Ultraviolet Glow from the Hydrogen Wall

International audienceWe compare new results of models of the interplanetary H Lyα intensity background in the outer heliosphere with scans performed by the Voyager 1/2 UV spectrometer (UVS) instruments between 1993 and 2003. This study shows that the excess intensity initially reported by Quémerais et al. can be explained by models of the hydrogen atom distribution including effects of the heliospheric interface. The models of the hydrogen atom distribution in the interplanetary medium used in this work have been developed following the numerical scheme presented by Baranov & Malama. Recent improvements are described by Izmodenov et al. Radiative transfer computations of the interplanetary Lyα intensity are made following a Monte Carlo approach presented by Quémerais and Quémerais & Izmodenov. We find that the upwind intensity excess observed in the outer heliosphere initially reported by Quémerais et al. can be explained by a full radiative transfer computation. This computation must include a full description of the velocity distributions of the different hydrogen populations that enter the heliosphere after crossing the interface. The excess upwind intensity observed by UVS on Voyager 1 and Voyager 2 can be explained as an emission of the decelerated hydrogen atoms near the stagnation point of the heliopause. Because those atoms are slowed down relative to the main hydrogen flow, photons they scatter suffer less absorption and are visible at a much larger distance than is the case for photons scattered by atoms in the main flow. The shape and extent of the excess emission gives information about the decelerated population near the heliopause stagnation point. A detailed comparison between the data and our present model does not show a complete agreement. The modeled intensity excess is larger than the observed one. We discuss possible improvements to the H distribution model in order to decrease the size of the excess in the model, for example, by decreasing the density of H atoms in the hydrogen wall