Radial distribution of the azimuthally averaged [C ii] emissivity at b = 0°. The shaded area represents the emissivity calculated considering all sampled Galactic longitudes. The connected boxes represent the radial emissivity distribution for 0°< l < 180°, while the connected circles represent that for 180° < l < 360°. The typical 1σ uncertainty in the [C ii] emissivity is 0.02 K km s^-1 kpc^-1.