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Direct numerical simulations of the galactic dynamo in the kinematic growing phase

ABSTRACT We present kinematic simulations of a galactic dynamo model based on the largescale diﬀerential rotation and the small scale helical ﬂuctuations due to supernova ex-plosions. We report for the ﬁrst time direct numerical simulations of the full galacticdynamo using an unparameterized global approach. We argue that the scale of helicityinjection is large enough to be directly resolved rather than parameterized. While theactual superbubble characteristics can only be approached, we show that numericalsimulations yield magnetic structures which are close both to the observations and tothe previous parameterized mean ﬁeld models. In particular, the quadrupolar symme-try and the spiraling properties of the ﬁeld are reproduced. Moreover, our simulationsshow that the presence of a vertical inﬂow plays an essential role to increase the mag-netic growth rate. This observation could indicate an important role of the downwardﬂow (possibly linked with galactic fountains) in sustaining galactic magnetic ﬁelds.Key words: Dynamo – Galaxies – Supernovae – Superbubbles.