Magnetic fields from cosmological bulk flows

We explore the possibility that matter bulk flows could generate the required vorticity in the electron-proton-photon plasma to source cosmic magnetic fields through the Harrison mechanism. We analyze the coupled set of perturbed Maxwell and Boltzmann equations for a plasma in which the matter and radiation components exhibit relative bulk motions at the background level. These background bulk motions induce a relative velocity between the matter and cosmic microwave background rest frames at the present time, i.e. a bulk flow, with an amplitude beta. We find that, to first order in cosmological perturbations, bulk flows with velocities compatible with current Planck limits (beta<8.5 x 10(-4) at 95% CL) could generate magnetic fields with an amplitude 10(-21) G on 10 kpc comoving scales at the time of completed galaxy formation which could be sufficient to seed a galactic dynamo mechanism.