On the complex nature of coronal heating

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.

💡 Research Summary

This paper investigates the relationship between magnetic field geometry, plasma properties, and the bright strands observed in coronal loops by means of a three‑dimensional resistive magnetohydrodynamic (MHD) simulation that includes a self‑consistent convection zone at both footpoints. The authors use a modified version of the radiative MHD code MURaM to model a straightened coronal loop of 50 Mm length, with 3.5 Mm deep convection zones at each end. Two simulations are performed: a low‑resolution (LR) run with 60 km grid spacing and a higher‑resolution (MR) run with 24 km spacing. The LR run provides a 1 s cadence over ~27 min, while the MR run supplies data every 96 s. Both runs incorporate gravity, Spitzer heat conduction, optically thick radiative losses in the lower atmosphere, and optically thin losses in the corona, assuming local thermodynamic equilibrium in the chromosphere and corona.

The simulation proceeds in stages: first a convection‑only box is relaxed; then a uniform vertical 60 G field is introduced and allowed to concentrate in intergranular lanes; finally the domain is extended into the corona using a tanh temperature profile and hydrostatic pressure, mirrored about the loop mid‑plane to create a closed loop. Open bottom boundaries mimic coupling to deeper convection, while the top boundary is open for outflows and vertical magnetic field is enforced.

Key findings concern the formation and location of current sheets, which are the sites of Ohmic heating. The authors develop an automated algorithm to identify current sheets and to map them onto the edges of magnetic flux bundles that connect discrete photospheric magnetic concentrations (kilogauss patches) to the coronal apex. The mapping reveals that current sheets preferentially appear at the interfaces between flux systems originating from different photospheric sources—i.e., at separatrix and quasi‑separatrix layers—consistent with the “flux‑tube tectonics” model. However, the analysis also shows that current sheets can form within a single flux bundle, driven by small‑scale vortical motions and vertical flows inside magnetic concentrations. These internal motions shear the field, enhance local magnetic gradients, and trigger reconnection, a process reminiscent of Parker’s braiding scenario.

The bright strands seen in synthetic emission are not simple, static cross‑sections of uniform temperature and density. Instead, their visibility results from a complex interplay of heating, conductive cooling, and chromospheric evaporation. Where a current sheet is strong, rapid heating raises temperature and drives evaporation, producing high‑density, high‑temperature plasma that emits strongly in EUV and X‑ray bands. Conversely, regions with weak or absent current sheets cool and become faint. Consequently, a strand can migrate, appear, or disappear over timescales comparable to the heating and cooling times, even within the same magnetic flux bundle.

Resolution tests demonstrate that the MR run resolves finer current sheets and thinner strands that are missed in the LR run, confirming that some heating occurs on scales below 60 km. Nevertheless, the qualitative picture—current sheets at flux‑system interfaces plus additional sheets generated internally—remains robust across resolutions.

The paper also follows a specific reconnection event in time, using field‑line tracing to show how magnetic connectivity changes abruptly when a current sheet forms, leading to localized heating and a transient brightening. This illustrates that the magnetic connectivity map evolves chaotically due to the turbulent convection driving at the footpoints.

In the discussion, the authors argue that modelling a coronal loop as a single cylindrical flux tube is useful for isolating particular heating mechanisms but fails to capture the true multi‑scale, multi‑connectivity nature of loops rooted in a realistic convection zone. Observed coronal loops likely consist of a network of dynamically evolving strands, each associated with distinct current sheets that arise both at inter‑bundle boundaries and within bundles. The results suggest that the solar corona operates under a hybrid heating regime that combines elements of flux‑tube tectonics (energy injection at separatrix layers) and Parker‑type braiding (internal shearing and reconnection), with the relative importance of each depending on the local magnetic topology and the spectrum of footpoint motions.

Overall, the study provides a self‑consistent numerical experiment that bridges the gap between idealised loop models and the complex reality of a convectively driven corona, offering new insights into why coronal loops display fine‑scale strand structure and how that structure is intimately linked to the underlying magnetic field dynamics.