Can Multi-threaded Flux Tubes in Coronal Arcades Support a Magnetohydrodynamic Avalanche?

Abstract

Magnetohydrodynamic (MHD) instabilities allow energy to be released from stressed magnetic fields, commonly modelled in cylindrical flux tubes linking parallel planes, but, more recently, also in curved arcades containing flux tubes with both footpoints in the same photospheric plane. Uncurved cylindrical flux tubes containing multiple individual threads have been shown to be capable of sustaining an MHD avalanche, whereby a single unstable thread can destabilise many. We examine the properties of multi-threaded coronal loops, wherein each thread is created by photospheric driving in a realistic, curved coronal arcade structure (with both footpoints of each thread in the same plane). We use three-dimensional MHD simulations to study the evolution of single- and multi-threaded coronal loops, which become unstable and reconnect, while varying the driving velocity of individual threads. Experiments containing a single thread destabilise in a manner indicative of an ideal MHD instability and consistent with previous examples in the literature. The introduction of additional threads modifies this picture, with aspects of the model geometry and relative driving speeds of individual threads affecting the ability of any thread to destabilise others. In both single- and multi-threaded cases, continuous driving of the remnants of disrupted threads produces secondary, aperiodic bursts of energetic release.

Self-consistent nanoflare heating in model active regions: MHD avalanches

“…Starting from a potential field, Reid et al ( 2018 ) introduce a model that self-consistently produces an MHD avalanche, with the generation of significant heating. Moving from a straightened model, with flux tubes connecting two parallel planes (in the style treated by Parker 1972 ), to a curved potential field, Threlfall, Reid & Hood ( 2021 ) extend this, confirming the viability of avalanches in the new, curved geometry. Here, that work is extended.…”

“…Volume-integrated magnetic, kinetic, and internal energies evolve in the manner outlined in Threlfall et al ( 2021 ), and as demonstrated by their respective evolutions o v er time in Fig. 2 .…”

“…Vortical motions are applied within the footpoints of each of seven flux tubes in the arcade at the lower z-boundary, according to the forms in equation (4) of Threlfall et al ( 2021 ), namely:…”

“…Further details of the model are discussed by Threlfall et al ( 2021 ). From that work, the third arrangement (Section 5 therein) is studied in greater depth here.…”

“…FollowingThrelfall et al ( 2021 ), the vortical motions at the footpoints of the coronal loop create smaller flux tubes out of the initial, near-potential arcade. Continuous twisting of each flux tube stresses Downloaded from https://academic.oup.com/mnras/article/518/1/1584/6807460 by University of St Andrews Library user on 23 November 2022…”