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 GO4: Basic: Magnetic Reconnection & Shocks |
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Chair: Scott Baalrud, Iowa Room: Grand A |
Tuesday, October 22, 2019 9:30AM - 9:42AM |
GO4.00001: Generation of the lower-hybrid wave inside a reconnecting current sheet in space and laboratory Jongsoo Yoo, S. Wang, M. Ambat, H. Ji, J. Jara-Almonte, L.-J. Chen, M. Yamada, W. Fox, S. Bose, A. Alt, A. Goodman Lower hybrid waves during magnetic reconnection with a guide field are studied with data from the Magnetospheric Multiscale (MMS) mission and the Magnetic Reconnection Experiment (MRX). The observed lower hybrid wave (LHW) propagates almost perpendicular to the magnetic field and is capable of inducing fluctuations in the electric and magnetic fields as well as density. A simple theoretical model has been developed to explain the excitation of the LHW, which shows that the high perpendicular electron current is the free energy source for the wave. The guide field is also required to keep the electron beta lower than the unity inside the current sheet. The parallel electron current affects the frequency of the most unstable mode. A magnetotail event and MRX data show that this wave exists in the electron diffusion region, possibly affecting the reconnection dynamics. [Preview Abstract] |
Tuesday, October 22, 2019 9:42AM - 9:54AM |
GO4.00002: Electron dynamics driven by nonlinear lower hybrid waves in a magnetic reconnection layer Li-Jen Chen, Shan Wang, Jonathan Ng, Olivier Le Contel, Naoki Bessho, Michael Hesse, Thomas Moore, BARBARA GILES, Roy Torbert Lower-hybrid waves although widely considered to be important in magnetic reconnection have not been observed experimentally in the core region of a reconnection layer. Here in-situ measurements from the terrestrial magnetotail are discussed to report nonlinear lower-hybrid waves, including solitary structures, driving electron vortical flows and demagnetizing electrons in a guide-field reconnection layer where correlated magnetic field and plasma jet reversals occur. The vortical flows generate new magnetic fields with strengths comparable to the guide field. Electrons form nongyrotropic distribution functions as they are accelerated by the wave electric field inside the flow vortices. The measurements reveal a regime of strong electron-wave interaction and how this interaction modifies the kinetic structure of the reconnection layer. [Preview Abstract] |
Tuesday, October 22, 2019 9:54AM - 10:06AM |
GO4.00003: The formation of power-law energy spectrum in 3D low-beta magnetic reconnection Fan Guo, Xiaocan Li, Hui Li, Adam Stanier, Patrick Kilian Magnetic reconnection has been proposed as a theory for explaining particle acceleration in solar flares. However, previous kinetic simulations of non-relativistic reconnection have not been able to obtain a power-law energy spectrum, which is a key observational feature of particle distribution. Here we present results from 3D fully kinetic particle-in-cell simulations of reconnection in the non-relativistic low-beta regime. We show that a clear power-law energy spectrum can form and sustain extensively during the simulation. Comparing with 2D simulations, where high-energy particles are trapped deep in magnetic islands, 3D simulations enable stronger acceleration for high-energy particles due to stochastic magnetic field lines and wave-particle scattering of high-energy particles. These effects lead to a nearly constant acceleration rate for particles at different energies. The power-law index is a balance of particle acceleration and particle escape from major acceleration region. This study clarifies the formation condition of power-law energy spectrum in a reconnection layer and has important implication for understanding particle energization during solar flares. [Preview Abstract] |
Tuesday, October 22, 2019 10:06AM - 10:18AM |
GO4.00004: Role of the Plasmoid Instability in 2D and 3D Magnetohydrodynamic Turbulence Chuanfei Dong, Liang Wang, Yi-Min Huang, Luca Comisso, Amitava Bhattacharjee Magnetohydrodynamic (MHD) turbulence plays a fundamental role in the transfer of energy in a wide range of space and astrophysical systems, from the solar corona and accretion disks, to the interstellar medium and galaxy clusters. An important feature of MHD turbulence is the tendency to develop sheets of strong electric current density. These current sheets are natural sites of magnetic reconnection, leading to the formation of plasmoids that eventually disrupt the sheet-like structures in which they are born. In this study, we investigate the role of the plasmoid instability in both 2D and 3D MHD turbulence by means of high-resolution direct numerical simulations. At sufficiently large magnetic Reynolds number, the combined effects of dynamic alignment and turbulent intermittency lead to a copious formation of plasmoids in a multitude of intense current sheets. The disruption of current sheet structures facilitates the energy cascade towards small scales, leading to the breaking and steepening of the energy spectrum in the plasmoid-mediated regime. [1] C. F. Dong, L. Wang, Y.-M. Huang, L. Comisso, A. Bhattacharjee, Role of the Plasmoid Instability in Magnetohydrodynamic Turbulence, Phys. Rev. Lett. 121, 165101 (2018). [Preview Abstract] |
Tuesday, October 22, 2019 10:18AM - 10:30AM |
GO4.00005: Magnetic reconnection in kink-unstable jets Bart Ripperda, Alexander Philippov, Jordy Davelaar, Lorenzo Sironi Compact objects like the black holes in the centers of galaxies are known to launch jets that reach highly relativistic speeds. The jet can efficiently accelerate particles to non-thermal energies, yet the acceleration mechanism is still under debate. The kink instability can generate macroscopic reconnecting current sheets which are liable to the plasmoid instability. Magnetic reconnection and subsequent plasmoid formation are conjectured to accelerate particles to non-thermal energies. In magnetohydrodynamics (MHD) models, magnetic reconnection occurs due to a finite resistivity breaking the frozen-in condition. We explore plasmoid formation in kink-unstable jets in the framework of relativistic resistive MHD with the Black Hole Accretion Code (BHAC). We model a simplified configuration by isolating the jet from its launching site. In this way the setup is controllable by setting typical plasma parameters, such that different magnetization and resistivity regimes can be explored. We analyze reconnection properties and compare to both ideal MHD results and full kinetic simulations. We show that the interaction between the kink instability and the reconnecting current sheets results in plasmoid formation and heating of the plasma. [Preview Abstract] |
Tuesday, October 22, 2019 10:30AM - 10:42AM |
GO4.00006: Magnetic reconnection in highly-extended current sheets at the National Ignition Facility W. Fox, D.B. Schaeffer, M. Rosenberg, G. Fiksel, H.S. Park, J. Matteucci, K. Lezhnin, A. Bhattacharjee, D. Uzdensky, C.K. Li, F. Seguin, S.X. Hu, A. Shvydky, D. Kalantar, B.A. Remington Magnetic reconnection enables the explosive conversion of magnetic field energy to plasma kinetic energy and energized particles in plasmas ranging from laboratory to astrophysical environments. A significant issue is understanding fast reconnection in systems much larger than intrinsic plasma scales. We present results from experiments at the National Ignition Facility to study reconnection in large and highly-extended current sheets. The magnetic fields are self-generated in two neighboring plasma plumes by the Biermann battery effect. By tiling a large number of NIF beams to create each plume, highly-elongated plasmas collide, producing well-controlled boundary conditions driven by 1-D flows. This allows detailed reconstruction of experimental magnetic fields from proton radiography data, obtained using mono-energetic protons from an imploded DHe3 capsule. We report observations from reconstructed magnetic fields, including the current sheet width, and the reconnection rate. Results are compared to particle-in-cell simulations which include the Biermann-battery generation self-consistently. [Preview Abstract] |
Tuesday, October 22, 2019 10:42AM - 10:54AM |
GO4.00007: Thomson scattering measurements of particle drift velocities in a reconnection current sheet Lee Suttle, J.D. Hare, J.W.D. Halliday, D. Russell, E. Tubman, V. Valenzuela-Villaseca, S.V. Lebedev The release of stored magnetic energy through magnetic reconnection often leads to imbalanced particle energy distributions, however the mechanisms producing nonequipartion are poorly understood. Thomson scattering measurements in pulsed-power experiments of driven, collisional magnetic reconnection have shown strong heating of ions, with Ti\textgreater \textgreater Te, much greater than can be accounted for by classical shock, resistive and viscous heating mechanisms [1]. We present newly performed measurements of the particle drift velocities in the direction of the reconnection electric field. These reveal ions acting as charge carriers for a significant fraction of the reconnection current, suggesting that the electrons remain magnetized to the reconnected field lines. We also observe relative electron-ion velocities exceeding the sound speed of the plasma, which could to lead to ion acoustic instability responsible for the anomalous heating. We assess the scattered signal strength for consistency with this hypothesis. [1] Suttle et al., PRL 116, 225001 (2016) [Preview Abstract] |
Tuesday, October 22, 2019 10:54AM - 11:06AM |
GO4.00008: Limits on the compression of magnetic islands in strongly radiative magnetic reconnection Kevin Schoeffler, Thomas Grismayer, Dmitri Uzdensky, Ricardo Fonseca, Luis Silva The evolution of magnetic islands generated in a reconnecting relativistic pair plasma is investigated using 2D and 3D particle-in-cell simulations in strong magnetic fields. For sufficiently strong fields (and a weak guide field), radiation cooling leads to compression of the magnetic islands, which amplifies fields and plasma density [1]. The quantum electrodynamic (QED) module [2] of the OSIRIS framework allows us to model the radiation as either classical radiation reaction or the QED emission of discrete photons according to non-linear Compton scattering, as well as single photon decay into pairs (non-linear Breit-Wheeler). These QED effects are important for the field strengths close to the critical (Schwinger) field occurring in magnetar magnetospheres, where gamma-ray flares occur. We show that the measured increases in density n and magnetic fields B due to compression are limited by power-laws in n-B space. In 3D, the magnetic flux ropes become kink-unstable, which effectively limits the compression of density. However, increasing upstream plasma magnetization leads to stronger magnetic compression, which in turn leads to increased pair production. [1] K. Schoeffler et al., ApJ, 870, 1 (2019) [2] T. Grismayer et al., Phys. Plasmas 23, 056706 (2016) [Preview Abstract] |
Tuesday, October 22, 2019 11:06AM - 11:18AM |
GO4.00009: Fusion Burning in Magnetically Confined Toroidal Plasmas R. Gatto, B. Coppi, A. Cardinali, B. Basu The thermonuclear instability in toroidal fusion burning plasmas [1] can manifest itself as a driving factor of modes that are radially localized around closed field lines on rational magnetic surfaces. The radial profile of the electron temperature perturbations can be of two parities: even and odd. In the first case the effective longitudinal thermal conductivity which can hinder the onset of the thermonuclear instability, for realistic values of relevant plasma parameters, can be reduced by the effects of modes involving magnetic reconnection when these have a radial transverse reconnected field with a odd (radial) profile. In the second case magnetic reconnection is shown to have a considerably different effect and is characterized by (reconnected) transverse fields that have an even radial profile. The existence of pairs of spatial ``thermonuclear'' singularities is pointed out.\\ $[1]$ B. Coppi and the Ignitor Program Members, Nucl. Fus., 55, 053011 (2015). [Preview Abstract] |
Tuesday, October 22, 2019 11:18AM - 11:30AM |
GO4.00010: Connection Theorem in General Relativity Felipe Asenjo The magnetohydrodynamic theorem on the conservation of the magnetic connections between plasma elements is generalized to relativistic plasmas in curved spacetime. This is achieved by using a generalized Ohm's law that contains Hall effect and electron pressure. The connections between plasma elements, which are established by a covariant connection equation, display a particularly complex structure in curved spacetime. It is shown that these connections can be interpreted in terms of magnetic field lines alone by adopting a 3 รพ 1 foliation of spacetime. The consequences of thermal-inertial effects are discussed.This theorem is a key step in order to define magnetic reconnection in high-energy plasmas in General Relativity. Implications are outlined [Preview Abstract] |
Tuesday, October 22, 2019 11:30AM - 11:42AM |
GO4.00011: Electron energy partition across interplanetary shocks near 1 AU Lynn Wilson III Analysis of 15,314 electron velocity distribution functions (VDFs) within $\pm$2 hours of 52 interplanetary (IP) shocks observed by the Wind spacecraft near 1 AU are presented. The electron VDFs are fit to the sum of three model functions for the cold dense core, hot tenuous halo, and field-aligned beam/strahl component. The halo and beam/strahl are always modeled as bi-kappa VDFs but the core is found to be best modeled by a bi-self-similar, not bi-Maxwellian, for nearly all cases and a bi-kappa for a small fraction of the events. The self-similar distribution deviation from a Maxwellian is a measure of inelasticity in particle scattering from waves and/or turbulence. The range of values defined by the lower and upper quartiles for the kappa exponents are $\kappa{\scriptstyle_{ec}}$ $\sim$ 5.40--10.2 for the core, $\kappa{\scriptstyle_{eh}}$ $\sim$ 3.58--5.34 for the halo, and $\kappa{\scriptstyle_{eb}}$ $\sim$ 3.40--5.16 for the beam/strahl. The lower-to-upper quartile range of symmetric bi-self-similar core exponents are $s{\scriptstyle_{ec}}$ $\sim$ 2.00--2.04, and asymmetric bi-self-similar core exponents are $p{\scriptstyle_{ec}}$ $\sim$ 2.20--4.00 for the parallel exponent, and $q{\scriptstyle_{ec}}$ $\sim$ 2.00--2.46 for the perpendicular exponent. The rest of the p [Preview Abstract] |
Tuesday, October 22, 2019 11:42AM - 11:54AM |
GO4.00012: Hybrid Simulations of Cosmic Ray Modified Shocks and Nonlinear Diffusive Shock Acceleration. Colby Haggerty, Damiano Caprioli We present simulations of collisionless plasma shocks performed with the first hybrid code to include relativistic ion dynamics (\textit{dHybridR}). In these simulations, we show evidence of modifications to the fluid shock jump conditions caused by the cosmic ray (CR) pressure. The rapid transition to CR modified shocks occurs soon after the onset of the shock. CR modified shocks are expected to have a harder power law slope, however we find a significantly steeper spectral index of nearly p\textasciicircum -5 for early times and hardening as the simulation progresses to approximately p\textasciicircum -4.3 as the compression ratio saturates. To understand these simulation results we present a non-linear theory of diffusive shock acceleration (DSA) which includes considerations for both the magnetic field and the CR modified jump conditions. The steep spectra is shown to be caused by a larger fraction of CRs escaping downstream than predicted by DSA. The enhanced escape rate of CRs is facilitated by the compressed magnetic field just downstream of the shock. This magnetic field was originally generated from the CR streaming instability upstream of the shock, and thus the compressed/enhanced magnetic field acts to regulate the energetic run-away problem that CR modified shocks presents for DSA. [Preview Abstract] |
Tuesday, October 22, 2019 11:54AM - 12:06PM |
GO4.00013: Non-relativistic collisionless shock formed by magnetic piston Quentin Moreno-Gelos, Anabella Araudo, Vladimir Tikhontchouk, Stefan Weber By using PIC simulations we study the collision of two fast plasma flows with one of them carrying a magnetic field. Ion interpenetration results in the formation of a magnetic piston with the magnetic field compression proportional to the density ratio of the colliding plasmas. The thickness of the piston increases with time and it turns into a reverse magnetized shock after less than one ion gyro period. The counter-propagating ions in the non-magnetized plasma upstream the piston excite the ion Weibel instability, which is gradually transforming in the magnetic turbulence, isotropize the particles on several ion trapping periods and eventually form a forward electromagnetic shock. The two shocks of different nature have a common downstream region and propagate in opposite directions. Ion gyration in the reverse shock results in a periodic ejection of fast ions from the Weibel mediated shock, forming jets and continuing their acceleration upstream. The Weibel filaments penetrate the magnetic piston and form magnetic cavities filled with a hot plasma downstream the reverse shock. These two localized structures -- jets and cavities -- contribute to the particle acceleration, which is more efficient than the one taking place in a simple Weibel-mediated shock. [Preview Abstract] |
Tuesday, October 22, 2019 12:06PM - 12:18PM |
GO4.00014: Prospects of multiple-ion-species shock studies in plasma-jet driven experiments Colin Adams, Ameer Mohammed, Maximilian Schneider An experimental campaign aims to generate multi-ion-species shocks during collisions of high-Mach-number plasma jets with stagnant plasma. In these experiments, the mean-free-path is small enough for the jet interaction to be collisional yet large enough to attempt diagnosis of the shock structure over a few hundred mean-free-paths. Shocks have been identified in these interactions and the parameter space is presently being investigated to confirm whether the observed shocks are present locally in multi-ion-species plasmas. Direct measurements and inferred plasma parameters are obtained using a suite of diagnostics which includes high spatial resolution spectroscopy and multi-chord interferometry. Preliminary results suggest a path toward experimentally resolving the structure of ion shock layers, with implications to the basic physics of Type II supernova explosions and shocks present in inertial-confinement fusion implosions. [Preview Abstract] |
Tuesday, October 22, 2019 12:18PM - 12:30PM |
GO4.00015: Study of self-generated magnetic field and electric field at the front of a strong shock in helium by multi-angle proton radiography Rui Hua, Joohwan Kim, Mark Sherlock, Mathieu Bailly-Grandvaux, Farhat Beg, Scott Wilks, Christopher McGuffey, Yuan Ping We report the measurement of both the magnetic field and electric field at a Mach 6 shock front in a low-density helium gas system. In the experiments, strong shocks were generated using two long pulse beams of 1 kJ total energy in 0.5 ns square pulse from the OMEGA EP laser system. A shorter pulse laser of 10 ps, 400 J was applied to generate TNSA protons for radiography. Given the difference in responses of charged particles to magnetic and electric fields, protons probed the shock front region from multiple angles in order to distinguish the magnetic field from the electric field. Probing obliquely, constraining deflection patterns identified a strong magnetic field on the order of $\sim$ 5-7 T, while orthogonal probing revealed an electric field corresponding to $\sim$ 300 V. Simulations indicate that the Biermann battery effect and the electron pressure gradient at the shock front account for the generation of the magnetic field and electric field, respectively. [Preview Abstract] |
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