UVS – Voyager 1 measurements in the outer heliosphere

Monday, 15 December 2014
Eric Quemerais, Univ Versailles St Quentin, Guyancourt, France, Bill R Sandel, University of Arizona, Tucson, AZ, United States, Rosine Lallement, Paris Observatory Meudon, GEPI, Meudon, France and Jean-Loup Bertaux, University of Versailles Saint-Quentin en Yvelines, Versailles, France
The UVS spectrometer on Voyager 1 is still active. The only line still observed in the data is the HI Lyman alpha transition due to backscattering of solar photons on H atoms in the outer heliosphere. The bakground intensity corresponds to a few 10s of rayleigh.

In 2003, movements of the V1 scan platforms were stopped and since then the line of sight of UVS has been pointing towards the same 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. However, a change of pattern started in 2011 and lasted until the end of 2013. Early measurements in early 2014 show a stabilization that need s to be confirmed in the next few months.

We will present the UVS-V1 data, with special attention to possible instrumental effects linked to thermal changes.To explain the UVS observations, it is necessary to modify our current picture of the distribution of hydrogen atoms in the outer heliosphere. These new assumptions will have an impact on our understanding of the heliospheric interface.