Counter-streaming heat-flux closure for electron-only collisionless magnetic reconnection

In electron-only collisionless magnetic reconnection (MR), a regime of growing importance in turbulent space plasmas, electrons develop strongly non-Maxwellian distributions that invalidate conventional fluid closures based on assumptions of near local thermodynamic equilibrium. Using particle-in-cell (PIC) simulations, we identify the physical origin of the electron heat-flux: counter-streaming between electron sub-populations originating from opposite sides of the current sheet, with each sub-population remaining approximately adiabatic. This insight yields a novel fluid closure, which we implement in fluid simulations using two adiabatic electron fluids initialized on opposite sides of the current sheet. The fluid simulations capture the heat-flux, reconnecting current density, thermal pressure, and bulk flows as observed in PIC, within a reduced fluid description that conventional single-electron-fluid models fundamentally cannot reproduce. The closure is most accurate at low $\beta_{\text{Reconn.}}$ and $B_{\text{Guide}}/B_{\text{Reconn.}}$, regimes relevant to Earth's magnetotail, where it establishes counter-streaming as the physical origin of heat-flux in electron-only collisionless MR and enables its computationally efficient fluid modeling.