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We examine the origin of electrons in a weakly outgassing comet, using Rosetta mission data and a 3D collisional model of electrons at a comet. We have calculated a new dataset of electron-impact ionization (EII) frequency throughout the Rosetta escort phase, with measurements of the Rosetta Plasma Consortium's Ion and Electron Sensor (RPC/IES). The EII frequency is evaluated in 15-minute intervals and compared to other Rosetta datasets. Electron-impact ionization is the dominant source of electrons at 67P away from perihelion and is highly variable (by up to three orders of magnitude). Around perihelion, EII is much less variable and less efficient than photoionization at Rosetta. Several drivers of the EII frequency are identified, including magnetic field strength and the outgassing rate. Energetic electrons are correlated to the Rosetta-upstream solar wind potential difference, confirming that the ionizing electrons are solar wind electrons accelerated by an ambipolar field. The collisional test particle model incorporates a spherically symmetric, pure water coma and all the relevant electron-neutral collision processes. Electric and magnetic fields are stationary model inputs, and are computed using a fully-kinetic, collisionless Particle-in-Cell simulation. Collisional electrons are modelled at outgassing rates of $Q=10^{26}$ s$^{-1}$ and $Q=1.5\times10^{27}$ s$^{-1}$. Secondary electrons are the dominant population within a weakly outgassing comet. These are produced by collisions of solar wind electrons with the neutral coma. The implications of large ion flow speed estimates at Rosetta, away from perihelion, are discussed in relation to multi-instrument studies and the new results of the EII frequency obtained in the present study., 27 Pages including Appendices, 24 Figures

The European Space Agency/Rosetta mission escorted comet 67P/Churyumov–Gerasimenko and witnessed the evolution of its coma from low activity (∼2.5–3.8 au) to rich ion-neutral chemistry (∼1.2–2.0 au). We present an analysis of the ion composition in the coma, focusing on the presence of protonated high proton affinity (HPA) species, in particular NH4+ ⁠. This ion is produced through the protonation of NH3 and is an indicator of the level of ion-neutral chemistry in the coma. We aim to assess the importance of this process compared with other NH4+ sources, such as the dissociation of ammonium salts embedded in dust grains. The analysis of NH4+ has been possible thanks to the high mass resolution of the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis/Double Focusing Mass Spectrometer (ROSINA/DFMS). In this work, we examine the NH4+ data set alongside data from the Rosetta Plasma Consortium instruments, and against outputs from our in-house ionospheric model. We show that increased comet outgassing around perihelion yields more detections of NH4+ and other protonated HPA species, which results from more complex ion-neutral chemistry occurring in the coma. We also reveal a link between the low magnetic field strength associated with the diamagnetic cavity and higher NH4+ counts. This suggests that transport inside and outside the diamagnetic cavity is very different, which is consistent with 3D hybrid simulations of the coma: non-radial plasma dynamics outside the diamagnetic cavity is an important factor affecting the ion composition.

International audience; Comets hold the key to the understanding of our Solar System, its formation and its evolution, and to the fundamental plasma processes at work both in it and beyond it. A comet nucleus emits gas as it is heated by the sunlight. The gas forms the coma, where it is ionised, becomes a plasma, and eventually interacts with the solar wind. Besides these neutral and ionised gases, the coma also contains dust grains, released from the comet nucleus. As a cometary atmosphere develops when the comet travels through the Solar System, large-scale structures, such as the plasma boundaries, develop and disappear, while at planets such large-scale structures are only accessible in their fully grown, quasi-steady state. In situ measurements at comets enable us to learn both how such large-scale structures are formed or reformed and how small-scale processes in the plasma affect the formation and properties of these large scale structures. Furthermore, a comet goes through a wide range of parameter regimes during its life cycle, where either collisional processes, involving neutrals and charged particles, or collisionless processes are at play, and might even compete in complicated transitional regimes. Thus a comet presents a unique opportunity to study this parameter space, from an asteroid-like to a Mars- and Venus-like interaction. The Rosetta mission and previous fast flybys of comets have together made many new discoveries, but the most important breakthroughs in the understanding of cometary plasmas are yet to come. The Comet Interceptor mission will provide a sample of multi-point measurements at a comet, setting the stage for a multi-spacecraft mission to accompany a comet on its journey through the Solar System. This White Paper, submitted in response to the European Space Agency’s Voyage 2050 call, reviews the present-day knowledge of cometary plasmas, discusses the many questions that remain unanswered, and outlines a multi-spacecraft European Space Agency mission to accompany a comet that will answer these questions by combining both multi-spacecraft observations and a rendezvous mission, and at the same time advance our understanding of fundamental plasma physics and its role in planetary systems.

Comet ATLAS disintegrated into several fragments while reaching its most recent perihelion at approximately 0.25AU in April 2020. Solar Orbiter is predicted to have crossed both the ion and dust tails of the comet between 31 May and 6 June 2020, when the spacecraft was close to 0.5AU. This constituted a unique opportunity to make in situ measurements of distinct cometary fragments at such a close distance from the Sun and to study the interaction of cometary pick-up ions with the solar wind plasma. In this study, we present and discuss possible signatures of this interaction as seen in various Solar Orbiter in situ sensors (MAG, RPW, SWA). We mainly focus on properties of a wide range of both structures and low-frequency electromagnetic waves that are supposedly driven by cometary pick-up ion instabilities and intermittently observed over several days during the encounter. These include trains of phase-steepened Alfvén waves propagating in both directions along the magnetic field, sharp discontinuities and current sheets, and precessing linearly polarised waves possibly suggesting the presence of non-gyrotropic sources of heavier pick-up ions. Observed wave properties are discussed and compared with expectations from linear theory and numerical simulations.

A dynamic plasma environment is created around the comet when its expanding atmosphere gets ionized by solar EUV radiation and electron-impact ionization. The characteristics of the near-cometary plasma vary with the cometary activity phase (i.e., heliocentric distance). Plasma instruments onboard the ESA/Rosetta spacecraft monitored the plasma environment near comet 67P/Churyumov-Gerasimenko and provided in situ observations of the plasma that enabled us to improve our understanding of different electron populations, their existence and characteristics in the near-comet environment at different steps of the comet activity cycle. High-energy electrons were observed near comet through out the Rosetta's escorting phase. These electrons are able to ionize cometary neutrals and thus they have a direct impact on the total plasma density around the comet. We have studied how the density and temperature of different energetic electron populations vary with heliocentric distance. The properties of these electron populations are estimated by fitting a double-kappa function to the measured electron phase space distribution. The results are compared with values that have been achieved by directly integrating over the phase space. The density and temperature values of the near-cometary electron populations are compared with the core and halo solar wind electron properties and their possible relationship is discussed. Acceleration and heating mechanisms for the electrons suggested in the literature are discussed and confronted to our results.

International audience; The French Plasma Physics Data Centre (CDPP, http://www.cdpp.eu/ ) has been addressing for almost the past 20 years all issues pertaining to natural plasma data distribution and valorization. Initially established by CNES and CNRS on the ground of a solid data archive, CDPP activities diversified with the advent of broader networks and interoperability standards, and through fruitful collaborations (e.g. with NASA/PDS). Providing access to remote data, designing and building science driven analysis tools then became at the forefront of CDPP developments. In the frame of data distribution, the CDPP has provided to the Rosetta Plasma Consortium (RPC), a suite of five different plasma sensors, with the possibility to visualize plasma data acquired by the Rosetta mission through its data analysis tool AMDA. AMDA was used during the operational phase of the Rosetta mission, facilitating data access between different Rosetta PI sensor teams, thus allowing 1/ a more efficient instruments operation planning and 2/ a better understanding of single instrument observations in the context of other sensor measurements and of more global observations. The data are now getting open to the public via the AMDA tool as they are released to the ESA/PSA. These in-situ data are complemented by model data, for instance, a solar wind propagation model (see http://heliopropa.irap.omp.eu ) or illumination maps of 67P (available through http://vespa.obspm.fr ). The CDPP also proposes 3D visualization tool for planetary / heliospheric environments which helps putting data in context (http://3dview.cdpp.eu ); for instance all comets and asteroids in a given volume and for a given time interval can be searched and displayed. From this fruitful experience the CDPP intends to play a similar role for the forthcoming data of the Solar Orbiter, Bepi-Colombo and JUICE missions as it is officially part of several instrument consortia. Beside highlighting the current database and products, the presentation will show how these future data could be presented and valorized through a combined use of the tools and models provided by the CDPP.

International audience; So far, Ganymede’s nearby plasma environment has been in part characterized only during a few flybys of the moon by the Galileo spacecraft at the end of the 1990s and through a few remote observations of auroral emissions by the Hubble Space Telescope. Our knowledge of the plasma composition, density and dynamics in Ganymede’s magnetosphere remains therefore limited. The JUICE spacecraft will characterize in detail the exosphere, ionosphere and particle environment around the moon. Prior to arrival, models have been developed to predict these regions and their interaction with the background Jovian particles and magnetic field.We have developed the first 3D test particle model of Ganymede’s ionosphere. The model is driven by: (1) the number densities of neutral species from the exospheric model of Leblanc et al. (Icarus, 2017), (2) solar extreme ultraviolet radiation (Woods et al. 2005), (3) electron fluxes coming from the Jovian plasma around the moon (Mauk et al., 2004) and (4) the electromagnetic field from the hybrid model of Leclercq et al. (PSS, in revision). In the simulation, the ionospheric ions are produced via photoionization and electron- impact ionization of the neutral exosphere. The test particles move under the influence of the magnetic and electric fields derived from the hybrid model.We will present the first three-dimensional maps of number densities and bulk speeds of the main ion species produced in Ganymede’s ionosphere. We will show and interpret our derived ion-impact 2D maps at the surface for both ionospheric ions and Jovian ions (coming from the Jovian plasma sheet), and provide sputtering rates of neutral molecule production resulting from these impacts. We will also quantify the importance of the charge-exchange process between the ions and exospheric species in terms of production of energetic neutrals, which is relevant for exospheric models. Finally, we will assess the variability of the ionosphere over a revolution of Ganymede around Jupiter, driven by the change in the neutral exosphere (Leblanc et al., 2017) and in the angle Sun-moon direction. We will evaluate its potential implications on the variability of Ganymede’s magnetic environment.

International audience; Context. Strong electron cooling on the neutral gas in cometary comae has been predicted for a long time, but actual measurements of low electron temperature are scarce. Aims. Our aim is to demonstrate the existence of cold electrons in the inner coma of comet 67P and show filamentation of this plasma.Methods. In situ measurements of plasma density, electron temperature and spacecraft potential were carried out by the Rosetta Langmuir probe instrument, LAP. We also performed analytical modelling of the expanding two-temperature electron gas.Results. LAP data acquired within a few hundred km from the nucleus are dominated by a warm component with electron temperature typically 5–10 eV at all heliocentric distances covered (1.25 to 3.83 AU). A cold component, with temperature no higher than about 0.1 eV, appears in the data as short (few to few tens of seconds) pulses of high probe current, indicating local enhancement of plasma density as well as a decrease in electron temperature. These pulses first appeared around 3 AU and were seen for longer periods close to perihelion. The general pattern of pulse appearance follows that of neutral gas and plasma density. We have not identified any periods with only cold electrons present. The electron flux to Rosetta was always dominated by higher energies, driving the spacecraft potential to order −10 V. Conclusions. The warm (5–10 eV) electron population observed throughout the mission is interpreted as electrons retaining the energy they obtained when released in the ionisation process. The sometimes observed cold populations with electron temperatures below 0.1 eV verify collisional cooling in the coma. The cold electrons were only observed together with the warm population. The general appearance of the cold population appears to be consistent with a Haser-like model, implicitly supporting also the coupling of ions to the neutral gas. The expanding cold plasma is unstable, forming filaments that we observe as pulses.

In this paper, we report the first in situ detection of the ammonium ion NH⁺₄ at 67P/Churyumov–Gerasimenko (67P/C-G) in a cometary coma, using the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA)/Double Focusing Mass Spectrometer (DFMS). Unlike neutral and ion spectrometers onboard previous cometary missions, the ROSINA/DFMS spectrometer, when operated in ion mode, offers the capability to distinguish NH⁺₄ from H₂O⁺ in a cometary coma. We present here the ion data analysis of mass-to-charge ratios 18 and 19 at high spectral resolution and compare the results with an ionospheric model to put these results into context. The model confirms that the ammonium ion NH⁺₄ is one of the most abundant ion species, as predicted, in the coma near perihelion.

The magnetometer onboard ESA's Rosetta orbiter detected its highest magnetic field magnitude of 250nT in July 2015, close to perihelion. This magnitude was an enhancement of a factor of five compared to normal values, which makes this the highest interplanetary magnetic field ever measured. We have examined the solar wind conditions at the time and found that a corotating interaction region (CIR), accompanied by a fast flow is the trigger for this unusual event. Because Rosetta does not have solar wind observations during the comet's active phase, we use ENLIL simulations as well as observations at Earth and Mars to constrain the solar wind parameters at the comet. Using a simple model for the magnetic field pile-up we can trace back the field in the coma to corresponding structures in the CIR. The large field is accompanied by a dramatic increase in electron and ion fluxes and energies. However, the electrons and ions in the field of view are not, as expected, increasing at the same time, instead the electrons follow the magnetic field, while the ion density increase is delayed. This is seen as evidence of the kinetic behaviour of the ions as opposed to a magnetized electron fluid. Combining the information on the plasma, we are able to identify at least three different regions in the plasma that have fundamentally different parameters. This allows us to separate the solar wind influence from the comet's effects on the plasma, a problem that is usually not solvable without a spacecraft monitoring the solar wind at the comet.

Cometary outbursts are one of the most spectacular aspects of comet behavior. They are characterized by an abrupt increase in cometary brightness followed by a gradual fall off to the pre-event brightness. Although there are several studies on outburst events, to our knowledge, no detailed analysis on the variation of the cometary plasma environment during an outburst has ever been reported. On 19 February 2016, when comet 67P/Churyumov-Gerasimenko was at a heliocentric distance of 2.4 AU, an outburst event, characterized by two orders of magnitude increase in coma surface brightness, took place. Rosetta was at a distance of 30 km from the comet nucleus, orbiting with a relative speed of 0.17 m/s. The Rosetta Plasma Consortium (RPC) provided in-situ measurements of the cometary plasma, embedded in the solar wind, and the associated magnetic field during this outburst, as the dust and gas expelled from the comet were passing by the spacecraft. While the neutral density (ROSINA/COPS) at the spacecraft position increased by a factor of 1.5, the local plasma density (RPC/MIP) was found to increase by a factor of 3 during the outburst event, driving the spacecraft potential more negative (RPC/LAP). The event was characterized by the energy degradation of energetic (10s of eV) electrons (RPC/IES). In response to the outburst, the local magnetic field exhibited a slight increase in amplitude and a slow rotation (RPC/MAG). A weakening of 10-100 mHz magnetic field fluctuations was also observed during the outburst. The RPC instruments show that the effects of the outburst on the plasma lasted for about 4 hours, from 1000 UT to 1400 UT. Detailed analyses of the observations made by RPC along with ROSINA/COPS will be presented in the paper.