Bulletin of the American Physical Society
56th Annual Meeting of the APS Division of Plasma Physics
Volume 59, Number 15
Monday–Friday, October 27–31, 2014; New Orleans, Louisiana
Session GM10: Mini-Conference: The Magnetic Universe -- A Mini-Conference in Honor of Stirling Colgate II |
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Chair: Eric Blackman, University of Rochester Room: Salon FGH |
Tuesday, October 28, 2014 9:30AM - 10:00AM |
GM10.00001: Stirling Colgate and Gamma-Ray Bursts Donald Lamb Even before the discovery of gamma-ray bursts (GRBs), Stirling Colgate proposed that bursts of x rays and gamma rays might be produced by a relativistic shock created in the supernova explosion of a massive star. We trace the scientific story of GRBs from their detection to the present, highlighting along the way Stirling's interest in them and his efforts to understand them. We summarize our current understanding that short, soft, repeating bursts are produced by magnetic neutron stars; short, hard bursts are produced by the mergers of neutron star-neutron star binaries; and long, hard bursts are produced by the core collapse of massive stars that have lost their hydrogen and helium envelopes. We then discuss some important open questions about GRBs and how they might be answered. We conclude by describing the recent serendipitous discovery of an x-ray burst of exactly the kind he proposed, and the insights into core collapse supernovae and GRBs that it provided. [Preview Abstract] |
Tuesday, October 28, 2014 10:00AM - 10:20AM |
GM10.00002: Generation and amplification of magnetic fields in laser-driven collisionless shocks Frederico Fiuza, Dmitri Ryutov, Anatoly Sptikovsky, Channing Huntington, Steven Ross, Luis Silva, Warren Mori, Bruce Remington, Hye-Sook Park Collisionless shocks are ubiquitous in astrophysical plasmas and are believed to play an important role in magnetic field amplification; however, the magnetic field dynamics in shocks is still poorly understood as in situ measurements are not available. Recent developments in high-power lasers are bringing the study of collisionless shocks into the realm of laboratory experiments. We have performed detailed 2D and 3D particle-in-cell simulations to explore the generation of collisionless shocks for laboratory conditions associated with counter-streaming high-velocity plasma flows. We capture all the relevant physics, which range from the generation of Biermann battery fields at the laser-foil region, to the micro-instabilities associated with the counter-streaming flows, and to the generation of turbulence at the shock. We show the generation of strong (\textgreater 1{\%} of equipartition) magnetic fields mediated by the Weibel instability and the conversion from well-defined filaments to magnetic turbulence as the shock is formed. We identify the conditions required to observe this magnetic field dynamics in shocks for the first time in the upcoming experiments at the National Ignition Facility. [Preview Abstract] |
Tuesday, October 28, 2014 10:20AM - 10:40AM |
GM10.00003: Could the universe get magnetized by galaxy cluster accretion shocks? Mikhail Medvedev The origin of the micro-Gauss magnetic fields in galaxy clusters is one of the outstanding problem of modern cosmology. We propose that accretion shocks on galaxy clusters accelerate cosmic rays, which in turn are natural and inevitably generate magnetic fields from scratch via a streaming, Weibel-type plasma instability. We develop a self-similar model of a cosmic-ray-modifies foreshock and demonstrate that, in contrast to the conventional lore, the generated magnetic fields (i) are large-scale (of order the shock curvature radius, $\sim$~tens of kpc or more) hence they are effectively decoupled from dissipation and are long-lived on the Hubble time and (ii) are strong enough, of the order of a fraction of the cosmic ray pressure, to meet observational constraints. Unlike other shock-related models of the field generation (e.g., via the Bell instability or the Richtmeyer-Meshkov vorticity instability), our model does not require preexisting seed fields; the fields are generated in the intracluster at essentially a few cluster light-crossing times. [Preview Abstract] |
Tuesday, October 28, 2014 10:40AM - 11:10AM |
GM10.00004: Magnetic Field Generation and Particle Energization in Relativistic Shear Flows Edison Liang, Wen Fu, Markus Boettcher, Parisa Roustazadeh This paper summarizes recent results obtained from 2 -and -3 D particle-in-cell (PIC) simulations of relativistic shear boundary layers (SBL). In addition to the creation of sustained, ordered magnetic fields due to counter-current instabilities, we find efficient energization of nonthermal electrons to high energies, making the SBL a strong candidate for enhanced synchrotron emission in relativistic jets, from blazars to gamma-ray bursts. The case of mixed electron-positron-ion shear flows is particularly interesting as it leads to the formation of an electron spectrum with both a high-energy peak near the ion kinetic energy, plus a hard power-law tail of slope near -- 3, which strongly resembles electron distributions responsible for the emissions of GRB and blazars. The electron momentum distribution exhibits extreme anisotropy, making the SBL a strong candidate for narrowly beamed synchrotron-self-Compton (SSC) radiation in some cases. [Preview Abstract] |
Tuesday, October 28, 2014 11:10AM - 11:30AM |
GM10.00005: Formation of Hard Power-laws in the Energetic Particle Spectra Resulting from Relativistic Magnetic Reconnection Fan Guo, Hui Li, William Daughton, Yi-Hsin Liu, Xiaocan Li Using fully kinetic simulations, we demonstrate that magnetic reconnection in relativistic plasmas is highly efficient at accelerating particles through a first-order Fermi process resulting from the curvature drift of particles in the direction of the electric field induced by the relativistic flows. This mechanism gives to the formation of hard power-law spectra in parameter regimes where the energy density in the reconnecting field exceeds the rest mass energy density and when the system size is sufficiently large. The power law slope approaches ``-1'' for closed systems and gets softer when particle loss from the acceleration region is included. A simple analytic model is proposed which explains these key features and predicts a general condition under which hard power-law spectra will be generated from magnetic reconnection. We demonstrate that both continuous inflow and Fermi-type acceleration lead to the power-law distributions. Finally, we discuss the role of particle anisotropy in particle acceleration during magnetic reconnection. The work shows that hard power-law distributions are a common feature in relativistic magnetic reconnection region, which may be important for explaining the high-energy emissions in systems like pulsars, jets from black holes, and gamma-ray bursts. [Preview Abstract] |
Tuesday, October 28, 2014 11:30AM - 11:50AM |
GM10.00006: Regular acceleration in magnetically-driven turbulence Andrey Beresnyak, Hui Li Astrophysics and space science knows many examples of magnetically-dominated environments. Often, due to large-scale motions or reconnection the free energy of magnetic field is converted into kinetic motions. We demonstrated that collisionless particles in such environments will be regularly accelerated while experiencing curvature drift. This could be applied, e.g. to the spontaneous reconnection above the solar surface that results in the heating of particles and production of non-thermal tails. Interestingly, the opposite processes, such as dynamo, will actually result in the net cooling of particles by the curvature drift. Being very generic, this acceleration mechanism is likely to be responsible in production of non-thermal particle distribution in many magnetized environments such as pulsar magnetospheres, jets from supermassive black holes, jets from imploding stars, etc. [Preview Abstract] |
Tuesday, October 28, 2014 11:50AM - 12:10PM |
GM10.00007: Guide Field Reconnection Turbulence and Coronal Heating M.J. Pueschel, D. Told, P.W. Terry, F. Jenko, E.G. Zweibel, V. Zhdankin, H. Lesch Magnetic reconnection is a prime contender for explaining plasma heating in the solar corona. This work focuses on turbulent reconnection simulations in the strong-guide-field limit, where the gyrokinetics both captures all relevant physical effects and is numerically efficient. Continuously replenished current sheets force a quasi-stationary turbulent state, where significant levels of $\mathbf{j} \cdot \mathbf{E}$ heating can be measured. In addition, plasmoids are observed to form in the turbulence, causing secondary reconnection events through mergers. Under coronal conditions, the volumetric heating rate is evaluated as $1.5 \times 10^{-3}$ erg cm$^{-3}$ s$^{-1}$, in good agreement with observations. This value scales as, in particular, the reconnecting field to the power of $1.8$, and the characteristic current sheet width to the power of $0.75$. Moreover, heating bursts associated with plasmoid mergers conform with time scales associated observationally with nanoflares. For further details on this work, as well as on the emergence of temperature anisotropies, see [M.J.~Pueschel et al., \textit{Magnetic Reconnection Turbulence in Strong Guide Fields: Basic Properties and Application to Coronal Heating}, accepted for publication in Astrophys.~J.~Suppl.~Ser.]. [Preview Abstract] |
Tuesday, October 28, 2014 12:10PM - 12:30PM |
GM10.00008: Ion Acceleration by Magnetic Pinch Instabilities- Powerful Neutron Sources Anna Hayes, Hui Li Since the 1950s pinch discharges with deuterium gas have been known to produce large neutron bursts. During these early quests for laboratory fusion it was initially believed that the heat produced in the pinch led to sufficently high temperatures that these neutrons resulted from thermonuclear (TN) burn. However, a series of careful measurements led by Stirling Colgate was carried out to show that these neutrons did not result form TN burn. Rather, they resulted from an m=0 sausage mode instability that accelerated the ions, causing beam-target interactions. Today, this same mechanism is used in dense plasma focus machines to generate intense neutron pulses for neutron activation experiments. One such experiment, to test the citicality of aging plutonium, is currently being planned at the Nevada Test Site. Helping to characterize the neutrons from the dense palsma focus to be used in this large experiment was the last applied physics project that Stirling work on. In this talk we will summarize the physics issues involved both in the original discovery in the 1950s and in today's experiments. [Preview Abstract] |
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