Samenvatting

From March 2014 to September 2016, the instruments in the Rosetta Plasma Consortium (RPC) onboard the European Space Agency's Rosetta spacecraft measured the plasma environment of comet 67P/Churyumov-Gerasimenko (67P/CG) in detail. Multiple datasets of these instruments show instances of asymmetric and irregular waves. These waves are characteristically non-linear, non-sinusoidal and periodically variable. This report presents a characterisation analysis of the available steepened wave data measured by the plasma instruments at comet 67P/CG, with a focus on data from the Ion Composition Analyser (ICA), developed at the Swedish institute of Space Physics (IRF). A database of steepened wave events is developed, with information about the availability and resolution of other datasets, like the Fluxgate Magnetometer (MAG) and the Dual Langmuir Probe (LAP). Next, a variation of the Maxwell-Boltzmann distribution function is derived and used as a model to fit the energy distributions measured by ICA. Following this, each manually selected steepened wave event is fitted with an exponential decay model for characterisation. Lastly, a statistical and correlation analysis is performed and the results are interpreted from a physical perspective.

A total of 113 hours of high time resolution ICA data contain qualitative steepened wave events, with 520 wave events selected. There is a clear correlation between the wave occurrences and comet 67P/CG’s closest approach to the Sun. A modulation is apparent in the distribution fit parameter thermal energy ????????????: It oscillates between ± 0.1−0.2 eV/q on top of a background value. This modulation has a rather specific frequency of (0.0313±0.0003) Hz. The specificity suggests a great robustness of the distribution model, and that its cause is likely not a physical but an instrumental occurrence. The expected correlation between distribution fit parameters drift energy ???????? and ion density ???????? is quantified with a correlation coefficient of ????=0.98 and reduced chi-squared of ????????2=2.15. The relationship can be roughly described by ????????=????0????????????/????????????????, with ????0=(89±16) cm−3 and ????????????????=(8.3±0.7) eV/q. A possible explanation for the spread in the data would be the dependence on other variables like the electron temperature or an electric field not induced by the spacecraft.

The steepened waves are characterised by exponential fit parameters amplitude ????, decay time ???? and rest energy ????????. Out of the 520 selected steepened wave events, 10 have a model efficiency value ????????????<0.8 and 36 have a negative rest energy. Removing these leaves a total of 475 steepened wave events, which suggests that the exponential decay model is well-suited for fitting the steepened waves. The spread of the exponential fit parameters is quantified, with average values of ????=(18.2±0.4) eV/q, ????=(60±2) s and ????????=(12.1±0.3) eV/q. The results suggest a moderate negative correlation between ???? and ???????? (Spearman’s correlation coefficient of ????=−0.42), which means that events with longer decay times tend to have lower energy levels. No clear correlation between exponential fit parameters and the comet’s gas production rate is found. Steepened waves are observed in all three datasets (ICA, MAG, LAP) and can be described by an exponential decay model in each.

The manual event selection process introduced a selection bias toward qualitative exemplary steepened wave shapes. As a result, this research characterises qualitative steepened wave events rather than the full expected wave population. The results have quantified the characteristics of the selected steepened wave events, though no definitive physical interpretation of the steepened waves can be concluded from this research. It is recommended that future research should focus on automatic event identification and increasing the sample size of steepened waves to reduce selection bias. A systemic analysis of Rosetta plasma datasets and relating steepened wave occurrences to cometary activity like bow shock evolution and the diamagnetic cavity may result in an explanation of the physical origin of the steepened waves.