Reconnection and particle acceleration in 3D current sheet evolution in moderately-magnetized astrophysical pair plasma

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 2D) nonlinear evolution of the tearing instability
disrupting a thin current sheet. To test how this process operates in 3D, we conduct a
comprehensive particle-in-cell simulation study comparing 2D and 3D 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 2D, 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 3D simulations of similar configurations exhibit
a diversity of behaviours, including some where energy release is determined by the
nonlinear relativistic drift-kink instability. Thus, 3D current-sheet evolution is not always
fundamentally classical reconnection with perturbing 3D 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 2D. Intriguingly, nonthermal particle acceleration is astonishingly robust,
depending on the upstream magnetization and guide field, but otherwise yielding similar
particle energy spectra in 2D and 3D. Though the variety of underlying current-sheet
behaviours is interesting, the similarities in overall energy release and particle spectra
may be more remarkable.