  Magnetic reconnection is an important source of energetic particles in
systems ranging from astrophysics to the laboratory. The large separation of
spatiotemporal scales involved makes it critical to determine the minimum
physical model containing the necessary physics for modeling particle
acceleration. By resolving the energy gain from ideal and nonideal
magnetohydrodynamic electric fields self-consistently in kinetic
particle-in-cell simulations of reconnection, we conclusively show the dominant
role of the nonideal field for the early stage of energization known as
injection. The importance of the nonideal field increases with magnetization,
guide field, and in three-dimensions, indicating its general importance for
reconnection in natural astrophysical systems. We obtain the statistical
properties of the injection process from the simulations, paving the way for
the development of extended MHD models capable of accurately modeling particle
acceleration in large-scale systems. The novel analysis method developed in
this study can be applied broadly to give new insight into a wide range of
processes in plasma physics.
