Preprocessing of solar vector magnetograms for force-free magnetic field extrapolation

¹ Institut für Physik, Universität Potsdam, Am Neuen Palais 10, 14469 Potsdam, Germany e-mail: fuhrmann@agnld.uni-potsdam.de;seehafer@uni-potsdam.de
² Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany e-mail: gvalori@aip.de

Received: 9 August 2007
Accepted: 7 September 2007

Abstract

Context.Reliable measurements of the solar magnetic field are restricted to the phoptosphere. As an alternative to measurements, the field in the higher layers of the atmosphere is calculated from the measured photospheric field, mostly under the assumption that it is force-free. However, the magnetic field in the photosphere is not force-free. Moreover, most methods for the extrapolation of the photospheric magnetic field into the higher layers prescribe the magnetic vector on the whole boundary of the considered volume, which overdetermines the force-free field. Finally, the extrapolation methods are very sensitive to small-scale noise in the magnetograph data, which, however, if sufficienly resolved numerically, should affect the solution only in a thin boundary layer close to the photosphere.

Aims.A new method for the preprocessing of solar photospheric vector magnetograms has been developed that, by improving their compatibility with the condition of force-freeness and removing small-scale noise, makes them more suitable for extrapolations into three-dimensional nonlinear force-free magnetic fields in the chromosphere and corona.

Methods.A functional of the photospheric field values is minimized whereby the total magnetic force and the total magnetic torque on the considered volume above the photosphere, as well as a quantity measuring the degree of small-scale noise in the photospheric boundary data, are simultaneously made small. For the minimization, the method of simulated annealing is used and the smoothing of noisy magnetograph data is attained by windowed median averaging.

Results.The method was applied to a magnetogram derived from a known nonlinear force-free test field to which an artificial noise had been added. The algorithm recovered all main structures of the magnetogram and removed small-scale noise. The main test was to extrapolate from the noisy photospheric vector magnetogram before and after the preprocessing. The preprocessing was found to significantly improve the agreement of the extrapolated with the exact field.