We propose to identify the main sources of ionization of the plasma in the coma of comet 67P/Churyumov-Gerasimenko at different locations in the coma and to quantify their relative importance, for the first time, for close cometocentric distances (<20 km) and large heliocentric distances (>3 au). The ionospheric model proposed is used as an organizing element of a multi-instrument data set from the Rosetta Plasma Consortium (RPC) plasma and particle sensors, from the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis and from the Microwave Instrument on the Rosetta Orbiter, all on board the ESA/Rosetta spacecraft. The calculated ionospheric density driven by Rosetta observations is compared to the RPC-Langmuir Probe and RPC-Mutual Impedance Probe electron density. The main cometary plasma sources identified are photoionization of solar extreme ultraviolet (EUV) radiation and energetic electron-impact ionization. Over the northern, summer hemisphere, the solar EUV radiation is found to drive the electron density -with occasional periods when energetic electrons are also significant. Over the southern, winter hemisphere, photoionization alone cannot explain the observed electron density, which reaches sometimes higher values than over the summer hemisphere; electron-impact ionization has to be taken into account. The bulk of the electron population is warm with temperature of the order of 7-10 eV. For increased neutral densities, we show evidence of partial energy degradation of the hot electron energy tail and cooling of the full electron population.

Context. The Rosetta spacecraft provided us with a unique opportunity to study comet 67P/Churyumov–Gerasimenko (67P) from a close perspective and over a 2-yr time period. Comet 67P is a weakly active comet. It was therefore unexpected to find an active and dynamic ionosphere where the cometary ions were largely dominant over the solar wind ions, even at large heliocentric distances. Aims. Our goal is to understand the different drivers of the cometary ionosphere and assess their variability over time and over the different conditions encountered by the comet during the Rosetta mission. Methods. We used a multi-instrument data-based ionospheric model to compute the total ion number density at the position of Rosetta. In-situ measurements from the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) and the Rosetta Plasma Consortium (RPC)–Ion and Electron Sensor (IES), together with the RPC–LAngmuir Probe instrument (LAP) were used to compute the local ion total number density. The results are compared to the electron densities measured by RPC–Mutual Impedance Probe (MIP) and RPC–LAP. Results. We were able to disentangle the physical processes responsible for the formation of the cometary ions throughout the 2-yr escort phase and we evaluated their respective magnitudes. The main processes are photo-ionization and electron-impact ionization. The latter is a significant source of ionization at large heliocentric distance (>2 au) and was predominant during the last 4 months of the mission. The ionosphere was occasionally subject to singular solar events, temporarily increasing the ambient energetic electron population. Solar photons were the main ionizer near perihelion at 1.3 au from the Sun, during summer 2015.

A major point of interest in cometary plasma physics has been the diamagnetic cavity, an unmagnetized region in the innermost part of the coma. Here we combine Langmuir and Mutual Impedance Probe measurements to investigate ion velocities and electron temperatures in the diamagnetic cavity of comet 67P, probed by the Rosetta spacecraft. We find ion velocities generally in the range 2–4 km/s, significantly above the expected neutral velocity ≲1 km/s, showing that the ions are (partially) decoupled from the neutrals, indicating that ion‐neutral drag was not responsible for balancing the outside magnetic pressure. Observations of clear wake effects on one of the Langmuir probes showed that the ion flow was close to radial and supersonic, at least with respect to the perpendicular temperature, inside the cavity and possibly in the surrounding region as well. We observed spacecraft potentials ≲−5 V throughout the cavity, showing that a population of warm (∼5 eV) electrons was present throughout the parts of the cavity reached by Rosetta. Also, a population of cold ( ≲0.11emeV) electrons was consistently observed throughout the cavity, but less consistently in the surrounding region, suggesting that while Rosetta never entered a region of collisionally coupled electrons, such a region was possibly not far away during the cavity crossings.

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The plasma environment has been measured for the first time near the surface of a comet. This unique data set has been acquired at 67P/Churyumov-Gerasimenko during ESA/Rosetta spacecraft's final descent on 2016 September 30. The heliocentric distance was 3.8 au and the comet was weakly outgassing. Electron density was continuously measured with Rosetta Plasma Consortium (RPC)-Mutual Impedance Probe (MIP) and RPC-LAngmuir Probe (LAP) during the descent from a cometocentric distance of 20 km down to the surface. Data set from both instruments have been cross-calibrated for redundancy and accuracy. To analyse this data set, we have developed a model driven by Rosetta Orbiter Spectrometer for Ion and Neutral Analysis-COmetary Pressure Sensor total neutral density. The two ionization sources considered are solar extreme ultraviolet radiation and energetic electrons. The latter are estimated from the RPC-Ion and Electron Sensor (IES) and corrected for the spacecraft potential probed by RPC-LAP. We have compared the results of the model to the electron densities measured by RPC-MIP and RPC-LAP at the location of the spacecraft. We find good agreement between observed and modelled electron densities. The energetic electrons have access to the surface of the nucleus and contribute as the main ionization source. As predicted, the measurements exhibit a peak in the ionospheric density close to the surface. The location and magnitude of the peak are estimated analytically. The measured ionospheric densities cannot be explained with a constant outflow velocity model. The use of a neutral model with an expanding outflow is critical to explain the plasma observations.

Four interplanetary corotating interaction regions (CIRs) were identified during 2016 June-September by the Rosetta Plasma Consortium (RPC) monitoring in situ the plasma environment of the comet 67P/Churyumov-Gerasimenko (67P) at heliocentric distances of ∼3-3.8 au. The CIRs, formed in the interface region between low-and high-speed solar wind streams with speeds of ∼320-400 km s −1 and ∼580-640 km s −1 , respectively, are characterized by relative increases in solar wind proton density by factors of ∼13-29, in proton temperature by ∼7-29, and in magnetic field by ∼1-4 with respect to the pre-CIR values. The CIR boundaries are well defined with interplanetary discontinuities. Out of 10 discontinuities, four are determined to be forward waves and five are reverse waves, propagating at ∼5-92 per cent of the magnetosonic speed at angles of ∼20 •-87 • relative to ambient magnetic field. Only one is identified to be a quasi-parallel forward shock with magnetosonic Mach number of ∼1.48 and shock normal angle of ∼41 •. The cometary ionosphere response was monitored by Rosetta from cometocentric distances of ∼4-30 km. A quiet time plasma density map was developed by considering dependences on cometary latitude, longitude, and cometocentric distance of Rosetta observations before and after each of the CIR intervals. The CIRs lead to plasma density enhancements of ∼500-1000 per cent with respect to the quiet time reference level. Ionospheric modelling shows that increased ionization rate due to enhanced ionizing (>12-200 eV) electron impact is the prime cause of the large cometary plasma density enhancements during the CIRs. Plausible origin mechanisms of the cometary ionizing electron enhancements are discussed.

Having a nucleus darker than charcoal, comets are usually detected from Earth through the emissions from their coma. The coma is an envelope of gas which forms through the sublimation of ices from the nucleus, as the comet gets closer to the Sun. In the far ultraviolet, observations of comae have revealed the presence of atomic hydrogen and oxygen emissions. When observed over large spatial scales as seen from Earth, such emissions are dominated by resonance fluorescence pumped by solar radiation. Here we analyse atomic emissions acquired close to the cometary nucleus by the Rosetta spacecraft. In order to identify their origin, we undertake a quantitative multi-instrument analysis of these emissions by combining coincident neutral gas, electron, and spectroscopic observations together. We establish that the atomic emissions detected from Rosetta around comet 67P/Churyumov-Gerasimenko at large heliocentric distances result from the dissociative excitation of cometary molecules by accelerated solar-wind electrons (and not electrons produced from photo-ionisation of cometary molecules as suggested in past studies). We reveal their auroral nature. Similar to the discrete aurorae at Earth and Mars, this newly-discovered cometary aurora is driven by the interaction of the solar wind with the local environment. We highlight how OI 1356Å could be used as a tracer of solar-wind electron variability. The Rosetta spacecraft escorted comet 67P/Churyumov-Gerasimenko (referred as 67P hereafter) for more than two years 1, 2. Onboard, the Alice ultraviolet imaging spectrograph 3 detected Far UltraViolet (FUV) atomic hydrogen and oxygen emissions 4-7 from the cometary coma. Spec

Context. The Rosetta spacecraft escorted comet 67P/Churyumov-Gerasimenko from 2014 to September 2016. The mission provided in situ observations of the cometary plasma during different phases of the cometary activity, which enabled us to better understand its evolution as a function of heliocentric distance. Aims. In this study, different electron populations, called warm and hot, observed by the Ion and Electron Sensor (IES) of the Rosetta Plasma Consortium (RPC) are investigated near the comet during the escorting phase of the Rosetta mission. Methods. The estimates for the suprathermal electron densities and temperatures were extracted using IES electron data by fitting a double-kappa function to the measured velocity distributions. The fitting results were validated using observations from other RPC instruments. We give upgraded estimates for the warm and hot population densities compared to values previously shown in literature. Results. The fitted density and temperature estimates for both electron populations seen by IES are expressed as a function of heliocentric distance to study their evolution with the cometary activity. In addition, we studied the dependence between the electron properties and cometocentric distance. Conclusions. We observed that when the neutral outgassing rate of the nucleus is high (i.e., near perihelion) the suprathermal electrons are well characterized by a double-kappa distribution. In addition, warm and hot populations show a significant dependence with the heliocentric distance. The populations become clearly denser near perihelion while their temperatures are observed to remain almost constant. Moreover, the warm electron population density is shown to be strongly dependent on the radial distance from the comet. Finally, based on our results we reject the hypothesis that hot electron population seen by IES consists of solely suprathermal (halo) solar wind electrons, while we suggest that the hot electron population mainly consists of solar wind thermal electrons that have undergone acceleration near the comet.