The Generation of Nonthermal Particles in the Relativistic Magnetic Reconnection of Pair Plasmas

Abstract: Particle acceleration in the magnetic reconnection of electron-positron plasmas is studied by using a particle-in-cell simulation. It is found that a significantly large number of nonthermal particles are generated by the inductive electric fields around an X-type neutral line when the reconnection outflow velocity, which is known to be an Alfv\'{e}n velocity, is on the order of the speed of light. In such a relativistic reconnection regime, we also find that electrons and positrons form a power-law-like energy distribution through their drift along the reconnection electric field under the relativistic Speiser motion. A brief discussion of the relevance of these results to the current sheet structure, which has an antiparallel magnetic field in astrophysical sources of synchrotron radiation, is presented.

• The paper demonstrates that high-resolution PIC simulations capture the efficient acceleration of nonthermal particles via inductive electric fields in pair plasmas.
• It reveals that relativistic Speiser motion produces power-law energy distributions, with particle energies scaling as mc²ΩcT.
• The study highlights the critical role of an extended acceleration region near the X-type neutral line in confining particles and boosting energy gains.

Overview of Particle Acceleration in Relativistic Magnetic Reconnection

The study by Zenitani and Hoshino explores a significant mechanism of particle acceleration in the context of relativistic magnetic reconnection within pair plasmas. Using high-resolution particle-in-cell (PIC) simulations, the authors examine the behaviors associated with non-thermal particle generation around an X-type neutral line in an environment where the reconnection outflow velocity approaches the speed of light. The research provides a quantitative evaluation of the acceleration process and highlights its implications for high-energy astrophysical phenomena.

At the core of the study is the investigation of particle acceleration due to inductive electric fields present near the X-type neutral lines in relativistic reconnection settings. The researchers found that both electrons and positrons formed a power-law-like energy distribution through the relativistic Speiser motion, facilitated by their drift along the reconnection electric field. This behavior reflects a significant transformation of magnetic energy to particle kinetic energy, a hallmark characteristic of magnetic reconnection processes.

Key Findings

• Non-Thermal Particle Generation: The simulations reveal that a remarkable number of non-thermal particles are generated by the dynamic inductive electric fields surrounding the X-type neutral line. The resultant energy spectra manifest as power-law distributions with an index of roughly 1, indicative of the efficient acceleration of particles to high energies.
• Relativistic Speiser Motion: The motion of particles within the reconnection region is effectively described by the relativistic Speiser orbit. The maximum energy attained by particles grows proportional to $mc^2\Omega_cT$, suggesting significant energy escalation through sustained exposure to the reconnection electric field.
• Size of Acceleration Region (AR): The AR, where $|E_y| \geq |B_{total}|$, plays a critical role in determining the extent of particle acceleration. The AR in these relativistic scenarios proves to be large relative to the plasma sheet thickness, suggesting effective particle confinement and prolonged acceleration.

Implications and Future Directions

Zenitani and Hoshino's findings have substantial implications for understanding high-energy astrophysical environments such as pulsar winds and active galactic nuclei, where relativistic pair plasmas are expected to arise naturally. The acceleration process demonstrates the potential significance of reconnection as a source of non-thermal and power-law distributions observed in the spectra of astrophysical sources.

Future studies should aim to incorporate additional complexities like Coulomb collisions, pair-production, and pair-annihilation to better emulate astrophysically accurate conditions. Moreover, extending simulation scale and incorporating multi-reconnection events could yield insights into higher energy accelerations beyond the current observed $\varepsilon_{max} \sim 38 mc^2$.

In summary, this research contributes critical insights into the particle acceleration processes intrinsic to relativistic magnetic reconnection, highlighting its relevance to cosmic particle energetics. By elucidating the dynamics and efficiencies of particle acceleration at scales commensurate with astrophysical phenomena, Zenitani and Hoshino pave the way for further explorations into high-energy plasma environments.