What can we learn from LFFF magnetic extrapolations

Observations of the Sun is done, up to now, in 2D and magnetic field is measured mainly in the photosphere. Magnetic extrapolation techniques allow us nevertheless to have a 3D view of the magnetic field. Different methods are available today. We present in this paper interesting and exploitable results obtained with linear force-free-field methods (lfff). The α parameter is assumed to be constant in the whole volume of computations. The results concern the global potential field in the corona (e.g. example of transequatorial loops), the magnetic topology of flaring active regions (e.g. flares and bald patch regions), twisted magnetic fields in filaments, and magnetic reconnection in emerging active region (e.g. Ellerman Bombs). We discuss on the limits of the validity of the methods. Published by Elsevier Ltd on behalf of COSPAR.