Particle acceleration in magnetic reconnection of electron-positron plasmas is studied by using a particle-in-cell simulation. It is found that significantly large number of non-thermal particles are generated by the inductive electric fields around an X-type neutral line when the reconnection outflow velocity, which is known to be an Alfvén velocity, is of the order of the speed of light. In such a relativistic reconnection regime, we also find that electrons and positrons form a power-law-like energy distribution through their drift along the reconnection electric field under the relativistic Speiser motion. A brief discussion of the relevance of these results to the current sheet structure, which has an anti-parallel magnetic field in astrophysical sources of synchrotron radiation, is presented.

A new measure to identify a small-scale dissipation region in collisionless magnetic reconnection is proposed. The energy transfer from the electromagnetic field to plasmas in the electron's rest frame is formulated as a Lorentz-invariant scalar quantity. The measure is tested by two-dimensional particle-in-cell simulations in typical configurations: symmetric and asymmetric reconnection, with and without the guide field. The innermost region surrounding the reconnection site is accurately located in all cases. We further discuss implications for nonideal MHD dissipation.

We study linear and nonlinear development of relativistic and ultrarelativistic current sheets of pair (e ± ) plasmas with antiparallel magnetic fields. Two types of two-dimensional problems are investigated by particle-in-cell simulations. First, we present the development of relativistic magnetic reconnection, whose outflow speed is on the of the light speed c. It is demonstrated that particles are strongly accelerated in and around the reconnection region and that most of the magnetic energy is converted into a "nonthermal" part of plasma kinetic energy. Second, we present another two-dimensional problem of a current sheet in a cross-field plane. In this case, the relativistic drift kink instability (RDKI) occurs. Particle acceleration also takes place, but the RDKI quickly dissipates the magnetic energy into plasma heat. We discuss the mechanism of particle acceleration and the theory of the RDKI in detail. It is important that properties of these two processes are similar in the relativistic regime of T mc 2 , as long as we consider the kinetics. Comparison of the two processes indicates that magnetic dissipation by the RDKI is a more favorable process in the relativistic current sheet. Therefore, the striped pulsar wind scenario should be reconsidered by the RDKI.

A review of present understanding of the dissipation region in magnetic reconnection is presented. The review focuses on results of the thermal inertia-based dissipation mechanism but alternative mechanisms are mentioned as well. For the former process, • combination of analytical theory and numerical modeling is presented. Furthermore, • new relation between the electric field expressions for anti-parallel and guide field reconnection is developed.