Long-term variation of the interplanetary H lyα glow: Voyager UVS measurements and implications for the solar H lyα irradiance

In this paper we study interplanetary (IP) Lyα data taken with the Voyager 1 and Voyager 2 spacecraft from 1980 to 1995. The coverage in time is equal to about 156 and 220 points yr^-1 for Voyager 1 and Voyager 2, respectively, with almost no gaps. The IP Lyα data are normalized for spatial changes in the emissivity, which arise from variations in observing geometry, by using a radiative transfer model. The normalized data show the variation of the solar H Lyα line-center flux during the solar cycle. We compare this variation with the solar H Lyα irradiance measurements of integrated flux from the Solar Mesosphere Explorer and the Upper Atmosphere Research Satellite/Solar-Stellar Irradiance Comparison Experiment (SOLSTICE), and, when direct solar measurements are not available, we use estimated irradiances from magnesium and helium indices. The comparison between Voyager IP data and solar data shows that the best agreement is found with the SOLSTICE set of measurements, when no differences in the variation of the line-center flux and the integrated flux are taken into account.