Evolution of three-dimensional coherent structures in Hall magnetohydrodynamics

TL;DR

This paper extends the EULAG-MHD model to include Hall MHD, revealing that Hall effects accelerate magnetic reconnection and induce three-dimensional structures like flux ropes, enhancing understanding of plasma phenomena in space and laboratory environments.

Contribution

The study introduces a Hall MHD extension to the EULAG-MHD model, demonstrating faster reconnection and 3D structure formation, which advances simulation capabilities for plasma physics.

Findings

Hall term accelerates magnetic reconnection onset

3D structures like flux ropes develop due to Hall effects

Hall influence leads to earlier flux rope breakage

Abstract

The work extends the computational model EULAG-MHD to include Hall magnetohydrodynamics (HMHD)---important to explore physical systems undergoing fast magnetic reconnection at the order of the ion inertial length scale. Examples include solar transients along with reconnections in magnetosphere, magnetotail and laboratory plasmas. The paper documents the results of two distinct sets of implicit large-eddy simulations in the presence and absence of the Hall forcing term, initiated with an unidirectional sinusoidal magnetic field. The HMHD simulation while benchmarking the code also emphasizes the complexity of three dimensional (3D) evolution over its two dimensional (2D) counterpart. The magnetic reconnections onset significantly earlier in HMHD. Importantly, the magnetic field generated by the Hall term breaks any inherent symmetry, ultimately making the evolution 3D. The resulting 3D reconnections develop magnetic flux ropes and magnetic flux tubes. Projected on the reconnection plane, the ropes and tubes appear as magnetic islands, which later break into secondary islands, and finally coalesce to generate an X-type neutral point. These findings are in agreement with the theory and contemporary simulations of HMHD, and thus verify our extension of the EULAG-MHD model. The second set explores the influence of the Hall forcing on generation and ascend of a magnetic flux rope from sheared magnetic arcades---a novel scenario instructive in understanding the coronal transients. The rope evolves through intermediate complex structures, ultimately breaking locally because of reconnections. Interestingly, the breakage occurs earlier in the presence of the Hall term, signifying faster dynamics leading to magnetic topology favorable for reconnections.