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 PO9: Space and Astrophysical Plasmas: Astrophysical Plasma, Laboratory Astrophysics |
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Chair: Hui Li Room: Grand C/E |
Wednesday, October 23, 2019 2:00PM - 2:12PM |
PO9.00001: Wakefield Acceleration in the M87 jet Yoshiaki Kato, Toshikazu Ebisuzaki, Toshiki Tajima We investigate electromagnetic pulses in one of the powerful radio jet in M87. The recent observations have revealed that the origin of its jet has been constrained within a few tens of Schwarzschild radii in the vicinity of the supermassive black hole. We therefore consider the strong Alfv\'{e}nic impulses generated by episodic eruptive accretions in the innermost region of a magnetized accretion disk, which give rise to the collective ponderomotive forces. Such a pondeoromotive force provides the so-called wakefield acceleration of the charged particles. This acceleration mechanism has advantages over the Fermi acceleration and therefore it is one of the most promising mechanisms for a central engine of the M87 jet. By measuring the collimation profile in the M87 jet from the latest observations, we examine the characteristic length scales of the wakefield acceleration under the astrophysical context. We also discuss implications of jets in the stellar-mass black holes. [Preview Abstract] |
Wednesday, October 23, 2019 2:12PM - 2:24PM |
PO9.00002: Magneto Gravitational Modes Driven by the Modulated Gravitational Field of Compact Collapsing Binaries Bruno Coppi, V. Ricci, R. Spigler A new theoretical process [1], to create high energy particle populations during the collapse of neutron star - neutron star or black hole - black hole binaries, has been identified. The oscillatory gravitational potential that is associated with the rotating binary is characterized by two frequencies, in the case where the masses of the two components are not equal, that reduce to one (the main) when the two masses are equal. Consequently the gravitationally confined plasma surrounding the considered binary will oscillate with the same frequencies. When one of these (e.g. the main) will become about equal to the frequency (about that of the compressional Alfv$\'e$n wave) of a newly identified vertically localized ballooning mode, the amplitude of this can be sustained by the gravitationally induced plasma density oscillations. Then the involved characteristic mode-particle resonances can raise the energy of a super-thermal fraction of the electron distribution up to relativistic values and lead to produce observable high energy radiation emission.\\ $[1]$ B. Coppi, Plasma Physics Reports, 45, 5 (2019). [Preview Abstract] |
Wednesday, October 23, 2019 2:24PM - 2:36PM |
PO9.00003: Structure of a collisionless pair jet in a magnetized electron-proton plasma Mark Eric Dieckmann, Doris Folini, Ingrid Hotz, Aida Nordman, Pierangelo Dell'Acqua, Anders Ynnerman, Rolf Walder We model with a PIC simulation the expansion of a pair cloud into an ambient magnetized electron-proton plasma at rest. The cloud temperature is 400 keV. It has the mean speed 0.9c along the magnetic field direction and a finite extent orthogonal to this direction. The pair cloud piles up the magnetic field into a piston that is strong enough to expel the protons. This piston becomes a discontinuity that separates the protons of the ambient plasma from the pair plasma. We thus observe the early stages of the formation of a pair jet in collisionless plasma. The magnetic field of the discontinuity, which is coherent along the sides of the jet, is in contact with relativistic electrons and positrons. This discontinuity thus becomes a source of synchrotron emissions. Protons are accelerated by the discontinuity to MeV energies and a fast magnetosonic shock forms that separates the outer cocoon of the jet from the pristine ambient plasma. The need to conserve the quasi-neutrality of the plasma lets the head of the jet become a source of energetic positrons. We discuss the implications of our findings for relativistic astrophysical pair jets like those that are emitted by accreting black holes. [Preview Abstract] |
Wednesday, October 23, 2019 2:36PM - 2:48PM |
PO9.00004: Characterising Diffusive Nonthermal Particle Acceleration in Kinetic Turbulence of Relativistic Pair Plasma Kai Wong, Vladimir Zhdankin, Dmitri Uzdensky, Gregory Werner, Mitchell Begelman Turbulent high-energy astrophysical systems such as pulsar wind nebulae and active galactic nuclei accelerate a substantial fraction of particles to nonthermal energies, as inferred from their emission spectra. Nonthermal particle acceleration in 3D particle-in-cell simulations of driven turbulence in relativistic pair plasmas is well-described by a Fokker-Planck energy diffusion-advection model in which particles receive stochastic energy kicks. This model is characterised by energy diffusion ($D$) and advection ($A$) coefficients, which are functions of particle energy $\epsilon$. By tracking particles in the simulation, we study the dependence of these coefficients on key physical parameters including plasma magnetisation and the turbulence driving scale. We consistently observe that, at nonthermal energies, $D \sim D_0 \epsilon^2$, in line with theoretical expectations. We find that the scaling factor $D_0$ increases as a power law function of magnetisation, and that it depends primarily on driving scale rather than system size. We also comment on the statistical properties of spatial transport. These results have implications for models of nonthermal particle acceleration in a broad range of high-energy astrophysical systems. [Preview Abstract] |
Wednesday, October 23, 2019 2:48PM - 3:00PM |
PO9.00005: ``Snowplow'' Model of Gamma-ray Bubbles in the Galaxy and Relevant Mode Particle Interactions A. Cardinali, B. Coppi A plasma outflow coming from the center of Our Galaxy is simulated by an ion ``beam'' reaching a ``plowed'' nearly stationary magnetic field at characteristic galactic distances. Then waves can be excited efficiently in the rarefied plasma ($10^{-2}-10^{-4}$ cm$^{-3}$) permeating the relevant magnetic field configuration. These are electrostatic lower hybrid modes driven to instability via Cerenkov interaction. By using a fluid model the relevant dispersion relation is derived and the growth rate is evaluated both analytically and numerically. This rate increases with the ``beam'' density and its maximum is found when the perpendicular (to the magnetic field) phase velocity of the mode is comparable to the velocity of the ``beam'' and the parallel phase velocity is comparable to electron thermal velocity. Then efficient energy transfer from the perpendicular ion ``beam'' to the electron population, via Landau damping, can be expected accelerating fast electrons, or heating the overall electron population. The radiation emission due to the energetic electron component could explain the observed X and gamma-ray spectra characterizing the Fermi Bubbles of our Galaxy [1].\\ $[1]$ H.-Y. K. Yang, M. Ruszkowski, E.G. Zweibel, Galaxies 6, 29 (2018). [Preview Abstract] |
Wednesday, October 23, 2019 3:00PM - 3:12PM |
PO9.00006: Measurements of growth and saturation of the Ion Weibel instability using Thomson scattering Colin Bruulsema, Seigfried Glenzer, Wojciech Rozmus, George Swadling, Frederico Fiuza The growth and saturation of the ion Weibel instability is typically described in a homogeneous plasma with isotropic electrons, growing due to the streaming velocity of the ions. The resulting saturation levels have compared well to experimental measurements. However, the saturation levels and growth rates observed in PIC simulations of truly homogeneous plasma do not agree with these predictions. We analyze these discrepancies in growth rate and saturation to determine the applicability of the PIC simulations. We also simulate the effects of inhomogeneous experimental plasmas on the growth and saturation of the Weibel instability. These effects inform a strategy to measure the growth rates of the Weibel instability in experimental plasma. Using Thomson scattering, we determine the ion and electron currents growing within the plasma produced at the OMEGA laser facility. This measures the resulting growth of the field and the filamentation of the plasma. This work was performed under the auspices of the Lawrence Livermore National Security, LLC, (LLNS) under Contract No. DE-AC52-07NA27344 [Preview Abstract] |
Wednesday, October 23, 2019 3:12PM - 3:24PM |
PO9.00007: Coherence constraints on physical parameters at bright radio sources: pulsars and FRBs Maxim Lyutikov Observations of high brightness temperature coherent radio emission, with brightness temperatures as high as $T_b \sim 10^{35}$ K, impose physical constraints on the plasma parameters at the emission sites, eg. some minimal plasma energy density. Additional important constraints come from the fact that resonantly emitting particles lose most of their energy to non-resonant inverse Compton and synchrotron processes. We list arguments that magnetospheres of neutron stars is the prefer loci for the generation of Fast Radio Burst. [Preview Abstract] |
Wednesday, October 23, 2019 3:24PM - 3:36PM |
PO9.00008: Particle-in-cell simulations of pair discharges at pulsar polar caps Fabio Cruz, Thomas Grismayer, Luis O Silva, Alexander Y Chen, Anatoly Spitkovsky When subject to the rotationally induced electric field of pulsar polar caps, electrons and positrons are accelerated along the magnetic field, producing gamma-ray curvature radiation. The emitted gamma-rays, in turn, are absorbed by the magnetic field, converting to new electron-positron pairs. The repetition of this process leads to a cascade of elementary particles that are the source of pulsar magnetospheric plasma. The final number of particles created in pair cascades and their connection with pulsar radio emission remains an open problem. Obtaining numerical models of pulsar pair discharges is a challenging endeavor and one that was only addressed in simplified one-dimensional simulations. In this work, we present two-dimensional particle-in-cell simulations of pair discharges near pulsar polar caps, including the Quantum Electrodynamics effects responsible for gamma-ray and pair production processes from first principles. These simulations allow studying the time dependence and distribution in altitudes and latitudes of pair cascades while resolving the relevant plasma electrodynamic scales. We analyze the particle spectra and discuss the constraints that our simulations put on pair production rates for use in global pulsar simulations, underlining the differences to previous models with simplified prescriptions. We also estimate the fraction of gamma-rays that escapes the polar cap and contributes to the flux of polar gamma-rays in Fermi data. [Preview Abstract] |
Wednesday, October 23, 2019 3:36PM - 3:48PM |
PO9.00009: Numerical Experiments on Magnetospheres of Weak Pulsars Yuran Chen, Fabio Cruz, Anatoly Spitkovsky Recent advances in numerical techniques and computational power have allowed us to simulate the pulsar magnetosphere from first principles using Particle-in-Cell techniques. These ab-initio simulations seem to indicate that pair creation through photon-photon collision at the light cylinder is required to sustain the pulsar engine. However for many rotation-powered pulsars, pair creation operates effectively only near the stellar surface where magnetic field is high. Without efficient photon-photon pair conversion, how these "weak pulsars" fill their magnetospheres and produce radio emission is still an open question. By pushing towards a parameter regime that was not studied in detail before, we discovered a range of self-consistent solutions to the pulsar magnetosphere that do not require pair production near the light cylinder. Depending on the electron-positron pairs produced, the pulsar transitions from a near-death state with little spin-down, through an highly time-dependent state where current is intermittent, to finally approaching a near force-free state with stable spin-down. We show the time evolution of all these different states, and attempt to compare these to the actual pulsar behaviors that we observe. [Preview Abstract] |
Wednesday, October 23, 2019 3:48PM - 4:00PM |
PO9.00010: Light curves and spectra of supernova shock breakout into inhomogenous winds Shane Coffing, Carolyn Kuranz, Chris Fryer Supernova explosions are primarily classified by the light curves they produce: the time evolution of their emission and spectral features. The earliest optical emission begins with shock breakout, when photons begin to pour out from the optically thick edge of the supernova shock into the surrounding circumstellar media. For massive stars exceeding roughly 10 times the mass of the sun, the circumstellar media is a strong, radial outflow known as a radiation driven wind. At early epochs, this wind may be very dense due to high mass loss rates at the end of the star's evolution and can be stratified due to episodic or unsteady mass loss. Furthermore, growing evidence supports that such outflows are inhomogeneous and clumpy, having large scale density perturbations. In this work, we present our first look at light curves and spectra of shock breakout into inhomogenous and clumpy winds, via multi-group radiation hydrodynamics simulations. [Preview Abstract] |
Wednesday, October 23, 2019 4:00PM - 4:12PM |
PO9.00011: The White Dwarf Photosphere Experiment Mike Montgomery, Don Winget Over 97{\%} of stars either are, or will become, white dwarf stars, giving them broad relevance. The astrophysical questions they can help us address include the age of the universe, the age and history of star formation of our Galaxy's various stellar populations, and aspects of the evolution of stars. The compact and dense nature of these ubiquitous stars means that their atomic physics is more difficult to model than other stars, even in the outermost layers. We briefly describe the astrophysical and physical problems associated with white dwarf photospheres (the plasma where the observed light originates) and assess the impact of these uncertainties. We establish the work on white dwarf stars in the larger context of the ``at-parameter'' experiments of the Wootton Center for Astrophysical Plasma Properties (WCAPP). The current experiments investigate macroscopic plasmas under the density and temperature conditions we find in the cosmos; we will briefly summarize the results of these ongoing experiments, with particular emphasis on recent results of the White Dwarf Photosphere Experiment (WDPE). [Preview Abstract] |
Wednesday, October 23, 2019 4:12PM - 4:24PM |
PO9.00012: Laboratory measurements of discrepancies between H$\beta $ and H$\gamma $ absorption line profiles at the conditions of White Dwarf photospheres Marc-Andre Schaeuble, J. E. Bailey, Taisuke Nagayama, T. A. Gomez, M. H. Montgomery, D. E. Winget Fits to stellar hydrogen Balmer absorption lines can be used to infer the masses of White Dwarf (WD) stars. The resulting masses depend on which Balmer series members are included in the spectral fit. Furthermore, studies have found this spectroscopic mass to be \textasciitilde 10{\%} lower than that determined by the independent gravitational redshift mass determination method. The combination of these trends cast doubt on the accuracy of spectroscopically determined masses. Here, we present laboratory experiments aimed at investigating weaknesses in the main component of the spectroscopic technique: hydrogen line-shape calculations. These experiments were performed at Sandia National Laboratories' Z-machine and allow for the recording of high-quality absorption spectra. Analysis of the experimental absorption spectra reveals that electron density (ne) values derived from the H$\gamma $ line are $\sim $34\textpm 6{\%} lower than from H$\beta $. To explore this difference, we investigated plasma gradients and errors in the data extraction and fitting methodology. We find that these components have a negligible impact on derived H$\beta $ and H$\gamma $ ne. This experimental evidence may suggest that the hydrogen line-shape calculations currently used in WD spectral fitting are not accurate enough to result in reliable WD masses. [Preview Abstract] |
Wednesday, October 23, 2019 4:24PM - 4:36PM |
PO9.00013: An experimental platform to study rotating plasma flows relevant to astrophysical disks and jets V. Valenzuela-Villaseca, S. V. Lebedev, J. P. Chittenden, F. Suzuki-Vidal, L. G. Suttle, E. R. Tubman, J. W. D. Halliday, D. Russell Disks are ubiquitous structures in the universe and are typically accompanied by outflows, seen in the form of highly collimated jets. We present first results of an experimental platform [1] developed on the MAGPIE pulsed-power facility (1 MA, 500 ns current pulse) which uses converging supersonic plasma flows to drive a differentially rotating disk together with an axial jet. The plasma flows are formed by ablation of 8 aluminium wires and are accelerated by the radial and azimuthal components of the JxB force. This produces an off-axis convergence that introduces rotation in the merging flows. The dynamics of the disk rotation and formation of an extended axial jet are observed by multi-frame XUV and optical imaging of the plasma self-emission. Laser interferometry and Thomson scattering allow us to map the plasma density and velocity distribution of the rotational plasma and its jet. The magnetic field in the plasma can be measured with Faraday rotation. The rotational characteristics of the flow are compared with simulations using the 3D MHD code Gorgon. Using this new platform, we expect to address questions regarding the interplay of rotation and magnetic fields frozen-in the flow and the acceleration of jets from the disk. [1] Bocchi et al., ApJ 767, 84 (2013). [Preview Abstract] |
Wednesday, October 23, 2019 4:36PM - 4:48PM |
PO9.00014: Design and Simulation of the Radishock Campaign on OMEGA. Suzannah Wood, Heather Johns, John Morton, Andy Liao, Ted Perry, Colin Brown, Nicholas Lanier, Pawel Kozlowski, Chris Fryer, Chris Fontes, Derek Schmidt, Alexandria Strickland, Todd Urbatsch Radishock is a high energy density radiation-hydrodynamic experiment fielded at the OMEGA laser facility. Radishock utilizes the non-invasive spectroscopic temperature diagnostic developed during the COAX campaign and allows us to study the spatial dependence of the interaction between a Marshak radiation wave and an ablatively driven counter-propagating shock. To determine temperature, a Ti-loaded SiO$_{\mathrm{2}}$ foam is backlit by a Kr-filled CH capsule, resulting in 1s-2p and 1s-3p K-shell absorption spectra after the Ti is ionized. The density measurement requires radiographing the target, for which we use a point-projection backlighter consisting of a V foil mounted to a substrate containing a pinhole. We have recently completed platform development on this experiment and will discuss preliminary experimental results along with challenges and successes in the design and simulation using CASSIO, an Eulerian AMR radiation-hydrodynamics code. Detailed comparisons between the measured temperature and density will constrain our models and validate our codes for radiation transport models. [Preview Abstract] |
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