 2019 2:20PM - 2:40PM |
UM9.00002: Radiative Relativistic Collisionless Plasma Turbulence as a Frontier of Extreme Plasma Astrophysics Dmitri Uzdensky, Vladimir Zhdankin, Gregory Werner, Mitchell Begelman We are entering a new era when first-principles computational and theoretical studies of complex collective plasma processes under extreme conditions—marked by the importance of relativistic, radiation, and pair-production effects—are becoming feasible and even routine. These studies, motivated by our desire to understand plasma behaviors around exotic astrophysical objects like neutron stars and black holes, form the new field of Extreme Plasma Astrophysics. One of the most exciting frontiers in this emerging field is radiative turbulence, where turbulent energy injection is balanced by radiative cooling. We present the results of our radiative particle-in-cell simulations of driven kinetic turbulence in a relativistic pair plasma with optically thin inverse-Compton cooling. We find that radiation quenches nonthermal particle acceleration, effectively thermalizing the plasma. The high-energy particle distributions are strongly anisotropic, leading to potentially observable intermittent beaming. The anisotropy, spatial inhomogeneity, and temporal variability of the high-energy emission are more extreme at high magnetizations, when bulk fluid motions become relativistic. These findings help us understand astrophysical gamma-ray flares and advance Extreme Plasma Astrophysics. [Preview Abstract] |
Thursday, October 24, 2019 2:40PM - 3:00PM |
UM9.00003: On QED plasmas Mikhail Medvedev Quantum electrodynamics (QED) effects are interesting phenomena that occur in strong electromagnetic fields. Advances in laser technology brought lab experiments close to the regime where they become important. Furthermore, astrophysical systems such as strongly magnetized neutrons stars and magnetars possess magnetic fields close to or even stronger than the Schwinger (critical) field. Whereas some QED effects are being incorporated in plasma codes, theoretical studies of QED plasmas are lacking. Here we derive the general equation describing QED plasma modes. We discuss the properties of the low-frequency modes, for which the transitions between the Landau levels can be neglected. [Preview Abstract] |
Thursday, October 24, 2019 3:00PM - 3:20PM |
UM9.00004: Multiscale simulations of particle acceleration in astrophysical shocks Anatoly Spitkovsky Particle acceleration in astrophysical shocks is central to the production of nonthermal radiation from a large variety of astrophysical sources, ranging from supernova remnants to GRB jets. The process of acceleration is an intrinsically multi-scale problem, connecting plasma microphysics at the shock to self-generated instabilities driven by accelerated particles far from the shock. While considerable progress has been made in studying acceleration with ab-initio particle-in-cell (PIC) simulations, future studies will need to address the range of scales with more computationally efficient methods. I will discuss the efforts at studying shock acceleration with fully kinetic and ``hybrid'' simulations, that combine the salient features of PIC schemes with computational efficiency of fluid methods. These methods allow the study of long-term back-reaction of accelerated particles on the shock structure, and the effects of global shock geometry on the local acceleration physics. [Preview Abstract] |
Thursday, October 24, 2019 3:20PM - 3:40PM |
UM9.00005: General Relativistic Particle-In-Cell Simulations of Pair Producing Gaps in Black Hole Magnetospheres Yajie Yuan, Yuran Chen In some low-luminosity accreting supermassive black hole systems, the supply of plasma in the jet funnel region can be a problem. It is believed that a local region with unscreened electric field can exist in the black hole magnetosphere, accelerating particles and producing high energy gamma-rays that can create $e^{\pm}$ pairs. We carry out time-dependent, self-consistent, 1D general relativistic PIC simulations of this process, including inverse Compton scattering, photon tracking, and $\gamma\gamma$ pair production. We find a highly time-dependent solution where a macroscopic gap opens quasi-periodically to create $e^{\pm}$ pairs and high energy radiation. We discuss possible implications for sources like M87 and Sgr A*, in particular the variable high energy emission from particles accelerated in the gap. [Preview Abstract] |
Thursday, October 24, 2019 3:40PM - 4:00PM |
UM9.00006: Particle Acceleration by Current-driven Instabilities Invited Speaker: Jonathan Zrake Many objects in the universe produce light by dissipating magnetic energy. Examples include solar flares, the nebulae surrounding pulsars, AGN jets, and gamma-ray bursts. Traditionally, magnetic energy dissipation and particle acceleration in these systems has been investigated from the microscopic point of view, focusing on magnetic reconnection around small-scale magnetic null points. I will present results from a research program that focuses instead on the macroscopic scales, paying particular attention to particle acceleration by unstable magnetic flux tubes. These structures are observed directly in the solar corona, and exist in magnetically accelerated outflows, including AGN jets and pulsar wind nebulae. We have found, using fully kinetic particle-in-cell simulations, a particular class of current-driven instability that is an exceptionally efficient accelerator of non-thermal particles. The mechanism is a first-order Fermi-like process, whereby plasma particles execute curvature-drifts in the direction of a coherent, inductive (MHD-like) electric field, while diffusing in a turbulent magnetic field. This mechanism is distinct from injection process observed near reconnecting current layers, where the dominant acceleration comes from the unscreened (parallel) electric field. [Preview Abstract] |
Thursday, October 24, 2019 4:00PM - 4:15PM |
UM9.00007: Particle motion and acceleration in accretion flows Fabio Bacchini, Bart Ripperda, Oliver Porth, Lorenzo Sironi, Alexander Philippov The recent reveal of the first direct black hole observations by the Event Horizon Telescope (EHT) collaboration has opened a new window on the physics of relativistic plasmas. For the first time, it is possible to verify theoretical predictions for plasma phenomena in accretion disks and jets in the surroundings of compact objects. In such environments, particles are accelerated to extremely high energies. Current ideal magnetohydrodynamic (MHD) models can reproduce the global structure of accretion flows and the related thermal physics of plasmas. However, no information on the mechanisms behind the acceleration of particles to nonthermal energies is included in such models. This lack of information on the microphysics represents the largest uncertainty in theoretical EHT results. The problem can be overcome by employing microphysical models based on particles. The physics of particle acceleration can be studied in MHD simulations with test particle approaches. The energy flows to and from single particles can be analysed in order to obtain realistic radiation models. These studies have important consequences for the interpretation of current and future observations of black hole environments. [Preview Abstract] |
Thursday, October 24, 2019 4:15PM - 4:30PM |
UM9.00008: Kinetic simulations of electron pre-energization by magnetized collisionless shocks in expanding laboratory plasmas Kirill Lezhnin, Will Fox, Derek Schaeffer, Jack Matteucci, Amitava Bhattacharjee, Anatoly Spitkovsky, Kai Germaschewski Collisionless shocks are common features in space and astrophysical systems where supersonic plasma flows interact. Recently experimental capabilities and diagnostics evolved sufficiently to allow detailed laboratory investigations of high-Mach number shocks [1]. Magnetized collisionless shocks are known to be responsible for the generation of energetic particles due to Fermi process, given enough pre-energization to enter the diffusive acceleration stage. Using 1D and 2D PIC simulations, we investigate particle acceleration mechanisms relevant to laboratory magnetized collisionless shocks. We consider two geometries: two colliding quasi-1D slabs, which can be cross-validated with previous numerical studies, and an ablation model which mimics plasma profiles observed in the expanding plasma experiments. With a parametric scan over shock parameters, we obtain predictions for the magnitude of shock-accelerated electron populations in the upstream and shock layer and their dependence on shock and plasma parameters. Near-future experiments appear capable of reaching these conditions, which will allow laboratory study of particle acceleration by shocks. [1] D.B. Schaeffer et al., Phys. Rev. Lett. 122, 245001 (2019) [Preview Abstract] |
Thursday, October 24, 2019 4:30PM - 4:45PM |
UM9.00009: A head-on collision between collisionless shock waves leads to strong magnetic fields and significant slowdown Elisabetta Boella, Kevin Schoeffler, Nitin Shukla, Giovanni Lapenta, Ricardo Fonseca, Luis Silva The interaction between multiple collisionless shocks is a fundamental process in plasma physics, playing a prominent role in recent observations [1]. However, to date, no detailed theoretical, numerical, or experimental studies exploring this interaction exist. Leveraging multi-dimensional particle-in-cell simulations, we identify a novel experimental setup that allows for investigating the process in the laboratory exploiting laser-driven electrostatic shocks. In order to study the microphysics of the interaction, we model the head-on collision of these nonlinear waves. Results indicate that the collision is highly inelastic, with the velocity of the shocks decreasing up to $50\%$ of the initial value. The slowdown is mainly due to magnetic fields generated by the Weibel instability, which is driven by a strong longitudinal electron heating occurring while the shocks approach. This setup could thus be also used to probe the Weibel instability and magnetic field generation in unmagnetized plasmas in the laboratory. [1] Meyer et al. Nature 2015 [Preview Abstract] |
Thursday, October 24, 2019 4:45PM - 5:00PM |
UM9.00010: Canonical Vorticity Framework for Magnetic Reconnection Young Dae Yoon, Paul M. Bellan Canonical vorticity $\mathbf{Q}_{\sigma}=m_{\sigma}\nabla\times\mathbf{u}_{\sigma}+q_{\sigma}\mathbf{B}$, the curl of the canonical momentum $\mathbf{P}_{\sigma}=m_{\sigma}\mathbf{u}_{\sigma}\mathbf{+}q_{\sigma}\mathbf{A}$, is an important ideal plasma parameter because $\mathbf{Q}_{\sigma}$ is perfectly frozen into the species fluid if the pressure is both isotropic and barotropic. We present a framework for reconnection where $\mathbf{Q}_{\sigma}$ is the main variable instead of $\mathbf{B}$. This framework shows that canonical vorticity evolution, i.e., $\partial\mathbf{Q}_{\sigma}/\partial t$, is driven by just two terms: a convective term which describes the frozen-in property of canonical vorticity and a “canonical battery” term which describes effects from the pressure tensor being non-isotropic or non-barotropic. This framework is simpler than the traditional framework based on the generalized Ohm's law where a multitude of terms give $\partial\mathbf{B}/\partial t$. To demonstrate the power of the canonical vorticity viewpoint, the growth, stability, morphology, and saturation of the magnetic reconnection electron-diffusion region are explained using the electron canonical vorticity framework. [Preview Abstract] |
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