Modeling the Emission of Energetic Neutral Atoms in Titan's Dynamic Magnetospheric Environment

Abstract

To provide context for Cassini observations of energetic neutral atoms (ENAs) generated by charge exchange between Titan's atmosphere and energetic magnetospheric ions, we compute synthetic ENA images for a variety of uniform and draped electromagnetic field configurations, flyby scenarios, and viewing geometries. Starting with the idealized case of the magnetospheric field near Titan pointing southward, we calculate the observable ENA emission morphology for a large set of detector locations, each mimicking the properties of Cassini's Ion and Neutral Camera. Regardless of whether the electromagnetic fields near Titan are treated as uniform or draped, ENA production occurs within a thin shell of radial thickness 85 km, located directly above the moon's exobase. The observable ENA morphology is largely controlled by the angle between the detector's boresight vector and Saturn's magnetospheric field. Both the radius and the sense of energetic ion gyration carve distinct gaps in the ENA emission pattern. Field line draping around Titan leaves a discernible imprint in the ENA emissions only for a narrow range of viewing geometries. We also apply the model to study the ENA images taken during the T20 and T24 flybys. During both encounters, the ambient magnetospheric field displayed frequent transitions between Saturn's northern and southern magnetodisk lobes. We show that the notion of a steady-state interaction between Titan and an averaged magnetospheric background field is not suitable to explain the observed ENA morphology. Instead, ambient field variations measured more than 1 hr before ENA image acquisition still leave an imprint in the emission pattern.