3D MHD simulations of coronal loops heated via magnetic braiding I. Continuous driving

TL;DR

This study investigates whether coronal heating can be sustained by nanoflares in braided magnetic fields using high-resolution 3D MHD simulations with continuous footpoint driving. The authors model four interacting flux tubes in a stratified solar atmosphere, including gravity, anisotropic conduction, optically thin radiative losses, and anomalous resistivity that triggers localized reconnection in narrow current sheets. They find a three-stage evolution: initial energy loading, a kink-induced avalanche producing intense heating, and a long post-avalanche phase where frequent, small-scale reconnection maintains a statistical steady state with $T \,\sim\,1$ MK and densities $\,\sim\,4\times10^{8}$ cm$^{-3}$; heating is highly intermittent and governed by current sheets, with a nanoflare-like energy distribution of index around $-1.8$. Forward-modelled AIA and MUSE emissions illustrate observable signatures of this heating, including footpoint brightenings and narrow hot-line brightenings, and reveal how diffusion and resolution control the heating intensity, informing interpretation of coronal-heating observations and guiding future active-region studies.

Abstract

The nature and detailed properties of the heating of the million-degree solar corona are important issues that are still largely unresolved. Nanoflare heating might be dominant in active regions and quiet Sun, although direct signatures of such small-scale events are difficult to observe in the highly conducting, faint corona. The aim of this work is to test the theory of coronal heating by nanoflares in braided magnetic field structures. We analyze a 3D MHD model of a multistrand flux tube in a stratified solar atmosphere, driven by twisting motions at the boundaries. We show how the magnetic structure is maintained at high temperature and for an indefinite time, by intermittent episodes of local magnetic energy release due to reconnection. We forward-modelled optically thin emission with SDO/AIA and MUSE and compared the synthetic observations with the intrinsic coronal plasma properties, focusing on the response to impulsive coronal heating. Currents build up and their impulsive dissipation into heat are also investigated through different runs. In this first paper, we describe the proliferation of heating from the dissipation of narrow current sheets in realistic simulations of braided coronal flux tubes at unprecedented high spatial resolutions.

Figures (9)

• Figure 1: 3D rendering of a simulation of a coronal loop in a box at three time snapshots times (before the avalanche: $t = -1000\,\mathrm{s}$; during the avalanche: $t=-740\,\mathrm{s}$; after the avalanche: $t = 1400\,\mathrm{s}$). Coloured field lines show the four magnetic field bundles subjected to continuous footpoints twisting. The first row shows the density stratification from the lower footpoint. The second row shows the current density magnitude in the corona. The third row shows the temperature distribution.
• Figure 2: Long-term evolution of the bundle of four flux tubes. From top to bottom: Maximum temperature, maximum velocity, and maximum current density, average coronal plasma density ($T > 10^6\,\mathrm{K}$) , total magnetic, internal, and kinetic energy as functions of time.
• Figure 3: Evolution of a field line and the neighbouring plasma. First row: x-position of the field line along the loop length ($z = 0\,\mathrm{Mm}$ is the apex) as function of time (left), trajectory of the upper footpoint motion (the bottom footpoint moves following the footpoints driver). Second row: distribution of the plasma temperature along the field line (left) and its maximum value (right). Third row: distribution of the ohmic heating along the field line (left) and its maximum value (right). Cyan arrows and circle highlight a reconnection event.
• Figure 4: Current sheets along the flux tube. Maps of the distribution of maximum ohmic heating and temperature (along the $\hat{y}$ direction), and a cut of the plasma density across the box at time $\sim 3000\,\mathrm{s}$ after the onset of the instability. White arrows in the first and second panels point at the location of two current sheets while in the third panel an upflow of plasma evaporating from the TR is shown.
• Figure 5: AIA synthetic maps, from the PLUTO 3D MHD simulation of the multi-threaded magnetic flux tube discussed in Section \ref{['sec:model']}, at time $\sim 3000\,\mathrm{s}$ after the onset of the instability and with the loop as in off-limb configuration (LoS along the $\hat{y}$ direction). From the left, intensity in the $94\,\AA$, $131\,\AA$, $171\,\AA$, $193\,\AA$, $211\,\AA$, and $335\,\AA$ channels, respectively. Arrows indicate the brightenings due to impulsive heating events. Movie I shows the temporal evolution of the six panels, across the six optically thin channels of AIA.