Outer heliosphere Ly-alpha measurements: 1993 to 1998

We present the Lyman alpha sky background data obtained by the UV spectrome ters on the Voyager 1 and 2 spacecraft between 1993 and early 1998. These d ata which consist of special maneuvers dedicated to the study of the Ly alp ha pattern in the outer heliosphere represent a unique opportunity to const rain the hydrogen distribution in the outer heliosphere. A first analysis o f the data is presented. This consists of a comparison with a Hot Model of the hydrogen distribution combined with a radiative transfer calculation in the hypothesis of Complete Frequency Redistribution. We confirm previous m easurements showing the existence of an excess in Ly alpha background in th e upwind direction. The radial variation of the upwind intensity measuremen ts found from the two Voyager data sets are different. The Voyager 1 data s how a radial decrease following r(-1.4) whereas the Voyager 2 data yield r( -0.79). These values have been derived after correction for solar flux vari ations using the SOLSTICE/UARS data set.