Outer heliosphere Lyman alpha background derived from two-shock model hydrogen distributions: Application to the Voyager UVS dataстатья
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Аннотация:In a previous paper [Quemerais et al. 1995], we have reported the results of Lyman alpha measurements made by the Ultraviolet Spectrometers aboard both Voyager 1 and 2 spacecraft since 1993. We showed that comparison of the data with a radiative transfer calculation for an unperturbed interstellar hydrogen flow into the solar system revealed a systematic excess in the upwind direction. An absolute estimate of this Lyman alpha intensity excess yielded 10 to 15 Rayleigh for each spacecraft. Results on the outer heliospheric hydrogen distribution obtained by Baranov and Malama [1993] have been used to estimate the Lyman alpha intensity backscattered by neutral hydrogen atoms at large distance from the Sun. The existence of the hydrogen wall near the heliopause implies a gradient of hydrogen at large distance from the Sun which significantly affects the Lyman alpha glow pattern. In these new computations we have also included variations of local bulk velocity and temperature of the hydrogen distribution in the radiative transfer calculation. Although it has not been possible to find a completely satisfactory fit between the data and the Baranov-Malama models, a better agreement was found than in the case of the standard hot model, which neglects effects of charge exchange between interstellar protons and hydrogen atoms near the heliopause. On the other hand, the remaining discrepancy between the models and the data, which is found near the upwind direction, suggests that the intensity excess does not follow the simple axial symmetry used here. Because the directions of the incoming hydrogen atoms and the galactic center nearly coincide, we must alternately consider the possibility that the Lyman alpha intensity excess may include a component of galactic origin.