Comets are the most active around their perigees. The increased outgassing can lead to a coma thick enough to effectively absorb the solar EUV radiation, which engenders a self-shielding comet nucleus and inner layers of the ionosphere. This effect of self-shielding can be calculated by the attenuation of the sunlight according to the Beer-Lambert law. Here we focus on the perihelion of comet 67P/Churyumov-Gerasimenko, the target comet of the ESA Rosetta mission. We calculate attenuated photoionization frequencies and implement these into an ionospheric model constructed in a recent project work (by the same author). The ionization frequencies and ion number densities are calculated as a function of cometocentric distance and compared with the latest published peer-reviewed article by Heritier et al. (2017). Overall, the agreement is fairly good. The most significant difference is the discrepancy of number densities of O2 ions, which is higher in our model by nearly an order of magnitude. This discrepancy is attributed to the fact that Heritier et al (2017) only considered charge transfer processes for the formation of O2+, while we identify photoionization of O2 as the main production mechanism.

Observations provided from RPC-ICA in combination with the data from RPC-MAG and ROSINA-COPS show that many aspects of the time variability of the detected ions is correlated with the magnetic field or -- to a smaller extent -- with neutral atmosphere density. We also show that not all changes in the cometary ion data reflect the nature of the plasma dynamics, but are a consequence of the instrumental limitations. The main outcome of the article in Appendix 1 is that the cometary ions can be divided into two populations with distinct characteristics. One population we termed the convecting population, is accelerated to higher energies through the interaction with the solar wind. The other population we termed the expanding population is moving radially away from the nucleus in the terminator plane. Both populations exhibit a significant anti-sunward component.In addition we present in this thesis a case with observations day-side of the terminator plane. There we show how the expanding population has a sunward component, consistent with initial radial expansion of the ions from the nucleus which gradually turn into an anti-sunward flow which is then observed in the terminator plane.

We analyse the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) - the Double Focusing Mass Spectrometer data between 2014 August and 2016 February to examine the effect of seasonal variations on the four major species within the coma of 67P/Churyumov-Gerasimenko (H2O, CO2, CO, and O-2), resulting from the tilt in the orientation of the comet's spin axis. Using a numerical data inversion, we derive the non-uniform activity distribution at the surface of the nucleus for these species, suggesting that the activity distribution at the surface of the nucleus has not significantly been changed and that the differences observed in the coma are solely due to the variations in illumination conditions. A three-dimensional Direct Simulation Monte Carlo model is applied where the boundary conditions are computed with a coupling of the surface activity distributions and the local illumination. The model is able to reproduce the evolution of the densities observed by ROSINA including the changes happening at equinox. While O-2 stays correlated with H2O as it was before equinox, CO2 and CO, which had a poor correlation with respect to H2O pre-equinox, also became well correlated with H2O post-equinox. The integration of the densities from the model along the line of sight results in column densities directly comparable to the VIRTIS-H observations. Also, the evolution of the volatiles' production rates is derived from the coma model showing a steepening in the production rate curves after equinox. The model/data comparison suggests that the seasonal effects result in the Northern hemisphere of 67P's nucleus being more processed with a layered structure while the Southern hemisphere constantly exposes new material.

An investigation is presented of the dust scattering in the coma of 67P/Churyumov-Gerasimenko for the dates of 2015 February 28, March 15 and April 27. A comparison of the morphology of dust continuum maps at 1.1 mu m and gas emission shows that for the above dates the spatial distribution of the dust is strongly correlated with H2O but not with CO2 emission. For April 27, the radial profile on the illuminated side of the nucleus in the inner coma agrees well with the direct simulation Monte Carlo (DSMC) calculations as the dust is accelerating and flows outwards distribution of the dust is narrower than the broader emission of the gas. Af. values are 1.13 m for 2015 February 28, 2.02 m for April 27, while local values for March 15 are 2.3-5.3 m, depending on the nucleus illumination. In the inner coma, the spectral reflectivity from 0.35 to 3.5 mu m displays a red slope with a change at around 1 mu m. From 0.35 to 0.8 mu m, the values range from 9 to 12 +/- 1 per cent per 100 nm both on the sunlit side and on the dark side. From 1 to 2.5 mu m, the values are 1.7 +/- 0.2 per cent per 100 nm on the sunlit side and 3 +/- 1 per cent per 100 nm on the dark side. For the August 26 jet, no significant increase of the colour gradient with distance from the nucleus could be observed, nor any significant difference detected between the jet and the background coma.

The plasma environment of an active comet provides a unique setting for plasma physics research. The complex interaction of newly created cometary ions with the flowing plasma of the solar wind gives rise to a plethora of plasma physics phenomena, that can be studied over a large range of activity levels as the distance to the sun, and hence the influx of solar energy, varies. In this thesis, we have used measurements of the spacecraft potential by the Rosetta Langmuir probe instrument (LAP) to study the evolution of activity of comet 67P/Churyumov-Gerasimenko as it approached the sun from 3.6 AU in August 2014 to 2.1 AU in March 2015. The measurements are validated by cross-calibration to a fully independent measurement by an electrostatic analyzer, the Ion Composition Analyzer (ICA), also on board Rosetta. The spacecraft was found to be predominantly negatively charged during the time covered by our investigation, driven so by a rather high electron temperature of ~5 eV resulting from the low collision rate between electrons and the tenuous neutral gas. The spacecraft potential exhibited a clear covariation with the neutral density as measured by the ROSINA Comet Pressure Sensor (COPS) on board Rosetta. As the spacecraft potential depends on plasma density and electron temperature, this shows that the neutral gas and the plasma are closely coupled. The neutral density and negative spacecraft potential were higher in the northern hemisphere, which experienced summer conditions during the investigated period due to the nucleus spin axis being tilted toward the sun. In this hemisphere, we found a clear variation of spacecraft potential with comet longitude, exactly as seen for the neutral gas, with coincident peaks in neutral density and spacecraft potential magnitude roughly every 6 h, when sunlit parts of the neck region of the bi- lobed nucleus were in view of the spacecraft. The plasma density was estimated to have increased during the investigated time period by a factor of 8-12 in the northern hemisphere and possibly as much as a factor of 20-44 in the southern hemisphere, due to the combined effects of seasonal changes and decreasing heliocentric distance. The spacecraft potential measurements obtained by LAP generally exhibited good correlation with the estimates from ICA, confirming the accuracy of both of these instruments for measurements of the spacecraft potential. / <p>QC 20200602</p>

Cette thèse s'inscrit dans le cadre de la mission spatiale Rosetta et porte sur les propriétés spectrophotométriques de la comète 67P/Churyumov-Gerasimenko à l’aide de l’instrument OSIRIS. Cet instrument est composé de deux caméras pour les observations du noyau et de la coma de la comète. Elles permettent d’acquérir des images avec des filtres qui opèrent dans la gamme du proche UV au proche IR. Dans un premier temps, j'ai analysé les courbes spectrophotométriques des taches claires qui sont apparues sur le noyau de la comète. Une étude comparative de celles-ci grâce aux données du spectro-imageur VIRTIS a ainsi permis de constater que les taches claires sont liées à la glace de H2O. Dans un second temps, j’ai entrepris une étude spectrophotométrique de la région Khonsu, qui a mis en évidence les variations saisonnières de la pente spectrale de différents terrains. Par la suite, j’ai élargi mon analyse des taches à tout le noyau de la comète. J’ai détecté plus de 50 taches claires dues à la présence de glace de H2O et j’ai produit une carte pour repérer leurs emplacements sur le noyau, afin d’étudier plus en détail leur répartition et leur évolution au cours de temps. Ceci m’a permis d’identifier quatre types de taches regroupés en fonction de leur morphologie et de constater qu'elles sont dues à différentes sources d'activité cométaire. / This thesis is based on the spectrophotometric properties of the comet 67P/Churyumov-Gerasimenko, using the OSIRIS instrument of Rosetta space mission. Composed of two scientific cameras to observe the nucleus and the coma of the comet, OSIRIS images are acquired with multiple filters, that span the near-UV to near-IR wavelength range. They were used to study the spectrophotometric curves of the exposed bright features that appeared on the surface of the cometary nucleus, leading to a comparative study, that was carried out in collaboration with the VIRTIS spectro-imager aboard Rosetta, that demonstrated, that these exposures are related to H2O ice, using its absorption band located at 2 microns. The thesis further details a spectrophotometric study of the Khonsu region in the southern latitudes of the comet, where the seasonal variation of the spectral slope of different types of terrains is explored. Finally, the results of an extended survey of exposed bright features are presented. More than 50 individual features are presented under four morphologies along with an albedo calculation, suggesting that different activity sources are responsible for their appearance on the nucleus.

Here I study the “plasma interactions with icy bodies in the solar system”, that is, my quest to understand the fundamental processes that govern such interactions. By using numerical modelling combined with in situ observations, one can infer the internal structure of icy bodies and their plasma environments. After a broad overview of the laws governing space plasmas a more detailed part follows. This contains the method on how to model the interaction between space plasmas and icy bodies. Numerical modelling of space plasmas is applied to the icy bodies Callisto (a satellite of Jupiter), the dwarf planet Ceres (located in the asteroid main belt) and the comet 67P/Churyumov-Gerasimenko. The time-varying magnetic field of Jupiter induces currents inside the electrically conducting moon Callisto. These create magnetic field perturbations thought to be related to conducting subsurface oceans. The flow of plasma in the vicinity of Callisto is greatly affected by these magnetic field perturbations. By using a hybrid plasma solver, the interaction has been modelled when including magnetic induction and agrees well with magnetometer data from flybys (C3 and C9) made by the Galileo spacecraft. The magnetic field configuration allows an inflow of ions onto Callisto’s surface in the central wake. Plasma that hits the surface knocks away matter (sputtering) and creates Callisto’s tenuous atmosphere. A long term study of solar wind protons as seen by the Rosetta spacecraft was conducted as the comet 67P/Churyumov-Gerasimenko approached the Sun. Here, extreme ultraviolet radiation from the Sun ionizes the neutral water of the comet’s coma. Newly produced water ions get picked up by the solar wind flow, and forces the solar wind protons to deflect due to conservation of momentum. This effect of mass-loading increases steadily as the comet draws closer to the Sun. The solar wind is deflected, but does not lose much energy. Hybrid modelling of the solar wind interaction with the coma agrees with the observations; the force acting to deflect the bulk of the solar wind plasma is greater than the force acting to slow it down. Ceres can have high outgassing of water vapour, according to observations by the Herschel Space Observatory in 2012 and 2013. There, two regions were identified as sources of water vapour. As Ceres rotates, so will the source regions. The plasma interaction close to Ceres depends greatly on the source location of water vapour, whereas far from Ceres it does not. On a global scale, Ceres has a comet-like interaction with the solar wind, where the solar wind is perturbed far downstream of Ceres. / Här studerar jag “plasmaväxelverkan med isiga kroppar i solsystemet”, det vill säga, min strävan är att förstå de grundläggande processerna som styr sådana interaktioner. Genom att använda numerisk modellering i kombination med observationer på plats vid himlakropparna kan man förstå sig på deras interna strukturer och rymdmiljöer. Efter en bred översikt över de fysiska lagar som styr ett rymdplasma följer en mer detaljerad del. Denna innehåller metoder för hur man kan modellera växelverkan mellan rymdplasma och isiga kroppar. Numerisk modellering av rymdplasma appliceras på de isiga himlakropparna Callisto (en måne kring Jupiter), dvärgplaneten Ceres (lokaliserad i asteroidbältet mellan Mars och Jupiter) och kometen 67P/Churyumov-Gerasimenko. Det tidsvarierande magnetiska fältet kring Jupiter inducerar strömmar inuti den elektriskt ledande månen Callisto. Dessa strömmar skapar magnetfältsstörningar som tros vara relaterade till ett elektriskt ledande hav under Callistos yta. Plasmaflödet i närheten av Callisto påverkas i hög grad av dessa magnetfältsstörningar. Genom att använda en hybrid-plasma-lösare har växelverkan modellerats, där effekten av magnetisk induktion har inkluderats. Resultaten stämmer väl överens med magnetfältsdata från förbiflygningarna av Callisto (C3 och C9) som gjordes av den obemannade rymdfarkosten Galileo i dess bana kring Jupiter. Den magnetiska konfigurationen som uppstår möjliggör ett inflöde av laddade joner på Callistos baksida. Plasma som träffar ytan slår bort materia och skapar Callistos tunna atmosfär. En långtidsstudie av solvindsprotoner sett från rymdfarkosten Rosetta utfördes då kometen 67P/Churyumov-Gerasimenko närmade sig solen. Ultraviolett strålning från solen joniserar det neutrala vattnet i kometens koma (kometens atmosfär). Nyligt joniserade vattenmolekyler plockas upp av solvindsflödet och tvingar solvindsprotonernas banor att böjas av, så att rörelsemängden bevaras. Denna effekt ökar stadigt då kometen närmar sig solen. Solvinden böjs av kraftigt, men förlorar inte mycket energi. Hybridmodellering av solvindens växelverkan bekräftar att kraften som verkar på solvinden till störst del får den att böjas av, medan kraften som verkar till att sänka dess fart är mycket lägre. Ceres har enligt observationer av rymdteleskopet Herschel under 2012 och 2013 haft högt utflöde av vattenånga från dess yta. Där har två regioner identifierats som källor för vattenångan. Eftersom Ceres roterar kommer källornas regioner göra det också. Plasmaväxelverkan i närheten av Ceres beror i hög grad på vattenångskällans placeringen, medan det inte gör det långt ifrån Ceres. På global nivå har Ceres en kometliknande växelverkan med solvinden, där störningar i solvinden propagerar långt nedströms från Ceres.