  Numerous studies have investigated the role of thermal instability in
regulating the phase transition between the cold cloudy and warm diffuse medium
of the interstellar medium. Considerable interest has also been devoted in
investigating the properties of turbulence in thermally unstable flows, special
emphasis on molecular clouds and the possibility of star formation. In this
study, we investigate another setting in which this instability may be
important, namely its effect on dynamo action in interstellar flows. The setup
we consider is a three dimensional periodic cube of gas with an initially weak
magnetic field, subject to heating and cooling, the properties of which are
such that thermal instability is provoked at certain temperature regime. Dynamo
action is established through external forcing on the flow field. By comparing
the results with a cooling function with exactly the same net effect but no
thermally unstable regime, we find the following. The critical Reynolds number
for the onset of the large-scale dynamo was observed to roughly double between
the thermally stable versus unstable runs, the conclusion being that the
thermal instability makes large-scale dynamo action more difficult. Whereas
density and magnetic fields were observed to be almost completely uncorrelated
in the thermally stable cases investigated, the action of thermal instability
was observed to produce a positive correlation of the form B propto rho^0.2.
This correlation is rather weak, and in addition it was observed to break down
at the limit of the highest densities.
