Multiple Scattering of Hydrogen Lyα Radiation in the Coma of Comet Hyakutake (C/1996 B2)

Abstract

The Goddard High Resolution Spectrograph on board the Hubble Space Telescope measured hydrogen Lyα (H Lyα) line profiles at different locations around the coma of comet Hyakutake (C/1996 B2) in 1996 April. The spectral resolution of about 4 km s-1 (Doppler velocity, FWHM) was significantly better (by a factor of 2-3) than any previous measurement and is sufficient to constrain models of atomic hydrogen production processes and inner coma thermodynamics. In a recent paper, we reported the line profile measurements and the results of an explicit model of the optically thin region of the coma. The spectrally integrated emission rates in all but the nucleus-centered spectrum were computed, but the inner coma line profiles were only discussed qualitatively. In the present paper, we investigate the details of the line profiles using a first-principles numerical model of the H coma and a new radiative transfer model based on the Monte Carlo method to account for the line radiation transport throughout the coma. This multiple scattering model uses number densities and velocity distributions of H atoms produced by the water dissociation processes, along with angle-dependent frequency redistribution to describe each scattering process. The computed spectra, when convolved with the instrument function, are in excellent agreement with the measured spectra. The model is able to reproduce features, such as the saturation of the line profiles in the optically deep regions, and the strong asymmetry of the isophotes due to shadowing of the nightside by H atoms of the inner coma. For the first time, a consistent and detailed radiative transfer treatment of a physically realistic cometary hydrogen density and velocity distribution confirms our overall understanding of water dissociation processes and partial thermalization in the coma.