Particle Injection and Nonthermal Particle Acceleration in Relativistic Magnetic Reconnection

Abstract

Magnetic reconnection in the relativistic regime has been proposed as an important process for the efficient production of nonthermal particles and high-energy emissions. Using fully kinetic particle-incell simulations, we investigate how guide-field strength and domain size affect characteristic spectral features and acceleration processes. We study two stages of acceleration: energization up until the injection energy γ inj and further acceleration that generates a power-law spectrum. Stronger guide fields increase the power-law index and γ inj , which suppresses acceleration efficiency. These quantities seemingly converge with increasing domain size, suggesting that our findings can be extended to largescale systems. We find that three distinct mechanisms contribute to acceleration during injection: particle streaming along the parallel electric field, Fermi reflection, and the pickup process. Fermi and pickup processes, related to the electric field perpendicular to the magnetic field, govern the injection for weak guide fields and larger domains. Meanwhile, parallel electric fields are important for injection in the strong guide field regime. In the post-injection stage, we find that perpendicular electric fields dominate particle acceleration in the weak guide field regime, whereas parallel electric fields control acceleration for strong guide fields. These findings will help explain the nonthermal acceleration and emissions in high-energy astrophysics, including black hole jets and pulsar wind nebulae.

Energy Partition of Thermal and Nonthermal Particles in Magnetic Reconnection

“…However, the efficiency was slightly enhanced in the case of a weak guide magnetic field. It is useful to mention that the similar result has been recently obtained by French et al (2022), in which the stronger guide fields suppress acceleration efficiency, and increase the power-law index and the injection energy defined by the transition energy from the thermal distribution to the power-law distribution.…”

“…Over time, we clearly observe that the Maxwellian plasmas are gradually heated owing to the reconnection heating process, and the high-energy components were further accelerated to form a nonthermal population. The behavior of these plasma heating and particle acceleration is basically same as many previous simulation studies (for example, Zenitani & Hoshino 2001;Hoshino et al 2001;Jaroschek et al 2004;Drake et al 2006;Liu et al 2011;Sironi & Spitkovsky 2011;Bessho & Bhattacharjee 2012;Cerutti et al 2012a;Guo et al 2014;Haggerty et al 2015;French et al 2022). Through examining of the energy interval of the time evolution of the energy spectra, it was found that the energy intervals were wider in the early acceleration stage compared with those in the later stage, suggesting that rapid energy gain occurs in the early stage.…”

Turbulence and Particle Acceleration in a Relativistic Plasma

“…Particles can also be stochastically accelerated by reflections from randomly moving magnetic mirrors, in the second-order Fermi acceleration process (e.g., Fermi 1949;Teller 1954;Kulsrud & Ferrari 1971;Selkowitz & Blackman 2004;Petrosian & Liu 2004). More recently, it has been realized that processes of magnetic reconnection may efficiently accelerate electrons, arguably due to non-ideal electric fields present in the reconnection layers, motional electric fields associated with plasma outflows, and particles being trapped in contracting and merging plasmoid islands formed in current sheets (e.g., Uzdensky et al 2011;Drake et al 2013;Sironi & Spitkovsky 2014;Guo et al 2020;French et al 2022;Sironi 2022). In a magnetically dominant plasma (where the magnetic energy exceeds the rest-mass energy of the particles), reconnection is able to produce non-thermal particles with hard ultra-relativistic energy spectra (e.g., Sironi & Spitkovsky 2014;Guo et al 2020).…”