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.

Three-dimensional (3D) particle-in-cell simulations of collisionless magnetic reconnection are presented. The initial equilibrium is a double Harris-sheet equilibrium and periodic boundary conditions are assumed in all three directions. No magnetic seed island is imposed initially, and no flow conditions are imposed. The current sheet width is assumed to be one ion inertial length calculated with the density in the center of the current sheet. The ion to electron mass ratio is mi/me=150, which suppresses the growth of the drift kink instability. Two different runs have been performed: a simulation with exactly antiparallel magnetic field and a simulation with a constant guide field of the same magnitude as the antiparallel field superimposed. In the antiparallel case the inductive field of the waves excited by the lower hybrid drift instability (LHDI) leads to rapid acceleration of the electrons in the center of the current sheet and subsequently to a current sheet thinning. The current increase in the center is balanced by reverse currents in the gradient region. In the thin current sheet rapid reconnection sets in which self-organizes into a two-dimensional structure with a single X line. However, ∼15% of the total flux is reconnected while reconnection is still patchy and 3D. In the guide field case the growth rate of the LHDI is reduced, but leads nevertheless after a considerably longer time to electron acceleration in the current sheet center and to a thinning of the current layer, followed by single X line reconnection. It is suggested that electron acceleration due to LHDI in current sheets of the order of the ion scale results in rapid onset of reconnection.

The appearance of generalized-Lorentzian distribution functions, a physical generalization of the so-called “κ-distributions,” in collisionless space plasmas is a frequent phenomenon. They represent “stationary states far from thermal equilibrium.” It is argued in which way they can be understood from kinetic theory of quasistationary highly correlated (presumably turbulent) states. Fits of model generalized-Lorentzian distributions to measured distributions in the magnetospheric tail and the auroral plasmas are presented. These fits provide reasonable values of density and temperature in the regions under consideration. Moreover, an expression for the Debye screening length is derived for such generalized-Lorentzian plasmas showing that the screening in correlated plasmas is reduced in comparison with Maxwellian plasmas.

Relativistic current sheets (RCSs) feature plasma instabilities considered as the potential key to magnetic energy dissipation in Poynting-flux-dominated plasma flows. Kinetic plasma simulations show that the physical nature of RCS evolution changes in the presence of radiation losses: In the ultrarelativistic regime (i.e., magnetization parameter sigma=10(4) defined as the ratio of magnetic to plasma rest frame energy density), the combined effect of nonlinear RCS dynamics and synchrotron emission introduces a temperature anisotropy triggering the growth of the relativistic tearing mode. In contrast to previous studies of the RCS with sigma approximately 1, the relativistic tearing mode then prevails over the drift kink mode. The ultrarelativistic RCS shows a typical life cycle from radiation-induced collapse towards a radiation-quiescent phase with topology analogous to that introduced by Sweet and Parker.