A Numerical Investigation of Unsheared Flux Cancelation

Abstract

Cancelation of magnetic flux in the solar photosphere and chromosphere has been linked observationally and theoretically to a broad range of solar activity phenomena, from filament channel formation to CME initiation. Because cancelation is typically measured at only a single layer in the atmosphere and only in the radial (line of sight) component of the magnetic field, the actual processes behind its observational signature are not fully understood. We have used our 3D MHD code with adaptive mesh refinement, ARMS, to investigate numerically the physics of flux cancelation, beginning with the simplest possible configuration: a subphotospheric Lundquist flux tube surrounded by a potential field in a gravitationally stratified atmosphere. Cancelation is driven by a two-cell circulation pattern imposed in the convection zone, in which the flows converge and form a downdraft at the polarity inversion line (PIL). We present and compare the results of 2D and 3D simulations of cancelation of initially unsheared flux – to our knowledge, these are the first such calculations in which the computational domain extends below the photosphere. The 2D simulation produces a flattened flux rope (plasmoid) whose axis remains centered along the PIL about 1650km above the photosphere, without rising higher into the corona by the end of the run (10,000 s). Our calculations also show that 3D cancelation in an arcade geometry does not produce a fully disconnected flux tube in the corona, in contrast to the 2D results. Rather, most of the reconnected field stays rooted in the photosphere and is gradually submerged by the downdrafts at the PIL. An interchange-like instability develops above the region where the converging flows are driven, breaking the horizontal symmetry along the PIL. This generates an alternating pattern of magnetic shear (magnetic field component aligned with the PIL), which ultimately produces systematic footpoint shuffling through reconnection across the folds of the convoluted PIL. These simulations demonstrate the importance of considering the effects of submergence, as well as the full 3D configuration of the magnetic field and atmosphere, in determining the physical processes behind flux cancelation on the Sun. A paper describing this work has been submitted to the Astrophysical Journal (January 2009).