Fast reconnection in relativistic pair plasmas: Analysis of particle acceleration in self-consistent full particle simulations

Abstract

Particle acceleration in collisionless magnetic reconnection is studied in the relativistic regime of an electron-positron plasma. For the first time, the highly dynamic late-time evolution of reconnection is simulated in two dimensions (2D) and the finite size of the acceleration region is resolved in 3D applying a fully electromagnetic relativistic particle-in-cell (PIC) code. The late-time evolution is extremely important with respect to particle acceleration, because thin current sheets show a highly dynamic late-time phase with instabilities evolving in the Alfvén velocity vA0 regime. Consequently, since c∼vA0 is valid as a peculiarity of pair plasmas, v×B-contributions become dominant in the accelerating electric field. Most remarkable: Though acceleration regions are highly variable at late times, the power-law shape of the particle energy distribution is smoothed compared to quasi-static reconnection configurations at early times [S. Zenitani and M. Hoshino, Astrophys. J. 562, L63 (2001)]. Spectral power indices of s∼−3 for the complete simulation box, s∼−1 within the X-zone, are preserved at late times and appear as a characteristic of pair plasma reconnection of thin current sheets! The spectral high-energy cut-off depends on the sheet width at late times and is most efficiently tuned by the ratio c/vA0. In 3D, sheet instabilities limit the acceleration potential of a single X-zone, but current driven instabilities like the relativistic drift kink mode can also significantly contribute to particle acceleration. Via the analysis of particle trajectories, the consequences of a finite 3D acceleration zone are resolved and efficient acceleration mechanisms in the context of dynamic X-points are identified.