Structure of Titan's low altitude ionized layer from the Relaxation Probe onboard HUYGENS

Some of the secrets of the atmosphere of Titan have been unveiled by the Huygens Probe. The Permitivity Wave and Altimetry system detected a hidden ionosphere much below the main ionosphere, that lies between 600 and 2000 km. Theoretical models predicted a low altitude ionosphere produced by cosmic rays that, contrary to magnetospheric particles and UV photons, are able to penetrate down in the atmosphere. Two sensors: Mutual Impedance (MI) and Relaxation Probe (RP) measured the conductivity of the ionosphere by two different methods and were able to discriminate the two branches of electrical conductivity due to the positive and negative charges. The measurements were made from 140 to 40 km and show a maximum of charge densities ≈2 × 109 m−3 positive ions and ≈450 × 106 m−3 electrons at around 65 km. Here we present the altitude distribution of the concentration of positive ions and electrons obtained from the RP and MI sensors.