Imperial College London

DrArnaudBeth

Faculty of Natural SciencesDepartment of Physics

Research Associate in Space
 
 
 
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Contact

 

+44 (0)20 7594 7661a.beth Website

 
 
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Location

 

H702Huxley BuildingSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

49 results found

Beth A, Galand M, Modolo R, Leblanc F, Jia X, Huybrighs H, Carnielli Get al., 2024, Ionospheric environment of Ganymede during the Galileo flybys

<jats:p>The Galileo spacecraft flew by Ganymede, down to 0.1 RG from the surface for the closest, six times giving us insight into its plasma environment. Its ionosphere, made of ions born from the ionisation of neutrals present in Ganymede&amp;#8217;s exosphere, represents the bulk of the plasma near the moon around closest approach. As it has been revealed by Galileo and Juno, near closest approach the ion population is dominated by low-energy ions from the water ion group (O+, HO+, H2O+) and O2+. However, little is known about their density, spatial distribution, and effect on the surface weathering of the moon itself. Galileo G2 flyby has been extensively studied. Based on a comparison between observations and 3D test-particle simulations, Carnielli et al. (2020a and 2020b) confirmed the ion composition (debated at the time), highlighted the inconsistency between the assumed exospheric densities and the observed ionospheric densities, and derived the contribution of ionospheric ions as an exospheric source. However, other flybys of Ganymede are also available (e.g. G1, G7, G8, G28, and G29) providing in-situ measurements at different phases of Ganymede around Jupiter or jovian magnetospheric conditions at the moon. We extend the original study by Carnielli et al. to other flybys, and compare our modelled ion moments (ion number density, velocity, and energy distribution) with Galileo in-situ data. We discuss our results and contrast them with those obtained for the G2 flyby.&amp;#160;&amp;#160;</jats:p>

Other

Lewis Z, Beth A, Galand M, Henri P, Rubin M, Stephenson Pet al., 2024, Constraining ion transport in the diamagnetic cavity of comet 67P

<jats:p>Comets are small icy bodies originating from the outer solar system that produce an increasingly dense gas coma through sublimation as they approach perihelion. Photoionisation of this gas results in a cometary ionosphere, which interacts with the impinging solar wind, leading to large scale plasma structures. One such structure is the diamagnetic cavity: the magnetic field-free inner region that the solar wind cannot penetrate. This region was encountered many times by the ESA Rosetta mission, which escorted comet 67P/Churyumov-Gerasimenko for a two-year section of its orbit.Within the diamagnetic cavity, high ion bulk velocities have been observed by the Rosetta Plasma Consortium (RPC) instruments. The fast ions are thought to have been accelerated by an ambipolar electric field, but the nature and strength of this field are difficult to determine analytically. Our study therefore aims to model the impact of various electric field profiles on the ionospheric density profile and ion composition. The 1D numerical model we have developed includes three key ion species (H2O+, H3O+, and NH4+) in order to assess the sensitivity of each to the timescale of plasma loss through transport. NH4+ is of particular interest, as it has been previously shown to be the dominant ion species at low cometocentric distances near perihelion. It is only produced through the protonation of NH3, a minor component of the neutral gas, and we show that this makes it particularly sensitive to the electric field.We also compare the simulated electron density to RPC datasets, to find the electric field strength and profile which best recreate the plasma densities measured inside the diamagnetic cavity near perihelion. From this, we also constrain the radial bulk ion speed that is required to explain the observations with the model.</jats:p>

Other

Jones GH, Snodgrass C, Tubiana C, Küppers M, Kawakita H, Lara LM, Agarwal J, André N, Attree N, Auster U, Bagnulo S, Bannister M, Beth A, Bowles N, Coates A, Colangeli L, Corral van Damme C, Da Deppo V, De Keyser J, Della Corte V, Edberg N, El-Maarry MR, Faggi S, Fulle M, Funase R, Galand M, Goetz C, Groussin O, Guilbert-Lepoutre A, Henri P, Kasahara S, Kereszturi A, Kidger M, Knight M, Kokotanekova R, Kolmasova I, Kossacki K, Kührt E, Kwon Y, La Forgia F, Levasseur-Regourd A-C, Lippi M, Longobardo A, Marschall R, Morawski M, Muñoz O, Näsilä A, Nilsson H, Opitom C, Pajusalu M, Pommerol A, Prech L, Rando N, Ratti F, Rothkaehl H, Rotundi A, Rubin M, Sakatani N, Sánchez JP, Simon Wedlund C, Stankov A, Thomas N, Toth I, Villanueva G, Vincent J-B, Volwerk M, Wurz P, Wielders A, Yoshioka K, Aleksiejuk K, Alvarez F, Amoros C, Aslam S, Atamaniuk B, Baran J, Barciński T, Beck T, Behnke T, Berglund M, Bertini I, Bieda M, Binczyk P, Busch M-D, Cacovean A, Capria MT, Carr C, Castro Marín JM, Ceriotti M, Chioetto P, Chuchra-Konrad A, Cocola L, Colin F, Crews C, Cripps V, Cupido E, Dassatti A, Davidsson BJR, De Roche T, Deca J, Del Togno S, Dhooghe F, Donaldson Hanna K, Eriksson A, Fedorov A, Fernández-Valenzuela E, Ferretti S, Floriot J, Frassetto F, Fredriksson J, Garnier P, Gaweł D, Génot V, Gerber T, Glassmeier K-H, Granvik M, Grison B, Gunell H, Hachemi T, Hagen C, Hajra R, Harada Y, Hasiba J, Haslebacher N, Herranz De La Revilla ML, Hestroffer D, Hewagama T, Holt C, Hviid S, Iakubivskyi I, Inno L, Irwin P, Ivanovski S, Jansky J, Jernej I, Jeszenszky H, Jimenéz J, Jorda L, Kama M, Kameda S, Kelley MSP, Klepacki K, Kohout T, Kojima H, Kowalski T, Kuwabara M, Ladno M, Laky G, Lammer H, Lan R, Lavraud B, Lazzarin M, Le Duff O, Lee Q-M, Lesniak C, Lewis Z, Lin Z-Y, Lister T, Lowry S, Magnes W, Markkanen J, Martinez Navajas I, Martins Z, Matsuoka A, Matyjasiak B, Mazelle C, Mazzotta Epifani E, Meier M, Michaelis H, Micheli M, Migliorini A, Millet A-L, Moreno F, Mottola S, Moutounet al., 2024, The Comet Interceptor Mission., Space Sci Rev, Vol: 220, ISSN: 0038-6308

Here we describe the novel, multi-point Comet Interceptor mission. It is dedicated to the exploration of a little-processed long-period comet, possibly entering the inner Solar System for the first time, or to encounter an interstellar object originating at another star. The objectives of the mission are to address the following questions: What are the surface composition, shape, morphology, and structure of the target object? What is the composition of the gas and dust in the coma, its connection to the nucleus, and the nature of its interaction with the solar wind? The mission was proposed to the European Space Agency in 2018, and formally adopted by the agency in June 2022, for launch in 2029 together with the Ariel mission. Comet Interceptor will take advantage of the opportunity presented by ESA's F-Class call for fast, flexible, low-cost missions to which it was proposed. The call required a launch to a halo orbit around the Sun-Earth L2 point. The mission can take advantage of this placement to wait for the discovery of a suitable comet reachable with its minimum ΔV capability of 600 ms-1. Comet Interceptor will be unique in encountering and studying, at a nominal closest approach distance of 1000 km, a comet that represents a near-pristine sample of material from the formation of the Solar System. It will also add a capability that no previous cometary mission has had, which is to deploy two sub-probes - B1, provided by the Japanese space agency, JAXA, and B2 - that will follow different trajectories through the coma. While the main probe passes at a nominal 1000 km distance, probes B1 and B2 will follow different chords through the coma at distances of 850 km and 400 km, respectively. The result will be unique, simultaneous, spatially resolved information of the 3-dimensional properties of the target comet and its interaction with the space environment. We present the mission's science background leading to these objectives, a

Journal article

Stephenson P, Beth A, Deca J, Galand M, Goetz C, Henri P, Heritier K, Lewis Z, Moeslinger A, Nilsson H, Rubin Met 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.

Journal article

Guilbert-Lepoutre A, Benseguane S, Martinien L, Lasue J, Besse S, Grieger B, Beth Aet al., 2023, Pits on Jupiter-family Comets and the Age of Cometary Surfaces, Planetary Science Journal, Vol: 4

Large and deep depressions, also known as pits, are observed at the surface of all Jupiter-family comets (JFCs) imaged by spacecraft missions. They offer the opportunity to glimpse the subsurface characteristics of comet nuclei and study the complex interplay between surface structures and cometary activity. This work investigates the evolution of pits at the surface of 81P/Wild 2, 9P/Tempel 1, and 103P/Hartley 2, in continuation of the work by Benseguane et al. on 67P/Churyumov-Gerasimenko. Pits are selected across the surface of each nucleus, and high-resolution shape models are used to compute the energy they receive. A thermal evolution model is applied to constrain how cometary activity sustained under current illumination conditions could modify them. Similar to what was found for 67P, we show that erosion resulting from water-driven activity is primarily controlled by seasonal patterns that are unique to each comet as a consequence of their shape and rotational properties. However, progressive erosion sustained after multiple perihelion passages is not able to carve any of the observed pits. Instead, cometary activity tends to erase sharp morphological features; they become wider and shallower over time. Our results reinforce the evolutionary sequence evidenced from independent measurables to transform “young” cometary surfaces, with sharp surface topography prone to outbursts, into “old” cometary surfaces. Finally, we suggest that the mechanism at the origin of the pits on JFCs should be able to carve these structures in a region of the solar system where water ice does not sublimate; the Centaur phase thus appears critical to understand JFC surface properties.

Journal article

Lewis ZM, Beth A, Altwegg K, Galand M, Goetz C, Heritier K, ORourke L, Rubin M, Stephenson Pet 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.

Journal article

Goetz C, Gunell H, Volwerk M, Beth A, Eriksson A, Galand M, Henri P, Nilsson H, Wedlund CS, Alho M, Andersson L, Andre N, De Keyser J, Deca J, Ge Y, Glassmeier K-H, Hajra R, Karlsson T, Kasahara S, Kolmasova I, LLera K, Madanian H, Mann I, Mazelle C, Odelstad E, Plaschke F, Rubin M, Sanchez-Cano B, Snodgrass C, Vigren Eet 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

Journal article

Benseguane S, Guilbert-Lepoutre A, Lasue J, Besse S, Leyrat C, Beth A, Costa Sitjà M, Grieger B, Capria MTet al., 2022, Evolution of pits at the surface of 67P/Churyumov-Gerasimenko, Astronomy and Astrophysics, Vol: 668, ISSN: 0004-6361

Context. The observation of pits at the surface of comets offers the opportunity to take a glimpse into the properties and the mechanisms that shape a nucleus through cometary activity. If the origin of these pits is still a matter of debate, multiple studies have recently suggested that known phase transitions (such as volatile sublimation or amorphous water ice crystallization) alone could not have carved these morphological features on the surface of 67P/Churyumov-Gerasimenko (hereafter 67P). Aims. We want to understand how the progressive modification of 67P' s surface due to cometary activity might have affected the characteristics of pits and alcoves. In particular, we aim to understand whether signatures of the formation mechanism of these surface morphological features can still be identified. Methods. To quantify the amount of erosion sustained at the surface of 67P since it arrived on its currently observed orbit, we selected 380 facets of a medium-resolution shape model of the nucleus, sampling 30 pits and alcoves across the surface. We computed the surface energy balance with a high temporal resolution, including shadowing and self-heating contributions. We then applied a thermal evolution model to assess the amount of erosion sustained after ten orbital revolutions under current illumination conditions. Results. We find that the maximum erosion sustained after ten orbital revolutions is on the order of 80 m, for facets located in the southern hemisphere. We thus confirm that progressive erosion cannot form pits and alcoves, as local erosion is much lower than their observed depth and diameter. We find that plateaus tend to erode more than bottoms, especially for the deepest depressions, and that some differential erosion can affect their morphology. As a general rule, our results suggest that sharp morphological features tend to be erased by progressive erosion. Conclusions. This study supports the assumption that deep circular pits, such as Seth_01, ar

Journal article

Yamauchi M, De Keyser J, Parks G, Oyama S-I, Wurz P, Abe T, Beth A, Daglis IA, Dandouras I, Dunlop M, Henri P, Ivchenko N, Kallio E, Kucharek H, Liu YC-M, Mann I, Marghitu O, Nicolaou G, Rong Z, Sakanoi T, Saur J, Shimoyama M, Taguchi S, Tian F, Tsuda T, Tsurutani B, Turner D, Ulich T, Yau A, Yoshikawa Iet al., 2022, Plasma-neutral gas interactions in various space environments: Assessment beyond simplified approximations as a Voyage 2050 theme, Experimental Astronomy: an international journal on astronomical instrumentation and data analysis, Vol: 54, Pages: 521-559, ISSN: 0922-6435

In the White Paper, submitted in response to the European Space Agency (ESA) Voyage 2050 Call, we present the importance of advancing our knowledge of plasma-neutral gas interactions, and of deepening our understanding of the partially ionized environments that are ubiquitous in the upper atmospheres of planets and moons, and elsewhere in space. In future space missions, the above task requires addressing the following fundamental questions: (A) How and by how much do plasma-neutral gas interactions influence the re-distribution of externally provided energy to the composing species? (B) How and by how much do plasma-neutral gas interactions contribute toward the growth of heavy complex molecules and biomolecules? Answering these questions is an absolute prerequisite for addressing the long-standing questions of atmospheric escape, the origin of biomolecules, and their role in the evolution of planets, moons, or comets, under the influence of energy sources in the form of electromagnetic and corpuscular radiation, because low-energy ion-neutral cross-sections in space cannot be reproduced quantitatively in laboratories for conditions of satisfying, particularly, (1) low-temperatures, (2) tenuous or strong gradients or layered media, and (3) in low-gravity plasma. Measurements with a minimum core instrument package (< 15 kg) can be used to perform such investigations in many different conditions and should be included in all deep-space missions. These investigations, if specific ranges of background parameters are considered, can also be pursued for Earth, Mars, and Venus.

Journal article

Goetz C, Behar E, Beth A, Bodewits D, Bromley S, Burch J, Deca J, Divin A, Eriksson AI, Feldman PD, Galand M, Gunell H, Henri P, Heritier K, Jones GH, Mandt KE, Nilsson H, Noonan JW, Odelstad E, Parker JW, Rubin M, Simon Wedlund C, Stephenson P, Taylor MGGT, Vigren E, Vines SK, Volwerk Met 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.

Journal article

Beth A, Galand M, Wedlund CS, Eriksson Aet al., 2022, Cometary Ionospheres: An Updated Tutorial

This chapter aims at providing the tools and knowledge to understand andmodel the plasma environment surrounding comets in the innermost part near thenucleus. In particular, our goal is to give an updated post-Rosetta view ofthis ionised environment: what we knew, what we confirmed, what we overturned,and what we still do not understand.

Book chapter

Beth A, Gunell H, Simon Wedlund C, Goetz C, Nilsson H, Hamrin Met al., 2022, First investigation of the diamagnetic cavity boundary layer with a 1D3V PIC simulation, Astronomy and Astrophysics: a European journal, Vol: 667, Pages: 1-16, ISSN: 0004-6361

Context. Amongst the different features and boundaries encountered around comets, one remains of particular interest to the plasma community: the diamagnetic cavity. Crossed for the first time at 1P/Halley during the Giotto flyby in 1986 and later met more than 700 times by the ESA Rosetta spacecraft around Comet 67P/Churyumov-Gerasimenko, this region, almost free of any magnetic field, surrounds nuclei of active comets. However, previous observations and modelling of this part of the coma have not yet provided a definitive answer as to the origin of such a cavity and on its border, the diamagnetic cavity boundary layer.Aims. We investigate which forces and equilibrium might be at play and balance the magnetic pressure at this boundary down to the spatial and temporal scales of the electrons in the 1D collisionless case. In addition, we scrutinise assumptions made in magneto-hydrodynamic and hybrid simulations of this environment and check for their validity.Methods. We simulated this region at the electron scale by means of 1D3V particle-in-cell simulations and SMILEI code.Results. Across this layer, depending on the magnetic field strength, the electric field is governed by different equilibria, with a thin double-layer forming ahead. In addition, we show that the electron distribution function departs from Maxwellian and/or gyrotropic distributions and that electrons do not behave adiabatically. We demonstrate the need to investigate this region at the electron scale in depth with fully kinetic simulations.

Journal article

Simon Wedlund CL, Volwerk M, Beth A, Mazelle C, Moestl C, Halekas JS, Gruesbeck JR, Rojas-Castillo Det al., 2022, A fast bow shock location predictor-estimator from 2D and 3D analytical models: Application to Mars and the MAVEN mission, Journal of Geophysical Research: Space Physics, ISSN: 2169-9380

Journal article

Bergman S, Wieser GS, Wieser M, Nilsson H, Vigren E, Beth A, Masunaga K, Eriksson Aet al., 2021, Flow directions of low-energy ions in and around the diamagnetic cavity of comet 67P, MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Vol: 507, Pages: 4900-4913, ISSN: 0035-8711

Journal article

Galand M, Feldman PD, Bockelee-Morvan D, Biver N, Cheng Y-C, Rinaldi G, Rubin M, Altwegg K, Deca J, Beth A, Stephenson P, Heritier KL, Henri P, Parker JW, Carr C, Eriksson AI, Burch Jet al., 2021, Far-ultraviolet aurora identified at comet 67P/Churyumov-Gerasimenko (vol 4, pg 1084, 2020), NATURE ASTRONOMY, ISSN: 2397-3366

Journal article

Stephenson P, Galand M, Feldman PD, Beth A, Rubin M, Bockelée-Morvan D, Biver N, -C Cheng Y, Parker J, Burch J, Johansson FL, Eriksson Aet 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

Journal article

Gunell H, Goetz C, Odelstad E, Beth A, Hamrin M, Henri P, Johansson FL, Nilsson H, Wieser GSet al., 2021, Ion acoustic waves near a comet nucleus: Rosetta observations at comet 67P/Churyumov-Gerasimenko, ANNALES GEOPHYSICAE, Vol: 39, Pages: 53-68, ISSN: 0992-7689

Journal article

Carnielli G, Galand M, Leblanc F, Modolo R, Beth A, Jia Xet 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.

Journal article

Beth A, Altwegg K, Balsiger H, Berthelier J-J, Combi MR, De Keyser J, Fiethe B, Fuselier SA, Galand M, Gombosi TI, Rubin M, Sémon Tet 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++.

Journal article

Galand M, Feldman PD, Bockelee-Morvan D, Biver N, Cheng Y-C, Rinaldi G, Rubin M, Altwegg K, Deca J, Beth A, Stephenson P, Heritier K, Henri P, Parker JW, Carr C, Eriksson AI, Burch Jet 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.

Journal article

Simon Wedlund C, Behar E, Nilsson H, Alho M, Kallio E, Gunell H, Bodewits D, Heritier K, Galand M, Beth A, Rubin M, Altwegg K, Volwerk M, Gronoff G, Hoekstra Ret 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

Journal article

Carnielli G, Galand M, Leblanc F, Modolo R, Beth A, Jia Xet 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.

Journal article

Wedlund Simon C, Behar E, Nilsson H, Alho M, Kallio E, Gunell H, Bodewits D, Heritier K, Galand M, Beth A, Rubin M, Altwegg K, Volwerk M, Gronoff G, Hoekstra Ret al., 2019, Solar wind charge exchange in cometary atmospheres III. Results from the Rosetta mission to comet 67P/Churyumov-Gerasimenko, Astronomy and Astrophysics, Vol: 630, ISSN: 0004-6361

Solar wind charge-changing reactions are of paramount importance to thephysico-chemistry of the atmosphere of a comet. The ESA/Rosetta mission tocomet 67P/Churyumov-Gerasimenko (67P) provides a unique opportunity to studycharge-changing processes in situ. To understand the role of these reactions inthe evolution of the solar wind plasma, and interpret the complex in-situmeasurements made by Rosetta, numerical or analytical models are necessary. Weuse an extended analytical formalism describing solar wind charge-changingprocesses at comets along solar wind streamlines. The model is driven by solarwind ion measurements from the Rosetta Plasma Consortium-Ion CompositionAnalyzer (RPC-ICA) and neutral density observations from the RosettaSpectrometer for Ion and Neutral Analysis-Comet Pressure Sensor (ROSINA-COPS),as well as charge-changing cross sections of hydrogen and helium particles in awater gas. A mission-wide overview of charge-changing efficiencies at comet 67Pis presented. Electron capture cross sections dominate and favor the productionof He and H energetic neutral atoms, with fluxes expected to rival those of H+and He2+ ions. Neutral outgassing rates are retrieved from local RPC-ICA fluxmeasurements, and match ROSINA's estimates very well. From the model, we findthat solar wind charge exchange is unable to fully explain the magnitude of thesharp drop of solar wind ion fluxes observed by Rosetta for heliocentricdistances below 2.5 AU. This is likely because the model does not take intoaccount the relative ion dynamics and, to a lesser extent, ignore the formationof bow shock-like structures upstream of the nucleus. This work also shows thatthe ionization by solar EUV radiation and energetic electrons dominates thesource of cometary ions, although solar wind contributions may be significantduring isolated events.

Journal article

Hoang M, Garnier P, Gourlaouen H, Lasue J, Reme H, Altwegg K, Balsiger H, Beth A, Calmonte U, Fiethe B, Galli A, Gasc S, Jaeckel A, Korth A, Le Roy L, Mall U, Rubin M, Semon T, Tzou C-Y, Waite JH, Wurz Pet al., 2019, Two years with comet 67P/Churyumov-Gerasimenko: H2O, CO2, and CO as seen by the ROSINA/RTOF instrument of Rosetta, Astronomy and Astrophysics: a European journal, Vol: 630, ISSN: 0004-6361

Journal article

Mandt KE, Eriksson A, Beth A, Galand M, Vigren Eet al., 2019, Influence of collisions on ion dynamics in the inner comae of four comets, ASTRONOMY & ASTROPHYSICS, Vol: 630, Pages: 1-8, ISSN: 1432-0746

Context. Collisions between cometary neutrals in the inner coma of a comet and cometary ions that have been picked up into the solar wind flow and return to the coma lead to the formation of a broad inner boundary known as a collisionopause. This boundary is produced by a combination of charge transfer and chemical reactions, both of which are important at the location of the collisionopause boundary. Four spacecraft measured ion densities and velocities in the inner region of comets, exploring the part of the coma where an ion-neutral collisionopause boundary is expected to form.Aims. The aims are to determine the dominant physics behind the formation of the ion-neutral collisionopause and to evaluate where this boundary has been observed by spacecraft.Methods. We evaluated observations from three spacecraft at four different comets to determine if a collisionopause boundary was observed based on the reported ion velocities. We compared the measured location of the ion-neutral collisionopause with measurements of the collision cross sections to evaluate whether chemistry or charge exchange are more important at the location where the collisionopause is observed.Results. Based on measurements of the cross sections for charge transfer and for chemical reactions, the boundary observed by Rosetta appears to be the location where chemistry becomes the more probable result of a collision between H2O and H2O+ than charge exchange. Comparisons with ion observations made by Deep Space 1 at 19P/Borrelly and Giotto at 1P/Halley and 26P/Grigg-Skjellerup show that similar boundaries were observed at 19P/Borrelly and 1P/Halley. The ion composition measurements made by Giotto at Halley confirm that chemistry becomes more important inside of this boundary and that electron-ion dissociative recombination is a driver for the reported ion pileup boundary.

Journal article

Wedlund CS, Behar E, Kallio E, Nilsson H, Alho M, Gunell H, Bodewits D, Beth A, Gronoft G, Hoekstra Ret al., 2019, Solar wind charge exchange in cometary atmospheres II. Analytical model, Astronomy and Astrophysics: a European journal, Vol: 630, ISSN: 0004-6361

Journal article

Wedlund CS, Bodewits D, Alho M, Hoekstra R, Behar E, Gronoff G, Gunell H, Nilsson H, Kallio E, Beth Aet al., 2019, Solar wind charge exchange in cometary atmospheres I. Charge-changing and ionization cross sections for He and H particles in H2O, Astronomy and Astrophysics: a European journal, Vol: 630, ISSN: 0004-6361

Journal article

Beth A, Galand M, Heritier K, 2019, Comparative study of photo-produced ionosphere in the close environment of comets, Astronomy & Astrophysics, Vol: 630, ISSN: 0004-6361

Context. The Giotto and Rosetta missions gave us the unique opportunity of probing the close environment of cometary ionospheres of 1P/Halley (1P) and 67P/Churyumov-Gerasimenko (67P). The plasma conditions encountered at these two comets were very different from each other, which mainly stem from the different heliocentric distances, which drive photoionization rates, and from the outgassing activities, which drive the neutral densities.Aims. We asses the relative contribution of different plasma processes that are ongoing in the inner coma: photoionization, transport, photoabsorption, and electron–ion dissociative recombination. The main goal is to identify which processes are at play to then quantitatively assess the ionospheric density.Methods. We provide a set of analytical formulas to describe the ionospheric number density profile for cometary environments that take into account some of these processes. We discuss the validity of each model in the context of the Rosetta and Giotto missions.Results. We show that transport is the dominant loss process at large cometocentric distances and low outgassing rates. Chemical plasma loss through e−-ion dissociative recombination matters around 67P near perihelion and at 1P during the Giotto flyby: its effects increase as the heliocentric distance decreases, that is, at higher outgassing activity and higher photoionization frequency. Photoabsorption is of importance for outgassing rates higher than 1028 s−1 and only close to the cometary nucleus, well below the location of both spacecraft. Finally, regardless of the processes we considered, the ion number density profile always follows a 1∕r law at large cometocentric distances.

Journal article

Luspay-Kuti A, Altwegg K, Berthelier JJ, Beth A, Dhooghe F, Fiethe B, Fuselier SA, Gombosi T, Hansen KC, Hassig M, Livadiotis G, Mall U, Mandt KE, Mousis O, Petrinec SM, Rubin M, Trattner KJ, Tzou C-Y, Wurz Pet al., 2019, Comparison of neutral outgassing of comet 67P/Churyumov-Gerasimenko inbound and outbound beyond 3 AU from ROSINA/DFMS, ASTRONOMY & ASTROPHYSICS, Vol: 630, ISSN: 1432-0746

Journal article

Carnielli G, Galand M, Leblanc F, Leclercq L, Modolo R, Beth A, Huybrighs HLF, Jia Xet al., 2019, First 3D test particle model of Ganymede's ionosphere, Icarus, Vol: 330, Pages: 42-59, ISSN: 0019-1035

We present the first three-dimensional multi-species ionospheric model for Ganymede, based on a test particle Monte Carlo approach. Inputs include the electromagnetic field configuration around the moon from the magnetospheric models developed by Leclercq et al. (2016) and by Jia et al. (2009), and the number density, bulk velocity and temperature distributions of the neutral exosphere simulated by Leblanc et al. (2017). According to our simulations, O2+ is the most abundant ion species, followed by O+, H2+ and H2O+. For O+ and O2+, the majority of ions produced impact the moon's surface, while for the other species the majority escapes Ganymede's magnetosphere. For all ion species, the escape occurs either in the direction of corotation of the Jovian plasma or through the Alfvén wings.To validate our model, the output of our simulations, performed under the Galileo G2 flyby conditions, are compared to the observations. These include the electron density derived by the plasma wave instrument (PWS), the ion energy spectrogram measured by the plasma analyzer (PLS) and the associated plasma moments (Frank et al., 1997a).On the one hand, the electron density found by our model is consistently underestimated throughout the flyby, being at least one order of magnitude lower compared to observations. We argue that the prime reason for this discrepancy comes from the exospheric density, which may be underestimated. On the other hand, we find a remarkably good agreement between the modeled ion energy spectrogram and that recorded by PLS, providing a validation of the test particle model. Finally, we compare the modeled plasma moments along the G2 flyby with those analyzed by Frank et al. (1997a). The data seems to be more consistent with an ionosphere dominated by O2+ instead of H+ or O+, as suggested previously in the literature. This supports our finding that O2+ is the dominant ion species close to the surface.

Journal article

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