Origin, evolution and impact of electrons at comet 67P
File(s)
Author(s)
Stephenson, Peter
Type
Thesis or dissertation
Abstract
The comet nucleus is a ball of ice and dust, heated through solar illumination. Near-surface ices sublimate under this heating and form the coma, an expanding envelope of neutral gas. These neutral molecules can be ionized through photoionization or by electron-impact, forming a cometary ionosphere. The ionosphere incorporates a variety of electron populations of different temperatures (cold, warm and hot) and origins (cometary or solar wind). My thesis focusses on identifying how these electrons are produced, on assessing how the various populations are formed, and on quantifying the impact the electrons have in the coma.
Rosetta became the first spacecraft to observe a comet from a close and continuous perspective. It escorted comet 67P/Churyumov-Gerasimenko for two years, from August 2014 (at 3.6 au), through perihelion (1.24 au), until Sept 2016 (3.8 au). Instruments onboard Rosetta probed the cometary environment as it evolved from weak activity far from the Sun to highly active around perihelion.
To understand the role of electron-neutral collisions in the coma, I have developed the first three dimensional collisional model of electrons at a comet. This kinetic test particle model incorporates electrons from the solar wind, photoelectrons, and electrons produced through electron-impact ionization. Electrons in the model move through complex and realistic electric and magnetic fields, inputted from a fully kinetic Particle-in-Cell (PiC) simulation, while subject to a variety of collisional processes.
The test particle model is applied to assess the formation, dynamics and loss of the cold population of electrons. These were observed throughout the Rosetta mission with Rosetta Plasma Consortium (RPC)’s Mutual Impedance Probe (MIP) and Langmuir Probe (LAP), even when the coma was not thought to be collisional. I demonstrate that the combination of realistic fields and the inclusion of collisions can sustain a cold electron population even when the coma is very
rarefied. Current estimates of collisionality in the coma are revised and can now explain many of the cold electron observations from the Rosetta missions.
The Alice FUV imaging spectrograph detected emissions from the coma in the far-ultraviolet (FUV, 700A-2050A). With a multi-instrument approach, I have demonstrated that FUV emissions in the CO2-rich southern hemisphere of 67P are driven by dissociative excitation of cometary neutrals by electron-impact. In-situ electron measurements with RPC’s Ion and Electron Sensor (IES) and neutral gas observations are compared to remote sensing measurements of FUV emissions from the coma. The FUV emissions are generated by accelerated solar wind electrons and are, therefore, aurorae.
The electron-impact ionization frequency is assessed from Rosetta dataset on the one hand and from collisional, test-particle simulations on the other, in order to determine the source of the ionizing electrons in the coma and to identify key drivers behind the ionization frequency they
induce.
Rosetta became the first spacecraft to observe a comet from a close and continuous perspective. It escorted comet 67P/Churyumov-Gerasimenko for two years, from August 2014 (at 3.6 au), through perihelion (1.24 au), until Sept 2016 (3.8 au). Instruments onboard Rosetta probed the cometary environment as it evolved from weak activity far from the Sun to highly active around perihelion.
To understand the role of electron-neutral collisions in the coma, I have developed the first three dimensional collisional model of electrons at a comet. This kinetic test particle model incorporates electrons from the solar wind, photoelectrons, and electrons produced through electron-impact ionization. Electrons in the model move through complex and realistic electric and magnetic fields, inputted from a fully kinetic Particle-in-Cell (PiC) simulation, while subject to a variety of collisional processes.
The test particle model is applied to assess the formation, dynamics and loss of the cold population of electrons. These were observed throughout the Rosetta mission with Rosetta Plasma Consortium (RPC)’s Mutual Impedance Probe (MIP) and Langmuir Probe (LAP), even when the coma was not thought to be collisional. I demonstrate that the combination of realistic fields and the inclusion of collisions can sustain a cold electron population even when the coma is very
rarefied. Current estimates of collisionality in the coma are revised and can now explain many of the cold electron observations from the Rosetta missions.
The Alice FUV imaging spectrograph detected emissions from the coma in the far-ultraviolet (FUV, 700A-2050A). With a multi-instrument approach, I have demonstrated that FUV emissions in the CO2-rich southern hemisphere of 67P are driven by dissociative excitation of cometary neutrals by electron-impact. In-situ electron measurements with RPC’s Ion and Electron Sensor (IES) and neutral gas observations are compared to remote sensing measurements of FUV emissions from the coma. The FUV emissions are generated by accelerated solar wind electrons and are, therefore, aurorae.
The electron-impact ionization frequency is assessed from Rosetta dataset on the one hand and from collisional, test-particle simulations on the other, in order to determine the source of the ionizing electrons in the coma and to identify key drivers behind the ionization frequency they
induce.
Version
Open Access
Date Issued
2022-03
Online Publication Date
2023-03-31T23:01:22Z
2023-06-21T10:43:00Z
Date Awarded
2022-10
Copyright Statement
Creative Commons Attribution NonCommercial NoDerivatives Licence
Advisor
Galand, Marina
Sponsor
Science and Technology Facilities Council (Great Britain)
Grant Number
PHSP G01221
Publisher Department
Physics
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)