Session TO05: Fundamental Plasmas: Reconnection II

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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 electron-only reconnection events in 2D numerical simulations of kinetic-Alfvén turbulence ran with particle-in-cell 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.   

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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 low-energy photons from an external source. Through the inverse Compton (IC) process, reconnection-energized particles upscatter these seed photons to the observed (e.g. X- and gamma-ray) flaring energies, simultaneously cooling down. When the energies of the scattering particles and seed photons are high enough, IC cooling transitions to the discrete Klein-Nishina (KN) regime, with particles delivering an order-unity fraction of their energies to single photons. In this limit, Comptonized photons may pair-produce 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 particle-in-cell 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.   

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Two-dimensional magnetohydrodynamics (MHD) simulations show that Petschek reconnection is unstable using a uniform resistivity. In this work, we show that a steady-state Petschek-like 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 Petschek-like reconnection accessible. Such a resistivity gradient may happen in the lower solar atmosphere where the plasma is highly stratified.   

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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 near-Earth and distant tail regions are still under debate. We perform two-dimensional (2D) and three-dimensional (3D) electromagnetic kinetic particle-in-cell simulations of substorm onset starting from an exact kinetic equilibirium that captures both the near-Earth 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 near-Earth region by a 3D cross-tail instability. We examine the roles of ballooning and drift-kink in the disruption, and compare with MMS and THEMIS observations. The disruption produces field-aligned currents and nonthermal particles, and we examine the acceleration mechanisms and connection to auroral signatures.   

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X-rays and gamma-rays from astrophysical sources, such as pulsar wind nebulae and blazar jets, are key observational probes that imply a source of high-energy electrons (and likely positrons) emitting via synchrotron and inverse Compton (IC) mechanisms. 2D particle-in-cell (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 non-radiative reconnection (in this large-magnetization 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.   

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Plasmoids -- quasi-circular 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 power-law distribution function, this energization process acts in such a way that the shape of the injected power law is conserved, while producing an additional non-thermal 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.   

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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 low-pressure 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.   

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We study the time-dependent formation and evolution of a current sheet (CS) in magnetized, collisionless, high-beta plasma using hybrid-kinetic particle-in-cell 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 field-biased 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 ion-Larmor 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).   

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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).   

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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).