Shane R. Carberry Mogan

The vertical propagation of wave energy into a planet’s exosphere, a process that affects atmospheric evolution, is calculated here using 1D molecular kinetic simulations. Effects sensitive to molecular interactions are examined by comparing simulation results to solutions of linear fluid models for steady wave activity using parameters associated with Mars’ upper atmosphere. In addition to correctly describing the wave behavior in the exobase region, these simulations directly yield nonlinear effects such as atmospheric heating. They also readily include the transient behavior due to the onset and decay of waves propagating into the rarefied region of a planet’s atmosphere. This is a first step in understanding the effects of variable wave activity in the region where the atmosphere evolves from collisional to collisionless.

The JUpiter ICy moons Explorer (JUICE) of the European Space Agency will investigate Jupiter and its icy moons Europa, Ganymede, and Callisto, with the aim to better understand the origin and evolution of our Solar System and the emergence of habitable worlds around gas giants. The Particle Environment Package (PEP) on board JUICE is designed to measure neutrals and ions and electrons at thermal, suprathermal, and radiation belt energies (eV to MeV). In the vicinity of Callisto, PEP will characterize the plasma environment, the outer parts of Callisto's atmosphere and ionosphere and their interaction with Jupiter's dynamic magnetosphere. Roughly 20 Callisto flybys with closest approaches between 200 and 5,000 km altitude are planned over the course of the JUICE mission. In this article, we review the state of the art regarding Callisto's ambient environment and magnetospheric interaction with recent modeling efforts for Callisto's atmosphere and ionosphere. Based on this review, we identify science opportunities for the PEP observations to optimize scientific insight gained from the foreseen JUICE flybys. These considerations will inform both science operation planning of PEP and JUICE and they will guide future model development for Callisto's atmosphere, ionosphere, and their interaction with the plasma environment.