Contribution of multiple scattering to the Lyman alpha radiance distribution in cometary comae

Observations of hydrogen Lyman Alpha (Ly-α) radiance distribution in cometary comae are important to deduce water production rates and understand the kinetics of coma gases. Here, we constructed a radiative transfer model including multiple scattering processes to reproduce Ly-α radiance distributions in cometary comae observed by the Hisaki satellite. Using the established model, we found that multiple scatterings should be considered when the hydrogen column density exceeds ∼3 × 10¹² /cm². Additionally, as deuterium Ly-α is almost optically thin unlike hydrogen Ly-α, the deuterium-to‑hydrogen (D/H) Ly-α radiance ratio was found to increase in inner comae. In the end, we calculated the Ly-α radiance profile expected to be observed by the Hydrogen Imager onboard the Comet Interceptor spacecraft. Overall, we quantitatively showed that it is necessary to consider multiple scattering when evaluating activities and D/H ratios of active comets with water production rates over 10²⁹ /s.