Rapid Change of Field Line Connectivity and Reconnection in Stochastic Magnetic Fields

TL;DR

This study investigates how stochastic magnetic fields influence magnetic reconnection, revealing that rapid changes in field line connectivity often lead to enhanced diffusion rather than true reconnection, with fast reconnection occurring mainly during instability onset.

Contribution

The paper demonstrates through simulations that stochasticity causes rapid connectivity changes mainly resulting in diffusion, and identifies conditions under which true reconnection occurs during instability.

Findings

Rapid connectivity change correlates with high field line exponentiation.

Overall field line mapping deforms gradually despite local rapid changes.

Fast reconnection occurs during instability onset, not just from stochasticity.

Abstract

Magnetic fields without a direction of continuous symmetry have the generic feature that neighboring field lines exponentiate away from each other and become stochastic, hence the ideal constraint of preserving magnetic field line connectivity becomes exponentially sensitive to small deviations from ideal Ohm's law. The idea of breaking field line connectivity by stochasticity as a mechanism for fast reconnection is tested with numerical simulations based on reduced magnetohydrodynamics equations with a strong guide field line-tied to two perfectly conducting end plates. Starting from an ideally stable force-free equilibrium, the system is allowed to undergo resistive relaxation. Two distinct phases are found in the process of resistive relaxation. During the quasi-static phase, rapid change of field line connectivity and strong induced flow are found in regions of high field line exponentiation. However, although the field line connectivity of individual field lines can change rapidly, the overall pattern of field line mapping appears to deform gradually. From this perspective, field line exponentiation appears to cause enhanced diffusion rather than reconnection. In some cases, resistive quasi-static evolution can cause the ideally stable initial equilibrium to cross a stability threshold, leading to formation of intense current filaments and rapid change of field line mapping into a qualitatively different pattern. It is in this onset phase that the change of field line connectivity is more appropriately designated as magnetic reconnection. Our results show that rapid change of field line connectivity appears to be a necessary, but not a sufficient condition for fast reconnection.