Are $E>B$ regions essential for particle injection in relativistic magnetic reconnection?

Abstract

During relativistic magnetic reconnection, X-points ($E>B$ regions at vanishing guide field) have been proposed to play an essential role in injecting particles into the nonthermal population. Using fully kinetic particle-in-cell simulations at a range of magnetizations $σ\equiv B_0^2/(4πn_0 m_e c^2) = 12$--$800$, we study in detail how the particle energy gains up to the injection threshold $γ_{\rm inj} = σ/4$ are partitioned into work done inside $E>B$ regions (unmagnetized), within one gyration after exit (re-magnetizing), and at all other times (magnetized). The magnetized channel supplies the largest part of the energy gain for $\sim 95\%$ of injected tracers at $σ= 12$ and $\sim 65\%$ at $σ= 800$. Most of the tracers in this magnetized majority never enter an $E>B$ region before injection. We also compare these injection shares with the earlier studies that proposed an essential role for $E>B$ regions. We conclude that $E>B$ regions are neither necessary for injection nor the dominant direct contributors to pre-injection work by injection share in two-dimensional, nonradiative pair-plasma reconnection at vanishing guide field.