Modeling the dispersal of an active region: quantifying energy input into the corona

In this paper, a new technique for modeling nonlinear force-free fields directly from line-of-sight magnetogram observations is presented. The technique uses sequences of magnetograms directly as lower boundary conditions to drive the evolution of coronal magnetic fields between successive force-free equilibria over long periods of time. It is illustrated by applying it to SOHO: MDI observations of a decaying active region, NOAA AR 8005. The active region is modeled during a four-day period around its central meridian passage. Over this time, the dispersal of the active region is dominated by random motions due to small-scale convective cells. Through studying the buildup of magnetic energy in the model, it is found that such small-scale motions may inject anywhere from (2.5-3) × 1025 erg s-1 of free magnetic energy into the coronal field. Most of this energy is stored within the center of the active region in the low corona, below 30 Mm. After four days, the buildup of free energy is 10% that of the corresponding potential field. This energy buildup is sufficient to explain the radiative losses at coronal temperatures within the active region. Small-scale convective motions therefore play an integral part in the energy balance of the corona. This new technique has wide ranging applications with the new high-resolution, high-cadence observations from the SDO:HMI and SDO:AIA instruments.