GALACTIC FOUNTAINS AS MAGNETIC PUMPS

We study a simple, kinematical model of a galactic fountain flow and s how that a horizontal field in the galactic plane can be pumped into t he halo to a height of several kiloparsecs. This pumping effect result s from the topological structure of the how in which the updraughts (r epresented by hot gas) form a connected network, whereas the downdraug hts (associated with isolated cool clouds) are disconnected from each other. Such a flow traps the large-scale magnetic held in the disc and deposits it at the top of the fountain flow. Unlike previously studie d models of topological pumping, in our case the flow is not constrain ed to a closed box and horizontal magnetic flux can leak out at the to p. We find significant pumping of mean magnetic held into the halo, wh ich can be parametrized by an advection velocity of order 1-10 km s(-1 ). The resulting magnetic field strength at a height of several kilopa rsecs above the galactic plane is comparable with that at the base of the flow.