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Abstract

We comment on a common practice to use pure Hund case (b) Zeeman parameters in planetary atmospheric radiative transfer applications involving molecular oxygen. A detailed theoretical formulation is presented for the Zeeman splitting coefficients of molecular oxygen in its ground vibronic state, taking also the relevant fine-structure interactions into account. The updated Zeeman parameters are compared to the simplified Hund case (b) approach used in earlier works. The biggest differences between the output of these formulations occur for states with low rotational energy, and the differences are greater for 16O18O than for 16O2. To get the order of magnitude error introduced by the simplification of the splitting coefficient computations, we perform a limited case study of forward simulations for a few space-borne and ground-based instruments. Our analysis shows that using simplified Zeeman coefficients introduces errors in the forward modelled radiation varying from insignificant up to 10 K.