Line formation region of the O vii resonance line. The left panels yield the variation of the intrinsic emission per solid angle (emissivity multiplied by the density squared) along the z axis of the (p,z) coordinates for different values of the impact parameter p. The middle panels provide the optical depth τ along the line of sight to the observer (located at z → ∞). The right panels yield the contribution to the observable emission, i.e. the product of the intrinsic emission per solid angle with exp(−τ). The morphology of this distribution results from the combination of the two plasma components at 0.10 and 0.20 keV that extend between 7.5 and 85 R_∗ for the former and 1.5 and 38 R_∗ for the latter.