Gyrokinetic and kinetic particle-in-cell simulations of guide-field reconnection. I: Macroscopic effects of the electron flows

TL;DR

This study compares gyrokinetic and fully kinetic PIC simulations of magnetic reconnection under strong guide fields, highlighting their similarities and differences in macroscopic electron flow effects across different plasma beta conditions.

Contribution

It provides the first detailed comparison of GK and PIC models focusing on macroscopic electron flow effects in guide-field reconnection, clarifying their applicability limits.

Findings

Both models show similar reconnection features at high guide fields ($b_g extgreater{}30$).

Differences arise at lower guide fields due to initial shear flows and fast outflows.

Secondary magnetic islands differ in core magnetic field and flow energy depending on guide field strength.

Abstract

In this work, we compare gyrokinetic (GK) and fully kinetic Particle-in-Cell (PIC) simulations of magnetic reconnection in the limit of strong guide field. In particular, we analyze the limits of applicability of the GK plasma model compared to a fully kinetic description of force free current sheets for finite guide fields ($b_g$). Here we report the first part of an extended comparison, focusing on the macroscopic effects of the electron flows. For a low beta plasma ($\beta_i=0.01$), it is shown that both plasma models develop magnetic reconnection with similar features in the secondary magnetic islands if a sufficiently high guide field ($b_g\gtrsim 30$) is imposed in the kinetic PIC simulations. Outside of these regions, in the separatrices close to the X points, the convergence between both plasma descriptions is less restrictive ($b_g\gtrsim 5$). Kinetic PIC simulations using guide fields $b_g \lesssim 30$ reveal secondary magnetic islands with a core magnetic field and less energetic flows inside of them in comparison to the GK or kinetic PIC runs with stronger guide fields. We find that these processes are mostly due to an initial shear flow absent in the GK initialization and negligible in the kinetic PIC high guide field regime, in addition to fast outflows on the order of the ion thermal speed that violate the GK ordering. Since secondary magnetic islands appear after the reconnection peak time, a kinetic PIC/GK comparison is more accurate in the linear phase of magnetic reconnection. For a high beta plasma ($\beta_i=1.0$) where reconnection rates and fluctuations levels are reduced, similar processes happen in the secondary magnetic islands in the fully kinetic description, but requiring much lower guide fields ($b_g\lesssim 3$).