Equilibria, Dynamics and Current Sheets Formation in Magnetically Confined Coronae

Original · EN

The dynamics of magnetic fields in closed regions of solar and stellar coronae are investigated with a reduced magnetohydrodynamic (MHD) model in the framework of Parker scenario for coronal heating. A novel analysis of reduced MHD equilibria shows that their magnetic fields have an asymmetric structure in the axial direction with variation length-scale zℓ ℓ B₀/b, where B₀ is the intensity of the strong axial guide field, b that of the orthogonal magnetic field component, and ℓ the scale of b. Equilibria are then quasi-invariant along the axial direction for variation scales larger than approximatively the loop length zℓ Lz, and increasingly more asymmetric for smaller variation scales zℓ Lz. The critical length zℓ Lz corresponds to the magnetic field intensity threshold b ℓ B₀/Lz. Magnetic fields stressed by photospheric motions cannot develop strong axial asymmetries. Therefore fields with intensities below such threshold evolve quasi-statically, readjusting to a nearby equilibrium, without developing nonlinear dynamics nor dissipating energy. But stronger fields cannot access their corresponding asymmetric equilibria, hence they are out-of-equilibrium and develop nonlinear dynamics. The subsequent formation of current sheets and energy dissipation is necessary for the magnetic field to relax to equilibrium, since dynamically accessible equilibria have variation scales larger than the loop length zℓ Lz, with intensities smaller than the threshold b ℓ B₀/Lz. The dynamical implications for magnetic fields of interest to solar and stellar coronae are investigated numerically and the impact on coronal physics discussed.