Bridging hybrid- and full-kinetic models with Landau-fluid electrons: I. 2D magnetic reconnection

Abstract

Context. Magnetic reconnection plays a fundamental role in plasma dynamics under many different conditions, from space and astrophysical environments to laboratory devices. High-resolution in-situ measurements from space missions allow to study naturally occurring reconnection processes in great detail. Alongside direct measurements, numerical simulations play a key role in investigating the fundamental physics underlying magnetic reconnection, also providing a test ground for current models and theory. The choice of an adequate plasma model to be employed in numerical simulations, while also compromising with their computational cost, is crucial to efficiently address the problem under study. Aims. We consider a new plasma model that includes a refined electron response within the hybrid-kinetic framework (fully kinetic ions and fluid electrons). The extent to which this new model can reproduce a full-kinetic description of 2D reconnection, with particular focus on its robustness during the non-linear stage, is evaluated. Methods. We perform 2D simulations of magnetic reconnection with moderate guide field by means of three different plasma models: (i) a hybrid-Vlasov-Maxwell (HVM) model with isotropic, isothermal electrons, (ii) a hybrid-Vlasov-Landau-fluid (HVLF) model where an anisotropic electron fluid is equipped with a Landau-fluid closure, and (iii) a full-kinetic model. Results. When compared to the full-kinetic case, the HVLF model effectively reproduces the main features of magnetic reconnection, as well as several aspects of the associated electron micro-physics and its feedback onto proton dynamics. This includes the global evolution of magnetic reconnection and the local physics occurring within the so-called electron-diffusion region, as well as the evolution of species' pressure anisotropy. In particular, anisotropy driven instabilities (such as fire-hose, mirror, and cyclotron instabilities) play a relevant role in regulating electrons' anisotropy during the non-linear stage of magnetic reconnection. As expected, the HVLF model captures all these features, except for the electron-cyclotron instability.