Magnetohydrodynamic models of solar coronal magnetic fields

We present some results concerning the possibility of determining the struc ture of solar active regions using measurements of the vector magnetic fiel d on the Sun surface as boundary conditions for the new numerical extrapola tion codes. I:rom these computations the main features of these configurati ons, as shear and twist (which are particular forms of magnetic helicity), are then used as ingredients to define model problems and solved for the ma gnetohydrodynamic (MHD) analysis of solar eruptive phenomena, in which ejec tion (or redistribution) of helicity occurs.