Cometary ionosphere analysis from Rosetta multi-instrument dataset

Abstract

The Rosetta spacecraft is the first spacecraft to ever escort a comet. It provided us with a unique opportunity to study comet Churyumov-Gerasimenko/67P from a close perspective and over a two-year time period. Comets are essentially composed of ice and dust. This ice sublimates and creates an envelop of gas around the nucleus, the so-called coma. This expanding gas is partially ionized through photo-ionization and electron-impact ionization, forming a cometary ionosphere, which is the object of my research.

The on-board Rosetta instruments were able to continuously monitor the ambient environment, from the rendez-vous in August 2014, to the crash-landing of the spacecraft in 2016. They surveyed the comet during its low outgassing periods at large heliocentric distances, as well as during its peak of activity, for near-perihelion conditions (1.3 au) in summer 2015. The Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) instrument set provided in-situ measurements of the neutral and ion composition, as well as the total neutral densities. The Rosetta Plasma Consortium (RPC) sensors returned in-situ measurements of the surrounding plasma, from which were derived physical parameters such as: plasma densities, ionizing electron fluxes, spacecraft potential and electron temperatures.

I was able to disentangle the physical processes responsible for the formation of the cometary plasma throughout the two-year escort phase and I evaluated their respective magnitudes. Electron-impact ionization, estimated from the RPC-Ion and Electron Sensor (IES) measurements, is a significant source of ionization at large heliocentric distance (>2 au). Photo-ionization by solar Extreme UltraViolet (EUV) radiation is the main ion source near perihelion.

For large heliocentric distance (>2 au) conditions during the escort phase, I have applied a multi-instrument data-based ionospheric model to compute the plasma densities at Rosetta. In-situ measurements from the ROSINA instruments and the RPC-IES, together with the RPC-LAngmuir Probes were used to compute the instantaneous total ion number density. Very good agreements between the modelled plasma density and that measured by RPC--Mutual Impedance Probe (MIP) and RPC--LAP attested of the correct identification of the plasma production and loss processes included in the model.

Near perihelion, the comet reached high outgassing rates and the medium was highly collisional at close cometocentric distances (<100km). Ion-neutral reactions and ion-electron dissociative recombinations are able to strongly affect the ion composition and densities. The ion chemical diversity and density profiles under these conditions were assessed using a numerical ionospheric model, which was also used to interpret the ion composition observed by ROSINA under different outgassing rates. These new findings have been contrasted with the ion composition observed at the very active comet Halley/1P, during the Giotto fly-by.