HPC MHD modelling of unstable reconnecting plasma in the solar corona and EUV diagnostics with the MUSE mission

The solar corona consists of plasma confined by, and interacting with, the coronal magnetic field. It displays extraordinarily high temperatures, reaching millions of degrees, which starkly contrast with the cold lower photosphere. The problem of coronal heating has been a long standing question in the solar community. The theory of nanoflares heating suggests that the magnetic field could gain free energy from photospheric motions and further dissipate it by impulsive and diffused heating events. The challenge arises from the fact that this individual reconnection events, called nanoflares, are too minuscule and rapid to be directly observed. However, their cumulative impact effectively maintains the corona at a temperature of millions of degrees, counteracting thermal conduction and radiative losses. Magnetic-stress-based energy release in the corona may involve MHD instabilities such as the kink instability in a single twisted magnetic flux tube. The initial helical current sheet progressively fragments in a turbulent way into smaller scale sheets. The turbulent dissipation of the magnetic structure into small-scale current sheets converts into a sequence of a-periodic, impulsive, heating events, similarly to nanoflare storm. Since single twisted magnetic filaments are generally embedded in a multi-threaded structure, an unstable strand could trigger a global MHD instability, and a nanoflare cascade. As coronal loops are heated to millions of kelvin degrees, the dense chromospheric plasma expands in their inside and reveals a filamentary structure. The reconfiguration of the magnetic structure and the resulting plasma dynamics are expected to occur on timescales of order 10 s and over spatial scales smaller than 1.0’’. Moreover, loop footpoints response is expected across the transition region. This magnetic processes are highly dynamic and non linear, and their description requires time-dependent magnetohydrodynamic modelling on high performance computing systems. Using full 3D MHD simulations with the PLUTO code we show that avalanches are a viable mechanism for the storing and release of magnetic energy in the solar corona, as a result of photospheric motions. We provide a synthetic perspective for the NASA MUSE forthcoming mission. Many fine scale features as well as rapid changes in emissivity and Doppler shifts might be accessible to the MUSE spectrometer and shed new light on coronal heating mechanisms.