Pioneer 10 and Voyagers 1 and 2 backscattered UV data and the nature of the interstellar medium

It has not yet been unambiguously established whether the bulk velocity of the very local interstellar medium (VLISM) relative to the Sun is subsonic or supersonic. Till now there has been no way to decide between the two possibilities although the backscattered UV data base obtained from the Pioneer 10 and Voyagers 1 and 2 missions in principle contain the required information. The principal difficulty in interpreting the UV data has been the lack of self-consistent heliospheric models. In recent years, however, several self-consistent models have become available. We have used two two-dimensional axisymmetric hydrodynamic models of the heliosphere developed by one of the authors for both supersonic and subsonic VLISM flow and compared their predictions with the observed backscattered Lyman-α data. We found that neither model can satisfactorily reproduce the observed Pioneer 10 and Voyager 2 UV data except for the large distance Pioneer 10 downstream UV data.