A Numerical Simulation for the Origins of Solar Magnetic Structure

We investigate numerically a new model for the origin of the solar coronal magnetic field structure observed in filament channels and in the complex structures of the slow wind. Using the Adaptively Refined Magnetohydrodynamic Solver (ARMS), we perform a series of numerical experiments to study the evolution of magnetic helicity injected into the solar corona by photospheric motions. Our simulation domain consists of a Cartesian box with an initially uniform vertical magnetic field and a low-beta plasma with uniform pressure and density. This system is driven by imposing flow patterns at the top and bottom boundary planes corresponding to the twisting motions expected from the quasi-random photospheric motions. We consider a variety of flow patterns made up of twist arranged in regular geometric orders (e.g. four twists in a quadrilateral arrangement, seven twists in a hexagonal), which generate sets of twisted flux tubes in the interior of the simulation box, the corona. This driving twist injects both energy and helicity into the coronal field. Depending upon the sense of the applied twist, we can inject either positive or negative helicity. If helicity of the same sign is injected into each of the flux tubes (the co-helicity case), we expect that the twist magnetic-field component of neighboring flux tubes will be oppositely directed and, therefore, will reconnect; on the other hand, if helicity of opposite signs is injected into neighboring flux tubes (the counter-helicity case), reconnection will not occur. This conjecture is confirmed by our simulations. We also have found generally that in co-helicity cases the reconnection indeed occurs and leads to a state in which the twist propagates to the largest scale: essentially, the individual flux tubes merge into one large twisted tube, with the twist concentrated at its outer boundary. We discuss the implications of our results for the evolution of coronal helicity and for the formation of filament channels on the Sun and slow-wind structures in the Heliosphere. Our research was sponsored by NASA.