SINGULAR POINTS OF THE POLARIZATION TENSOR

The problem to compute the magnetic field above the chromosphere using data of the vector tau = B-t/B-t that gives the projected field direc tion can be solved with different approximations. The field of directi on vectors tau is, however, not the only field accessible to observati ons. The Stokes parameters, which are components of the radiation tens or; can be measured at each paint of the image plane. The directions o f the eigenvectors of the radiation tensor define two mutually orthogo nal systems of integral curves in the image plane. These families of c urves have singular points, which are generally of different type than those of the vector field. When the morphology of H alpha chromospher ic fibrils are used to infer the topology of the magnetic field, a sim ilar problem is met, suggesting that singular points should also be pr esent there.