Reconnection and particle acceleration in three-dimensional current sheet evolution in moderately magnetized astrophysical pair plasma

Abstract

Magnetic reconnection, a plasma process converting magnetic energy to particle kinetic energy, is often invoked to explain magnetic energy releases powering high-energy flares in astrophysical sources including pulsar wind nebulae and black hole jets. Reconnection is usually seen as the (essentially two-dimensional) nonlinear evolution of the tearing instability disrupting a thin current sheet. To test how this process operates in three dimensions, we conduct a comprehensive particle-in-cell simulation study comparing two- and three-dimensional evolution of long, thin current sheets in moderately magnetized, collisionless, relativistically hot electron–positron plasma, and find dramatic differences. We first systematically characterize this process in two dimensions, where classic, hierarchical plasmoid-chain reconnection determines energy release, and explore a wide range of initial configurations, guide magnetic field strengths and system sizes. We then show that three-dimensional (3-D) simulations of similar configurations exhibit a diversity of behaviours, including some where energy release is determined by the nonlinear relativistic drift-kink instability. Thus, 3-D current sheet evolution is not always fundamentally classical reconnection with perturbing 3-D effects but, rather, a complex interplay of multiple linear and nonlinear instabilities whose relative importance depends sensitively on the ambient plasma, minor configuration details and even stochastic events. It often yields slower but longer-lasting and ultimately greater magnetic energy release than in two dimensions. Intriguingly, non-thermal particle acceleration is astonishingly robust, depending on the upstream magnetization and guide field, but otherwise yielding similar particle energy spectra in two and three dimensions. Although the variety of underlying current sheet behaviours is interesting, the similarities in overall energy release and particle spectra may be more remarkable.

Particle Injection and Nonthermal Particle Acceleration in Relativistic Magnetic Reconnection

“…In the relativistic regime relevant to AGN jets and PWNe, the upstream ambient "hot" magnetization parameter σ h ≡ B 2 0 /4πh (i.e., the enthalpy density of the reconnecting magnetic field divided by the relativistic enthalpy density h of the upstream plasma) is often very large (σ h 1), leading to strong nonthermal particle acceleration (see Hoshino & Lyubarsky 2012;Ka-gan et al 2015;Guo et al 2020, for focused reviews). Numerous PIC simulation studies of collisionless relativistic reconnection have found normalized reconnection rates of η rec ≡ v in /v out 0.1 (Liu et al 2015(Liu et al , 2017Liu et al 2020;Werner et al 2018) and efficient particle acceleration to high energies (Zenitani & Hoshino 2001;Jaroschek & Treumann 2004;Zenitani & Hoshino 2007, 2008Lyubarsky & Liverts 2008;Cerutti et al 2013;Cerutti et al 2014a;Sironi & Spitkovsky 2014;Melzani et al 2014;Guo et al 2014Guo et al , 2015Nalewajko et al 2015;Sironi et al 2016;Werner et al 2016;Werner & Uzdensky 2017;Werner et al 2018;Schoeffler et al 2019;Mehlhaff et al 2020;Hakobyan et al 2021). In particular, several studies conducted over the last two decades have shown that magnetic reconnection in the magnetically-dominated (σ h 1) regime robustly produces power-law energy distributions of energetic particles f ∝ γ −p (e.g., Zenitani & Hoshino 2001, 2008Jaroschek & Treumann 2004;Lyubarsky & Liverts 2008;Guo et al 2014Guo et al , 2015Guo et al , 2019…”

Non-thermal particle acceleration from maximum entropy in collisionless plasmas

“…For example, PIC simulations find comparable non-thermal particle acceleration from magnetic dissipation with different current sheet geometries and ensuing dynamics (e.g. Werner & Uzdensky 2021). PIC simulations of relativistic turbulence find that non-thermal particle distributions have a similar shape for different driving mechanisms (electromagnetic, solenoidal, compressive, imbalanced), despite different time scales to arrive at those distributions (Zhdankin 2021 b ; Hankla et al.…”

Fully Kinetic Shearing-box Simulations of Magnetorotational Turbulence in 2D and 3D. I. Pair Plasmas

“…These mechanisms can rapidly convert builtup magnetic energy to plasma energy, accelerating a significant fraction of particles to very high (nonthermal) energies (as shown by kinetic simulations, e.g., Daly et al 2008;Guo et al 2014;Sironi & Spitkovsky 2014;Werner et al 2016;Zhdankin et al 2017Zhdankin et al , 2019Comisso & Sironi 2018, 2019Werner et al 2018;Wong et al 2020). In particular, the tearing instability tends to disrupt thin current sheets; concurrently, the DKI can cause a current sheet to kink (ripple) and become distorted, dissipating magnetic energy as the sheet turbulently folds over on itself (Werner & Uzdensky 2021). These processes play an important role in dissipating magnetic fields generated by the MRI, and require a kinetic treatment to be captured selfconsistently.…”