Bootstrapping the Coronal Magnetic Field Withstereo: Unipolar Potential Field Modeling

We investigate the recently quantified misalignment of α mis ≈ 20°–40° between the three-dimensional geometry of stereoscopically triangulated coronal loops observed with STEREO /EUVI (in four active regions (ARs)) and theoretical (potential or nonlinear force-free) magnetic field models extrapolated from photospheric magnetograms. We develop an efficient method of bootstrapping the coronal magnetic field by forward fitting a parameterized potential field model to the STEREO -observed loops. The potential field model consists of a number of unipolar magnetic charges that are parameterized by decomposing a photospheric magnetogram from the Michelson Doppler Imager. The forward-fitting method yields a best-fit magnetic field model with a reduced misalignment of α PF ≈ 13°–20°. We also evaluate stereoscopic measurement errors and find a contribution of α SE ≈ 7°–12°, which constrains the residual misalignment to α NP ≈ 11°–17°, which is likely due to the nonpotentiality of the ARs. The residual misalignment angle, α NP , of the potential field due to nonpotentiality is found to correlate with the soft X-ray flux of the AR, which implies a relationship between electric currents and plasma heating.