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Sunday, September 10

  1. page 2017 Archive edited Sept 12: Bart Ripperda and Fabio Bacchini (KU Leuven) "Numerical Comparison between Relati…

    Sept 12: Bart Ripperda and Fabio Bacchini (KU Leuven)
    "Numerical Comparison between Relativistic Particle Pushers for Astrophysics"
    Plasma processes cover a large range of energy and length scales, from the global fluid scalesto the fundamental particle scales. While the large scales can be covered within the magnetohydrodynamics (MHD) approximation, the small scales cannot. The macroscopic evolution of a plasma often develops relatively slowly, even in relativistic regimes. The macroscopic scale is however tightly coupled to faster phenomena occurring at smaller scales. In astrophysics many of these phenomena occur in the setting of relativistic magnetized plasmas. Around compact objects like black holes or neutron stars relativistic effects have to be taken into account for both the global flow and the particles. However, even in the solar
    corona or the Earth's magnetosphere particles can accelerate to mildly relativistic energies. At relativistic energies the particle equations of motion become nonlinear due to the occurrence of the Lorentz factor. There are several numerical methods to treat particle motion accurately. Here we aim to test a selection of available methods applied to known tests for which analytic solutions are available. The accuracy and performance of the particle pushers will be tested for various regimes, from Newtonian to highly relativistic energies in idealized setups relevant in astrophysics. Accuracy is assessed by determining how well conserved quantities are evolved. This study focuses on the particle pusher and therefore only static,
    spatially uniform and non-uniform electromagnetic fields are considered. The pushers considered are commonly used in MHD codes to evolve test particles in a global (magnetized) fluid flow and in particle-in-cell (PIC) codes to evolve both particles and electromagnetic fields (cite codes). In both methods the electromagnetic fields have to be interpolated to the particle position typically. Interpolation errors are tested by feeding the
    pusher with an analytic spatially varying field and comparing to the results with an interpolated field. We also show the extension of the tested schemes to a covariant form allowing to resolve particle dynamics in general relativistic plasma dynamics.

    July 7: Bart Ripperda and Fabio Bacchini (KU Leuven)
    "Particle acceleration in relativistic plasmas"
    (view changes)
    7:08 am

Saturday, September 9

  1. 6:29 am

Monday, July 3

  1. page 2017 Archive edited July 7: Bart Ripperda and Fabio Bacchini (KU Leuven) "Particle acceleration in relativisti…

    July 7: Bart Ripperda and Fabio Bacchini (KU Leuven)
    "Particle acceleration in relativistic plasmas"
    We analyze particle acceleration in explosive reconnection events in magnetically dominated proton-electron plasmas. Reconnection is driven by large-scale magnetic stresses in interacting current-carrying flux tubes. Our model relies on development of current-driven instabilities on macroscopic scales. These tilt-kink instabilities develop in an initially force-free equilibrium of repelling current channels. Using MHD methods we study a 3D model of repelling and interacting flux tubes in which we simultaneously evolve test particles, guided by electromagnetic fields obtained from MHD. We identify two stages of particle acceleration; Initially particles accelerate in the current channels, after which the flux ropes start tilting and kinking and particles accelerate due to reconnection processes in the plasma. The explosive stage of reconnection produces non-thermal energy distributions with slopes that depend on plasma resistivity and the initial particle velocity. We also discuss the influence of the length of the flux ropes on particle acceleration and energy distributions. This study extends previous 2.5D results to 3D setups, providing all ingredients needed to model realistic scenarios like solar flares, black hole flares and particle acceleration in pulsar wind nebulae: formation of strong resistive electric fields, explosive reconnection and non-thermal particle distributions. By assuming initial energy equipartition between electrons and protons, applying low resistivity in accordance with solar corona conditions and limiting the flux rope length to a fraction of a solar radius we obtain realistic energy distributions for solar flares with non-thermal power law tails and maximum electron energies up to 11 MeV and maximum proton energies up to 1 GeV.

    June 9: David Eichler (Ben Gurion University)
    "An Alternative Explanation for the Energy Dependent Boron to Carbon Ratio in Cosmic Rays"
    (view changes)
    7:02 am

Thursday, June 8

  1. page 2017 Archive edited June 9: David Eichler (Ben Gurion University) "An Alternative Explanation for the Energy D…

    June 9: David Eichler (Ben Gurion University)
    "An Alternative Explanation for the Energy Dependent Boron to Carbon Ratio in Cosmic Rays"

    May 19: Bei Wang (Princeton PICSciE & IPCC)
    "Particle-In-Cell Optimization on Multi/Many-core Architectures"
    (view changes)
    11:06 am

Monday, May 15

  1. page 2017 Archive edited May 19: Bei Wang (Princeton PICSciE & IPCC) "Particle-In-Cell Optimization on Multi/Ma…

    May 19: Bei Wang (Princeton PICSciE & IPCC)
    "Particle-In-Cell Optimization on Multi/Many-core Architectures"
    We have witnessed a rapid evolution of computing architectures due to power constrains in the last decade. Understanding how to efficiently utilize these systems in the context of demanding numerical algorithms is an urgent task for many application scientists. In this talk, we describe approaches we use to develop a highly scalable particle-in-cell (PIC) code across one of the broadest sets of computer architectures, including multicore CPU, GPU and Intel Xeon Phi. In particular, we describe our “lessons learned” and “best practices” in optimizing PIC algorithm on Knights Landing (KNL), the 2nd generation Intel Xeon Phi processor.

    May 12: Russell Kulsrud (Princeton) and Rashid Sunyaev (MPA & IAS)
    "Diffusion of mass through tera gauss fields on the surface of neutron stars in HXRBs"
    (view changes)
    8:49 am

Tuesday, May 9

  1. page 2017 Archive edited May 12: Russell Kulsrud (Princeton) and Rashid Sunyaev (MPA & IAS) "Resistive Diffusio…

    May 12: Russell Kulsrud (Princeton) and Rashid Sunyaev (MPA & IAS)
    "Resistive Diffusion"Diffusion of mass through tera gauss fields on the Neutron Star Mound"
    When
    surface of neutron stars in HXRBs"
    A large amount of
    mass falls
    ...
    the polar regionsregion of a neutron star in a binary X-ray source system it tends to spread out overXray binaries and the entire surface. A long standing question in research on this problemis, is will the mass be anchoredcompletely frozen on the magnetic field lines and drag the field with it, or is there iscan it diffuse through them? In this talk we present a special mechanism that allowsfor the mass to slip throughlatter possibility. A strong MHD instability occurs in the magnetic field lines leads to much less distortion? Since the amounttop layers of mass falling on the neutron star can actually be comparable withdriven by the neutron star mass,incoming mass. This instability has the question of which alternative holds is very important. In this paper we suggest an efficient mechanism that will allow the mass to slip through the lines while leaving the neutron star’s field largely unchanged. The mechanism is based on a strong ideal instability very similar tosame properties as the Schwarzschild
    ...
    solar convection zone, (Schwarschild 1957). Since the instability itself is an ideal one, it can not directly force the masszone. It gives rise to slip through the lines, However, it can create a turbulent cascade of eddies whose scale extends down to a resistive scale, at the same time mixingwhich mixes up the field lines up without breaking them. On this scale the mass can cross the lines. This instability is efficient enoughso that itlines originally far apart can producecome with a mass flow inresistivity diffusion distance and transfer the outer neutron star plasma without growingmasses between them. However, the lines of force themselves are not disrupted. This leads to a large amplitude. Thean equilibrium which is marginal amplitude saturates at a few percent of its full growth. Thewith respect to the instability setsjust as happens in the mass per flux distribution of the accumulated material on different lines. We believe with this mechanism, the neutron star magnetic moment is not greatly changed even after a fraction of a solar mass has fallen on it.Schwarzschild case.
    April 28: Luca Comisso (Princeton)
    "Relativistic Reconnection: from flat to curved spacetime"
    (view changes)
    6:56 am

Wednesday, May 3

  1. page 2017 Archive edited May 12: Russell Kulsrud (Princeton) and Rashid Sunyaev (MPA & IAS) "Resistive Diffusio…

    May 12: Russell Kulsrud (Princeton) and Rashid Sunyaev (MPA & IAS)
    "Resistive Diffusion of the Neutron Star Mound"
    When mass falls on the polar regions of a neutron star in a binary X-ray source system it tends to spread out over the entire surface. A long standing question in research on this problem is will the mass be anchored on the magnetic field lines and drag the field with it, or is there is a special mechanism that allows the mass to slip through the magnetic field lines leads to much less distortion? Since the amount of mass falling on the neutron star can actually be comparable with the neutron star mass, the question of which alternative holds is very important. In this paper we suggest an efficient mechanism that will allow the mass to slip through the lines while leaving the neutron star’s field largely unchanged. The mechanism is based on a strong ideal instability very similar to the Schwarzschild instability in the solar convection zone, (Schwarschild 1957). Since the instability itself is an ideal one, it can not directly force the mass to slip through the lines, However, it can create a cascade of eddies whose scale extends down to a resistive scale, at the same time mixing the field lines up without breaking them. On this scale the mass can cross the lines. This instability is efficient enough that it can produce a mass flow in the outer neutron star plasma without growing to a large amplitude. The marginal amplitude saturates at a few percent of its full growth. The instability sets the mass per flux distribution of the accumulated material on different lines. We believe with this mechanism, the neutron star magnetic moment is not greatly changed even after a fraction of a solar mass has fallen on it.

    April 28: Luca Comisso (Princeton)
    "Relativistic Reconnection: from flat to curved spacetime"
    (view changes)
    12:26 pm

Friday, April 21

  1. page 2017 Archive edited April 28: Luca Comisso (Princeton) "Relativistic Reconnection: from flat to curved spaceti…

    April 28: Luca Comisso (Princeton)
    "Relativistic Reconnection: from flat to curved spacetime"
    In recent years, the classical Sweet-Parker and Petschek models have been extended in the special relativistic regime, both for MHD plasmas [1] and two-fluid electron-positron plasmas [2]. Nevertheless, there could be situations, like in the vicinity of black holes, where also general relativistic effects can become important. Here, we present a two-fluid description of the relativistic reconnection process for pair plasmas in the flat spacetime limit [2], and then we analyze the reconnection process in the MHD approximation for plasmas around rotating black holes [3]. A simple generalization of the Sweet-Parker model is used as a first approximation to the problem, and the reconnection rate, as well as other important properties of the reconnection layer, has been calculated taking into account the effect of spacetime curvature.
    [1] Y. E. Lyubarsky, Mon. Not. R. Astron. Soc. 358, 113 (2005)
    [2] L. Comisso and F.A. Asenjo, Phys. Rev. Lett. 113, 045001 (2014)
    [3] F.A. Asenjo and L. Comisso, Phys. Rev. Lett. 118, 055101 (2017)

    April 21: Jonathan Zrake (Columbia)
    "Magnetic relaxation and turbulence in pulsar wind nebulae"
    (view changes)
    8:48 am

Sunday, April 16

  1. page 2017 Archive edited April 21: Jonathan Zrake (Columbia) "Magnetic relaxation and turbulence in pulsar wind neb…

    April 21: Jonathan Zrake (Columbia)
    "Magnetic relaxation and turbulence in pulsar wind nebulae"
    Pulsar wind nebulae (PWNe) are energized by the electromagnetic spin-down power of a rapidly rotating neutron star. Their emission is primarily synchrotron, produced by relativistic electrons radiating in a sub-equipartition magnetic field. The processes by which a pulsar wind, which is born in a strongly magnetized state, eventually shares its energy with electrons, have been a long-standing question in the theory of pulsars and their nebulae (sometimes referred to as the sigma-problem). I will discuss how dissipation in PWNe may be understood in terms of a process known as magnetic relaxation, and give an overview in general physics terms of recent advances in this topic. MHD simulations reveal the process is generally turbulent, and that magnetic field structures tend to organize themselves spatially, even when the field lacks net magnetic helicity. I will discuss how this process helps to explain the magnetization level of the Crab's synchrotron nebula.

    April 14: Massimo Cappi (INAF/IASF-Bologna)
    "The Athena X-ray Observatory: scientific objectives and mission study"
    (view changes)
    6:00 pm

Tuesday, April 4

  1. page 2017 Archive edited ... Athena is a large X-ray Observatory proposed to address the Science Theme “The Hot and Energet…
    ...
    Athena is a large X-ray Observatory proposed to address the Science Theme “The Hot and Energetic Universe”, which has been selected by ESA in its Cosmic Vision program.
    After reviewing its core science goals, the astrophysics and cosmic evolution of large-scale hot structures and black holes in the Universe, and (some of) its Observatory capabilities, I will present the mission telescope and instruments, to be implemented as a Large mission planned for launch in 2028.
    April 7: Zhenyu Wang (Princeton)
    "Generation of Collisionless Shocks in Laser-Produced Plasmas"

    March 17: Andrei Beloborodov (Columbia)
    "Magnetic flares near accreting black holes"
    (view changes)
    5:58 am

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