Auroral Lyman alpha and H-2 bands from the giant planets 3. Lyman alpha spectral profile including charge exchange and radiative transfer effects andH-2 color ratios

In this paper, third of a series of three dealing with a model of auroral H and H-2 emission in the giant planets, we focus on the characteristics of the emergent emission, the only one which can be compared with observations . As the Jovian atmosphere is optically thick at 1215.67 Angstrom, modeling of emergent auroral Lyman alpha line profiles requires the use of a radiat ive transfer code to model the transport of photons from the auroral source to the top of the atmosphere. Here, radiative transfer effects are modeled using the "doubling and adding" method. This radiative transfert code is s elf-consistently coupled with the energy degradation code used in the first two papers to compute the excitation rate along the path of precipitating particles as a function of wavelength. Input parameters are the identity an d the energy of the incoming particles. We find that the auroral Lyman a li ne profile shows a central reversal due to the atmospheric H overlying the emitting layer. The shape of the emergent line is almost only sensitive to the column of H in the line of sight to the emission, related, via the atmo spheric model used, to the the particle penetration depth (i.e. their energ y). In addition, in the case of proton precipitation, charge exchange produ ces fast H atoms (H-f) which precipitate with the protons. H-f can also be excited and radiate Lyman alpha photons. This produces a second, Doppler sh ifted, component, of the Lyman alpha profile. This component may represent as much as 77.4% of the total Lyman alpha intensity for 10 keV protons,and it decreases with incident proton energy. It also extends over a broad wave length range (up to 56 Angstrom for 1 MeV proton). Detection of this compon ent would unambiguously identify protons as the particles responsible for t he Jovian aurorae. However, for high proton energies, the escaping flux may be too weak to be detected. Finally, following earlier analyses of IUE aur oral spectra, we compute the color ratio C between the fluxes escaping in t wo particular wavelength ranges of the H-2 Werner and Lyman bands, 1230-130 0 Angstrom and 1557-1619 Angstrom. We also compute the ratio C-Ly alpha bet ween the Ha short-wavelength range and the line integrated Lyman alpha flux . C and C-Ly alpha are sensitive to the CH4 and the H column densities, res pectively, overlying the auroral source along the line of sight. Once an at mospheric model is assumed, energies of the precipitating particles can be derived. Each of these ratios shows a specific variation with the energy of the particles. In addition, C-Ly alpha is sensitive to the identity of pre cipitating particles as well, so that combined together, they can, in princ iple, provide a unique diagnostic of the Jovian aurorae.