Plasma environment of an intermediately active comet : Evolution and dynamics observed by ESA's Rosetta spacecraft at 67P/Churyumov-Gerasimenko

Abstract: The subject of this thesis is the evolution and dynamics of the plasma environment of a moderately active comet before, during and after its closest approach to the Sun. For over 2 years in 2014-2016, the European Space Agency’s Rosetta spacecraft followed the comet 67P/Churyumov-Gerasimenko at distances typically between a few tens and a few hundred kilometers from the nucleus, the longest and closest inspection of a comet ever made. Its payload included a suite of five plasma instruments (the Rosetta Plasma Consortium, RPC), providing unprecedented in-situ measurements of the plasma environment in the inner coma of a comet.In the first two studies, we use spacecraft potential measurements by the Langmuir probe instrument (LAP) to study the evolving cometary plasma environment. The spacecraft potential was mostly negative, often below -10 V and sometimes below -20 V, revealing the presence of warm (around 5-10 eV) coma photoelectrons, not effectively cooled by collisions with the relatively tenuous coma gas. The magnitude of the negative spacecraft potential depends on the electron density and traced heliocentric, cometocentric, seasonal and diurnal variations in cometary outgassing, consistent with production at or inside the cometocentric distance of the spacecraft as the dominant source of the observed plasma.In the third study, we investigate ion velocities and electron temperatures in the diamagnetic cavity of the comet, combining LAP and Mutual Impedance Probe (MIP) measurements. Ion velocities were generally in the range 2-4 km/s, well above the expected neutral velocity of at most 1 km/s. Thus, the ions were (at least partially) decoupled from the neutrals already inside the diamagnetic cavity, indicating that ion-neutral drag was not responsible for balancing the outside magnetic pressure. The spacecraft potential was around -5 V throughout the cavity, showing that warm electrons were consistently present inside the cavity, at least as far in as Rosetta reached. Also, cold (below about 0.1 eV) electrons were consistently observed throughout the cavity, but less consistently in the surrounding region, suggesting that while Rosetta never entered a region of efficient collisional cooling of electrons, such a region was possibly not far away during the cavity crossings. Also, it reinforces the idea of previous authors that the intermittent nature of the cold electron component was due to filamentation of this cold plasma at or near the cavity boundary, possibly related to an instability of this boundary.Finally, we report the detection of large-amplitude, quasi-harmonic density-fluctuations with associated magnetic field oscillations in association with asymmetric plasma and magnetic field enhancements previously found in the region surrounding the diamagnetic cavity, occurring predominantly on their descending slopes. Typical frequencies are around 0.1 Hz, i.e. about ten times the water and half the proton gyro-frequency, and the associated magnetic field oscillations, when detected, have wave vectors perpendicular to the background magnetic field. We suggest that they are Ion Bernstein waves, possibly excited by the drift-cyclotron instability resulting from the strong plasma inhomogeneities this region.