Constraining transport in the diamagnetic cavity of comet 67P

The ESA Rosetta mission escorted comet 67P for a two-year section of its six-year orbit around the Sun. By perihelion in August 2015, the neutral and plasma data obtained by the spacecraft instruments showed that the comet had transitioned to a dynamic object with large scale plasma structures and a rich ion environment. One such plasma structure is the diamagnetic cavity: a magnetic field-free region formed by the interaction between the un-magnetised cometary plasma and the impinging solar wind. Within this region, unexpectedly high ion bulk velocities have been observed, which have been postulated to be caused by acceleration by an ambipolar electric field. In this study we use a 1D numerical model of the cometary ionosphere to constrain the impact of various electric field profiles on the ion density profile. In the model we include three ion species, H₂O⁺, H₃O⁺ and NH₄⁺. The latter is only produced through the protonation of NH₃ and only lost through ion-electron dissociative recombination and thus particularly sensitive to the timescale of plasma loss through transport. We therefore aim to constrain the minimum strength of an electric field that would allow NH₄⁺ to still be produced in significant enough quantities to explain its detection by the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis/Double Focussing Mass Spectrometer (ROSINA/DFMS) instrument. We also assess the modelled total plasma density profile for different outgassing and electric field conditions and compare this to the electron density from the Rosetta Plasma Consortium (RPC) dataset.