= Saha half-ionization temperature
{c}
{title2=$y-\tfrac32\ln y=\ln[2/(3\times10^{-16})]$}
With the rounded <hydrogen> <Saha equation> coefficient, the condition $X_e=1/2$ gives the displayed equation for $y=\mathcal B/T$. Its low-temperature root is about $42$, hence $T\simeq0.32$ eV for <hydrogen> binding energy $13.6$ eV. Dropping the slowly varying logarithm gives the rough leading-log estimate $T\sim0.4$ eV. This equilibrium convention is distinct from <photon decoupling> and from a full kinetic recombination calculation.
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