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Spectral observations of the massive colliding wind binary Eta Carinae showphase-dependent variations, in intensity and velocity, of numerous heliumemission 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 withinthe primary's dense wind photosphere. The combination of these effects isthought to be the cause of the particularly interesting features observed inthe helium lines at periastron. We perform 3D radiative transfer simulations of$\eta$ Car's interacting winds at periastron. Using the SimpleX radiativetransfer algorithm, we post-process output from 3D smoothed particlehydrodynamic simulations of the inner 150 au of the $\eta$ Car system for twodifferent primary star mass-loss rates ($\dot{M}_{\eta_{\mathrm{A}}}$). Usingprevious results from simulations at apastron as a guide for the initialconditions, we compute 3D helium ionization maps. We find that, for higher$\dot{M}_{\eta_{\mathrm{A}}}$, $\eta_{\mathrm{B}}$ He$^{0+}$-ionizing photonsare 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 ofthe WWIR and in a small region close to the stars. Lowering$\dot{M}_{\eta_{\mathrm{A}}}$ allows $\eta_{\mathrm{B}}$'s ionizing photons toreach the expanding unshocked secondary wind on the apastron side of thesystem, and create a low fraction of He$^{+}$ in the pre-shock primary wind.With apastron on our side of the system, our results are qualitativelyconsistent with the observed variations in strength and radial velocity of$\eta$ Car's helium emission and absorption lines, which helps better constrainthe regions where these lines arise.

3D radiative transfer simulations of Eta Carinae's inner colliding winds - II: Ionization structure of helium at periastron