and the second term corresponds to the diffuse ionized gas. For the neutral Hydrogen, we use the formula $$n_{\rm HI, H_2}(z) = n_0 \,{\rm sech}^2 \left[ \frac{z}{z_{1/2}}\ln(1+\sqrt{2}) \right]$$ with $n_0=0.6$ cm$^{-3}$ and $z_{1/2}=125$ pc for the atomic component [@NakanishiSofue03], whereas $n_0=0.09$ cm$^{-3}$ and $z_{1/2}=93$ pc describe the molecular gas [@NakanishiSofue06]. The corresponding density distributions are shown in Figure \[figObs\] by dashed lines, and the effect of 20 per cent errors in each normalization and scale length is represented by shaded areas enclosed by dotted lines. This estimate roughly corresponds to the actual errors quoted in @Reynolds91, and it is substantially smaller than those in @NakanishiSofue06. No information about the observational errors is provided in @NakanishiSofue03, but, given the large dispersion in the Hydrogen column densities along different lines of sight [@DickeyLockman90], as well as the amplitude of the fluctuations in the azimuthally-averaged surface density profiles, 20 per cent is arguably a realistic estimate.

The results of a model with central density $n_0=1$ cm$^{-3}$, central magnetic field $B_0=5~\mu$G, and a stellar distribution given by equation (\[eqRhoStar\]) are also shown in Figure \[figObs\] as black solid lines. Although the model gives a reasonable order-of-magnitude estimate (in agreement with the observed values within 