The cometary plasma environment is formed through the ionisation of the neutral gas coma, which is mostly water. Directly produced H2O+ can go on to interact with other neutral gas molecules to produce a rich and variable cometary ionosphere. However, the cometary ionosphere is not gravitationally bound, so the possibility for ion-neutral chemical reactions is counter-balanced by transport into space. This thesis focusses on the balance between these processes, by evaluating the response of the composition and density of the ionosphere to the changing plasma dynamics through a range of heliocentric distances. The work is underpinned by data from the Rosetta mission, which escorted comet 67P/Churyumov-Gerasimenko for two years.
Data from the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) Double Focussing Mass Spectrometer (DFMS) instrument is exploited using a multi-instrument approach. This allows assessment of the variation in high proton affinity ion species, primarily NH4+, which require ion-neutral interactions to be produced and are therefore particularly sensitive to ion transport. It is shown for the first time that the ion-neutral chemistry is more complex inside the diamagnetic cavity, a magnetic field-free region surrounding the nucleus at high activity.
A 1D fluid ionospheric model is built to test the impact of acceleration by an ambipolar electric field on the ionospheric composition, bulk velocity, and total density inside the diamagnetic cavity. The model is used to constrain the electric field strength through comparison with electron density data from the Rosetta Plasma Consortium (RPC) instruments. The study is then expanded to lower cometary activity, by adapting a 3D collisional test-particle model to simulate the reaction of cometary ions to electric and magnetic fields from a hybrid simulation. In doing so, we highlight potential improvements of hybrid simulations and advance our ability to explain the plasma density observed by Rosetta at 67P.