scale lengths and the total gas mass obtained for the system.

On the other hand, magnetic fields tend to counteract the action of gravity. Due to the effect of line dragging, the intensity of a tangled field increases with gas density as $B \propto n^{2/3}$ [observationally, $B \propto n^\kappa$, with $0.5<\kappa<1$, see e.g. @Crutcher99; @Henney+05], and the associated magnetic pressure varies as $P_{\rm mag} \propto B^2 \propto n^{4/3}$. This equation of state is stiffer than that of an ideal gas, and therefore a much milder change in gas density is necessary in order to achieve hydrostatic equilibrium.

The second panel of Figure \[figRhoStar\_B\] shows how the inclusion of a tangled magnetic field with mid-plane intensity $B_0=5~\mu$G [@Beck01; @HeilesCrutcher05] changes the structure of our ISM model. As can be inferred from the bottom panels, the magnetic field provides most of the pressure support against gravity. The extent of the cold phase increases by roughly an order of magnitude, reaching now hundreds of pc. In the outer, hot layer, pressure is dominated by the thermal component, and the magnetic field (for our adopted value of $B_0$) does not play a significant role.

Comparison with observations
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![ Comparison between the model predictions 