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In the absence of an initial seed, the Biermann battery term of a non-idealinduction equation acts as a source that generates weak magnetic fields. Thesefields are then amplified via a dynamo mechanism. The Kelvin-Helmholtzinstability is a fluid phenomenon that takes place in many astrophysicalscenarios and can trigger the action of the Biermann battery and dynamoprocesses. We aim to investigate the effect that the ionisation degree of theplasma and the interaction between the charged and neutral species have on thegeneration and amplification of magnetic fields during the different stages ofthe instability. We use the two-fluid model implemented in the numerical codeMancha-2F. We perform 2D simulations starting from a configuration with noinitial magnetic field and which is unstable due to a velocity shear. We varythe ionisation degree of the plasma and we analyse the role that the differentcollisional terms included in the equations of the model play on the evolutionof the instability and the generation of magnetic field. We find that when nocollisional coupling is considered between the two fluids, the effect of theBiermann battery mechanism does not depend on the ionisation degree. However,when elastic collisions are taken into account, the generation of magneticfield is increased as the ionisation degree is reduced. This behaviour isslightly enhanced if the process of charge-exchange is also considered. We alsofind a dependence on the total density of the plasma related to the dependenceon the coupling degree between the two fluids. As the total density isincreased, the results from the two-fluid model converge to the predictions ofsingle-fluid models.

Simulations of the Biermann battery mechanism in two-fluid partially ionised plasmas