Fig. 9. Dependence of the irreducible source vector components on the magnetic field strength parameter . Same display and same atmospheric parameters as in Fig. 5. Magnetic field parameters (, )=(, ). The numbers near the curves are the values of . The case refers to the non-magnetic resonance scattering polarization. At small optical depths, reach their maxima for , and do so for . Thus a narrow range represents the value of peak sensitivity of a line to the Hanle effect. The effects of on emergent polarization at line centre are shown in Fig. 15