Detection of Titan's ionosphere from Voyager 1 radio occultation observations

Evidence for a marginal detection of the Titan ionosphere has been obtained from a new analysis of the dual-frequency Doppler data recorded during the Voyager 1 occultation in 1980. The original report by Lindal et al. (1983, Icarus 53, 348-363) gave only upper bounds on the peak electron density of 3000 cm(-3) during ingress, (evening terminator) and 5000 cm(-3) during egress (morning terminator). The dual-frequency ingress data imply a maximum electron density of 2400 +/-: 1100 cm(-3) for Titan's upper ionosphere at an altitude of 1180 +/- 150 km, The egress data were determined to be of limited use for this analysis because the X-band signal was received for only a few seconds. Nevertheless, a distinct ionospheric peak is revealed in the S-band data for both ingress and ,egress. The height and peak density of this ionized layer are in good agreement with expectations from numerical models that invoke photoionization and energetic electron impacts, (C) 1997 Academic Press.