A one-dimensional chemical-diffusive model of the Jovian ionosphere, in conjunction with measured upper atmospheric temperatures, is used to analyze the upper ionospheric electron densities on Jupiter measured by the Galileo RSS instrument on December 8, 1995. The analyses of these measurements have yielded quite different ionospheric properties at ingress and egress in terms of both the magnitude and the altitude of the peak electron density (ne). At ingress, the peak ne was ~105 cm-3 at an altitude of ~900 km. However, at egress the ne peak was ~5 times smaller than at ingress and was located at ~1800 km altitude. As with our previous studies, we find it necessary to invoke a combination of vibrationally excited H2 and vertical plasma flow to explain the measured ionospheric structure. The most interesting conclusion of this study is that a downward drift of plasma is required to fit the peak altitude of the ingress ne profile. The direction of the vertical flow is most likely determined by the horizontal neutral wind. At egress, the situation is quite different because a strong field-aligned drift of 90 m/s, most likely caused by the meridional component of the neutral wind, is required to maintain the peak ne at the measured altitude. The role of vibrationally excited H2 in determining the magnitude of the measured ne appears to be less important at ingress than at egress. ¿ 1999 American Geophysical Union