A Two-Fluid Study of Oblique Tearing Modes in a Force-Free Current Sheet

TL;DR

This study compares two-fluid and kinetic models to understand oblique tearing modes in force-free current sheets, finding that two-fluid models can approximate the most unstable modes and extend tearing theory to oblique angles.

Contribution

It extends two-fluid tearing theory to oblique modes and compares its predictions with kinetic simulations in a force-free current sheet.

Findings

Oblique modes are most unstable for guide fields larger than the reconnecting field.

Two-fluid models approximate the growth of oblique modes similarly to kinetic simulations.

The extended theory predicts a flat oblique spectrum and underestimates growth rates.

Abstract

Kinetic simulations have demonstrated that three-dimensional reconnection in collisionless regimes proceeds through the formation and interaction of magnetic flux ropes, which are generated due to the growth of tearing instabilities at multiple resonance surfaces. Since kinetic simulations are intrinsically expensive, it is desirable to explore the feasibility of reduced two-fluid models to capture this complex evolution, particularly, in the strong guide field regime, where two-fluid models are better justified. With this goal in mind, this paper compares the evolution of the collisionless tearing instability in a force-free current sheet with a two-fluid model and fully kinetic simulations. Our results indicate that the most unstable modes are oblique for guide fields larger than the reconnecting field, in agreement with the kinetic results. The standard two-fluid tearing theory is extended to address the tearing instability at oblique angles. The resulting theory yields a flat oblique spectrum and underestimates the growth of oblique modes in a similar manner to kinetic theory relative to kinetic simulations.