Bulletin of the American Physical Society
60th Annual Meeting of the APS Division of Plasma Physics
Volume 63, Number 11
Monday–Friday, November 5–9, 2018; Portland, Oregon
Session JO7: Reconnection, Shocks, Jets, and Other Astrophysical Topics |
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Chair: Yue Zhang, University of Washington Room: OCC B117-119 |
Tuesday, November 6, 2018 2:00PM - 2:12PM |
JO7.00001: Relativistic Nonthermal Particle Acceleration in Plasmoid-Mediated Magnetic Reconnection Dmitri A Uzdensky Relativistic magnetic reconnection has been shown numerically to be an efficient and powerful energetic particle accelerator, providing an appealing physical explanation for nonthermal high-energy radiation observed in many astrophysical sources. Here, I present an analytical model aiming to elucidate the key physical processes responsible for relativistic nonthermal particle acceleration (NTPA) by collisionless reconnection in the large-system, plasmoid-mediated regime, and to explain the main features of reconnection-driven NTPA seen in simulations. These features include the dependence of the power-law index α and high-energy cutoff γc of the resulting nonthermal particle energy spectrum f(γ) on the ambient plasma magnetization and guide magnetic field, and (for γc) on the system size. In this model, energetic particles are continuously accelerated by the main reconnection electric field Erec until they become magnetized by the reconnected magnetic field and eventually trapped in plasmoids large enough to confine them. I argue that the balance between electric acceleration and magnetization controls the power-law index, while trapping by plasmoids governs γc, thus tying the particle energy spectrum to the plasmoid distribution function. |
Tuesday, November 6, 2018 2:12PM - 2:24PM |
JO7.00002: Time Resolved Measurements of Electron Acceleration in Pulsed Power Driven Magnetic Reconnection Jack W. D. Halliday, Jack D. Hare, Lee G. Suttle, Sergey V. Lebedev, Simon N. Bland, Eleanor R. Tubman, Daniel R. Russell, Francisco Suzuki-Vidal, Sergey A. Pikuz, Tania A. Shelkovenko Magnetic reconnection is a relaxation mechanism through which energy stored in magnetic flux is dissipated, leading to plasma heating and acceleration, and the generation of fast particles. Historically, particle acceleration has been a key signature of reconnection in astrophysical observations, however it is typically not diagnosed in laboratory experiments.
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Tuesday, November 6, 2018 2:24PM - 2:36PM |
JO7.00003: Relativistic Reconnection with Inverse Compton Radiaction Gregory R Werner, Alexander A Philippov, Dmitri A Uzdensky Relativistic reconnection converts magnetic energy to particle energy, giving rise to heating and nonthermal particle acceleration (NTPA). The resulting highly relativistic particles can subsequently convert their energy to synchrotron and inverse Compton radiation; thus reconnection-driven NTPA may ignite spectacular high-energy flares in exotic astrophysical environments. However, radiation reaction forces (`radiaction' for short) may impede NTPA. With particle-in-cell simulation, we investigate relativistic reconnection in collisionless electron-positron plasmas with self-consistent inverse Compton radiaction. Unsurprisingly, weak radiaction scarcely alters NTPA, yielding a now-familiar power-law particle spectrum that radiates long after reconnection. With strong radiaction, however, a broken power-law spectrum forms as dissipated magnetic energy is rapidly converted from particle to photon energy, giving off a luminosity proportional to the reconnection rate. The particle spectrum has an unaltered power law at lower energies, and, above the break, a steeper power law that fluctuates in time due to the stochastic influence of plasmoids. The time-integrated photon spectrum roughly reflects the time-averaged luminosity-weighted particle spectrum. |
Tuesday, November 6, 2018 2:36PM - 2:48PM |
JO7.00004: Endogenous Magnetic Reconnection and Associated Processes in Weakly Collisional Regimes Bamandas Basu, Bruno Coppi The driving factor an endogenous magnetic reconnection process lays within the layer where a drastic change of magnetic field topology occurs. For this a significant electron temperature gradient is required to be present in a magnetically confined plasma. The associated electron temperature fluctuations are anisotropic [1]. A localized class of mode involves a reconnected field ${\tilde{B}}_{x}$ of odd parity (as a function of the radial variable), contrary to the commonly considered reconnecting modes [2] associated with a finite effective resistivity. Since there are plasmas in the Universe with considerable electron thermal energy contents, the considered class of modes could be candidates to produce generation or conversion of magnetic energy and high energy particle populations [3]. With their excitation, these modes acquire momentum [3] in the direction of the main magnetic field and the body of the plasma column should recoil in the opposite direction. Purely resistive endogenous modes are identified in the limit where the “diamagnetic” frequencies are not important. [1] B. Coppi,B. Basu, Phys. Lett. A, 382, 400 (2018). [2] B. Coppi, Phys. Fluids, 8, 2273 (1965). [3] B. Coppi, B. Basu, and A. Fletcher, Nucl. Fus., 57, 7 (2017). |
Tuesday, November 6, 2018 2:48PM - 3:00PM |
JO7.00005: Magnetic island merger as a mechanism for inverse magnetic energy transfer Muni Zhou, Pallavi Bhat, Nuno F Loureiro, Dmitri A Uzdensky How large scale magnetic fields in the universe are formed is unclear. One possible mechanism is the transfer of magnetic energy from seed fields generated at kinetic scales to system scales. We investigate the mechanism of magnetic energy transfer to larger spatial scales in a system of parallel current filaments (corresponding to flux ropes or magnetic islands). We introduce a magnetic-reconnection-based model to describe the hierarchical coalescence of current filaments and analyze the self-similar properties and basic scalings of the system. A statistical model describing the coalescence in phase space is developed to confirm the local interaction approximation of the hierarchical model. Reduced MHD simulations confirm the existence of self-similar properties. Scaling exponents from simulation converge to the theoretical predictions as the resistivity is decreased, confirming that magnetic reconnection is the mechanism of inverse transfer of magnetic energy. |
Tuesday, November 6, 2018 3:00PM - 3:12PM |
JO7.00006: Anomalous resistivity generated by lower hybrid drift waves in the current sheet of a laboratory plasma in the presence of a guide field Jongsoo Yoo, Xuehan Guo, Hantao Ji, Masaaki Yamada, Jonathan Marc Jara-Almonte, William Randolph Fox, Fulvia Pucci The lower hybrid drift wave (LHDW) has been a candidate for anomalous resistivity in the current sheet of magnetic reconnection. Although LHDW has been observed in both laboratory ([e.g.] Carter et al. 2002) and space ([e.g.] Norgren et al. 2012), its role in fast reconnection has not been verified. In the Magnetic Reconnection Experiment (MRX), the effect of LHDW on anomalous resistivity is addressed by measuring fluctuations of the density and electric field with a four-tip Langmuir probe. Laboratory measurements indicate that LHDW is not stabilized near the X-line with a high guide field, generating anomalous resistivity there. The wave number vector determined by the hodogram technique shows that LHDW propagates mostly perpendicular to the magnetic field. The wave frequency is near or below the local lower hybrid frequency (fLH). This wave propagates mostly perpendicular to the background magnetic field, which is different from the electromagnetic fluctuations that have been recently reported as a whistler mode [Stechow et al. 2018]. |
Tuesday, November 6, 2018 3:12PM - 3:24PM |
JO7.00007: Extracting the reconnection electric field from electron distribution functions Naoki Bessho, Li-Jen Chen, Shan Wang, Michael Hesse Electron velocity distribution functions (VDFs) in the electron diffusion region (EDR) in magnetic reconnection are in a crescent shape in the velocity plane perpendicular to the magnetic field, due to meandering motion across a current sheet. In reconnection in Earth’s magnetotail, however, the crescent shape is significantly modified because of the reconnection electric field. We study the formation of multiple striations in an electron crescent VDF in magnetotail reconnection, by means of 2-D particle-in-cell simulations, theory, and space observations by Magnetospheric Multiscale (MMS). During the meandering motion in the EDR, electrons are accelerated by the reconnection electric field, which makes the crescent asymmetric in the velocity plane perpendicular to the magnetic field, and generates multiple striations. Analyzing electron motion in a simplified 1-D current sheet, we derive an equation to describe multiple striations of electron VDF, as a function of the distance from the magnetic neutral line and the strength of the reconnection electric field. The theory and simulations show excellent agreement. Applying the theory to VDF data obtained in observations by MMS, we derive the amplitude of the reconnection electric field in Earth’s magnetotail. |
Tuesday, November 6, 2018 3:24PM - 3:36PM |
JO7.00008: Suppression of Inverse Compton Radiation from Relativistic Magnetic Reconnection by Seed Photon Anisotropy John M. Mehlhaff, Gregory R. Werner, Dmitri A. Uzdensky Relativistic magnetic reconnection powers high-energy flares in astrophysical systems such as blazar jets and accreting black hole coronae. The inverse Compton (IC) process, where relativistic electrons upscatter soft seed photons to higher energies, is responsible for the most energetic component of the flaring spectra. We investigate how seed-photon anisotropy modifies the IC output of relativistic reconnection. For this task, we developed a code that calculates the angular and spectral content of Compton emission from arbitrary distributions of relativistic electrons and ambient photons. We take electron distributions from snapshots of particle-in-cell reconnection simulations and specify various functional forms for the background radiation. Reconnection-generated electron distributions develop focused beams of energetic particles. Illuminated isotropically, each beam produces an equally focused IC flare. However, when a particle beam is aligned even modestly with a directed seed photon field, we find the power and mean spectral energy of its flare can be suppressed by orders of magnitude. This result may have implications for reconnection-powered flares in, e.g., blazar jets, where the external seed photons can be highly anisotropic. |
Tuesday, November 6, 2018 3:36PM - 3:48PM |
JO7.00009: Kinetic simulations of magnetic reconnection: from collisionless to collisional regimes Samuel R Totorica, Tom Abel, Frederico Fiuza Magnetic reconnection is a fundamental plasma process that plays an important role in the evolution of magnetized plasmas in space physics, astrophysics, and the laboratory. The dynamics of magnetic reconnection are controlled by the dissipation region. Particle-in-cell (PIC) simulations with binary Monte Carlo Coulomb collisions allow the study of the interplay between collisional and kinetic effects during magnetic reconnection from first principles. We present the results of PIC simulations of magnetic reconnection for a large parameter scan over system size and collisionality. We will discuss how these parameters control the reconnection rate, particle acceleration, and formation of plasmoids from the collisionless to MHD regimes. The results will then be connected to recent experiments studying magnetic reconnection in laboratory high-energy-density plasmas. |
Tuesday, November 6, 2018 3:48PM - 4:00PM |
JO7.00010: Triggering QED processes by reconnection in near critical magnetic fields Kevin Schoeffler, Thomas Grismayer, Dmitri A Uzdensky, Ricardo Fonseca, Luis O Silva
Magnetic reconnection in a relativistic pair plasma is investigated using both 2D and 3D particle-in-cell simulations, where the magnetic field approaches the critical (Schwinger) field, allowing for quantum electrodynamic (QED) radiation and pair-creation. The simulations are performed with the QED module [1] of the OSIRIS framework that includes single photon emission by electrons and positrons (non-linear Compton scattering) and single photon decay into pairs (non-linear Breit-Wheeler). We show that even when the magnetic fields initially do not exhibit QED effects, the reconnection leads to both energetic particles, and locally enhanced magnetic fields, such that radiative cooling and pair-production processes begin to play an important role. We highlight that the radiation spectra that may be observable from such events differ strongly from the classical cases with much steeper spectra and the differences between 2D and 3D models. This sets the stage for testing if magnetic reconnection may be a mechanism powering gamma-ray flares in magnetar magnetospheres. [1] T. Grismayer et al., Physics of Plasmas 23, 056706 (2016) |
Tuesday, November 6, 2018 4:00PM - 4:12PM |
JO7.00011: Multi-plasmoid magnetic reconnection initiated by kinetic instabilities in colliding laser-produced plasmas William Fox, Gennady Fiksel, Daniel J Haberberger, Derek Schaeffer, Jackson Matteucci, Michael J Rosenberg, Suxing Hu, Andrew Howard, Dmitri A Uzdensky, Hye-Sook Park, Kirill Lezhnin, Amitava Bhattacharjee, Kai Germaschewski Magnetic reconnection enables the explosive conversion of magnetic field energy to plasma kinetic energy in plasmas ranging from laboratory to astrophysical environments. A challenge has been to understand how reconnection proceeds rapidly in large systems, though theory and simulations have shown reconnection rates to be greatly enhanced by plasmoid instabilities, which break a long current sheet into a hierarchy of shorter reconnection layers. Here we show experiments with colliding magnetized laser-produced plasmas obtaining extended current sheets (L/di ~ 100, where di is the ion skin depth) and low dissipation (high Lundquist number S= μ0LVA/η ~ 1000). The current sheet is observed to break up into a chain of a large number (~20) of magnetic islands. Proton radiography and optical shadowgraphy observe the size and temporal growth of island structures, which merge into larger structures over the course of the interaction. We discuss astrophysical implications of these observations and reconnection experiments at NIF at even larger system size. |
Tuesday, November 6, 2018 4:12PM - 4:24PM |
JO7.00012: Investigation of ion heating/transport process during high guide field reconnection using full-2D ultra high resolution ion Doppler tomography Hiroshi Tanabe, Qinghong Cao, Moe Akimitsu, Asuka Sawada, Haruaki Tanaka, Shun Kamiya, Setthivoine You, Michiaki Inomoto, Yasushi Ono We present detailed full 2D imaging measurement of ion heating during high guide field merging/reconnection experiment in TS-3U. We have found two types of fine structure both around X-point and downstream under the influence of better toroidal confinement. The high temperature region around the X-point is affected by Hall current jHall from the decoupling of ions and electrons, the characteristic heating profile rotates poloidally toward jHall×Bt direction. This characteristic is clearer in high field side (Bt depends on major radius for tokamak) and with higher mass ratio (enhancement of jHall due to the larger scale length than current sheet width). While at the end of merging, ion heating downstream is surrounded by closed flux surface formed by reconnected field lines and forms another fine structure. The high temperature profile downstream propagates vertically and finally forms poloidally double-ring-like structure under the influence of better toroidal confinement. Higher guide field strongly suppresses perpendicular heat transport (χ///χ⏊∼2(ωciτii)2>>10) and the double-peak ion temperature profile by outflow heating was transported to field line direction, finally forms hollow profile. |
Tuesday, November 6, 2018 4:24PM - 4:36PM |
JO7.00013: Finite collisional age effects on electrostatic shocks Istvan Pusztai, Andréas Sundström, Ian Abel, James L. Juno, Jason M TenBarge, Ammar Hakim In strictly collisionless electrostatic shocks the ion distribution function can develop discontinuities along phase space separatrices, due to the ion reflection. However the ions in laser-plasma experiments operating with dense and/or high-Z targets are not completely collisionless. In a semi-analytical orbit-averaged kinetic time-dependent treatment we consider the effect of a small but finite collisionality, which acts to smoothen these discontinuities into growing boundary layers. Ions diffusing into the previously empty trapped regions of phase space upset the charge balance, and affect the downstream properties of the shock. Importantly, this is a cumulative process, thus the collisional age of the shock, which can become significant during the lifetime of the shock, is more relevant for the process than the collision frequency, even if the latter is small. After comparisons to the theory, simulations with the continuum solver Gkeyll [J. Juno et al 2018 J. Comp. Phys. 353, 110] are used to study the evolution of weakly collisional shocks, at order unity collisional age. |
Tuesday, November 6, 2018 4:36PM - 4:48PM |
JO7.00014: Plasma dynamics of laser produced annular plasmas expanding in neutral gas backgrounds Mario Favre, Vicente Valenzuela-Villaseca, Wilmer Useche, Felipe Veloso, Edmund Wyndham, Heman Bhuyan We present further studies on the dynamics and overall plasma properties of an annular plasma. A ring shaped plasma is produced by focusing a Nd:YAG laser beam (1064 nm, 3.5 ns FWHM, ~2×109 W/cm2) onto a flat graphite target employing a combination of a 10 mrad axicon prism and a converging lens, which results in an initial ring-like shape plasma of 1 mm radius and ~150 mm thickness. The initial plasma expands either in Argon back ground, up to atmospheric pressure, and/or in the presence of a static axial magnetic field of a few kGauss characteristic strength. Both, the expansion dynamics and spatially resolved plasma density evolution are characterized using shadowgraphy and schlieren imaging, and Mach-Zehnder interferometry, with a Nd:YAG laser pulse (532 nm, 3.5 ns), and ns time resolved visible spectroscopy. As a main feature, it is observed that the radially expanding annular plasma implodes on axis, leading to the formation of a jet-like structure, which later evolves into a planar blast with a shock front. The comprehensive experimental data allows the effect of non-parallel temperature and density gradients on plasma dynamics to be evaluated. |
Tuesday, November 6, 2018 4:48PM - 5:00PM |
JO7.00015: e- e+ plasma-dark electromagnetism similarity establishes a (nearly) weaker-than-gravity bound on long-range dark matter self-interactions Nitin Shukla, Kevin Schoeffler, Jorge Vieira, Jeremy Mardon, Brain Feldstein, Ricardo Fonseca, Luis O Silva Dark matter (DM) has been theorized to be charged under its own ``dark electromagnetism" (DEM). Under this hypothesis, DM can behave like a cold collisionless plasma of self-interacting DM particles, and exhibit plasma-like instabilities with observational consequences [1]. Using PIC simulations [2], the nonlinear evolution of such instabilities driven by the interpenetration of two e- e+ plasma clouds that mimic the ``dark plasma" are explored. We show that the clouds slow down due to both oblique and Weibel generated electromagnetic fields, which deflect the particle trajectories, transferring bulk forward momentum into transverse momentum and thermal velocity spread. This process causes the flow velocity (vfl) to decrease by a factor of √3 in a time interval Δt ωp∼1/√α (c/vfl), close to 10 × the instability growth time, where α is the equipartition parameter determined by the non-linear saturation of the instabilities, and ωp is the plasma frequency. We show that if the typical DM slab length L > vfl Δt, this slowdown is always expected. Comparison with astronomical observations reveals strong new constraints on DEM with the dark electromagnetic self-interaction ΔαD < 4 × 10-25. [1] Heikinheimo et. al PRB 749 7 (2015) [2] Fonseca et al., PPCF 50, 124034 (2008) |
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