26 day modulation of the sky background lyα brightness: Estimating the interplanetary hydrogen density

We present here a series of sky background observations, all having a fixed pointing direction for at least one solar rotation, made with both Voyager UVS instruments between 1981 and 1993. These data show modulations with a 26 day period, which we have used to estimate the interplanetary hydrogen density. A comparison of these data with multiple scattering computations of the damping of the solar Lyα flux modulation yields an interplanetary hydrogen density of 0.15 ± 0.10 cm^-3. This result is independent of instrument calibration, but its accuracy is limited by the difficulty of estimating the solar Lyα line flux in every direction at the time of observations. To estimate the solar H Lyα line flux, we have used the Solar Mesosphere Explorer H Lyα flux measurements between 1981 and 1988 and the He I 1083 nm equivalent width data after 1988. We find also that a linear relation between the integrated H Lyα line and the line center does not apply near the solar maximum. Voyager 2 data obtained in 1989 are in better agreement with an empirical nonlinear relation derived by Vidal-Madjar. Finally, for the present position of the Voyager space-craft, we estimate that the factor by which the modulation is damped is at least 0.4, which means that the antisolar sky background modulations have an amplitude less than 40% of the amplitude of the subspacecraft solar flux at line center.