Publications
158 results found
Stephenson P, Beth A, Deca J, et al., 2023, The source of electrons at comet 67P, Monthly Notices of the Royal Astronomical Society, Vol: 525, Pages: 5041-5065, ISSN: 0035-8711
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 data set 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-min intervals and compared to other Rosetta data sets. EII 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, collision-less Particle-in-Cell simulation. Collisional electrons are modelled at outgassing rates of Q = 1026 s−1 and Q = 1.5 × 1027 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 this study.
Lewis ZM, Beth A, Altwegg K, et al., 2023, Origin and trends in NH4+ observed in the coma of 67P, Monthly Notices of the Royal Astronomical Society, Vol: 523, Pages: 6208-6219, ISSN: 0035-8711
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.
Leblanc F, Roth L, Chaufray JY, et al., 2023, Ganymede's atmosphere as constrained by HST/STIS observations, Icarus, Vol: 399, ISSN: 0019-1035
A new analysis of aurora observations of Ganymede's atmosphere on the orbital leading and trailing hemispheres has been recently published by Roth et al. (2021), suggesting that water is its main constituent near noon. Here, we present two additional aurora observations of Ganymede's sub-Jovian and anti-Jovian hemispheres, which suggest a modulation of the atmospheric H2O/O2 ratio on the moon's orbital period, and analyze the orbital evolution of the atmosphere. For this, we propose a reconstruction of aurora observations based on a physical modelling of the exosphere taking into account its orbital variability (the Exospheric Global Model; Leblanc et al., 2017). The solution described in this paper agrees with Roth et al. (2021) that Ganymede's exosphere should be dominantly composed of water molecules. From Ganymede's position when its leading hemisphere is illuminated to when it is its trailing hemisphere, the column density of O2 may vary between 4.3 × 1014 and 3.6 × 1014 cm−2 whereas the H2O column density should vary between 5.6 × 1014 and 1.3 × 1015 cm−2. The water content of Ganymede's atmosphere is essentially constrained by its sublimation rate whereas the O2 component of Ganymede's atmosphere is controlled by the radiolytic yield. The other species, products of the water molecules, vary in a more complex way depending on their sources, either as ejecta from the surface and/or as product of the dissociation of the other atmospheric constituents. Electron impact on H2O and H2 molecules is shown to likely produce H Lyman-alpha emissions close to Ganymede, in addition to the observed extended Lyman-alpha corona from H resonant scattering. All these conclusions being highly dependent on our capability to accurately model the origins of the observed Ganymede auroral emissions, modelling these emissions remains poorly constrained without an accurate knowledge of the Jovian magnetospheric and Ganymede ionospheric electron popul
Moses JI, Brown ZL, Koskinen TT, et al., 2023, Saturn’s atmospheric response to the large influx of ring material inferred from Cassini INMS measurements, Icarus, Vol: 391, Pages: 1-40, ISSN: 0019-1035
During the Grand Finale stage of the Cassini mission, organic-rich ring material was discovered to be flowing into Saturn’s equatorial upper atmosphere at a surprisingly large rate. Through a series of photochemical models, we have examined the consequences of this ring material on the chemistry of Saturn’s neutral and ionized atmosphere. We find that if a substantial fraction of this material enters the atmosphere as vapor or becomes vaporized as the solid ring particles ablate upon atmospheric entry, then the ring-derived vapor would strongly affect the composition of Saturn’s ionosphere and neutral stratosphere. Our surveys of Cassini infrared and ultraviolet remote-sensing data from the final few years of the mission, however, reveal none of these predicted chemical consequences. We therefore conclude that either (1) the inferred ring influx represents an anomalous, transient situation that was triggered by some recent dynamical event in the ring system that occurred a few months to a few tens of years before the 2017 end of the Cassini mission, or (2) a large fraction of the incoming material must have been entering the atmosphere as small dust particles less than 100 nm in radius, rather than as vapor or as large particles that are likely to ablate. Future observations or upper limits for stratospheric neutral species such as HCN, HCN, and CO at infrared wavelengths could shed light on the origin, timing, magnitude, and nature of a possible vapor-rich ring-inflow event.
Bockelée-Morvan D, Filacchione G, Altwegg K, et al., 2022, AMBITION – comet nucleus cryogenic sample return, Experimental Astronomy, Vol: 54, Pages: 1077-1128, ISSN: 0922-6435
We describe the AMBITION project, a mission to return the first-ever cryogenicallystored sample of a cometary nucleus, that has been proposed for the ESA ScienceProgramme Voyage 2050. Comets are the leftover building blocks of giant planetcores and other planetary bodies, and fingerprints of Solar System’s formation processes. We summarise some of the most important questions still open in cometaryscience and Solar System formation after the successful Rosetta mission. We showthat many of these scientific questions require sample analysis using techniques thatare only possible in laboratories on Earth. We summarize measurements, instrumentation and mission scenarios that can address these questions. We emphasize the needfor returning a sample collected at depth or, still more challenging, at cryogenic temperatures while preserving the stratigraphy of the comet nucleus surface layers. Weprovide requirements for the next generation of landers, for cryogenic sample acquisition and storage during the return to Earth. Rendezvous missions to the main beltcomets and Centaurs, expanding our knowledge by exploring new classes of comets,are also discussed. The AMBITION project is discussed in the international contextof comet and asteroid space exploration.
Goetz C, Gunell H, Volwerk M, et al., 2022, Cometary plasma science Open science questions for future space missions, Experimental Astronomy: an international journal on astronomical instrumentation and data analysis, Vol: 54, Pages: 1129-1167, ISSN: 0922-6435
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
Rodriguez S, Vinatier S, Cordier D, et al., 2022, Science goals and new mission concepts for future exploration of Titan's atmosphere geology and habitability: Titan POlar Scout/orbitEr and In situ lake lander and DrONe explorer (POSEIDON), Experimental Astronomy: an international journal on astronomical instrumentation and data analysis, Vol: 54, Pages: 911-973, ISSN: 0922-6435
In response to ESA’s “Voyage 2050” announcement of opportunity, we propose an ambitious L-class mission to explore one of the most exciting bodies in the Solar System, Saturn’s largest moon Titan. Titan, a “world with two oceans”, is an organic-rich body with interior-surface-atmosphere interactions that are comparable in complexity to the Earth. Titan is also one of the few places in the Solar System with habitability potential. Titan’s remarkable nature was only partly revealed by the Cassini-Huygens mission and still holds mysteries requiring a complete exploration using a variety of vehicles and instruments. The proposed mission concept POSEIDON (Titan POlar Scout/orbitEr and In situ lake lander DrONe explorer) would perform joint orbital and in situ investigations of Titan. It is designed to build on and exceed the scope and scientific/technological accomplishments of Cassini-Huygens, exploring Titan in ways that were not previously possible, in particular through full close-up and in situ coverage over long periods of time. In the proposed mission architecture, POSEIDON consists of two major elements: a spacecraft with a large set of instruments that would orbit Titan, preferably in a low-eccentricity polar orbit, and a suite of in situ investigation components, i.e. a lake lander, a “heavy” drone (possibly amphibious) and/or a fleet of mini-drones, dedicated to the exploration of the polar regions. The ideal arrival time at Titan would be slightly before the next northern Spring equinox (2039), as equinoxes are the most active periods to monitor still largely unknown atmospheric and surface seasonal changes. The exploration of Titan’s northern latitudes with an orbiter and in situ element(s) would be highly complementary in terms of timing (with possible mission timing overlap), locations, and science goals with the upcoming NASA New Frontiers Dragonfly mission that will provide in situ exploration o
Goetz C, Behar E, Beth A, et al., 2022, The plasma environment of comet 67P/Churyumov-Gerasimenko, Space Science Reviews, Vol: 218, Pages: 1-120, ISSN: 0038-6308
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.
Beth A, Galand M, Simon Wedlund C, et al., 2022, Cometary Ionospheres: An Updated Tutorial, Comets III, Editors: Meech, Combi, Publisher: University of Arizona Press
This chapter aims at providing the tools and knowledge to understand and model the plasma environment surrounding comets in the innermost part near the nucleus. In particular, our goal is to give an updated post-Rosetta view of this ionised environment: what we knew, what we confirmed, what we overturned, and what we still do not understand.
Stephenson P, Galand M, Deca J, et al., 2022, Cold electrons at a weakly outgassing comet, Europlanet Science Congress 2022, Publisher: Copernicus GmbH
Stephenson P, Altwegg K, Beth A, et al., 2022, The source of electrons at a weakly outgassing comet, Publisher: Copernicus GmbH
<jats:p>&lt;p&gt;The Rosetta spacecraft escorted comet 67P/Churyumov-Gerasimenko for two years along its orbit, from Aug 2014 to Sep 2016, observing the evolution of the comet from a close perspective. The Rosetta Plasma Consortium (RPC) monitored the plasma environment at the spacecraft throughout the escort phase.&lt;/p&gt;&lt;p&gt;Cometary electrons are produced by ionization of the neutral gas coma. This occurs through photoionization by extreme ultraviolet photons, and through electron-impact ionization (EII) by collisions of energetic electrons with the coma. Far from perihelion, EII is, at times, more prevalent than photoionization (Galand et al., 2016; Heritier et al., 2018), but the EII frequency has not been assessed across the whole mission. The source of the cometary electrons, and the origin of the ionizing electrons is still unclear.&lt;/p&gt;&lt;p&gt;We have calculated the electron impact ionization (EII) frequency throughout the Rosetta mission and at its location from measurements of RPC&amp;#8217;s Ion and Electron Sensor (RPC/IES). EII ionization is confirmed as the dominant source of cometary electrons and ions when far from perihelion but is much more variable than photoionization. We compare the EII frequency with properties of the neutral coma and cometary plasma to identify key drivers of the energetic electron population. The EII frequency is structured by outgassing rate and magnetic field strength.&lt;/p&gt;&lt;p&gt;The first 3D collision model of electrons at a comet (Stephenson et al. 2022) is also utilised to assess the origin of electrons within the coma. The model uses self-consistently calculated electric and magnetic fields from a fully-kinetic and collisionless Particle-in-Cell model (Deca et al. 2017, 2019)as an input. The modelling approach confirms cometary electrons are produced by impacts of energetic e
Lewis Z, Beth A, Altwegg K, et al., 2022, Ionospheric composition of comet 67P near perihelion with multi-instrument Rosetta datasets, Publisher: Copernicus GmbH
<jats:p>&lt;p&gt;The European Space Agency Rosetta mission escorted comet 67P/Churyumov-Gerasimenko for two years, during which it acquired an extensive dataset, revealing unprecedented detail about the neutral and plasma environment of the coma. The measurements were made over a large range of heliocentric distances, and therefore of outgassing activities, as Rosetta witnessed 67P evolve from a low-activity icy body at 3.8 AU to a dynamic object with large-scale plasma structures and rich ion and neutral chemistry near perihelion at 1.2 AU. One such plasma structure is the diamagnetic cavity, a region of negligible magnetic field surrounding the comet nucleus. It is formed through the interaction of the unmagnetized outwardly expanding cometary plasma with the incoming solar wind. This region was encountered many times by Rosetta between April 2015 and February 2016, as the comet moved towards and away from perihelion.&lt;/p&gt;&lt;p&gt;In this study, we focus on the changing role of chemistry during the escort phase, particularly on trends in the detection of high proton affinity species near perihelion and within the diamagnetic cavity. NH&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;+&lt;/sup&gt; is produced through the protonation of NH&lt;sub&gt;3&lt;/sub&gt; which has the highest proton affinity of the neutral species and is therefore the terminal ion. The ratio of this species to the major ion species H&lt;sub&gt;3&lt;/sub&gt;O&lt;sup&gt;+&lt;/sup&gt; can then be an indicator of the importance of ion-neutral chemistry as an ion loss process compared to transport. We use data from the high resolution mode of the ROSINA (Rosetta Orbital Spectrometer for Ion-Neutral Analysis)/DFMS (Double Focussing Mass Spectrometer) instrument, which allows certain ions of the
Deca J, Stephenson P, Divin A, et al., 2022, A Fully Kinetic Perspective on Weakly Active Comets: Symmetric versus Asymmetric Outgassing, Publisher: Copernicus GmbH
<jats:p>&lt;p&gt;For more than two years, ESA&amp;#8217;s Rosetta mission measured the complex and ever-evolving plasma environment surrounding comet 67P/Churyumov-Gerasimenko. In this work, we explore the structure and dynamics of the near-comet plasma environment at steady state, comparing directly the results of a spherically symmetric Haser model and an asymmetric outgassing profile based on the measurements from the ROSINA instrument onboard Rosetta during 67P&amp;#8217;s weakly outgassing stages. Using a fully kinetic semi-implicit particle-in-cell code, we are able to characterise (1) the various ion and electron populations and their interactions, and (2) the implications to the mass-loading process caused by taking into account asymmetric outgassing. Our model complements observations by providing a full 3D picture that is directly relevant to help interpret the measurements made by the Rosetta Plasma Consortium instruments. In addition, understanding such details better is key to help disentangle the physical drivers active in the plasma environment of comets visited by future exploration missions.&lt;/p&gt;</jats:p>
Stephenson P, Galand M, Deca J, et al., 2022, A collisional test particle model of electrons at a comet, Monthly Notices of the Royal Astronomical Society, Vol: 511, Pages: 4090-4108, ISSN: 0035-8711
We have developed the first 3D collisional model of electrons at a comet, which we use to examine the impact of electron-neutral collisions in the weakly outgassing regime. The test-particle Monte Carlo model uses electric and magnetic fields from a fully kinetic Particle-in-Cell (PiC) model as an input. In our model, electrons originate from the solar wind or from ionization of the neutral coma, either by electron impact or absorption of an extreme ultraviolet photon. All relevant electron-neutral collision processes are included in the model including elastic scattering, excitation, and ionization. Trajectories of electrons are validated against analytically known drifts and the stochastic energy degradation used in the model is compared to the continuous slowing down approximation. Macroscopic properties of the solar wind and cometary electron populations, such as density and temperature, are validated with simple known cases and via comparison with the collisionless PiC model. We demonstrate that electrons are trapped close to the nucleus by the ambipolar electric field, causing an increase in the efficiency of electron-neutral collisions. Even at a low-outgassing rate (Q = 1026 s−1), electron-neutral collisions are shown to cause significant cooling in the coma. The model also provides a multistep numerical framework that is used to assess the influence of the electron-to-ion mass ratio, enabling access to electron dynamics with a physical electron mass.
Chadney JM, Koskinen TT, Hu X, et al., 2022, Energy deposition in Saturn's equatorial upper atmosphere, Icarus, Vol: 372, Pages: 1-16, ISSN: 0019-1035
We construct Saturn equatorial neutral temperature and density profiles of H, H2, He, and CH4, between 10−12 and 1 bar using measurements from Cassini’s Ion Neutral Mass Spectrometer (INMS) taken during the spacecraft’s final plunge into Saturn’s atmosphere on 15 September 2017, combined with previous deeper atmospheric measurements from the Cassini Composite InfraRed Spectrometer (CIRS) and from the UltraViolet Imaging Spectrograph (UVIS). These neutral profiles are fed into an energy deposition model employing soft X-ray and Extreme UltraViolet (EUV) solar fluxes at a range of spectral resolutions (∆λ = 4×10−3 nm to 1 nm) assembled from TIMED/SEE, from SOHO/SUMER, and from the Whole Heliosphere Interval (WHI) quiet Sun campaign. Our energy deposition model calculates ion production rate profiles through photo-ionisation and electron-impact ionisation processes, as well as rates of photo-dissociation of CH4. The ion reaction rate profiles we determine are important to obtain accurate ion density profiles, meanwhile methane photo-dissociation is key to initiate complex organic chemical processes. We assess the importance of spectral resolution in the energy deposition model by using a high-resolution H2 photo-absorption cross section, which has the effect of producing additional ionisation peaks near 800 km altitude. We find that these peaks are still formed when using low resolution (∆λ = 1 nm) or mid-resolution (∆λ = 0.1 nm) solar spectra, as long as high-resolution cross sections are included in the model.
Matteini L, Laker R, Horbury T, et al., 2021, Solar Orbiter's encounter with the tail of comet C/2019 Y4 (ATLAS): Magnetic field draping and cometary pick-up ion waves, Astronomy and Astrophysics: a European journal, Vol: 656, ISSN: 0004-6361
ontext. Solar Orbiter is expected to have flown close to the tail of comet C/2019 Y4 (ATLAS) during the spacecraft’s first perihelion in June 2020. Models predict a possible crossing of the comet tails by the spacecraft at a distance from the Sun of approximately 0.5 AU.Aims. This study is aimed at identifying possible signatures of the interaction of the solar wind plasma with material released by comet ATLAS, including the detection of draped magnetic field as well as the presence of cometary pick-up ions and of ion-scale waves excited by associated instabilities. This encounter provides us with the first opportunity of addressing such dynamics in the inner Heliosphere and improving our understanding of the plasma interaction between comets and the solar wind.Methods. We analysed data from all in situ instruments on board Solar Orbiter and compared their independent measurements in order to identify and characterize the nature of structures and waves observed in the plasma when the encounter was predicted.Results. We identified a magnetic field structure observed at the start of 4 June, associated with a full magnetic reversal, a local deceleration of the flow and large plasma density, and enhanced dust and energetic ions events. The cross-comparison of all these observations support a possible cometary origin for this structure and suggests the presence of magnetic field draping around some low-field and high-density object. Inside and around this large scale structure, several ion-scale wave-forms are detected that are consistent with small-scale waves and structures generated by cometary pick-up ion instabilities.Conclusions. Solar Orbiter measurements are consistent with the crossing through a magnetic and plasma structure of cometary origin embedded in the ambient solar wind. We suggest that this corresponds to the magnetotail of one of the fragments of comet ATLAS or to a portion of the tail that was previously disconnected and advected past the spacec
Stephenson P, Galand M, Deca J, et al., 2021, Forming a cold electron population at a weakly outgassing comet&#160;
<jats:p>&lt;p&gt;The Rosetta Mission rendezvoused with comet 67P/Churyumov-Gerasimenko in August 2014 and escorted it for two years along its orbit. The Rosetta Plasma Consortium (RPC) was a suite of instruments, which observed the plasma environment at the spacecraft throughout the escort phase. The Mutual Impedance Probe (RPC/MIP; Wattieaux et al, 2020; Gilet et al., 2020) and Langmuir Probe (RPC/LAP; Engelhardt et al., 2018), both part of RPC, measured the presence of a cold electron population within the coma.&lt;/p&gt;&lt;p&gt;Newly born electrons, generated by ionisation of the neutral gas, form a warm population within the coma at ~10eV. Ionisation is either through absorption of extreme ultraviolet photons or through collisions of energetic electrons with the neutral molecules. The cold electron population is formed by cooling the newly born, warm electrons via electron-neutral collisions. Assuming the radial outflow of electrons, the cold population was only expected at comet 67P close to perihelion, where outgassing rate from the nucleus was at its highest (Q &gt; 10&lt;sup&gt;28&lt;/sup&gt; s&lt;sup&gt;-1&lt;/sup&gt;). However, cold electrons were observed until the end of the Rosetta mission at 3.8au when the outgassing was weak (Q&lt;10&lt;sup&gt;26&lt;/sup&gt; s&lt;sup&gt;-1&lt;/sup&gt;). Under the radial outflow assumption, there should not have been sufficient neutral gas to efficiently degrade the electron energies.&lt;/p&gt;&lt;p&gt;We have developed the first 3D collision model of electrons at a comet. Self-consistently calculated electric and magnetic fields from a collisionless and fully-kinetic Particle-in-Cell model (Deca et al., 2017; 2019) are used as a stationary input for the test particle simulations. We model th
Rothkaehl H, Andre N, Auster U, et al., 2021, Dust, Field and Plasma instrument onboard ESA&#8217;s Comet Interceptor &#160;mission&#160;
<jats:p>&lt;p&gt;The main goal of ESA&amp;#8217;s F-1 class Comet Interceptor mission is to characterise, for the first time, a long period comet; preferably a dynamically-new or an interstellar object. The main spacecraft, will have its trajectory outside of the inner coma, whereas two sub-spacecrafts will be targeted inside the inner coma, closer to the nucleus. The flyby of such a comet &amp;#160;will offer unique multipoint measurement opportunity to study the comet's dusty and ionised environment in ways exceeding that of the previous cometary missions, including Rosetta.&lt;br /&gt;&amp;#160;&lt;br /&gt;The Dust Field and Plasma (DFP) instruments located on both the main spacecraft A and on the sub-spacecraft B2, is a combined experiment dedicated to the in situ, multi-point study of the multi-phased ionized and dusty environment in the coma of the target and &amp;#160;its interaction with the surrounding space environment and the Sun.&lt;br /&gt;&amp;#160;&lt;br /&gt;The DFP instruments will be present in different configurations on the Comet Interceptor spacecraft A and B2. To enable the measurements on spacecraft A, the DFP is composed of 5 sensors; Fluxgate magnetometer DFP-FGM-A, Plasma instrument with nanodust and E-field measurements capabilities DFP-COMPLIMENT, Electron spectrometer DFP-LEES, Ion and energetic neutrals spectrometer DFP-SCIENA &amp;#160;and Dust detector DFP-DISC. On board of spacecraft B2 the DFP is composed of 2 sensors: Fluxgate magnetometer DFP-FGM-B2 and Cometary dust detector DFP-DISC.&lt;br /&gt;&amp;#160;&lt;br /&gt;The DFP instrument will measure magnetic field, the electric field, plasma parameters (density, temperature, speed), the distribution functions of electrons, ions and energetic neutrals, spacecraft potential, mass, number and spatial density of c
Galand M, Feldman PD, Bockelee-Morvan D, et al., 2021, Far-ultraviolet aurora identified at comet 67P/Churyumov-Gerasimenko (vol 4, pg 1084, 2020), NATURE ASTRONOMY, ISSN: 2397-3366
Stephenson P, Galand M, Feldman PD, et al., 2021, Multi-instrument analysis of far-ultraviolet aurora in the southern hemisphere of Comet 67P/Churyumov-Gerasimenko, Astronomy and Astrophysics: a European journal, Vol: 647, Pages: 1-19, ISSN: 0004-6361
Aims. We aim to determine whether dissociative excitation of cometary neutrals by electron impact is the major source of far ultraviolet (FUV) emissions at comet 67P/Churyumov-Gerasimenko in the southern hemisphere at large heliocentric distances, bothduring quiet conditions and impacts of corotating interaction regions observed in the summer of 2016.Methods. We combined multiple datasets from the Rosetta mission through a multi-instrument analysis to complete the first forwardmodelling of FUV emissions in the southern hemisphere of comet 67P and compared modelled brightnesses to observations with theAlice FUV imaging spectrograph. We modelled the brightness of OI1356, OI1304, Lyman-β, CI1657, and CII1335 emissions, whichare associated with the dissociation products of the four major neutral species in the coma: CO2, H2O, CO, and O2. The suprathermalelectron population was probed by the Ion and Electron Sensor of the Rosetta Plasma Consortium (RPC/IES) and the neutral col umn density was constrained by several instruments: the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA), theMicrowave Instrument for the Rosetta Orbiter (MIRO) and the Visual InfraRed Thermal Imaging Spectrometer (VIRTIS).Results. The modelled and observed brightnesses of the FUV emission lines agree closely when viewing nadir and dissociativeexcitation by electron impact is shown to be the dominant source of emissions away from perihelion. The CII1335 emissions areshown to be consistent with the volume mixing ratio of CO derived from ROSINA. When viewing the limb during the impactsof corotating interaction regions, the model reproduces brightnesses of OI1356 and CI1657 well, but resonance scattering in theextended coma may contribute significantly to the observed Lyman-β and OI1304 emissions. The correlation between variationsin the suprathermal electron flux and the observed FUV line brightnesses when viewing the comet’s limb suggests electrons areaccelerated on
Baran J, Rothkaehl H, Andre N, et al., 2021, The challenges of&#160; the Dust-Field-Plasma&#160; (DFP) instrument onboard ESA &#160;Comet Interceptor mission&#160;
<jats:p>&lt;p&gt;The&amp;#160;flyby of a dynamically new comet by ESA-F1 Comet Interceptor spacecraft offers unique multi-point&amp;#160;opportunities for studying the comet's dusty and ionised cometary &amp;#160;environment in ways that were not possible with previous missions, including Rosetta. As Comet Interceptor is an F-class mission, the payload is limited in terms of mass, power, and heritage. Most in situ science sensors therefore have been tightly integrated into a single Dust-Field-Plasma (DFP) instrument on the main spacecraft A and on the ESA sub-spacecraft B2, while there is&amp;#160;a Plasma Package suite on the&amp;#160;JAXA second sub-spacecraft B1. The advantage of tight integration is an important reduction of mass, power, and especially complexity, by keeping the electrical and data interfaces of the sensors internal to the DFP instrument.&lt;/p&gt;&lt;p&gt;The full diagnostics located on the board of the 3 spacecrafts will allow&amp;#160; to modeling the comet environment and described the complex physical processes around the comet and on their surface including also the&amp;#160; description of wave particle&amp;#160; interaction in dusty cometary plasma.&amp;#160;&lt;/p&gt;&lt;p&gt;The full set of DFP instrument&amp;#160;on &amp;#160;board the Comet Interceptor &amp;#160;spacecraft will allow&amp;#160;to model &amp;#160;the comet plasma&amp;#160;environment and&amp;#160;its interaction with the solar wind.&amp;#160;It will also allow to&amp;#160;describe&amp;#160;the complex physical processes taking place including wave particle&amp;#160;&amp;#160;interaction in dusty cometary plasma .&amp;#160;&lt;/p&gt;&lt;p&gt;On spacecraft A, DFP consists of a magne
Stephenson P, Galand M, Deca J, et al., 2021, Electron cooling at a weakly outgassing comet
<jats:p>&lt;p&gt;The Rosetta spacecraft arrived at comet 67P in August 2014 and then escorted it for 2 years along its orbit. Throughout this escort phase, two plasma instruments (Mutual Impedance Probe, MIP; and Langmuir Probe, LAP) measured a population of cold electrons (&lt; 1 eV) within the coma of 67P (Engelhardt et al., 2018; Wattieaux et al, 2020; Gilet et al., 2020). These cold electrons are understood to be formed by cooling warm electrons through collisions with the neutral gas. The warm electrons are primarily newly-born and produced at roughly 10eV within the coma through ionisation. While it was no surprise that cold electrons would form near perihelion given the high density of the neutral coma, the persistence of the cold electrons up to a heliocentric distance of 3.8 au was highly unexpected. With the low outgassing rates observed at such large heliocentric distances (Q &lt; 10&lt;sup&gt;26&lt;/sup&gt; s&lt;sup&gt;-1&lt;/sup&gt;), there should not be enough neutral molecules to cool the warm electrons efficiently before they ballistically escape the coma.&lt;/p&gt;&lt;p&gt;We use a collisional test particle model to examine the formation of the cold electron population at a weakly outgassing comet. The electrons are subject to stochastic collisions with the neutral coma which can either scatter or cool the electrons. Multiple electron neutral collision processes are included such that the electrons can undergo elastic scattering as well as collisions inducing excitation and ionisation of the neutral species. The inputted electric and magnetic fields, which act on the test particles, are taken from a 3D fully-kinetic, collisionless Particle-in-Cell (PiC) model of the solar wind and cometary ionosphere (Deca et al., 2017; 2019), with the same neutral coma as used in our model. We use a pure water coma with spherical sym
Nilsson H, Behar E, Burch JL, et al., 2021, Birth of a Magnetosphere, MAGNETOSPHERES IN THE SOLAR SYSTEM, Editors: Maggiolo, Andre, Hasegawa, Welling, Zhang, Paxton, Publisher: AMER GEOPHYSICAL UNION, Pages: 427-439, ISBN: 978-1-119-50752-9
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Carnielli G, Galand M, Leblanc F, et al., 2020, Simulations of ion sputtering at Ganymede, Icarus, Vol: 351, Pages: 1-11, ISSN: 0019-1035
Ganymede's surface is subject to constant bombardment by Jovian magnetospheric and Ganymede's ionospheric ions. These populations sputter the surface and contribute to the replenishment of the moon's exosphere.Thus far, estimates for sputtering on the moon's surface have included only the contribution from Jovian ions. In this work, we have used our recent model of Ganymede's ionosphere Carnielli et al., 2019 to evaluate the contribution of ionospheric ions for the first time. In addition, we have made new estimates for the contribution from Jovian ions, including both thermal and energetic components.For Jovian ions, we find a total sputtering rate of 2.2 × 1027 s−1, typically an order of magnitude higher compared to previous estimates. For ionospheric ions, produced through photo- and electron-impact ionization, we find values in the range 2.7 × 1026–5.2 × 1027 s−1 when the moon is located above the Jovian plasma sheet. Hence, Ganymede's ionospheric ions provide a contribution of at least 10% to the sputtering rate, and under certain conditions they dominate the process. This finding indicates that the ionospheric population is an important source to consider in the context of exospheric models.
Stephenson P, Galand M, Deca J, et al., 2020, Cooling of Electrons in a Weakly Outgassing Comet
<jats:p>&lt;p&gt;The plasma instruments, Mutual Impedance Probe (MIP) and Langmuir Probe (LAP), part of the Rosetta Plasma Consortium (RPC), onboard the Rosetta mission to comet 67P revealed a population of cold electrons (&lt;1eV) (Engelhardt et al., 2018; Wattieaux et al, 2020; Gilet et al., 2020). This population is primarily generated by cooling warm (~10eV) newly-born cometary electrons through collisions with the neutral coma. What is surprising is that the cold electrons were detected throughout the escort phase, even at very low outgassing rates (Q&lt;1e26 s&lt;sup&gt;-1&lt;/sup&gt;) at large heliocentric distances (&gt;3 AU), when the coma was not thought to be dense enough to cool the electron population significantly.&lt;/p&gt;&lt;p&gt;&amp;#160;Using a collisional test particle model, we examine the behaviour of electrons in the coma of a weakly outgassing comet and the formation of a cold population through electron-neutral collisions. The model incorporates three electron sources: the solar wind, photo-electrons produced through ionisation of the cometary neutrals by extreme ultraviolet solar radiation, and secondary electrons produced through electron-impact ionisation.&lt;/p&gt;&lt;p&gt;The model includes different electron-water collision processes, including elastic, excitation, and ionisation collisions.&lt;/p&gt;&lt;p&gt;&amp;#160;The electron trajectories are shaped by electric and magnetic fields, which are taken from a 3D collisionless fully-kinetic Particle-in-Cell (PIC) model of the solar wind and cometary plasma&amp;#160; (Deca 2017, 2019). We use a spherically symmetric coma of pure water, which gives a r&lt;sup&gt;-2&lt;/sup&gt; profile in the neutral density. Throughout their lifetime, electrons undergo stochast
Beth A, Altwegg K, Balsiger H, et al., 2020, ROSINA ion zoo at Comet 67P, Astronomy and Astrophysics: a European journal, Vol: 642, Pages: 1-23, ISSN: 0004-6361
Context. The Rosetta spacecraft escorted Comet 67P/Churyumov-Gerasimenko for 2 yr along its journey through the Solar System between 3.8 and 1.24 au. Thanks to the high resolution mass spectrometer on board Rosetta, the detailed ion composition within a coma has been accurately assessed in situ for the very first time.Aims. Previous cometary missions, such as Giotto, did not have the instrumental capabilities to identify the exact nature of the plasma in a coma because the mass resolution of the spectrometers onboard was too low to separate ion species with similar masses. In contrast, the Double Focusing Mass Spectrometer (DFMS), part of the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis on board Rosetta (ROSINA), with its high mass resolution mode, outperformed all of them, revealing the diversity of cometary ions.Methods. We calibrated and analysed the set of spectra acquired by DFMS in ion mode from October 2014 to April 2016. In particular, we focused on the range from 13–39 u q−1. The high mass resolution of DFMS allows for accurate identifications of ions with quasi-similar masses, separating 13C+ from CH+, for instance.Results. We confirm the presence in situ of predicted cations at comets, such as CHm+ (m = 1−4), HnO+ (n = 1−3), O+, Na+, and several ionised and protonated molecules. Prior to Rosetta, only a fraction of them had been confirmed from Earth-based observations. In addition, we report for the first time the unambiguous presence of a molecular dication in the gas envelope of a Solar System body, namely CO2++.
Galand M, Feldman PD, Bockelee-Morvan D, et al., 2020, Far-ultraviolet aurora identified at comet 67P/ Churyumov-Gerasimenko, Nature Astronomy, Vol: 4, Pages: 1084-1091, ISSN: 2397-3366
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 that forms through the sublimation of ices from the nucleus as the comet gets closer to the Sun. In the far-ultraviolet portion of the spectrum, 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 and reveal their auroral nature. To identify their origin, we undertake a quantitative multi-instrument analysis of these emissions by combining coincident neutral gas, electron and far-ultraviolet observations. 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 by electrons produced from photo-ionization of cometary molecules). Like the discrete aurorae at Earth and Mars, this cometary aurora is driven by the interaction of the solar wind with the local environment. We also highlight how the oxygen line O I at wavelength 1,356 Å could be used as a tracer of solar-wind electron variability.
Madanian H, Burch JL, Eriksson AI, et al., 2020, Electron dynamics near diamagnetic regions of comet 67P/Churyumov- Gerasimenko, Planetary and Space Science, Vol: 187, ISSN: 0032-0633
The Rosetta spacecraft detected transient and sporadic diamagnetic regions around comet 67P/Churyumov-Gerasimenko. In this paper we present a statistical analysis of bulk and suprathermal electron dynamics, as well as a case study of suprathermal electron pitch angle distributions (PADs) near a diamagnetic region. Bulk electron densities are correlated with the local neutral density and we find a distinct enhancement in electron densities measured over the southern latitudes of the comet. Flux of suprathermal electrons with energies between tens of eV to a couple of hundred eV decreases each time the spacecraft enters a diamagnetic region. We propose a mechanism in which this reduction can be explained by solar wind electrons that are tied to the magnetic field and after having been transported adiabatically in a decaying magnetic field environment, have limited access to the diamagnetic regions. Our analysis shows that suprathermal electron PADs evolve from an almost isotropic outside the diamagnetic cavity to a field-aligned distribution near the boundary. Electron transport becomes chaotic and non-adiabatic when electron gyroradius becomes comparable to the size of the magnetic field line curvature, which determines the upper energy limit of the flux variation. This study is based on Rosetta observations at around 200 km cometocentric distance when the comet was at 1.24 AU from the Sun and during the southern summer cometary season.
Simon Wedlund C, Behar E, Nilsson H, et al., 2020, Solar wind charge exchange in cometary atmospheresII. Analytical model, Astronomy and Astrophysics: a European journal, Vol: 640, Pages: C3-C3, ISSN: 0004-6361
Carnielli G, Galand M, Leblanc F, et al., 2020, Constraining Ganymede's neutral and plasma environments through simulations of its ionosphere and Galileo observations, Icarus, Vol: 343, Pages: 1-11, ISSN: 0019-1035
Ganymede's neutral and plasma environments are poorly constrained by observations. Carnielli et al. (2019) developed the first 3D ionospheric model aimed at understanding the dynamics of the present ion species and at quantifying the presence of each component in the moon's magnetosphere. The model outputs were compared with Galileo measurements of the ion energy flux, ion bulk velocity and electron number density made during the G2 flyby. A good agreement was found in terms of ion energy distribution and bulk velocity, but not in terms of electron number density. In this work, we present some improvements to our model Carnielli et al. (2019) and quantitatively address the possible sources of the discrepancy found in the electron number density between the Galileo observations and our ionospheric model. We have improved the ion model by developing a collision scheme to simulate the charge-exchange interaction between the exosphere and the ionosphere. We have simulated the energetic component of the O$_2$ population, which is missing in the exospheric model of Leblanc et al. (2017) and added it to the original distribution, hence improving its description at high altitudes. These improvements are found to be insufficient to explain the discrepancy in the electron number density. We provide arguments that the input O$_2$ exosphere is underestimated and that the plasma production acts asymmetrically between the Jovian and anti-Jovian hemispheres. In particular, we estimate that the O$_2$ column density should be greater than $10^{15}$~cm$^{-2}$, i.e., higher than previously derived upper limits (and a factor 10 higher than the values from Leblanc et al. (2017)), and that the ionization frequency from electron impact must be higher in the anti-Jovian hemisphere for the G2 flyby conditions.
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