  Topological insulators (TIs) exhibit a quantized magnetoelectric response
when time-reversal symmetry is broken on its surface. This unusual
electromagnetic (EM) response is a unique macroscopic manifestation of the
quantum Hall effect on the TI surface and it is described by a quantized
Chern-Simons theory. In this paper, we construct the Green's function (GF)
describing the EM response of two topological media separated by a cylindrical
interface. This GF, in the appropriate limits, describes the magnetoelectric
effect of both i) a cylindrical TI surrounded by a dielectric fluid, and ii) a
TI with a cylindrical dielectric-filled cavity. We calculate the EM fields
produced by a line charge and a line current near the TI surface and we show
that, in addition to the standard image electromagnetic sources, charge and
current densities of magnetic monopoles will also appear. We discuss some
experimental setups which could be used to test these magnetic monopole fields.
