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Abstract While relativistic magnetic reconnection in pair plasmas has emerged in recent years as a candidate for the origin of radiation from extreme astrophysical environments, the corresponding subrelativistic pair-plasma regime has remained less explored, leaving open the question of how relativistic physics affects reconnection. In this paper, we investigate the differences between these regimes by contrasting two-dimensional particle-in-cell simulations of reconnection in pair plasmas with relativistic magnetization (σ≫ 1) and subrelativistic magnetization (σ< 1). By utilizing unprecedentedly large domain sizes and outflow boundary conditions, we demonstrate that lowering the magnetization results in a change in the reconnection geometry from a plasmoid chain to a Petschek geometry, where laminar exhausts bounded by slow-mode shocks emanate from a single diffusion region. We attribute this change to the reduced plasmoid production rate in the low-σcase: When the secondary tearing rate is sufficiently low, plasmoids are too few in number to prevent the system from relaxing into a stable Petschek configuration. This geometric change also affects particle energization: We show that while high-σplasmoid chains generate power-law energy spectra, low-σPetschek exhausts merely heat incoming plasma and yield negligible nonthermal acceleration. These results have implications for predicting the global current sheet geometry and the resulting energy spectra in a variety of systems.
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This content will become publicly available on October 15, 2026
Driven Collisionless Reconnection of Force-free Flux Tubes: From Onset to Coalescence
Abstract We investigate the onset of driven collisionless reconnection and plasmoid formation in a magnetically dominated pair plasma, using 2D particle-in-cell simulations. Two force-free flux tubes of radiusRare initially pushed together with a prescribed velocity, forming a current sheet whose width shrinks until reconnection sets in. Even in our largest simulation withR ≈ 1600 plasma skin depths, the sheet thickness at reconnection onset is comparable to the skin depth. Plasmoid chains develop when the sheet length-to-width aspect ratioA ≳ 30. In the strongly magnetized limit, the onset of reconnection occurs in roughly 2–6 light-crossing times, depending on the imposed driving timescale, which controls the duration of the thinning phase. In the subsequent nonlinear merging phase, the evolution becomes effectively independent of the initially imposed velocity, leading to magnetic-energy dissipation consistent with a normalized reconnection rate ∼0.1. Our results have important implications for explosive release of magnetic energy in magnetospheres of astrophysical compact objects and their surroundings.
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- Award ID(s):
- 2308944
- PAR ID:
- 10692468
- Publisher / Repository:
- American Astronomical Society
- Date Published:
- Journal Name:
- The Astrophysical Journal
- Volume:
- 992
- Issue:
- 2
- ISSN:
- 0004-637X
- Page Range / eLocation ID:
- 193
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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