  Spectral observations of the massive colliding wind binary Eta Carinae show
phase-dependent variations, in intensity and velocity, of numerous helium
emission and absorption lines throughout the entire 5.54-year orbit.
Approaching periastron, the 3D structure of the wind-wind interaction region
(WWIR) gets highly distorted due to the eccentric ($e \sim 0.9$) binary orbit.
The secondary star ($\eta_{\mathrm{B}}$) at these phases is located deep within
the primary's dense wind photosphere. The combination of these effects is
thought to be the cause of the particularly interesting features observed in
the helium lines at periastron. We perform 3D radiative transfer simulations of
$\eta$ Car's interacting winds at periastron. Using the SimpleX radiative
transfer algorithm, we post-process output from 3D smoothed particle
hydrodynamic simulations of the inner 150 au of the $\eta$ Car system for two
different primary star mass-loss rates ($\dot{M}_{\eta_{\mathrm{A}}}$). Using
previous results from simulations at apastron as a guide for the initial
conditions, we compute 3D helium ionization maps. We find that, for higher
$\dot{M}_{\eta_{\mathrm{A}}}$, $\eta_{\mathrm{B}}$ He$^{0+}$-ionizing photons
are not able to penetrate into the pre-shock primary wind. He$^{+}$ due to
$\eta_{\mathrm{B}}$ is only present in a thin layer along the leading arm of
the WWIR and in a small region close to the stars. Lowering
$\dot{M}_{\eta_{\mathrm{A}}}$ allows $\eta_{\mathrm{B}}$'s ionizing photons to
reach the expanding unshocked secondary wind on the apastron side of the
system, and create a low fraction of He$^{+}$ in the pre-shock primary wind.
With apastron on our side of the system, our results are qualitatively
consistent with the observed variations in strength and radial velocity of
$\eta$ Car's helium emission and absorption lines, which helps better constrain
the regions where these lines arise.
