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We discuss the rest-frame optical emission line spectra of a large (~50)sample of z=3.1 Lyman alpha emitters (LAEs) whose physical properties suggestsuch sources are promising analogs of galaxies in the reionization era.Reliable Lyman continuum escape fractions have now been determined for a largesample of such LAEs from the Lyman Continuum Escape Survey (LACES) undertakenvia deep HST imaging in the SSA22 survey area reported in Fletcher et al.(2019). Using new measures of [OII] emission secured from Keck MOSFIRE spectrawe re-examine, for a larger sample, earlier claims that Lyman continuumleakages may correlate with the nebular emission line ratio [OIII]/[OII] asexpected for density-bound HII regions. We find that a large [OIII]/[OII] lineratio is indeed a necessary condition for Lyman continuum leakage,strengthening earlier claims made using smaller samples at various redshifts.However, not all LAEs with large [OIII]/[OII] line ratios are leakers andleaking radiation appears not to be associated with differences in otherspectral diagnostics. This suggests the detection of leaking radiation ismodulated by an additional property, possibly the viewing angle for porous HIIregions. We discuss our new results in the context of the striking bimodalityof LAE leakers and non-leakers found in the LACES program and the implicationsfor the sources of cosmic reionization.

The Lyman Continuum Escape Survey -- II: Ionizing Radiation as a Function of the [OIII]/[OII] Line Ratio