Callisto: Signatures of Plasma Interaction, Induction, and Energetic Particle Dynamics at the Galilean Moon

Lucas Liuzzo
Friday, October 19, 2018 - 3:00pm
Ford ES&T L1205
Dr. Sven Simon (Advisor), Dr. Annalisa Bracco, Dr. James Wray, Dr. Alexander Robel, and Dr. Christopher Paranicas

We present results from a three‐dimensional simulation model of the complex plasma environment near the Jovian moon Callisto. Beneath Callisto’s icy crust possibly lies a liquid saltwater ocean, properties of which can be constrained through magnetic induction signatures generated by Jupiter’s time‐varying magnetospheric field. The first two Callisto flybys of NASA’s Galileo mission (1995­‐2003) occurred under rare and unusual conditions where currents in the ambient plasma were weak, so the unobscured signature of Callisto’s induced field was detected. However, such currents are not generally negligible: magnetic field line pileup and draping generate strong plasma currents as Callisto is continuously bombarded by Jovian magnetospheric plasma, thereby partially obscuring the moon’s inductive signature. Indeed, Galileo never again detected Callisto’s induced field in isolation from plasma interaction signatures during the remainder of the mission. Accounting for the plasma interaction is therefore crucial to identify the moon’s inductive response, but is particularly challenging at Callisto: large ionospheric pickup ion gyroradii generate substantial asymmetries in the plasma flow and magnetic field patterns, so ion dynamics must be represented kinetically.

We apply a hybrid (kinetic ions, fluid electrons) model of the moon’s plasma interaction that accurately represents the resulting asymmetries. By modeling Callisto’s plasma environment during all Galileo flybys, we identify a region near Callisto’s wakeside surface where the induced field is decoupled from the magnetic perturbations associated with Alfvén wings and ionospheric mass loading. We also model the dynamics of energetic magnetospheric ions in the highly perturbed electromagnetic environment near Callisto. We demonstrate that energetic ion observations can be used as an auxiliary tool to identify Callisto’s inductive signature and independently confirm the existence of a subsurface ocean, as the induced field leaves a distinct, ring­‐like imprint on the  precipitation patterns of energetic ions onto the moon. These two independent methods for identifying induction signatures provide a framework that can also be applied to study the magnetic environments of Europa, Ganymede, and Callisto during the upcoming JUpiter ICy moons Explorer mission (2022-‐2033), and to constrain the inductive responses of other potentially habitable ocean worlds in the solar system like Titan or Triton.