Disc-like magneto-gravitational equilibria

Models are constructed of cool, disc-like clouds in magneto-gravitational equilibrium, immersed in a warm, low-density inter-cloud medium, and with the magnetic field a locally distorted part of the local galactic field. In each case, an area density distribution is adopted, and the detailed flux-to-mass distribution constructed from the electromagnetic and dynamical equations. As expected from simple virial theorem arguments, for each density distribution the non-dimensional total flux/mass parameter $$\textit {f}$$ cannot be less than a critical value $$\textit {f}_\text c$$ if mechanical equilibrium is to be maintained. Each such limiting model (with $$\textit {f}=\textit {f}_\text c$$) has a magnetic neutral point at the edge. It is found that in the various limiting models the flux-to-mass distribution has a nearly constant form (with $$\textit {f}_\text c$$ correspondingly nearly constant), in spite of the very different spatial distributions of both mass-density and flux-density. The consequences of plasma-plus-field diffusion are briefly discussed.