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The Ion Composition Analyzer (ICA) on the Rosetta spacecraft observed both the solar wind and the cometary ionosphere around comet 67P/Churyumov–Gerasimenko for nearly two years. However, observations of low energy cometary ions were affected by a highly negative spacecraft potential, and the ICA ion density estimates were often much lower than plasma densities found by other instruments. Since the low energy cometary ions are often the highest density population in the plasma environment, it is nonetheless desirable to understand their properties. To do so, we select ICA data with densities comparable to those of Rosetta’s Langmuir Probe (LAP)/Mutual Impedance Probe (MIP) throughout the mission. We then correct the cometary ion energy distribution of each energy-angle scan for spacecraft potential and fit a drifting Maxwell–Boltzmann distribution, which gives an estimate of the drift energy and temperature for 3521 scans. The resulting drift energy is generally between 11–18 eV and the temperature between 0.5–1 eV. The drift energy shows good agreement with published ion flow speeds from LAP/MIP during the same time period and is much higher than the cometary neutral speed. We see additional higher energy cometary ions in the spectra closest to perihelion that would be well described by a second Maxwellian-like distribution. The energy and temperature are negatively correlated with heliocentric distance, with a stronger dependence on heliocentric distance for temperature. It cannot be quantitatively determined whether this trend is primarily due to heliocentric distance or spacecraft distance to the comet, which increased with decreasing heliocentric distance.

At a low activity comet the plasma is distributed in an asymmetric way. The hybrid simulation code Amitis is used to look at the spatial evolution of ion velocity distribution functions (VDFs), from the upstream solar wind (SW) to within the comet magnetosphere where the SW is heavily mass-loaded by the cometary plasma. We find that the spatial structures of the ions and fields form a highly asymmetric induced magnetosphere. The VDFs of SW and cometary ions vary drastically for different locations in the comet magnetosphere. The shape of the VDFs differ for different species. The SW protons show high anisotropies that occasionally resemble partial rings, in particular at small cometocentric distances. A second, decoupled, proton population is also found. Solar wind alpha particles show similar anisotropies, although less pronounced and at different spatial scales. The VDFs of cometary ions are mostly determined by the structure of the electric field. We perform supplementary dynamic particle backtracing to understand the flow patterns of SW ions that lead to these anisotropic distributions. This tracing is needed to understand the origin of cometary ions in a given part of the comet magnetosphere. The particle tracing also aids in interpreting observed VDFs and relating them to spatial features in the electric and magnetic fields of the comet environment.

The morphology of induced comet magnetospheres varies greatly over a comet's orbit. Far away from the Sun, the cometary activity, quantified by the outgassing rate, is very low and the cometary ion density is smaller than the solar wind ion density. In this case the interaction between the cometary plasma and the solar wind is characterised by a simple deflection of the solar wind as it gets mass-loaded by the cometary plasma. In contrast, near perihelion the cometary activity and outgassing rate increase by several orders of magnitude. A fully developed bow shock forms as part of the interaction between the cometary plasma and the solar wind, and extends tens of thousands to millions of km from the comet nucleus into the upstream solar wind. At low-to-intermediate cometary activity the spatial scales of the induced comet magnetosphere are similar to the gyroradii of the ions and no fully developed bow shock is formed. The comet–solar wind interaction in this transition period is strongly influenced by kinetic effects acting on both cometary and solar wind ions. The physical processes in the plasma environment near such low- activity comets are the subject of this thesis. Its main focus is on the shape, origin, and evolution of the ion velocity distributions (VDFs), as well as the energy transfer between the solar wind and the cometary magnetosphere. Furthermore, a combined approach utilising both in-situ measurements as well as numerical modelling provides a complete 3D picture of the interaction. We use observations from the Rosetta mission and 3D global kinetic hybrid modelling to study the interaction. 

The European Space Agency mission Rosetta to comet 67P/Churyumov-Gerasimenko was equipped with a broad spectrum of scientific payloads, including two ion spectrometers: the Ion Composition Analyzer (ICA), and the Ion and Electron Spectrometer (IES) as part of the Rosetta Plasma Consortium (RPC). On 19 April 2016, at a heliocentric distance of 2.8 au, ICA and IES detected partial ring-like VDFs of solar wind protons along with approximately isotropic distributions of the solar wind alpha particles. These observations stand in contrast to the expected and previously observed Maxwellian distributions of deflected solar wind ions usually associated with low cometary activity. The fitted velocity components of the partial rings show a significant deceleration of the proton bulk velocity, potentially connected to the initial stages of bow shock formation. The lack of partial ring formation for alpha particles is attributed to their larger gyroradii compared to protons. The observed cometary pickup ions during this time period also show the initial stages of partial ring formation. The fitted velocities in the case of the cometary ions are much lower compared to those of the solar wind ions, which suggests a strong shielding of the inner coma from the undisturbed solar wind electric field. 

The formation of non-Maxwellian ion VDFs as a result of comet–solar wind interaction is subsequently studied using the kinetic hybrid model code Amitis. Partial ring distributions are found to form in large parts of the comet magnetosphere, including close to the nucleus where they have been observed by Rosetta. The shapes of solar wind proton VDFs continuously evolve as the solar wind traverses the cometary plasma environment and are non-Maxwellian throughout most of the magnetosphere. As a result of the solar wind interaction with the cometary ions, the plasma forms a magnetic pile-up layer in the -E-hemisphere, the hemisphere which the solar wind is deflected towards. In this magnetic pile-up layer the magnetic field strength is stronger compared to the interplanetary magnetic field, and the solar wind proton density is increased. Upstream of and in the magnetic pile-up layer, secondary populations of protons resemble reflected ions found at planetary bow shocks. The solar wind alpha particles show a different spatial evolution of their VDFs due to the larger gyroradii. Additional simulations show that the composition of the solar wind affects the size and shape of the induced comet magnetosphere. The large inertia of alpha particles makes them less efficient in transferring energy to the cometary ions and electromagnetic fields upstream of the nucleus. Therefore, a larger proportion of alpha particles at a given solar wind dynamic pressure and cometary activity leads to a relatively larger mass loading of the solar wind protons. This in turn results in an expansion of the magnetic pile-up layer, along with a decrease in magnetic field strength. 

No spacecraft visiting a comet has been equipped with instruments to directly measure the static electric field. However, the electric field can occasionally be estimated indirectly by observing its effects on the ion velocity distribution. We present such observations made by the Rosetta spacecraft on 19 April 2016, 35 km from the nucleus. At this time comet 67P was at a low outgassing rate and the plasma environment was relatively stable. The ion velocity distributions show the cometary ions on the first half of their gyration. We estimate the bulk drift velocity and the gyration speed from the distributions. By using the local measured magnetic field and assuming an E × B drift of the gyrocentre, we get an estimate for the average electric field driving this ion motion. We analyze a period of 13 hr, during which the plasma environment does not change drastically. We find that the average strength of the perpendicular electric field component is 0.21 mV/m. The direction of the electric field is mostly anti-sunward. This is in agreement with previous results based on different methods.

We present partial ring distributions of solar wind protons observed by the Rosetta spacecraft at comet 67P/Churyumov-Gerasimenko. The formation of ring distributions is usually associated with high activity comets, where the spatial scales are larger than multiple ion gyroradii. Our observations are made at a low-activity comet at a heliocentric distance of 2.8 AU on 19 April 2016, and the partial rings occur at a spatial scale comparable to the ion gyroradius. We use a new visualization method to simultaneously show the angular distribution of median energy and differential flux. A fitting procedure extracts the bulk speed of the solar wind protons, separated into components parallel and perpendicular to the gyration plane, as well as the gyration velocity. The results are compared with models and put into context of the global comet environment. We find that the formation mechanism of these partial rings of solar wind protons is entirely different from the well-known partial rings of cometary pickup ions at high-activity comets. A density enhancement layer of solar wind protons around the comet is a focal point for proton trajectories originating from different regions of the upstream solar wind. If the spacecraft location coincides with this density enhancement layer, the different trajectories are observed as an energy-angle dispersion and manifest as partial rings in velocity space.

We examine the origin of electrons in a weakly outgassing comet, using Rosetta mission data and a 3D collisional model of electrons at a comet. We have calculated a new data set of electron-impact ionization (EII) frequency throughout the Rosetta escort phase, with measurements of the Rosetta Plasma Consortium's Ion and Electron Sensor (RPC/IES). The EII frequency is evaluated in 15-min intervals and compared to other Rosetta data sets. EII is the dominant source of electrons at 67P away from perihelion and is highly variable (by up to three orders of magnitude). Around perihelion, EII is much less variable and less efficient than photoionization at Rosetta. Several drivers of the EII frequency are identified, including magnetic field strength and the outgassing rate. Energetic electrons are correlated to the Rosetta-upstream solar wind potential difference, confirming that the ionizing electrons are solar wind electrons accelerated by an ambipolar field. The collisional test particle model incorporates a spherically symmetric, pure water coma and all the relevant electron-neutral collision processes. Electric and magnetic fields are stationary model inputs, and are computed using a fully kinetic, collision-less Particle-in-Cell simulation. Collisional electrons are modelled at outgassing rates of Q = 1026 s-1 and Q = 1.5 × 1027 s-1. Secondary electrons are the dominant population within a weakly outgassing comet. These are produced by collisions of solar wind electrons with the neutral coma. The implications of large ion flow speed estimates at Rosetta, away from perihelion, are discussed in relation to multi-instrument studies and the new results of the EII frequency obtained in this study.

Context: The ionosphere of a comet is known to deflect the solar wind through mass loading, but the interaction is dependent on cometary activity. We investigate the details of this process at comet 67P using the Rosetta Ion Composition Analyzer.

Aims: This study aims to compare the interaction of the solar wind and cometary ions during two different time periods in the Rosetta mission.

Methods: We compared both the integrated ion moments (density, velocity, and momentum flux) and the velocity distribution functions for two days, four months apart. The velocity distribution functions were projected into a coordinate system dependent on the magnetic field direction and averaged over three hours.

Results: The first case shows highly scattered H⁺ in both ion moments and velocity distribution function. The He²⁺ ions are somewhat scattered, but less so, and appear more like those of H₂O⁺ pickup ions. The second case shows characteristic evidence of mass-loading, where the solar wind species are deflected, but the velocity distribution function is not significantly changed.

Conclusions. The distributions of H⁺ in the first case, when compared to He²⁺ and H₂O⁺ pickup ions, are indicative of a narrow cometosheath on the scale of the H⁺ gyroradius. Thus, He²⁺ and H₂O⁺, with larger gyroradii, are largely able to pass through this cometosheath. An examination of the momentum flux tensor suggests that all species in the first case have a significant non-gyrotropic momentum flux component that is higher than that of the second mass-loaded case. Mass loading is not a sufficient explanation for the distribution functions and momentum flux tensor in the first case, and so we assume this is evidence of bow shock formation.

Context: Rosetta followed comet 67P at heliocentric distances from 1.25 to 3.6 au. The solar wind was observed for much of this time, but was significantly deflected and to some extent slowed down by the interaction with the coma.

Aims: We use the different changes in the speed of H⁺ and He²⁺ when they interact with the coma to estimate the upstream speed of the solar wind. The different changes in the speed are due to the different mass per charge of the particles, while the electric force per charge due to the interaction is the same. A major assumption is that the speeds of H⁺ and He²⁺ were the same in the upstream region. This is investigated.

Methods: We derived a method for reconstructing the upstream solar wind from H⁺ and He²⁺ observations. The method is based on the assumption that the interaction of the comet with the solar wind can be described by an electric potential that is the same for both H⁺ and He²⁺. This is compared to estimates from the Tao model and to OMNI and Mars Express data that we propagated to the observation point.

Results: The reconstruction agrees well with the Tao model for most of the observations, in particular for the statistical distribution of the solar wind speed. The electrostatic potential relative to the upstream solar wind is derived and shows values from a few dozen volts at large heliocentric distances to about 1 kV during solar events and close to perihelion. The reconstructed values of the solar wind for periods of high electrostatic potential also agree well with propagated observations and model results.

Conclusions: The reconstructed upstream solar wind speed during the Rosetta mission agrees well with the Tao model. The Tao model captures some slowing down of high-speed streams as compared to observations at Earth or Mars. At low solar wind speeds, below 400 km s^-1, the agreement is better between our reconstruction and Mars observations than with the Tao model. The magnitude of the reconstructed electrostatic potential is a good measure of the slowing-down of the solar wind at the observation point.

Context. The solar wind affects the plasma environment around all solar system bodies. A strong solar wind dynamic pressure pushes plasma boundaries closer to these objects. For small objects kinetic effects on scales smaller than an ion gyroradius play an important role, and species with various mass-per-charge may act differently. In this case the solar wind composition can be important.

Aims. Protons are the dominant ion species in the solar wind; however, sometimes the density of alpha particles increases significantly. We analyse the effect of different solar wind alpha-to-proton ratios on the plasma boundaries of the induced cometary magnetosphere. In addition, we investigate the energy transfer between the solar wind ions, the cometary ions, and the electromagnetic fields.

Methods. Using the hybrid model Amitis, we simulate two different alpha-to-proton ratios and analyse the resulting plasma structures. We calculate the power density (E · J) of all three ion species (solar wind protons and alphas, and cometary ions) to identify load and generator regions. The integrated 1D power density shows the evolution of the power density from the upstream solar wind to downstream of the nucleus. 

Results. A higher alpha-to-proton ratio leads to a larger comet magnetosphere but weaker magnetic field pile-up. The protons transfer energy to the fields and the cometary ions in the entire upstream region and the pile-up layer. Upstream of the nucleus, alphas are inefficient in transferring energy and can act as a load, especially for low alpha-to-proton ratios. The transfer of energy from alphas to cometary ions happens further downstream due to their larger inertia. 

Conclusions. For a multi-species solar wind the mass loading and energy transfer upstream of the pile-up layer will be most efficient for the species with the lowest inertia, typically protons, since different ion gyroradii give different flow patterns for the individual species.