Speaker
Prof.
Maarit Mantere
Description
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
usntable 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.
