MHD MODELLING OF ENERGY RELEASE IN CORONAL CLOSED MAGNETIC FLUX TUBES

In this thesis I address the release of magnetic energy into heating in the million degreessolar corona, through detailed MHD modelling of single or interacting closed magneticflux tubes. According to a commonly accepted scenario, these tubes are stressed byprogressive twisting at their footpoints driven by slow photospheric motions. Particularattention is devoted to the observational implications and therefore to the plasma responseto the energy release in a realistic solar atmosphere.As a detailed mechanism to convert the magnetic energy into heat I explore thereconnection events triggered by the kink instability of continuously twisted flux tubes.I performed time-dependent 2.5D and 3D MHD simulations to model the evolution ofclosed magnetic tubes, the coronal loops, subjected to footpoint motions and eventuallyMHD instabilities. Our model includes a stratified, magnetised atmosphere extendingfrom the chromosphere to the corona, and accounts for key physical processes suchas thermal conduction, optically thin radiation, and transitions from high- to low-betaregions, including magnetic field expansion from the footpoints. I use the stat-of-artMHD numerical code PLUTO, tailored to describe solar coronal conditions. As apreliminary step, which allowed me to tune up the numerical tool, I explored the effectof asymmetric heating release in coronal loop, and found that the high Alfv´ en speedin the corona levels out possible asymmetries and explain the observed symmetry ofcoronal loops, even under prolonged asymmetric footpoint motions.Most of the work is devoted to study twisted interacting flux tubes subject to kinkinstability. The study confirms that kink instabilities can lead to MHD avalanches, evenin a realistic solar atmosphere, driving significant heating up to microflare temperatures(∼10 MK) and inducing chromospheric evaporation. As a next step, spectral data inextreme-ultraviolet (EUV) lines are synthesized for comparison with the anticipatedobservational capabilities of the forthcoming MUltislit Solar Explorer (MUSE) NASA mission. Footpoints EUV emission in the MUSE Fe ix 1 MK channel will mark earlyplasma responses to heating, while Fe xv 2.5 MK will track denser plasma at intermediateheights, and Fe xix 10 MK will reveal hot plasma within current sheets. Further effortwas devoted to possible signatures of magnetic reconnection. A nanojet — a small,high-velocity reconnection outflow — was identified as key observable signature of thisimpulsive energy release, with temperature reaching 8 MK, outflow velocity of severalhundred km s−1, and duration of less than one minute. The simulations suggest thatMUSE will be able to detect these features, providing crucial insights into the heatingmechanisms of the solar corona.As a final step, which is an initial step for future developments, we addressed theeffects of prolonged footpoints rotation, until a statistical energy balance is achieved,to investigate DC coronal heating in multi-stranded coronal loops. A machine-learningbased algorithm for automatic detection of nanojets is presented as promising tool toinvestigate the nanoflare phenomenon and its observational signatures, based on physicalassumptions.