Quadrupolar Density Structures in Driven Magnetic Reconnection Experiments with a Guide Field

TL;DR

This study experimentally investigates how guide magnetic fields influence the structure and dynamics of magnetic reconnection layers, revealing quadrupolar density and magnetic field structures at large scales and high plasma beta.

Contribution

It provides the first detailed experimental observation of quadrupolar density structures associated with guide fields in reconnection layers at large scales and high beta.

Findings

Quadrupolar magnetic field structures are observed along the reconnection separatrices.

Line-integrated electron density shows quadrupolar patterns consistent with Hall effects.

Reconnection layers are less than the ion skin depth, emphasizing Hall physics importance.

Abstract

Magnetic reconnection is a ubiquitous process in plasma physics, driving rapid and energetic events such as coronal mass ejections. Reconnection between magnetic fields with arbitrary shear can be decomposed into an anti-parallel, reconnecting component, and a non-reconnecting guide-field component which is parallel to the reconnecting electric field. This guide field modifies the structure of the reconnection layer and the reconnection rate. We present results from experiments on the MAIZE pulsed-power generator (500 kA peak current, 200 ns rise-time) which use two exploding wire arrays, tilted in opposite directions, to embed a guide field in the plasma flows with a relative strength $b\equiv B_g/B_{rec}=\text{0, 0.4, or 1}$. The reconnection layers in these experiments have widths which are less than the ion skin depth, $d_i=c/\omega_{pi}$, indicating the importance of the Hall term, which generates a distinctive quadrupolar magnetic field structure along the separatrices of the reconnection layer. Using laser imaging interferometry, we observe quadrupolar structures in the line-integrated electron density, consistent with the interaction of the embedded guide field with the quadrupolar Hall field. Our measurements extend over much larger length scales ($40 d_i$) at higher $\beta$ ($\sim 1$) than previous experiments, providing an insight into the global structure of the reconnection layer.