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 eld-free model corona. We simulate the buoyant rise of twisted magnetic flux tubes initially positioned near the base of a stable, stratied model convection zone, and use the results of these calculations to drive a 3-D MHD model corona. The simulations show that time-dependent sub-surface flows are an important component of the dynamic evolution and sub- sequent morphology of an emerging magnetic structure. During the initial stages of the flux emergence process, the overlying magnetic eld diers signicantly from a force-free state. However, as the runs progress, and boundary flows ad- just, most of the coronal eld | with the exception of those structures located relatively close to the model photosphere | relaxes to a more force-free cong- uration. Potential eld extrapolations do not adequately represent the magnetic structure when emerging active region elds are twisted. In the dynamic models, if arched flux ropes emerge with non-zero helicity, the overlying eld readily forms sigmoid-shaped structures. However, the chirality of the sigmoid, and other de- tails of its structure, depends 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.