A Coronal Loop in a Box: Energy Generation, Heating and Dynamics

In our study we aim at an understanding how the energy to heat the upper atmosphere is generated by the photospheric magneto-convection, transported into the upper atmosphere, and how its dissipation governs the formation of the internal structure of a coronal magnetic loop. In a 3D MHD model we study a coronal loop that is rooted with both footpoints in a shallow convection zone layer. Therefore the driving at the coronal base arises self-consistently from magneto-convection in plage-type areas. To fit into a cartesian box, we straighten the coronal loop. This allows a high spatial resolution within the loop that cannot be achieved in a model of a whole active region. To conduct the numerical experiments we employ the MURaM code that includes heat conduction, radiative transfer and optically thin radiative losses. We find that the Poynting flux into the loop is generated by small-scale photospheric motions within strong magnetic flux concentrations. Turbulent behaviour develops in the upper layers of the atmosphere as a response to the footpoint motions. Vortex flows are found at various heights within the loop. These are organised in swirls that form coherent structures with a magnetic connection from the intergranular lanes in the photosphere through the chromosphere up to several megameters into the corona. In the coronal part of the loop plasma motions perpendicular to the magnetic axis of the swirl are associated with an increased heating rate and thus enhanced temperatures. At any given time, only part of the loop is filled with swirls which leads to a substructure of the loop in terms of temperature and density. Consequently the emission as it would be observed by AIA or XRT reveals transient bright strands that form in response to the heating events related to the swirls. With this model we can build a coherent picture of how the energy flux to heat the upper atmosphere is generated near the solar surface and how this process drives and governs the heating and dynamics of a coronal loop