Negative ion densities in the ionosphere of Titan–Cassini RPWS/LP results

• Negative and positive ion and electron charge density are mapped in Titan's ionosphere.
• Neg. ion charge densities reach up to 2500 cm−3, exp. increasing to lower altitudes.
• Below 1000 km ions dominate over electrons and the ionosphere behaves as dusty plasma.
• Our results support a scenario of day-to-night transport of long-lived heavy negative ions.

The Cassini spacecraft Radio and Plasma Wave Science (RPWS) Langmuir Probe (LP) provides in-situ measurements of Titan's ionosphere. We present here data from 47 deep flybys in the time period October 2004–July 2012 of charge densities of positive and negative ions as well as electrons. These densities have been mapped with respect to altitude and solar zenith angle (SZA) in an altitude range of 880–1400 km. The inferred electron number densities are consistent with earlier presented observational results. Negative ion charge densities exhibit a trend that exponentially increases towards lower altitudes within the covered altitude range. This is especially evident on the nightside of Titan (SZA >110°). The negative ion charge densities at the lowest traversed altitudes (near 960 km) are inferred to be in the range 300–2500 cm−3. The results show that very few free electrons (ne/ni∼0.1–0.7) exist in the deepest regions (880–1050 km) of Titan's nightside ionosphere. Instead the deep nightside part of Titan's ionosphere is dominated by both negatively and positively charged heavy (>100 amu) organic ions. We therefore believe a dust/aerosol-ion plasma exists here, similar to what is found in noctilucent clouds in Earth's mesosphere.