UVS-Voyager 1 measurements in the outer heliosphere

The UVS spectrometer on Voyager 1 is still active in 2014. The only line still observed in the spectral range covered by UVS is the HI Lyman alpha transition due to the backscattering of solar photons by H atoms in the outer heliosphere. The background intensity corresponds to a few tens of rayleigh. In 2003, movements of the Voyager 1 scan platforms were stopped and since then the line of sight of UVS has been fixed. It is pointing towards a direction close to the upwind direction. Over the 2003-2010 period, IPH background data have been very constant and followed the variations of the solar illuminating flux at lyman alpha, smoothed by multiple scattering efftects between the sun and the outer heliosphere. After corrrection from solar flux variations, data obtained in this period changed by less than 10% over a distance of more than 20 AU which is difficult to explain with radiative transfer models. However, a change in the radial dependence of the data started in 2011 and lasted until the end of 2013. New measurements in early 2014 show a stabilization and a return to the previous pattern. This needs to be confirmed in the next few months. We will present the UVS-V1 data, and give special attention to possible instrumental effects linked to thermal changes in the Voyager 1 spacecraft as various heaters are switched off. We think that the changes observed since 2011 are not due to instrumental effects. We will also present some ideas that can help us to explain the UVS observations. To do this, it is necessary to modify our current picture of the distribution of hydrogen atoms in the outer heliosphere. These new results have an impact on our understanding of the heliospheric interface