Capturing kinetic counter-streaming effects in magnetic reconnection using 4-fluid plasma simulations

The development of fluid plasma models capable of capturing important kinetic microphysical processes is a key component in the design of computationally efficient algorithms of multi-scale plasma dynamics. In the context of collisionless magnetic reconnection, intra-species counter-streaming is an important kinetic feature which we show is associated with breakdown of commonly used fluid closures; traditional 2-fluid simulations cannot resolve this feature. Here, we introduce a method to resolve counter-streaming in fluid simulations by including four overlapping fluids, two for electrons and two for ions. We demonstrate that these four-fluid simulations reproduce counter-streaming and significant aspects of field and plasma structure from fully kinetic simulations of magnetic reconnection, including the reconnection rate. In addition to improving the accuracy of fluid simulations, our 4-fluid approach highlights the prominent role of counter-streaming in magnetic reconnection and offers a new lens into its investigation.