Inflexion method to define limb. The intensity profile along straight line perpendicular to the limb in Mg II continuum (panel a) and the intensity-position derivative in the function of position (panel b) along the same profile line. The limb is defined as the strongest derivative extremum and highlighted by a red line (panel a and b). The intensity profile parallel to the limb in H-α continuum (panel c) and the intensity-position derivative in the function of position (panel d). The limb is defined as the strongest derivative extremum and highlighted by a red line (panel c and d).