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We developed a new nonlinear force-free magnetic field (NLFFF)forward-fitting algorithm based on an analytical approximation of force-freeand divergence-free NLFFF solutions, which requires as input a line-of-sightmagnetogram and traced 2D loop coordinates of coronal loops only, in contrastto stereoscopically triangulated 3D loop coordinates used in previous studies.Test results of simulated magnetic configurations and from four active regionsobserved with STEREO demonstrate that NLFFF solutions can be fitted with equalaccuracy with or without stereoscopy, which relinquishes the necessity ofSTEREO data for magnetic modeling of active regions (on the solar disk). The 2Dloop tracing method achieves a 2D misalignment of $\mu_2=2.7^\circ\pm1.3^\circ$ between the model field lines and observed loops, and an accuracy of$\approx 1.0%$ for the magnetic energy or free magnetic energy ratio. The threetimes higher spatial resolution of TRACE or SDO/AIA (compared with STEREO)yields also a proportionally smaller misalignment angle between model fit andobservations. Visual/manual loop tracings are found to produce more accuratemagnetic model fits than automated tracing algorithms. The computation time ofthe new forward-fitting code amounts to a few minutes per active region.

Nonlinear Force-Free Magnetic Field Fitting to Coronal Loops with and without Stereoscopy