Coronal Loops Dynamics and Photospheric Forcing Patterns

We present a series of numerical simulations aimed at understanding the nature of the dynamics and the magnetic reconnection taking place in the Parker model for coronal heating. A coronal loop is studied via reduced magnetohydrodynamics simulations in Cartesian geometry. A uniform and strong magnetic field threads the volume between the two photospheric planes, where a forcing in the form of a velocity field is applied. It is commonly thought that the topology of the photospheric driver should strongly influence the dynamics of a coronal loop, and that the magnetic field lines anchored to the photospheric planes should passively follow their footpoints motions. In this picture the electric currents should develop along neighboring field lines whose footpoints have a relative shear motion. In previous works we have identified MHD turbulence as the physical process that transports energy from the scale of photospheric motions to the small dissipative scales where magnetic reconnection takes place. Here we present a series of simulations aimed at understanding if the MHD turbulent dynamics are due to the complexity of the imposed photospheric forcing or if they rather originate from the intrinsic nonlinear properties of the system. To this effect we apply a few ``ordered'' photospheric forcings in the form of a 1D shear flow pattern and various combinations of symmetric vortices. In all cases initially the magnetic field that develops in the coronal loop is a simple map of the photospheric velocity field. This initial configuration is unstable to some kind of instability (a multiple tearing, a kink, etc.) that develops islands with X and O points in the planes orthogonal to the axial field. Once the nonlinear stage sets in the system evolution is characterized by a regime of MHD turbulence dominated by magnetic energy. A well developed power law in energy spectra is observed and the magnetic field never returns to the simple initial state mapping the photospheric flow. The formation of X and O points in the planes orthogonal to the axial field allows the continued and repeated formation and dissipation of small scale current sheets where the plasma is heated. We conclude that the observed turbulent dynamics are not induced by the complexity of the pattern that the magnetic field lines footpoints follow but they rather stem from the inherent nonlinear nature of the system. Adding that the total dissipation rate is independent from the Reynolds number at sufficiently high values indicates that the magnetic reconnection taking place is very likely turbulent and its properties will be analyzed more in depth in future works.