GALACTIC FOUNTAINS AS MAGNETIC PUMPS

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