Bulletin of the American Physical Society
62nd Annual Meeting of the APS Division of Plasma Physics
Volume 65, Number 11
Monday–Friday, November 9–13, 2020; Remote; Time Zone: Central Standard Time, USA
Session TO05: Fundamental Plasmas: Reconnection IILive

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Chair: Sayak Bose, PPPL 
Thursday, November 12, 2020 9:30AM  9:42AM Live 
TO05.00001: Particle Acceleration in Electrononly Reconnection Cristian Vega, Vadim Roytershteyn, Gian Luca Delzanno, Stanislav Boldyrev Observations of the Earth's magnetosheath by the NASA Magnetospheric Multiscale (MMS) mission have drawn attention to reconnection events in which only electron outflows were detected, without accompanying ion jets [1]. In [2] we detected these electrononly reconnection events in 2D numerical simulations of kineticAlfvén turbulence ran with particleincell code VPIC and found their sizes to be smaller than the ion inertial scale and the reconnection rate to be close to 0.1. In this presentation we study particle heating in these novel reconnection events both numerically and analytically. [1] – Phan, T. D., Eastwood, J. P., Shay, M. A., et al. 2018, Nature, 557, 202. [2] – Vega, C., Roytershteyn, V., Delzanno, G. L., Boldyrev, S., 2020, The Astrophysical Journal Letters, 893, L10. [Preview Abstract] 
Thursday, November 12, 2020 9:42AM  10:06AM Live 
TO05.00002: KleinNishina: A Qualitatively New Regime of Radiative Relativistic Magnetic Reconnection (PhD Oral24) John Mehlhaff, Gregory Werner, Dmitri Uzdensky, Mitchell Begelman Astrophysical plasmas commonly exhibit a number of exotic characteristics as fascinating as the environments that host them. In many systems, magnetic field lines snap through relativistic reconnection, powering spectacular flares. Sometimes, such as in blazar jets and accretion disk coronae, the reconnection region is bathed in lowenergy photons from an external source. Through the inverse Compton (IC) process, reconnectionenergized particles upscatter these seed photons to the observed (e.g. X and gammaray) flaring energies, simultaneously cooling down. When the energies of the scattering particles and seed photons are high enough, IC cooling transitions to the discrete KleinNishina (KN) regime, with particles delivering an orderunity fraction of their energies to single photons. In this limit, Comptonized photons may pairproduce with their seed photon population, providing another channel for feedback on the evolving plasma (besides the IC cooling itself). Using a new module for the particleincell code Zeltron, we ran a series of relativistic magnetic reconnection simulations in this qualitatively distinct IC regime. We report initial results from these simulations, studying radiative feedback on reconnection in the previously unexplored KN limit. [Preview Abstract] 
Thursday, November 12, 2020 10:06AM  10:18AM Live 
TO05.00003: Petscheklike reconnection in the transition region of a hyperbolic tangent resistivity profile ShanChang Lin, YiHsin Liu Twodimensional magnetohydrodynamics (MHD) simulations show that Petschek reconnection is unstable using a uniform resistivity. In this work, we show that a steadystate Petscheklike reconnection develops in the transition region of a hyperbolic tangent resistivity profile. Our study suggests that the gradient of resistivity along the outflow direction is crucial in making Petscheklike reconnection accessible. Such a resistivity gradient may happen in the lower solar atmosphere where the plasma is highly stratified. [Preview Abstract] 
Thursday, November 12, 2020 10:18AM  10:30AM Live 
TO05.00004: Kinetic Simulations of Substorm Onset From the Coupling of Reconnection and ThreeDimensional CrossTail Instabilities Samuel Totorica, Amitava Bhattacharjee Substorms are explosive disruptions that play a critical role in the response of the magnetosphere to the solar wind. Observations show a slow growth phase involving magnetic flux loading and current sheet thinning, followed by a rapid onset. Models involving various instabilities have been proposed, however the onset mechanisms and interplay between nearEarth and distant tail regions are still under debate. We perform twodimensional (2D) and threedimensional (3D) electromagnetic kinetic particleincell simulations of substorm onset starting from an exact kinetic equilibirium that captures both the nearEarth and distant tail regions. Our simulations reproduce features of observations including reconnection onset, plasmoid formation, current sheet flapping, and dipolarization fronts. Comparing 2D and 3D allows the isolation of reconnection from 3D instabilities. A critical difference between 2D and 3D is an explosive disruption of dipolarization fronts in the nearEarth region by a 3D crosstail instability. We examine the roles of ballooning and driftkink in the disruption, and compare with MMS and THEMIS observations. The disruption produces fieldaligned currents and nonthermal particles, and we examine the acceleration mechanisms and connection to auroral signatures. [Preview Abstract] 
Thursday, November 12, 2020 10:30AM  10:42AM Live 
TO05.00005: Radiative Relativistic Reconnection in 3D Gregory Werner, John Mehlhaff, Dmitri Uzdensky Xrays and gammarays from astrophysical sources, such as pulsar wind nebulae and blazar jets, are key observational probes that imply a source of highenergy electrons (and likely positrons) emitting via synchrotron and inverse Compton (IC) mechanisms. 2D particleincell (PIC) simulation has shown that relativistic magnetic reconnection converts magnetic energy to plasma energy, powering nonthermal particle acceleration (NTPA) that might explain observations. Using PIC simulation including the radiation reaction force, we study the effects of synchrotron and IC cooling on 3D reconnection in collisionless relativistic pair plasma with large upstream magnetization and weak guide field. We find that 2D and 3D reconnection with radiative cooling yield similar reconnection rates and NTPA, just as in nonradiative reconnection (in this largemagnetization regime). While the emitted IC spectra are also similar, the synchrotron spectra differ: in 3D fewer particles accelerate beyond the synchrotron burnoff limit. Importantly, kinetic beaming (which requires radiative cooling) degrades in 3D, likely reducing observed intensity and variability. Thus 2D simulation accurately yields global reconnection rates and NTPA, but 3D simulation is required to obtain observational signatures. [Preview Abstract] 
Thursday, November 12, 2020 10:42AM  10:54AM Live 
TO05.00006: Secondary Energization in Compressing Plasmoids during Magnetic Reconnection Hayk Hakobyan, Maria Petropoulou, Anatoly Spitkovsky, Lorenzo Sironi Plasmoids  quasicircular structures formed in reconnecting current sheets  were previously considered to be the graveyards of energetic particles. We demonstrate the important role of plasmoids in shaping the particle energy spectrum in relativistic reconnection. Using 2D PIC simulations in pair plasmas, we study a secondary particle energization process that takes place inside compressing plasmoids. We demonstrate that plasmoids grow in time, while their interiors compress, amplifying the internal magnetic field. The magnetic field felt by particles injected in an isolated plasmoid increases linearly with time, which leads to particle energization as a result of magnetic moment conservation. For particles injected with a powerlaw distribution function, this energization process acts in such a way that the shape of the injected power law is conserved, while producing an additional nonthermal tail $E^{3}$ at higher energies followed by an exponential cutoff. The cutoff energy, which increases with time as $E_{\rm cut}\propto\sqrt{t}$, can greatly exceed $\sigma m_e c^2$. We analytically predict the secondary acceleration timescale and the shape of the emerging particle energy spectrum, which can be of major importance in certain astrophysical systems, such as blazar jets. [Preview Abstract] 
Thursday, November 12, 2020 10:54AM  11:06AM Live 
TO05.00007: Connecting XLine Heating Efficiency to the Reconnection Rate Problem YiHsin Liu, ShanChang Lin, Michael Hesse, Fan Guo, Xiaocan Li, Haocheng Zhang, Sarah Peery During magnetically dominated relativistic reconnection, inflowing plasma depletes the initial relativistic pressure at the x line, and collisionless plasma heating inside the diffusion region appears to be insufficient to overcome this pressure loss. The resulting significant pressure drop causes a collapse at the x line, essentially a localization mechanism of the diffusion region necessary for fast reconnection. The extension of this lowpressure region (into the outflow) further explains the bursty nature of antiparallel reconnection because a once opened outflow exhaust can also collapse, which repeatedly triggers secondary tearing islands. However, a stable single x line reconnection can be achieved when an external guide field exists, since the reconnecting magnetic field component rotates out of the reconnection plane at outflows, providing additional magnetic pressure to keep the exhaust open. [Preview Abstract] 
Thursday, November 12, 2020 11:06AM  11:18AM Live 
TO05.00008: Tearing and Reconnection in a Mirrorinfested Current Sheet Himawan Winarto, Matthew Kunz We study the timedependent formation and evolution of a current sheet (CS) in magnetized, collisionless, highbeta plasma using hybridkinetic particleincell simulations. The CS is thinned using a persistently driven incompressible shear flow, which makes the CS increasingly unstable to tearing. This thinning also increases the strength of the magnetic field seen by inflowing fluid elements, which by adiabatic invariance produces a fieldbiased pressure anisotropy in the plasma. At large values of the plasma beta, this pressure anisotropy is large enough to excite the mirror instability, which deforms the reconnecting field on ionLarmor scales. The effect of this instability on the structure of the CS and on the onset of magnetic reconnection is studied, making contact with the theory proposed by Alt & Kunz (2019, J. Plasma Phys. 85, 764850101). [Preview Abstract] 
Thursday, November 12, 2020 11:18AM  11:30AM Live 
TO05.00009: Magnetothermal Reconnection: Concept of Substance and Definitive Proof B. Coppi, B. Basu A purely oscillatory mode is identified that involves magnetic reconnection in the presence of a significant electron temperature gradient, of a finite transverse electron thermal conductivity $(\propto D_{\bot }^{e} )$ and of a relatively large longitudinal thermal conductivity $(\propto D_{\parallel }^{e} )$. The presence of a finite electrical resistivity, electron inertia, or of an inductivity, that is of a conventional Ohm's law, is not required although the mode is influenced by the relevant effects. The simplest unperturbed confinement configuration in which these modes can be sustained is a plane geometry with a sheared magnetic field. The mode frequency depends on the electron temperature gradient and the phase velocity is in the direction of the electron diamagnetic velocity. Since the width of the layer where reconnection takes place is proportional to $\left( {D_{\bot }^{e} /D_{\parallel }^{e} } \right)^{1/4}L_{\bot } $, where $L_{\bot } $ is a macroscopic length, and there is ample experimental evidence of the fact that $D_{\bot }^{e} $ can be considerable the issue of dealing with reconnection layers that can be unrealistically small can be avoided. [1] B. Coppi and B. Basu, Phys. Plasmas \textbf{26}, 042115 (2019). [Preview Abstract] 
Thursday, November 12, 2020 11:30AM  11:42AM Live 
TO05.00010: Role of Magnetic Reconnection and of Thermonuclear Electron Heating in Fusion Burning Plasmas B. Coppi, R. Gatto, B. Basu The approach to ignition conditions involves the role of tridimensional modes and processes. A class of these concerns the case where most of the energy of the generated fusion reaction products is deposited on the electron population and radially extended or localized modes involving magnetic reconnection [1] can be excited. Regimes with a relatively large longitudinal electron thermal conductivity and a significant local electron temperature gradient are considered. Then a pair of singularities of the perturbed electron temperature associated with the rate of (thermonuclear) heating of the electron population are found to emerge in the vicinity of the rational surfaces around which magnetic reconnection can take place. The analysis of the perturbed electron temperature profile involves four radial asymptotic regions: an outer ideal MHD region, a `thermal' region related to the thermal conductivities, a `thermonuclear region', and an innermost region. [1] B. Coppi and B. Basu, Phys. Plasmas \textbf{26}, 042115 (2019). [Preview Abstract] 
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