A model analysis of Galileo electron densities on Jupiter

A one-dimensional chemical-diffusive model of the Jovian ionosphere, in con junction with measured upper atmospheric temperatures, is used to analyse t he 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 te rms of both the magnitude and the altitude of the peak electron density (n( e)). At ingress, the peak n(e) was similar to 10(5) cm(-3) at an altitude o f similar to 900 km. However, at egress the n(e) peak was similar to 5 time s smaller than at ingress and was located at similar to 1800 km altitude. A s with our previous studies, we find it necessary to invoke a combination o f vibrationally excited H-2 and vertical plasma flow to explain the measure d ionospheric structure. The most interesting conclusion of this study is t hat a downward drift of plasma is required to fit the peak altitude of the ingress n(e) profile. The direction of the vertical flow is most likely det ermined by the horizontal neutral wind. At egress, the situation is quite d ifferent 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 t he peak n(e) at the measured altitude. The role of vibrationally excited H- 2 in determining the magnitude of the measured n(e) appears to be less impo rtant at ingress than at egress.