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Extrapolations of solar photospheric vector magnetograms intothree-dimensional magnetic fields in the chromosphere and corona are usuallydone under the assumption that the fields are force-free. The fieldcalculations can be improved by preprocessing the photospheric magnetograms. Wecompare two preprocessing methods presently in use, namely the methods ofWiegelmann et al. (2006) and Fuhrmann et al. (2007). The two preprocessingmethods were applied to a recently observed vector magnetogram. We examine thechanges in the magnetogram effected by the two preprocessing algorithms.Furthermore, the original magnetogram and the two preprocessed magnetogramswere each used as input data for nonlinear force-free field extrapolations bymeans of two different methods, and we analyze the resulting fields. Bothpreprocessing methods managed to significantly decrease the magnetic forces andmagnetic torques that act through the magnetogram area and that can causeincompatibilities with the assumption of force-freeness in the solution domain.Both methods also reduced the amount of small-scale irregularities in theobserved photospheric field, which can sharply worsen the quality of thesolutions. For the chosen parameter set, the Wiegelmann et al. method led togreater changes in strong-field areas, leaving weak-field areas mostlyunchanged, and thus providing an approximation of the magnetic field vector inthe chromosphere, while the Fuhrmann et al. method weakly changed the wholemagnetogram, thereby better preserving patterns present in the originalmagnetogram. Both preprocessing methods raised the magnetic energy content ofthe extrapolated fields to values above the minimum energy, corresponding tothe potential field. Also, the fields calculated from the preprocessedmagnetograms fulfill the solenoidal condition better than those calculatedwithout preprocessing.

A comparison of preprocessing methods for solar force-free magnetic field extrapolation