A Coupled Model for the Emergence of Active Region Magnetic Flux into the Solar Corona

Abstract

We present a set of numerical simulations that model the emergence of active region magnetic flux into an initially field-free model corona. We simulate the buoyant rise of twisted magnetic flux tubes initially positioned near the base of a stable stratified model convection zone and use the results of these calculations to drive a three-dimensional magnetohydrodynamic model corona. The simulations show that time-dependent subsurface flows are an important component of the dynamic evolution and subsequent morphology of an emerging magnetic structure. During the initial stages of the flux emergence process, the overlying magnetic field differs significantly from a force-free state. However, as the runs progress and boundary flows adjust, most of the coronal field—with the exception of those structures located relatively close to the model photosphere—relaxes to a more force-free configuration. Potential field extrapolations do not adequately represent the magnetic structure when emerging active region fields are twisted. In the dynamic models, if arched flux ropes emerge with nonzero helicity, the overlying field readily forms sigmoid-shaped structures. However, the chirality of the sigmoid and other details of its structure depend on the observer's vantage point and the location within a given loop of emitting plasma. Thus, sigmoids may be an unreliable signature of the sign and magnitude of magnetic twist.