Title:    Modeling the Emission of Energetic Neutral Atoms at Titan

Abstract:    Saturn's largest moon Titan orbits near the outer edge of the planet's magnetosphere, where conditions vary erratically on timescales ranging from years to tens of minutes. Magnetospheric plasma that rotates with the planet's magnetic field continuously overtakes the moon. Titan's ionosphere causes this magnetic field to pile up and drape around the moon, forming a localized induced magnetosphere. Our present characterization of Titan's induced magnetosphere is largely based on plasma and magnetic field data collected in situ, along the one-dimensional trajectory of Cassini during these flybys. However, it is difficult to place such measurements within the context of the full three-dimensional interaction due to the rapid oscillation of Saturn's magnetosphere. Charge exchange between energetic magnetospheric ions and the moon's neutral atmosphere generates energetic neutral atoms (ENAs), which can be imaged in a manner largely analogous to traditional photography. The Cassini spacecraft took numerous such "photos" of Titan's ENA signature across 126 close flybys. ENA images constitute snapshots of the moon's entire interaction region, observed simultaneously, making these remote measurements advantageous in such a variable environment. It remains challenging to interpret such observations, however, as ENA images contain information on the ambient energetic ion distribution, the electromagnetic environment near Titan, and the moon's atmosphere. A successful disentangling of these influences, which does not yet exist, would provide a major scientific advantage over in-situ measurements alone. Specifically, understanding exactly how the energetic ion dynamics in Titan's induced magnetosphere shape ENA observations is key to deciphering the information embedded in ENA images taken by Cassini. For this purpose, we have developed two new simulation codes which calculate ENA production and detection. Each of our models utilizes a different detection scheme for capturing modeled ENA emissions: first, a hypothetical spherical detector which reveals the entire ENA population, and second, a point-like detector with a finite field of view closely resembling that of Cassini's ENA camera. Using the first scheme, we identify a "belt" of elevated ENA flux that forms a great circle in the plane perpendicular to the ambient magnetospheric field vector. Field line draping attenuates the intensity of ENA emissions into this belt, but does not strongly alter the belt morphology. Using the second model, we generate and compare over 1000 synthetic ENA images of Titan's magnetospheric interaction with data from several Cassini flybys. We find that both the ambient field vector and field line draping can strongly influence the observed ENA signature, and the visibility of the moon's plasma interaction in ENA images is highly dependent on the viewing geometry.

Date:    November 8, 2024
Time:    3:30 pm
Location:    Ford ES&T L1114
Committee:    Dr. Sven Simon, Dr. James Wray, Dr. Annalisa Bracco, Dr. Nepomuk Otte, Dr. Lucas Liuzzo