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The environment of a comet is a fascinating and unique laboratory to study plasma processes and the formation of structures such as shocks and discontinuities from electron scales to ion scales and above. The European Space Agency's Rosetta mission collected data for more than two years, from the rendezvous with comet 67P/Churyumov-Gerasimenko in August 2014 until the final touch-down of the spacecraft end of September 2016. This escort phase spanned a large arc of the comet's orbit around the Sun, including its perihelion and corresponding to heliocentric distances between 3.8 AU and 1.24 AU. The length of the active mission together with this span in heliocentric and cometocentric distances make the Rosetta data set unique and much richer than sets obtained with previous cometary probes. Here, we review the results from the Rosetta mission that pertain to the plasma environment. We detail all known sources and losses of the plasma and typical processes within it. The findings from in-situ plasma measurements are complemented by remote observations of emissions from the plasma. Overviews of the methods and instruments used in the study are given as well as a short review of the Rosetta mission. The long duration of the Rosetta mission provides the opportunity to better understand how the importance of these processes changes depending on parameters like the outgassing rate and the solar wind conditions. We discuss how the shape and existence of large scale structures depend on these parameters and how the plasma within different regions of the plasma environment can be characterised. We end with a non-exhaustive list of still open questions, as well as suggestions on how to answer them in the future. [ABSTRACT FROM AUTHOR]

Moment analysis of ion spectrograms measured by the Ion Composition Analyser (ICA) in the coma of comet 67P typically produces an ion number density estimate markedly lower than the number density of free electrons as measured by the Mutual Impedance Probe and the dual Langmuir Probe. While there are good reasons to distrust the ion density moment estimate in these circumstances, the issue cannot yet be considered fully understood and it is of interest to see whether any natural non-instrumental cause is possible. An obvious such cause would be whether a significant fraction of the positive charge density resides in positively charged dust grains that are not measured by the ICA. Here, we show that this scenario is highly unlikely, even near perihelion where photoemission is the strongest. In our semi-analytical grain charging model, we balance the current contributions to grains of photoelectron emission and electron attachment so as to find the expected charge state for a grain of a given radius. The charge state is affected by the ambient electron number density, the electron temperature, and the heliocentric distance. While at times the bulk of the dust population around comet 67P could be charged positive, dust charging, including photoelectron emission, should have a negligible influence on the overall ionization balance in the cometary coma simply because the dust particles are not ubiquitous enough. [ABSTRACT FROM AUTHOR]

On 6–8 June 2015, the Ion and Electron Sensor on board Rosetta observed keV‐range water‐group pickup ions arriving from the solar direction. Based on magnetic field intensification and variations, the appearance of the ions was likely to have been caused by a coronal mass ejection. During the 3‐day period when Rosetta was 200 km from the comet, peak ion energy/charge (E/q) varied over a range from 50 eV to 1 keV in concert with neutral gas density variations caused by the rotation of the comet and its variable solar illumination. Thermal ion densities showed the same variations. The neutral density variations provided a unique opportunity to observe the repeated slowing of the solar wind by mass loading caused by charge exchange between energetic water‐group ions and thermal water‐group molecules. Such solar wind slowing was observed previously only by flyby missions that provided single events. Plain Language Summary: The Rosetta orbiter carried a number of instruments to measure the properties of the neutral and ionized gas surrounding the nucleus. Included in these were plasma instruments to measure the characteristics of the charged particles. The Ion and Electron Sensor was one of them. Also on board were the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis, the Ion Composition Analyzer, and the Langmuir Probe. This paper discusses some of the results of measurements by these instruments and their relation to each other. It was found that the neutral gas emitted by the comet nucleus and the resulting positively charged ions interact in such a way to produce slowing down of the solar wind as a result of what is called "charge exchange", in which an electron is transferred from a neutral molecule to an ion. Key Points: Charge exchange of energetic water group pickup ions with coma water molecules is observed within the solar wind cavity of Comet 67P Churyumov‐GerasimenkoRotation of the comet produced variations in the neutral density at the spacecraft causing strong charge‐exchange deceleration of the incoming ions within the density peaksRecovery to the initial water‐group ion beam as minimum neutral densities, again appeared, was shown to result from pickup of thermal ions in the coma [ABSTRACT FROM AUTHOR]

We report the detection of large‐amplitude, quasi‐harmonic density fluctuations with associated magnetic field oscillations in the region surrounding the diamagnetic cavity of comet 67P. Typical frequencies are ∼0.1 Hz, corresponding to ∼10 times the water and ≲0.5 times the proton gyro‐frequencies, respectively. Magnetic field oscillations are not always clearly observed in association with these density fluctuations, but when they are, they consistently have wave vectors perpendicular to the background magnetic field, with the principal axis of polarization close to field‐aligned and with a ∼90° phase shift with respect to the density fluctuations. The fluctuations are observed in association with asymmetric plasma density and magnetic field enhancements previously found in the region surrounding the diamagnetic cavity, occurring predominantly on their descending slopes. This is a new type of waves not previously observed at comets. They are likely ion Bernstein waves, and we propose that they are excited by unstable ring, ring‐beam, or spherical shell distributions of cometary ions just outside the cavity boundary. These waves may play an important role in redistributing energy between different particle populations and reshape the plasma environment of the comet. Key Points: Large‐amplitude low‐frequency (around 0.1 Hz) plasma density fluctuations are observed at comet 67PThey coincide with the plasma density and magnetic field enhancements surrounding the diamagnetic cavityThis is a new type of waves at comets, probably ion Bernstein waves, possibly driven by velocity space anisotropies arising near the cavity [ABSTRACT FROM AUTHOR]

Average flow patterns of ions around comet 67P detected by the RPC-ICA instrument onboard Rosetta are presented both as a time series and as a spatial distribution of the average flow in the plane perpendicular to the comet – Sun direction (Y–Z plane in the coordinate systems used). Cometary ions in the energy range up to 60 eV flow radially away from the nucleus in the Y–Z plane, irrespective of the direction of the magnetic field, throughout the mission. These ions may however be strongly affected by the spacecraft potential, the uncertainty due to this is briefly discussed. Inside the solar wind ion cavity and in the periods just before and after, the cometary pick up ions moving antisunward are deflected against the inferred solar wind electric field direction. This is opposite to what is observed for lower levels of mass-loading. These pick up ions are behaving in a similar way to the solar wind ions and are deflected due to mass-loading. A spatial asymmetry can be seen in the observations of deflected pick up ions, with motion against the electric field primarily within a radius of 200 km of the nucleus and also in the negative electric field hemisphere. Cometary ions observed by RPC-ICA typically move in the antisunward direction throughout the mission. These are average patterns, full-resolution data show very much variability. [ABSTRACT FROM AUTHOR]

We calculate the momentum flux and pressure of ions measured by the Ion Composition Analyzer (ICA) on the Rosetta mission at comet 67P/Churyumov‐Gerasimenko. The total momentum flux stays roughly constant over the mission, but the contributions of different ion populations change depending on heliocentric distance. The magnetic pressure, calculated from Rosetta magnetometer measurements, roughly corresponds with the cometary ion momentum flux. When the spacecraft enters the solar wind ion cavity, the solar wind fluxes drop drastically, while the cometary momentum flux becomes roughly 10 times the solar wind fluxes outside of the ion cavity, indicating that pickup ions behave similarly to the solar wind ions in this region. We use electron density from the Langmuir probe to calculate the electron pressure, which is particularly important close to the comet nucleus where flow changes from antisunward to radially outward. Plain Language Summary: To understand the atmosphere of a comet, we must understand how its charged particles interact with the solar wind. Here we look at momentum flux, the amount of momentum carried by particles flowing through a certain area, measured at the comet 67P/Churyumov‐Gerasimenko by the Rosetta spacecraft. We find that the total momentum flux changes the amount we would expect from the comet getting closer to the Sun. When in the part of the atmosphere with no solar wind, the ions coming from the comet have more momentum flux than the solar wind ions elsewhere in the comet atmosphere. The increase matches what we expect from the comet getting closer to the Sun, which increases the total density of the comet atmosphere. The cometary ions replace the solar wind ions in the atmosphere, so the total stays the same. The magnetic field varies with the cometary ions. We find electron pressure with measured electron data. It is higher than the momentum flux, especially in the part of the atmosphere close to the comet nucleus. This means that the atmosphere here is primarily moving outward from the comet, while farther away from the nucleus it is mainly flowing away from the Sun. Key Points: Momentum flux of Rosetta ion data shows the evolution of a cometary ionosphere, including boundary regionsMagnetic pressure is on the order of the total ion momentum flux and roughly corresponds with cometary ion momentum fluxThe cometary pickup ions in the solar wind ion cavity take over the role of the solar wind ions outside the cavity [ABSTRACT FROM AUTHOR]

We present a Rosetta Plasma Consortium (RPC) case study based on four events in 2015 autumn at various radial distances, phase angles and local times, just after the perihelion of comet 67P/Churyumov-Gerasimenko. Pulse-like (high-amplitude, up to minutes in time) signatures are seen with several RPC instruments in the plasma density (with the LAngmuir Probe, LAP and Mutual Impedance Probe, MIP), ion energy and flux (with the Ion Composition Analyzer, ICA) and the magnetic field intensity (with the magnetometer, MAG). Furthermore, the cometocentric distance relative to the electron exobase is seen to be a good organizing parameter for the measured plasma variations. The closer Rosetta is to this boundary, the more pulses are measured. This is consistent with the pulses being filaments of plasma originating from the diamagnetic cavity boundary, as predicted by simulations. [ABSTRACT FROM AUTHOR]

The Rosetta orbiter witnessed several hundred diamagnetic cavity crossings (unmagnetized regions) around comet 67P/Churyumov-Gerasimenko during its two year survey of the comet. The characteristics of the plasma environment inside these diamagnetic regions are studied using in situ measurements by the Rosetta Plasma Consortium instruments. Although the unmagnetized plasma density has been observed to exhibit little dynamics compared to the very dynamical magnetized cometary plasma, we detected several localized dynamic plasma structures inside those diamagnetic regions. These plasma structures are not related to the direct ionization of local cometary neutrals. The structures are found to be steepened, asymmetric plasma enhancements with typical rising-to-descending slope ratio of ~2.8 (±1.9), skewness ~0.43 (±0.36), mean duration of ~2.7 (±0.9) min and relative density variation ΔN/N of ~0.5 (±0.2), observed close to the electron exobase. Similar steepened plasma density enhancements were detected at the magnetized boundaries of the diamagnetic cavity as well as outside the diamagnetic region. The plausible scalelength and propagation direction of the structures are estimated from simple plasma dynamics considerations. It is suggested that they are large-scale unmagnetized plasma enhancements, transmitted from the very dynamical outer magnetized region to the inner magnetic field-free cavity region. [ABSTRACT FROM AUTHOR]

We present initial results on how the solar wind convective electric field E = -v x B is affecting the cold plasma of comet 67P/Churyumov-Gerasimenko. The solar wind velocity v is assumed to be directed radially away from the Sun, and the magnetic field B is obtained from measurements of the magnetometer (MAG), within the coma of 67P. The presence of a cold plasma is determined by the Langmuir probe (LAP) instrument and is defined as when the derivative of the measured electron current with respect to the probe bias voltage exceeds 70 nA/V. We find that a significant fraction of the cold plasma is found in the -E hemisphere of the comet. Sources of errors include e.g. a draping of the magnetic field leading to the measured B inside the coma not being the same as the solar wind B, as well as a deflection of the solar wind. Despite this, the results agrees with previous simulation results on how the cometary coma becomes asymmetric with respect to the convective electric field. [ABSTRACT FROM AUTHOR]