Reappraisal of thePioneer 10andVoyager 2Lyα Intensity Measurements

The Pioneer 10 (P10) and Voyager 2 (V2) calibration difference of 4.4 at Lyα has made it difficult to interpret the Lyα data and also to resolve the outer planetary upper atmosphere excess Lyα glow problem. We have carried out radiative transfer calculations using an improved radiative transfer code and six heliosphere neutral-plasma density models to study the calibration of P10 and V2 at Lyα and found that both P10 and V2 intensity measurements are in need of revision. The intercalibration difference is discussed using our model calculations, recent large-distance neutral hydrogen density determinations obtained from pickup-ion and solar wind slow-down data, the recent change in the estimate of the solar Lyα flux values, and Voyager 1 energetic particle measurements. These recent heliospheric measurements and Lyα glow model calculations support the need for an upward revision of P10 and a downward revision of V2 Lyα intensity. It is not yet possible to give a definitive estimate of the required revision because of lack of knowledge of the very local interstellar medium neutral hydrogen density. The calibration revision is found to reduce the range of variation of Jovian dayglow.