  Magnetic reconnection in the relativistic and trans-relativistic regimes is
able to accelerate particles to hard power law energy spectra $f \propto
\gamma^{-p}$ (approaching $p=1$). The underlying acceleration mechanism that
determines the spectral shape is currently a topic of intense investigation. By
means of fully kinetic plasma simulations, we carry out a study of particle
acceleration during magnetic reconnection in the trans-relativistic regime of a
proton-electron plasma. While earlier work in this parameter regime has focused
on the effects of electric field parallel to the local magnetic field on the
particle injection (from thermal energy to the lower energy bound of the
power-law spectrum), here we examine the roles of both parallel and
perpendicular electric fields to gain a more complete understanding on the
injection process and further development of a power-law spectrum. We show that
the parallel electric field does contribute significantly to particle
injection, and is more important in the initial phase of magnetic reconnection.
However, as the simulation proceeds, the acceleration by the perpendicular
electric field becomes more important for particle injection and completely
dominates the acceleration responsible for the high-energy power-law spectrum.
This holds robustly, in particular for longer reconnection times and larger
systems, i.e. in simulations that are more indicative of the processes in
astrophysical sources.
