Fig. 3: Velocity-space structure and formation of smile-shaped electron gradient distributions in the simulated electron diffusion region of asymmetric magnetic reconnection.
a Zoom-in view of the particle-in-cell (PIC) simulation domain shown in Fig. 2h, here showing the z-component of the electron pressure divergence, \({(\nabla \cdot {{{{\bf{P}}}}}_{e})}_{z}\), with a yellow region of size Δx × Δz = 30de × 2de highlighting roughly where smile-shaped gradient distributions are found on the magnetospheric (low-density) side of the X-line. b Electron velocity distributions, fe (right column), and their corresponding gradient distributions, ∇zfe (left column), taken at five varying z-locations as indicated by the small white boxes in (a). (c) Schematic representation of the smile-shaped velocity-space structure of ∇zfe near z = 2de. The blue (red) regions are where electron phase space density decreases (increases) while moving spatially in the + z-direction. The blue (red) arrows further indicate how these coherent regions of the gradient distribution move in velocity space as ∇zfe is sampled at increasing values of z, as shown by the spatial evolution evident in (b) and discussed in the Results section. The dashed outlines indicate representative model contours of fe1 taken near z = 2de, while the solid outlines denote contours of fe2 taken near z = 3de. The prominent blue `eyes' form near the ends of the crescent-shaped population in fe as the crescent’s angular gyrophase extent decreases. The red `smile' forms at the center of the crescent population as electron phase space density concentrates there, leaving a blue, beard-like `halo' region around the crescent. The more isotropic red `core' of the gradient distribution forms as phase space density in the low-energy core population increases while moving away from the electron diffusion region (EDR) acceleration region.
