Interpretation of Pioneer 10 heliospheric Ly α glow data obtained beyond 30 AU

Abstract

The Pioneer 10 (P10) Ly dataset is the only dataset that is available for the study of the very local in- terstellar medium (VLISM) in the downstream direction rel- ative to the incoming interstellar neutral hydrogen flow, at very large distances from the sun. Selected P10 data ob- tained in 1984 and 1986 at distances between 31.81 to 32.25 and 37.29 to 37.74 AU have been used to estimate the local interstellar neutral hydrogen and proton densities. State of the art, stationary neutral-plasma and radiative transfer mod- els have been used in the interpretation of the data. No sta- tionary VLISM heliospheric model was found that best fitted both the P10 1984 data and the 1986 data. The failure to find a single best fit model is most probably due to the fact that the heliospheric model used here did not incorporate time- dependence and interstellar magnetic field effects. The photometer on-board P10 has measured the interplan- etary Ly background radiation for more than twenty years and has made a quantitative study of the downstream neu- tral hydrogen density possible. This analysis uses the lat- est state of the art neutral hydrogen-plasma and radiative transfer models outlined in the later sections to calculate Ly glow intensities and these intensities are compared with the P10 data. The new results are presented and the implications discussed.