Diurnal variations of Titan's ionosphere

We present our analysis of the diurnal variations of Titan's ionosphere (between 1000 and 1300 km) based on a sample of Ion Neutral Mass Spectrometer (INMS) measurements in the Open Source Ion (OSI) mode obtained from eight close encounters of the Cassini spacecraft with Titan. Although there is an overall ion depletion well beyond the terminator, the ion content on Titan's nightside is still appreciable, with a density plateau of ∼700 cm−3 below ∼1300 km. Such a plateau is a combined result of significant depletion of light ions and modest depletion of heavy ones on Titan's nightside. We propose that the distinctions between the diurnal variations of light and heavy ions are associated with their different chemical loss pathways, with the former primarily through “fast” ion‐neutral chemistry and the latter through “slow” electron dissociative recombination. The strong correlation between the observed night‐to‐day ion density ratios and the associated ion lifetimes suggests a scenario in which the ions created on Titan's dayside may survive well to the nightside. The observed asymmetry between the dawn and dusk ion density profiles also supports such an interpretation. We construct a time‐dependent ion chemistry model to investigate the effect of ion survival associated with solid body rotation alone as well as superrotating horizontal winds. For long‐lived ions, the predicted diurnal variations have similar general characteristics to those observed. However, for short‐lived ions, the model densities on the nightside are significantly lower than the observed values. This implies that electron precipitation from Saturn's magnetosphere may be an additional and important contributor to the densities of the short‐lived ions observed on Titan's nightside.