Constraining 3D Magnetic Field Extrapolations Using The Twin Perspectives of STEREO

The 3D magnetic topology of a solar active region (NOAA 10956) was recon-structed using a linear force-free field extrapolation constrained using the twin perspectives of STEREO. A set of coronal field configurations was initially gen-erated from extrapolations of the photospheric magnetic field observed by the Michelson Doppler Imager (MDI) on SOHO. Using an EUV intensity-based cost function, the extrapolated field lines that were most consistent with 171 A ̊ pass-band images from the Extreme UltraViolet Imager (EUVI) on STEREO were identified. This facilitated quantitative constraints to be placed on the twist (α) of the extrapolated field lines, where ∇×B = αB. Using the constrained values of α, the evolution in time of twist, connectivity, and magnetic energy were then studied. A flux emergence event was found to result in significant changes in the magnetic topology and total magnetic energy of the region