Remote diagnostic of the hydrogen wall through measurements of the backscattered solar Lyman alpha radiation by Voyager 1/UVS in 1993-2003

Abstract : We perform a new analysis of the Lyman-alpha data obtained by Voyager 1 during the spatial scans in 1993-2003 while Voyager 1 was at 53-88 AU from the Sun. These data are the important source of information on the hydrogen distribution in the outer heliosphere. A sophisticated global kinetic-MHD model of the heliospheric interface and radiative transfer are used for the analysis. It is shown for the first time that the ratio of the Lyman-alpha intensities detected in the downwind and upwind lines-of-sight in the outer heliosphere is sensitive to the configuration (peak value and location) of the hydrogen wall. The hydrogen wall is a source of Doppler shifted backscattered Lyman-alpha photons, so it can be seen from inside the heliosphere. Therefore, Voyager 1/UVS Lyman-alpha data can be used for remote sensing of the hydrogen wall. We show that our current global model of the outer heliosphere, which is consistent with many other measurements including Lyman-alpha data from both Voyagers in 1980-1993, provides a systematically larger downwind to upwind intensity ratio compared with the UVS data in 1993-2003. In order to decrease the ratio, a higher and/or closer hydrogen wall is needed.