Heliographic coordinates: line emission used for spectropolarimetry comes from one layer of the Sun (circle). The magnetic field and electric current density are then calculated in the heliographic coordinates, as the drawing shows: the horizontal component is in the heliographic plane, which is tangent to the considered layer of the Sun, and the vertical component is perpendicular to that along the (Oz) axis. The X-ray emissions are taken in the plane-of-sky (POS), i.e. in the plane perpendicular to the line of sight (LOS).