Multiple scattering of hydrogen Ly alpha radiation in the coma of comet Hyakutake (C/1996 B2)

The Goddard High Resolution Spectrograph on board the Hubble Space Telescop e measured hydrogen Ly alpha (H Ly alpha) line profiles at different locati ons around the coma of comet Hyakutake (C/1996 B2) in 1996 April. The spect ral resolution of about 4 km s(-1) (Doppler velocity, FWHM) was significant ly better (by a factor of 2-3) than any previous measurement and is suffici ent to constrain models of atomic hydrogen production processes and inner c oma thermodynamics. In a recent paper, we reported the line profile measure ments 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-c entered spectrum were computed, but the inner coma line profiles were only discussed qualitatively. In the present paper, we investigate the details o f 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 accou nt for the line radiation transport throughout the coma. This multiple scat tering model uses number densities and velocity distributions of H atoms pr oduced by the water dissociation processes, along with angle-dependent freq uency redistribution to describe each scattering process. The computed spec tra, when convolved with the instrument function, are in excellent agreemen t 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 t he 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 rad iative transfer treatment of a physically realistic cometary hydrogen densi ty and velocity distribution confirms our overall understanding of water di ssociation processes and partial thermalization in the coma.