Contribution of multiple scattering to the distribution of Lyman alpha emission in comet comae

Comets are important in understanding the material balance of current and past planets. Water production rates from comet nuclei have been evaluated using various instruments including a Japanese satellite, Hisaki. In case of UV observations, water production rates are generally evaluated through the comparison of the observations of the Lyman alpha radiance distribution and kinetic model of hydrogen atoms generated by photodissociation of water molecules. However, dynamics of hydrogen atoms in comae in the vicinity of nuclei has not been well understood especially for long period comets with large water production rates.

In this study, we obtained the spatial distributions of Lyman alpha radiance through the analysis of spectroscopic data of long period comets such as C/2013 US_{10} (Catalina) observed by Hisaki. As a results, inclination of the Lyman alpha radiance was found to become flatter below the impact parameter of 5 × 10^{4} km. We attributed this variation of inclination to multiple scattering, and established a radiative transfer model considering multiple scattering of photons. Then we successfully reproduced the Lyman alpha radiance distributions in comae observed by Hisaki.

According to calculations using this model, multiple scattering becomes effective in comae when the hydrogen column density exceeds approximately 5 × 10^{22} /km^{2}. Additionally, multiple scattering was found to cause the sunward/anti-sunward radiance asymmetry less than 3 %, and the apparent increase of D/H ratio around the nuclei by a factor of more than 10.

Using these results, the feasibility of detecting deuterium and evaluating the D/H ratio via Hydrogen Imager (HI) onboard the Comet Interceptor spacecraft was discussed. It was found that deuterium could be detected with a sufficient S/N ratio and that the D/H ratio could be evaluated with relative error less than 60 % using the model. The largest error factor in D/H ratio calculation is the dependence of the results of radiative transfer model on hydrogen temperature. Constraint on observations by HI or calculation by models such as DSMC will enable the calculation of the D/H ratio with higher accuracy.