An estimate of the H₂ density in the atomic hydrogen cloud of Titan

Charged particle experiments on the Voyager spacecraft at Saturn can be used to provide some useful estimates on the charge exchange loss rate of magnetospheric particles in the atomic hydrogen cloud of Titan. The thermal plasma instrument measured the number density and ion and electron temperatures of the corotating plasma and thus the charge exchange loss time scale of the neutral gas; the low‐energy particle detectors measured the flux of the energetic neutrals generated by charge exchange recombination of the hot magnetospheric plasma. These observational results together with known reaction rate coefficients can be used to compute the total H + H 2 density in the hydrogen torus. As the Voyager UV spectrometer experiment determined the average number density of hydrogen atoms independently, a limit on the H 2 density in the neutral torus region can be estimated. This method leads to an H 2 density value of ≲10 cm −3 , considerably less than the limiting value for the ballistic motion of the neutral particles to be collisionally dominated.