On the complex nature of coronal heating

TL;DR

The paper tackles how coronal heating relates to the 3D magnetic topology of loops and the appearance of bright strands. It employs a self-consistent 3D radiative MHD simulation with convection-zone anchoring to analyze where heating occurs, how current sheets form, and how emission relates to magnetic connectivity. The findings reveal that loops lack a single coherent flux-tube structure; heating concentrates in current sheets at interfaces between flux sources and within braided regions, with heating and evaporation dynamics shaping observed strands. The results support a hybrid tectonics-braiding picture, showing that resolution and complex flows influence heating sites and that interpreting observations requires accounting for dynamic topology and emission history.

Abstract

A large part of the hot corona consists of magnetically confined, bright plasma loops. These observed loops are in turn structured into bright strands. We investigate the relationship between magnetic field geometry, plasma properties and bright strands with the help of a 3D resistive MHD simulation of a coronal loop rooted in a self-consistent convection zone layer. We find that it is impossible to identify a loop as a simple coherent magnetic flux tube that coincides with plasma of nearly uniform temperature and density. The location of bright structures is determined by a complex interplay between heating, cooling and evaporation timescales. Current sheets form preferentially at the interfaces of magnetic flux from different sources. They may also form within bundles of magnetic field lines since motions within magnetic concentrations drive plasma flows on a range of timescales that provide further substructure and can locally enhance magnetic field gradients and thus facilitate magnetic reconnection. The numerical experiment therefore possesses aspects of both the flux tube tectonics and flux braiding models. While modelling an observed coronal loop as a cylindrical flux tube is useful to understand the physics of specific heating mechanisms in isolation, it does not describe well the structure of a coronal loop rooted in a self-consistently evolving convection zone.

Figures (18)

• Figure 1: Simulation setup. Three-dimensional magnetic field lines at t=12.36 min. The slices show maps of the vertical magnetic field component $B_{z}$ at the photosphere at each foodpoint.
• Figure 2: Flux tube connectivity for run LR at time $t=12.36\; \rm{min}$. Top row: Vertical magnetic field at footpoints 1 and 2. The colored dots show the footpoints of the magnetic field lines traced from the apex that intersect one of the 1 kG magnetic patches. Middle row: Vertical component of the current density at the apex, overlayed with field line seeds colored according to the magnetic concentrations in which they are rooted. The black contours showcase current sheets with current densities two standard deviations above the mean current density. Bottom row: Mapping of magnetic field lines to the opposite loop footpoint. The blue and pink box in the lower right panel mark the positions of the close-ups of the footpoints shown in Fig. \ref{['fig:footpoint_zoom']}.
• Figure 3: Fragmentation of magnetic flux bundles in the corona for run LR and the same snapshot as Fig. \ref{['fig:connect_fp']}. Mapping of field lines from the photosphere to the mid-plane for FP 1 (left) and FP 2 (right). Both panels are overlayed with the edges of the flux bundle maps from the opposite footpoint (dark red contours).
• Figure 4: Distribution of current sheet length for different thresholds imposed on the current density for run LR.
• Figure 5: Flux tube connectivity for a snapshot from simulation run MR with a higher resolution, using a grid size of $\Delta x$= 24 km. Top row: Vertical magnetic field at footpoint 1 and 2. The colored dots show the footpoints of the magnetic field lines traced from the apex. Bottom row: Vertical component of the current density at the apex, overlayed with field line seeds colored according to the magnetic concentrations in which they are rooted. The black contours showcase current sheets with current densities two standard deviations above the mean current density.