A new multifluid cometary plasma model

The target of the Comet Interceptor mission, aiming to explore a dynamically new comet, has not yet been identified. The two best-studied comets to date are comet 1P/Halley, visited by the Giotto mission via flyby, and comet 67P/Churyumov-Gerasimenko, visited by the Rosetta mission for an extended period of two years. When the comets are close enough to the Sun, neutral gas sublimates off the cometary nucleus and is ionized by solar radiation and energetic particles, forming the cometary ionosphere. This ionosphere interacts with the solar wind, forming a structured plasma environment. The plasma environment of 1P during the Giotto flyby and 67P at perihelion can both be described applying a fluid approach. However, even at perihelion, where outgassing is highest, the outgassing rate of 67P (~10^28 s-1) was lower than that of Halley during the Giotto flyby (~8x10^29 s-1); because of this, the structural scales and the dominant physical processes of the plasma differ between these two scenarios. They form the bounds of a parameter space that will most likely contain the target of the Comet Interceptor mission. Predicting the properties of the plasma environment of the new comet requires global modelling of the cometary ionosphere and its interaction with the solar wind; any modelling approach should accurately describe both the state of 1P during the Giotto mission and 67P at perihelion.

 

We present a new 3D multi-fluid magnetohydrodynamics (MHD) model, with heritage from the laboratory plasma MHD modelling code Gorgon. The model solves the coupled continuity, momentum, and energy equations for cometary and solar wind ions as separate fluids, as well as the electron energy equation. Ionization, electron-ion dissociative recombination, and collisional effects are included.

 

Our first test scenario corresponds to the conditions encountered by Giotto during the flyby of 1P, as it has been used as a reference case for Comet Interceptor during the instrument development phase. We compare model outputs along the trajectory of the Giotto mission to on-board observations. This validation will allow for future parameter space studies in support of Comet Interceptor.