LONG-TERM VARIATION OF THE INTERPLANETARY H-LY-ALPHA GLOW - VOYAGER UVS MEASUREMENTS AND IMPLICATIONS FOR THE SOLAR H-LY-ALPHA IRRADIANCE

In this paper we study interplanetary (IP) Ly alpha data taken with th e Voyager 1 and Voyager 2 spacecraft from 1980 to 1995. The coverage i n time is equal to about 156 and 220 points yr(-1) for Voyager 1 and V oyager 2, respectively, with almost no gaps. The IP Ly alpha data are normalized for spatial changes in the emissivity, which arise from var iations in observing geometry, by using a radiative transfer model. Th e normalized data show the variation of the solar H Ly alpha line-cent er flux during the solar cycle. We compare this variation with the sol ar H Ly alpha irradiance measurements of integrated flux from the Sola r Mesosphere Explorer and the Upper Atmosphere Research Satellite/Sola r-Stellar Irradiance Comparison Experiment (SOLSTICE), and, when direc t solar measurements are not available, we use estimated irradiances f rom magnesium and helium indices. The comparison between Voyager IP da ta and solar data shows that the best agreement is found with the SOLS TICE set of measurements, when no differences in the variation of the line-center flux and the integrated flux are taken into account.