Galactic Fountains as Magnetic Pumps

We study a simple, kinematical model of a galactic fountain ow and show that a horizontal eld in the galactic plane can be pumped into the halo to a height of several kiloparsecs. This pumping eeect results from the topological structure of the ow 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 ow traps the large scale magnetic eld in the disc and deposits it at the top of the fountain ow. Unlike previously studied models of topological pumping, in our case the ow is not constrained to a closed box and horizontal magnetic ux can leak out at the top. We nd signiicant pumping of mean magnetic eld into the halo, that can be parameterised by an advection velocity of order 1{10 km s ?1. The resulting magnetic eld strength at a height of several kiloparsecs above the galactic plane is (at least) comparable with that at the base of the ow.