  We study the propagation of the ionizing radiation emitted by the secondary
star in Eta Carinae. We find that a large fraction of this radiation is
absorbed by the primary stellar wind, mainly after it encounters the secondary
wind and passes through a shock wave. The amount of absorption depends on the
compression factor of the primary wind in the shock wave. We build a model
where the compression factor is limited by the magnetic pressure in the primary
wind. We find that the variation of the absorption by the post-shock primary
wind with orbital phase changes the ionization structure of the circumbinary
gas and can account for the radio light curve of Eta Car. Fast variations near
periastron passage are attributed to the onset of the accretion phase.
