Electron flat-top distributions in magnetic reconnection simulations

Collisionless magnetic reconnection hosts electron velocity distribution functions (VDFs; f) that are far from local thermodynamic equilibrium, as represented by the Maxwell-Boltzmann distribution function. One important example of such VDFs is the flat-top distribution, which is characterized by ∂f/∂v=0 in the VDF core. Simulations have shown that electron flat-top VDFs develop in the ion diffusion region of magnetic reconnection. There, large-scale parallel electric fields (E) trap the electrons that convect into the reconnection region with a small parallel velocity, leading to the formation of flat-top VDFs. During this process, the E strongly heats the trapped electrons parallel to the magnetic field, resulting in very large parallel temperature anisotropies. The formation of flat-tops is therefore believed to be an important contributor to electron heating and energization during reconnection. Spacecraft observations of electron flat-top distributions have recently provided indirect measurements of the total work done by E on the electrons. However, questions regarding the spatial distribution of flat-top VDFs and their role in electron energization during reconnection remain. Simulations are an essential complement to spacecraft observations, as they provide us with additional information about the spatial structure and temporal evolution of the reconnection event. 

Here, we will present results from 2D particle-in-cell simulations investigating electron flat-top distributions in symmetric collisionless reconnection. In particular, we will focus on where the flat-tops are generated, and on the energization mechanisms underlying their formation. We find that electron flat-top VDFs are most commonly found near the central reconnection region and in the outflow, correlating with the large-scale E present there. By decomposing the electric field into potential and solenoidal (inductive) parts, we find that the large-scale E is, to a large extent, due to an electric potential associated with charge separation in the diffusion region. The energy that the electrons gain from the potential part of the electric field as they enter the reconnection site, is lost as they exit it. Our results thus suggest that a large fraction of the heating associated with the formation of electron flat-tops should be considered temporary, as only the inductive part of the electric field can yield persistent energization.