Constraints on Titan's ionosphere

The near flyby of Saturn's moon Titan by Voyager 1 revealed a Venus‐like interaction between the moon and Saturn's magnetospheric plasma. Although neither the radio science experiment occultation observation nor the in situ measurements directly detected the ionosphere, plasma of ionospheric origin was observed as Voyager 1 passed through Titan's wake. Balancing the magnetic pressure in this low‐β region of Saturn's magnetosphere with ionospheric particle pressure yields an upper limit on the ionospheric density. Using an ionospheric temperature equal to the exospheric temperature of 200°K yields a charge density of about 3000 cm −3 , which is consistent with the peak ionospheric electron density inferred from a balance of electron impact ionization of molecular nitrogen and recombination loss. Both of these quantities are consistent with limits derived from Voyager 1 observations. Good constraints on these quantities are important in planning the Cassini mission to orbit Saturn and probe Titan's ionosphere and atmosphere at the beginning of the next century.