Capturing kinetic counter-streaming effects in magnetic reconnection with multi-fluid plasma simulations

The development of fluid plasma models capable of capturing important kinetic microphysical processes is a key component in the design of physically accurate and computationally efficient algorithms of multi-scale plasma dynamics. In the context of collisionless magnetic reconnection (MR) in a force-free current sheet, electron-electron counter-streaming is an important kinetic feature which we show is associated with the breakdown of commonly used fluid closures; standard 2-fluid simulations with a single electron species cannot resolve this phenomenon. Here, we introduce a method to resolve electron counter-streaming in fluid simulations by including two overlapping electron-fluids. We demonstrate that these multi-electron fluid simulations reproduce counter-streaming and significant aspects of field, flow, and plasma structure from fully kinetic simulations of MR across a wide range of plasma beta and mass-ratios. In particular, we show that the physics of electron counter-streaming plays an important role in the development of secondary instabilities that lead to the break up of the current sheet. In addition to improving the accuracy of fluid simulations, our multi-fluid approach highlights the important role of electron counter-streaming in magnetic reconnection and offers a new lens into its investigation.