Elementary heating events – Magnetic interactions between two flux sources

Observations taken by the SoHO MDI instrument have revealed that the quiet photospheric magnetic flux is, on average, recycled within a few days. As new flux emerges from the convection zone into the photosphere it is moved around by horizontal motions resulting from overshoots of convection cells. These motions cause the magnetic fields extending from flux fragments to tangle, forcing different magnetic flux systems to interact. Only the process of magnetic reconnection limits the complexity of magnetic field line connectivity. The energy liberated by these detangling or destressing processes act as a natural energy source which may heat the solar coronal plasma. In this paper, we use a numerical approach to solve the MHD equations in a three-dimensional domain to examine the dynamical behaviour of one simple magnetic flux interaction. The model consists of a uniform magnetic field overlying two flux sources of opposite polarity that are initially unconnected and are forced to interact as they are driven passed each other. We find that the development from initially unconnected sources to connected sources proceeds quite quickly and simply. This change takes place through driven separator reconnection in a systematically twisted current sheet. The out flow velocity from the reconnection is highly asymmetric with much higher velocities in the region defined by the field lines connected to both sources. However, the change back to two independent sources after the nearest approach has past takes place on a much longer time scale even though the distance between the sources increases significantly. This is because the opening of the field has to take place through separatrix reconnection and at this phase of the development there are no forcing of the fluxes to drive a fast opening of the magnetic field.