Thermal instability revisited
  Field's linear analysis of thermal instability is repeated using methods
related to Whitham's theory of wave hierarchies, which brings out the
physically relevant parameters in a much clearer way than in the original
analysis. It is also used for the stability of non-equilibrium states and we
show that for gas cooling behind a shock, the usual analysis is only
quantitatively valid for shocks that are just able to trigger a transition to
the cold phase. A magnetic field can readily be included and we show that this
does not change the stability criteria. By considering steady shock solutions,
we show that almost all plausible initial conditions lead to a magnetically
dominated state on the unstable part of the equilibrium curve. These results
are used to analyse numerical calculations of perturbed steady shock solutions
and of shocks interacting with a warm cloud.
