The locus of particle acceleration in simulation S2K025 in the interval 47 τ _c ⩽ t ⩽ 70 τ _c corresponding to the primary particle energization phase. The contours show the logarithm of the y-averaged energy gain per particle per unit volume ( x, z) for the ≈2000 highest-energy particles in the simulation in this interval (see text); the lowest contour corresponds to 0.04 times the peak value of . The underlying color image shows the projection in the xz plane of the current density J _y averaged over the interval; the current density is scaled linearly from light purple to dark red.