Imperial College London

DrMarinaGaland

Faculty of Natural SciencesDepartment of Physics

Reader in Planetary Science
 
 
 
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Huxley BuildingSouth Kensington Campus

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Summary

 

Publications

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117 results found

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

© 2019 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, O 2+ is the most abundant ion species, followed by O + , H 2+ and H 2 O + . For O + and O 2+ , 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 O 2+ instead of H + or O + , as suggested previously in the literature. This supports our finding that O 2+ is the dominant ion species close to the surface.

JOURNAL ARTICLE

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

JOURNAL ARTICLE

Tinetti G, Drossart P, Eccleston P, Hartogh P, Heske A, Leconte J, Micela G, Ollivier M, Pilbratt G, Puig L, Turrini D, Vandenbussche B, Wolkenberg P, Beaulieu J-P, Buchave LA, Ferus M, Griffin M, Guedel M, Justtanont K, Lagage P-O, Machado P, Malaguti G, Min M, Norgaard-Nielsen HU, Rataj M, Ray T, Ribas I, Swain M, Szabo R, Werner S, Barstow J, Burleigh M, Cho J, du Foresto VC, Coustenis A, Decin L, Encrenaz T, Galand M, Gillon M, Helled R, Carlos Morales J, Munoz AG, Moneti A, Pagano I, Pascale E, Piccioni G, Pinfield D, Sarkar S, Selsis F, Tennyson J, Triaud A, Venot O, Waldmann I, Waltham D, Wright G, Amiaux J, Augueres J-L, Berthe M, Bezawada N, Bishop G, Bowles N, Coffey D, Colome J, Crook M, Crouzet P-E, Da Peppo V, Sanz IE, Focardi M, Frericks M, Hunt T, Kohley R, Middleton K, Morgante G, Ottensamer R, Pace E, Pearson C, Stamper R, Symonds K, Rengel M, Renotte E, Ade P, Affer L, Alard C, Allard N, Altieri F, Andre Y, Arena C, Argyriou I, Aylward A, Baccani C, Bakos G, Banaszkiewicz M, Barlow M, Batista V, Bellucci G, Benatti S, Bernardi P, Bezard B, Blecka M, Bolmont E, Bonfond B, Bonito R, Bonomo AS, Brucato JR, Brun AS, Bryson I, Bujwan W, Casewell S, Charnay B, Pestellini CC, Chen G, Ciaravella A, Claudi R, Cledassou R, Damasso M, Damiano M, Danielski C, Deroo P, Di Giorgio AM, Dominik C, Doublier V, Doyle S, Doyon R, Drummond B, Duong B, Eales S, Edwards B, Farina M, Flaccomio E, Fletcher L, Forget F, Fossey S, Fraenz M, Fujii Y, Garcia-Piquer A, Gear W, Geoffray H, Gerard JC, Gesa L, Gomez H, Graczyk R, Griffith C, Grodent D, Guarcello MG, Gustin J, Hamano K, Hargrave P, Hello Y, Heng K, Herrero E, Hornstrup A, Hubert B, Ida S, Ikoma M, Iro N, Irwin P, Jarchow C, Jaubert J, Jones H, Julien Q, Kameda S, Kerschbaum F, Kervella P, Koskinen T, Krijger M, Krupp N, Lafarga M, Landini F, Lellouch E, Leto G, Luntzer A, Rank-Luftinger T, Maggio A, Maldonado J, Maillard J-P, Mall U, Marquette J-B, Mathis S, Maxted P, Matsuo T, Medvedev A, Miguel Y, Minier V, Moreet al., 2018, A chemical survey of exoplanets with ARIEL, Experimental Astronomy, Vol: 46, Pages: 135-209, ISSN: 0922-6435

Thousands of exoplanets have now been discovered with a huge range of masses, sizes and orbits: from rocky Earth-like planets to large gas giants grazing the surface of their host star. However, the essential nature of these exoplanets remains largely mysterious: there is no known, discernible pattern linking the presence, size, or orbital parameters of a planet to the nature of its parent star. We have little idea whether the chemistry of a planet is linked to its formation environment, or whether the type of host star drives the physics and chemistry of the planet’s birth, and evolution. ARIEL was conceived to observe a large number (~1000) of transiting planets for statistical understanding, including gas giants, Neptunes, super-Earths and Earth-size planets around a range of host star types using transit spectroscopy in the 1.25–7.8 μm spectral range and multiple narrow-band photometry in the optical. ARIEL will focus on warm and hot planets to take advantage of their well-mixed atmospheres which should show minimal condensation and sequestration of high-Z materials compared to their colder Solar System siblings. Said warm and hot atmospheres are expected to be more representative of the planetary bulk composition. Observations of these warm/hot exoplanets, and in particular of their elemental composition (especially C, O, N, S, Si), will allow the understanding of the early stages of planetary and atmospheric formation during the nebular phase and the following few million years. ARIEL will thus provide a representative picture of the chemical nature of the exoplanets and relate this directly to the type and chemical environment of the host star. ARIEL is designed as a dedicated survey mission for combined-light spectroscopy, capable of observing a large and well-defined planet sample within its 4-year mission lifetime. Transit, eclipse and phase-curve spectroscopy methods, whereby the signal from the star and planet are differentiated using know

JOURNAL ARTICLE

Hajra R, Henri P, Myllys M, Heritier KL, Galand M, Wedlund CS, Breuillard H, Behar E, Edberg NJT, Goetz C, Nilsson H, Eriksson AI, Goldstein R, Tsurutani BT, More J, Vallieres X, Wattieauxu Get al., 2018, Cometary plasma response to interplanetary corotating interaction regions during 2016 June-September: a quantitative study by the Rosetta Plasma Consortium, Monthly Notices of the Royal Astronomical Society, Vol: 480, Pages: 4544-4556, ISSN: 0035-8711

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

JOURNAL ARTICLE

Heritier KL, Altwegg K, Berthelier J-J, Beth A, Carr CM, De Keyser J, Eriksson AI, Fuselier SA, Galand M, Gombosi TI, Henri P, Johansson FL, Nilsson H, Rubin M, Wedlund CS, Taylor MGGT, Vigren Eet al., 2018, On the origin of molecular oxygen in cometary comae, NATURE COMMUNICATIONS, Vol: 9, ISSN: 2041-1723

JOURNAL ARTICLE

Heritier K, Galand M, Henri P, Johansson FL, Beth A, Eriksson AI, Vallières X, Altwegg K, Burch JL, Carr C, Ducrot E, Hajra R, Rubin Met al., 2018, Plasma source and loss at comet 67P during the Rosetta mission, Astronomy and Astrophysics, ISSN: 0004-6361

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

JOURNAL ARTICLE

Moore L, Galand M, Kliore AJ, Nagy AF, O'Donoghue Jet al., 2018, Saturn's Ionosphere, Saturn in the 21st Century, Editors: Baines, Publisher: Cambridge University Press

This chapter summarizes our current understanding of the ionosphere ofSaturn. We give an overview of Saturn ionospheric science from the Voyager erato the present, with a focus on the wealth of new data and discoveries enabledby Cassini, including a massive increase in the number of electron densityaltitude profiles. We discuss recent ground-based detection of the effect of"ring rain" on Saturn's ionosphere, and present possible model interpretationsof the observations. Finally, we outline current model-data discrepancies andindicate how future observations can help in advancing our understanding of thevarious controlling physical and chemical processes.

BOOK CHAPTER

Beth A, Galand MIF, 2018, Effects of the convective field on weakly outgassing comets., Monthly Notices of the Royal Astronomical Society, Vol: 469, Pages: S824-S841, ISSN: 0035-8711

By applying a kinetic approach, we have developed two models in order to assess the influence of one main driver of plasma acceleration, the convective electric field, on the cometary ion distribution at 67P/Churyumov-Gerasimenko (67P/C-G). This electric field is carried by the solar wind and corresponds to the acceleration undergone by cometary ions ultimately picked up. We have quantified its contribution on ion number density and mean velocity profiles, supported by an intercomparison with the recent literature. We found that the ion number density should reflect a departure from the observed ∼1/r law. We discuss reasons for this discrepancy.

JOURNAL ARTICLE

Chadney JM, Koskinen TT, Galand M, Unruh YC, Sanz-Forcada Jet al., 2017, Effect of stellar flares on the upper atmospheres of HD 189733b and HD 209458b, Astronomy and Astrophysics, Vol: 608, ISSN: 0004-6361

Stellar flares are a frequent occurrence on young low-mass stars around whichmany detected exoplanets orbit. Flares are energetic, impulsive events, andtheir impact on exoplanetary atmospheres needs to be taken into account wheninterpreting transit observations. We have developed a model to describe theupper atmosphere of Extrasolar Giant Planets (EGPs) orbiting flaring stars. Themodel simulates thermal escape from the upper atmospheres of close-in EGPs.Ionisation by solar radiation and electron impact is included and photochemicaland diffusive transport processes are simulated. This model is used to studythe effect of stellar flares from the solar-like G star HD209458 and the youngK star HD189733 on their respective planets. A hypothetical HD209458b-likeplanet orbiting the active M star AU Mic is also simulated. We find that theneutral upper atmosphere of EGPs is not significantly affected by typicalflares. Therefore, stellar flares alone would not cause large enough changes inplanetary mass loss to explain the variations in HD189733b transit depth seenin previous studies, although we show that it may be possible that an extremestellar proton event could result in the required mass loss. Our simulations dohowever reveal an enhancement in electron number density in the ionosphere ofthese planets, the peak of which is located in the layer where stellar X-raysare absorbed. Electron densities are found to reach 2.2 to 3.5 times pre-flarelevels and enhanced electron densities last from about 3 to 10 hours after theonset of the flare. The strength of the flare and the width of its spectralenergy distribution affect the range of altitudes that see enhancements inionisation. A large broadband continuum component in the XUV portion of theflaring spectrum in very young flare stars, such as AU Mic, results in a broadrange of altitudes affected in planets orbiting this star.

JOURNAL ARTICLE

Hajra R, Henri P, Vallières X, Galand M, Héritier K, Eriksson AI, Odelstad E, Edberg NJT, Burch JL, Broiles T, Goldstein R, Glassmeier KH, Richter I, Goetz C, Tsurutani BT, Nilsson H, Altwegg K, Rubin Met al., 2017, Impact of a cometary outburst on its ionosphere: Rosetta Plasma Consortium observations of the outburst exhibited by comet 67P/Churyumov-Gerasimenko on 19 February 2016, Astronomy and Astrophysics, Vol: 607, ISSN: 0004-6361

© ESO, 2017. We present a detailed study of the cometary ionospheric response to a cometary brightness outburst using in situ measurements for the first time. The comet 67P/Churyumov-Gerasimenko (67P) at a heliocentric distance of 2.4 AU from the Sun, exhibited an outburst at ∼1000 UT on 19 February 2016, characterized by an increase in the coma surface brightness of two orders of magnitude. The Rosetta spacecraft monitored the plasma environment of 67P from a distance of 30 km, orbiting with a relative speed of ∼0.2 m s-1. The onset of the outburst was preceded by pre-outburst decreases in neutral gas density at Rosetta, in local plasma density, and in negative spacecraft potential at ∼0950 UT. In response to the outburst, the neutral density increased by a factor of ∼1.8 and the local plasma density increased by a factor of ∼3, driving the spacecraft potential more negative. The energetic electrons (tens of eV) exhibited decreases in the flux of factors of ∼2 to 9, depending on the energy of the electrons. The local magnetic field exhibited a slight increase in amplitude (~5 nT) and an abrupt rotation (∼36.4°) in response to the outburst. A weakening of 10-100 mHz magnetic field fluctuations was also noted during the outburst, suggesting alteration of the origin of the wave activity by the outburst. The plasma and magnetic field effects lasted for about 4 h, from ∼1000 UT to 1400 UT. The plasma densities are compared with an ionospheric model. This shows that while photoionization is the main source of electrons, electron-impact ionization and a reduction in the ion outflow velocity need to be accounted for in order to explain the plasma density enhancement near the outburst peak.

JOURNAL ARTICLE

Mendillo M, Narvaez C, Vogt MF, Mayyasi M, Forbes J, Galand M, Thiemann E, Benna M, Eparvier F, Chamberlin P, Mahaffy P, Andersson Let al., 2017, Sources of ionospheric variability at Mars, Journal of Geophysical Research: Space Physics, Vol: 122, Pages: 9670-9684, ISSN: 2169-9380

During the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission's deep-dip #2 campaign of 17–22 April 2015, spacecraft instruments observed all of the physical parameters needed to assess the photo-chemical-equilibrium (PCE) explanation for ionospheric variability at a fixed altitude (135 km) near the peak of the Martian ionosphere. MAVEN measurements of electron density, electron temperature, neutral CO2 density, and solar irradiance were collected during 28 orbits. When inserted into the PCE equation, the measurements of varying PCE drivers correlated with the observed electron density variations to within instrumental uncertainty levels. The dominant source of this positive correlation was the variability of CO2 densities associated with the longitudinal wave-2 component of nonmigrating tides in the Martian thermosphere.

JOURNAL ARTICLE

Eriksson AI, Engelhardt IAD, Andre M, Bostrom R, Edberg NJT, Johansson FL, Odelstad E, Vigren E, Wahlund J-E, Henri P, Lebreton J-P, Miloch WJ, Paulsson JJP, Wedlund CS, Yang L, Karlsson T, Jarvinen R, Broiles T, Mandt K, Carr CM, Galand M, Nilsson H, Norberg Cet al., 2017, Cold and warm electrons at comet 67P/Churyumov-Gerasimenko, Astronomy and Astrophysics, Vol: 605, ISSN: 0004-6361

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

JOURNAL ARTICLE

Heritier KL, Altwegg K, Balsiger H, Berthelier J-J, Beth A, Bieler A, Biver N, Calmonte U, Combi MR, De Keyser J, Eriksson AI, Fiethe B, Fougere N, Fuselier SA, Galand M, Gasc S, Gombosi TI, Hansen KC, Hassig M, Kopp E, Odelstad E, Rubin M, Tzou C-Y, Vigren E, Vuitton Vet al., 2017, Ion composition at comet 67P near perihelion: Rosetta observations and model-based interpretation, Monthly Notices of the Royal Astronomical Society, Vol: 469, Pages: S427-S442, ISSN: 0035-8711

We present the ion composition in the coma of comet 67P with newly detected ion species over the 28–37 u mass range, probed by Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA)/Double Focusing Mass Spectrometer (DFMS). In summer 2015, the nucleus reached its highest outgassing rate and ion-neutral reactions started to take place at low cometocentric distances. Minor neutrals can efficiently capture protons from the ion population, making the protonated version of these neutrals a major ion species. So far, onlyNH+4has been reported at comet 67P. However, there are additional neutral species with proton affinities higher than that of water (besides NH3) that have been detected in the coma of comet 67P: CH3OH, HCN, H2CO and H2S. Their protonated versions have all been detected. Statistics showing the number of detections with respect to the number of scans are presented. The effect of the negative spacecraft potential probed by the Rosetta Plasma Consortium/LAngmuir Probe on ion detection is assessed. An ionospheric model has been developed to assess the different ion density profiles and compare them to the ROSINA/DFMS measurements. It is also used to interpret the ROSINA/DFMS observations when different ion species have similar masses, and their respective densities are not high enough to disentangle them using the ROSINA/DFMS high-resolution mode. The different ion species that have been reported in the coma of 67P are summarized and compared with the ions detected at comet 1P/Halley during the Giotto mission.

JOURNAL ARTICLE

Hajra R, Henri P, Vallières X, Galand MIF, Heritier K, Eriksson E, Odelstad Eet al., 2017, Impact of a cometary outburst on its ionosphere: Rosetta Plasma Consortium observations of the comet 67P/Churyumov-Gerasimenko outburst on 19 February 2016, Astronomy & Astrophysics, Vol: 607, ISSN: 1432-0746

We present a detailed study of the cometary ionospheric response to a cometary brightness out-burst using in-situ measurements for the first time. The comet 67P/ Churyumov-Gerasimenko (67P) at a heliocentric distance of 2.4 AU from the Sun, exhibited an outburst at ∼1000 UT on 19February 2016, characterized by two orders of magnitude increase in the coma surface brightness.The Rosetta spacecraft monitored the plasma environment of 67P from a distance of 30 km, orbiting with a relative speed of 0.2 m s -1 The onset of the outburst was preceded by “pre-outburstdecreases” in neutral gas density at Rosetta, in local plasma density and in negative spacecraft potential at ∼0950 UT. In response to the outbust, the neutral density increased by a factor of ∼1.8, the local plasma density increased by a factor of ∼3, driving the spacecraft potential morenegative. The energetic (10s of eV) electrons exhibited decreases in the flux by factors of ∼2 to 9 depending on the energy of the electrons. The local magnetic field exhibited a slight increase (∼5 nT) in amplitude and an abrupt rotation (∼36.4) in response to the outburst. A weakening of 0–100 mHz magnetic field fluctuations was also noted during the outburst, suggesting alteration of the origin of the wave activity by the outburst. The plasma and magnetic field effects lasted forabout 4 h, from ∼1000 UT to 1400 UT. The plasma densities are compared with an ionospheric model. This shows that while photoionization is the main source of electrons, electron-impactionization and a reduction in the ion outflow velocity need to be accounted for in order to explain the plasma density enhancement near the outburst peak.

JOURNAL ARTICLE

Nilsson H, Wieser GS, Behar E, Gunell H, Wieser M, Galand M, Wedlund CS, Alho M, Goetz C, Yamauchi M, Henri P, Odelstad E, Vigren Eet al., 2017, Erratum: Evolution of the ion environment of comet 67P during the Rosetta mission as seen by RPC-ICA, Monthly Notices of the Royal Astronomical Society, Vol: 469, Pages: S804-S804, ISSN: 0035-8711

JOURNAL ARTICLE

Henri P, Vallières X, Hajra R, Goetz C, Richter I, Glassmeier K-H, Galand M, Rubin M, Eriksson AI, Nemeth Z, Vigren E, Beth A, Burch JL, Carr C, Nilsson H, Tsurutani B, Wattieaux Get al., 2017, Diamagnetic region(s): structure of the unmagnetized plasma around Comet 67P/CG, Monthly Notices of the Royal Astronomical Society, Vol: 469, Pages: S372-S379, ISSN: 0035-8711

The ESA’s comet chaser Rosetta has monitored the evolution of the ionized atmosphere of comet 67P/Churyumov–Gerasimenko (67P/CG) and its interaction with the solar wind, during more than 2 yr. Around perihelion, while the cometary outgassing rate was highest, Rosetta crossed hundreds of unmagnetized regions, but did not seem to have crossed a large-scale diamagnetic cavity as anticipated. Using in situ Rosetta observations, we characterize the structure of the unmagnetized plasma found around comet 67P/CG. Plasma density measurements from RPC-MIP are analysed in the unmagnetized regions identified with RPC-MAG. The plasma observations are discussed in the context of the cometary escaping neutral atmosphere, observed by ROSINA/COPS. The plasma density in the different diamagnetic regions crossed by Rosetta ranges from ∼100 to ∼1500 cm−3. They exhibit a remarkably systematic behaviour that essentially depends on the comet activity and the cometary ionosphere expansion. An effective total ionization frequency is obtained from in situ observations during the high outgassing activity phase of comet 67P/CG. Although several diamagnetic regions have been crossed over a large range of distances to the comet nucleus (from 50 to 400 km) and to the Sun (1.25–2.4 au), in situ observations give strong evidence for a single diamagnetic region, located close to the electron exobase. Moreover, the observations are consistent with an unstable contact surface that can locally extend up to about 10 times the electron exobase.

JOURNAL ARTICLE

Nilsson H, Wieser GS, Behar E, Gunell H, Wieser M, Galand M, Simon Wedlund C, Alho M, Goetz C, Yamauchi M, Henri P, Odelstad E, Vigren Eet al., 2017, Evolution of the ion environment of comet 67P during the Rosetta mission as seen by RPC-ICA, Monthly Notices of the Royal Astronomical Society, Vol: 469, Pages: S252-S261, ISSN: 0035-8711

Rosetta has followed comet 67P from low activity at more than 3.6 au heliocentric distance to high activity at perihelion (1.24 au) and then out again. We provide a general overview of the evolution of the dynamic ion environment using data from the RPC-ICA ion spectrometer. We discuss where Rosetta was located within the evolving comet magnetosphere. For the initial observations, the solar wind permeated all of the coma. In 2015 mid-April, the solar wind started to disappear from the observation region, to re-appear again in 2015 December. Low-energy cometary ions were seen at first when Rosetta was about 100 km from the nucleus at 3.6 au, and soon after consistently throughout the mission except during the excursions to farther distances from the comet. The observed flux of low-energy ions was relatively constant due to Rosetta's orbit changing with comet activity. Accelerated cometary ions, moving mainly in the antisunward direction gradually became more common as comet activity increased. These accelerated cometary ions kept being observed also after the solar wind disappeared from the location of Rosetta, with somewhat higher fluxes further away from the nucleus. Around perihelion, when Rosetta was relatively deep within the comet magnetosphere, the fluxes of accelerated cometary ions decreased, as did their maximum energy. The disappearance of more energetic cometary ions at close distance during high activity is suggested to be due to a flow pattern where these ions flow around the obstacle of the denser coma or due to charge exchange losses.

JOURNAL ARTICLE

Vigren E, André M, Edberg NJT, Engelhardt IAD, Eriksson AI, Galand M, Goetz C, Henri P, Heritier K, Johansson FL, Nilsson H, Odelstad E, Rubin M, Stenberg-Wieser G, Tzou C-Y, Vallières Xet al., 2017, Effective ion speeds at ∼200–250 km from comet 67P/Churyumov–Gerasimenko near perihelion, Monthly Notices of the Royal Astronomical Society, Vol: 469, Pages: S142-S148, ISSN: 0035-8711

In 2015 August, comet 67P/Churyumov–Gerasimenko, the target comet of the ESA Rosetta mission, reached its perihelion at ∼1.24 au. Here, we estimate for a three-day period near perihelion, effective ion speeds at distances ∼200–250 km from the nucleus. We utilize two different methods combining measurements from the Rosetta Plasma Consortium (RPC)/Mutual Impedance Probe with measurements either from the RPC/Langmuir Probe or from the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA)/Comet Pressure Sensor (COPS) (the latter method can only be applied to estimate the effective ion drift speed). The obtained ion speeds, typically in the range 2–8 km s−1, are markedly higher than the expected neutral outflow velocity of ∼1 km s−1. This indicates that the ions were de-coupled from the neutrals before reaching the spacecraft location and that they had undergone acceleration along electric fields, not necessarily limited to acceleration along ambipolar electric fields in the radial direction. For the limited time period studied, we see indications that at increasing distances from the nucleus, the fraction of the ions’ kinetic energy associated with radial drift motion is decreasing.

JOURNAL ARTICLE

Heritier KL, Henri P, Vallières X, Galand M, Odelstad E, Eriksson AI, Johansson FL, Altwegg K, Behar E, Beth A, Broiles TW, Burch JL, Carr CM, Cupido E, Nilsson H, Rubin M, Vigren Eet al., 2017, Vertical structure of the near-surface expanding ionosphere of comet 67P probed by Rosetta, Monthly Notices of the Royal Astronomical Society, Vol: 469, Pages: S118-S129, ISSN: 0035-8711

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

JOURNAL ARTICLE

Beth A, Altwegg K, Balsiger H, Berthelier J-J, Calmonte U, Combi MR, De Keyser J, Dhooghe F, Fiethe B, Fuselier SA, Galand M, Gasc S, Gombosi TI, Hansen KC, Hassig M, Heritier KL, Kopp E, Le Roy L, Mandt KE, Peroy S, Rubin M, Semon T, Tzou C-Y, Vigren Eet al., 2017, First in situ detection of the cometary ammonium ion NH4+ (protonated ammonia NH3) in the coma of 67P/C-G near perihelion, Monthly Notices of the Royal Astronomical Society, Vol: 462, Pages: S562-S572, ISSN: 0035-8711

In this paper, we report the first in situ detection of the ammonium ion NH+44+ at 67P/Churyumov–Gerasimenko (67P/C-G) in a cometary coma, using the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA)/Double Focusing Mass Spectrometer (DFMS). Unlike neutral and ion spectrometers onboard previous cometary missions, the ROSINA/DFMS spectrometer, when operated in ion mode, offers the capability to distinguish NH+44+ from H2O+ in a cometary coma. We present here the ion data analysis of mass-to-charge ratios 18 and 19 at high spectral resolution and compare the results with an ionospheric model to put these results into context. The model confirms that the ammonium ion NH+44+ is one of the most abundant ion species, as predicted, in the coma near perihelion.

JOURNAL ARTICLE

Vigren E, Altwegg K, Edberg NJT, Eriksson AI, Galand M, Henri P, Johansson F, Odelstad E, Tzou C-Y, Vallieres Xet al., 2017, Erratum: “Model–observation comparisons of electron number densities in the coma of 67P/Churyumov-Gerasimenko during 2015 January” (2016, AJ, 152, 59), Astronomical Journal, Vol: 153, Pages: 50-50, ISSN: 0004-6256

JOURNAL ARTICLE

Galand M, Héritier KL, Odelstad E, Henri P, Broiles TW, Allen AJ, Altwegg K, Beth A, Burch JL, Carr CM, Cupido E, Eriksson AI, Glassmeier K-H, Johansson FL, Lebreton J-P, Mandt KE, Nilsson H, Richter I, Rubin M, Sagnières LBM, Schwartz SJ, Sémon T, Tzou C-Y, Vallières X, Vigren E, Wurz Pet al., 2016, Ionospheric plasma of comet 67P probed by Rosetta at 3 AU from the Sun, Monthly Notices of the Royal Astronomical Society, Vol: 462, Pages: S331-S351, ISSN: 1365-2966

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

JOURNAL ARTICLE

Fuselier SA, Altwegg K, Balsiger H, Berthelier JJ, Beth A, Bieler A, Briois C, Broiles TW, Burch JL, Calmonte U, Cessateur G, Combi M, De Keyser J, Fiethe B, Galand M, Gasc S, Gombosi TI, Gunell H, Hansen KC, Hässig M, Heritier KL, Korth A, Le Roy L, Luspay-Kuti A, Mall U, Mandt KE, Petrinec SM, Rème H, Rinaldi M, Rubin M, Sémon T, Trattner KJ, Tzou C-Y, Vigren E, Waite JH, Wurz Pet al., 2016, Ion chemistry in the coma of comet 67P near perihelion, Monthly Notices of the Royal Astronomical Society, Vol: 462, Pages: S67-S77, ISSN: 1365-2966

The coma and the comet–solar wind interaction of comet 67P/Churyumov–Gerasimenko changed dramatically from the initial Rosetta spacecraft encounter in 2014 August through perihelion in 2015 August. Just before equinox (at 1.6 au from the Sun), the solar wind signal disappeared and two regions of different cometary ion characteristics were observed. These ‘outer’ and ‘inner’ regions have cometary ion characteristics similar to outside and inside the ion pileup region observed during the Giotto approach to comet 1P/Halley. Rosetta/Double-Focusing Mass Spectrometer ion mass spectrometer observations are used here to investigate the H3O+/H2O+ ratio in the outer and inner regions at 67P/ Churyumov–Gerasimenko. The H3O+/H2O+ ratio and the H3O+ signal are observed to increase in the transition from the outer to the inner region and the H3O+ signal appears to be weakly correlated with cometary ion energy. These ion composition changes are similar to the ones observed during the 1P/Halley flyby. Modelling is used to determine the importance of neutral composition and transport of neutrals and ions away from the nucleus. This modelling demonstrates that changes in the H3O+/H2O+ ratio appear to be driven largely by transport properties and only weakly by neutral composition in the coma.

JOURNAL ARTICLE

Grün E, Agarwal J, Altobelli N, Altwegg K, Bentley MS, Biver N, Della Corte V, Edberg N, Feldman PD, Galand M, Geiger B, Götz C, Grieger B, Güttler C, Henri P, Hofstadter M, Horanyi M, Jehin E, Krüger H, Lee S, Mannel T, Morales E, Mousis O, Müller M, Opitom C, Rotundi A, Schmied R, Schmidt F, Sierks H, Snodgrass C, Soja RH, Sommer M, Srama R, Tzou C-Y, Vincent J-B, Yanamandra-Fisher P, A'Hearn MF, Erikson AI, Barbieri C, Barucci MA, Bertaux J-L, Bertini I, Burch J, Colangeli L, Cremonese G, Da Deppo V, Davidsson B, Debei S, De Cecco M, Deller J, Feaga LM, Ferrari M, Fornasier S, Fulle M, Gicquel A, Gillon M, Green SF, Groussin O, Gutiérrez PJ, Hofmann M, Hviid SF, Ip W-H, Ivanovski S, Jorda L, Keller HU, Knight MM, Knollenberg J, Koschny D, Kramm J-R, Kührt E, Küppers M, Lamy PL, Lara LM, Lazzarin M, Lòpez-Moreno JJ, Manfroid J, Epifani EM, Marzari F, Naletto G, Oklay N, Palumbo P, Parker JW, Rickman H, Rodrigo R, Rodrìguez J, Schindhelm E, Shi X, Sordini R, Steffl AJ, Stern SA, Thomas N, Tubiana C, Weaver HA, Weissman P, Zakharov VV, Taylor MGGTet al., 2016, The 2016 Feb 19 outburst of comet 67P/CG: an ESA Rosetta multi-instrument study, Monthly Notices of the Royal Astronomical Society, Vol: 462, Pages: S220-S234, ISSN: 1365-2966

On 2016 Feb 19, nine Rosetta instruments serendipitously observed an outburst of gas and dust from the nucleus of comet 67P/Churyumov-Gerasimenko. Among these instruments were cameras and spectrometers ranging from UV over visible to microwave wavelengths, in situ gas, dust and plasma instruments, and one dust collector. At 09:40 a dust cloud developed at the edge of an image in the shadowed region of the nucleus. Over the next two hours the instruments recorded a signature of the outburst that significantly exceeded the background. The enhancement ranged from 50 per cent of the neutral gas density at Rosetta to factors >100 of the brightness of the coma near the nucleus. Dust related phenomena (dust counts or brightness due to illuminated dust) showed the strongest enhancements (factors >10). However, even the electron density at Rosetta increased by a factor 3 and consequently the spacecraft potential changed from ∼−16 V to −20 V during the outburst. A clear sequence of events was observed at the distance of Rosetta (34 km from the nucleus): within 15 min the Star Tracker camera detected fast particles (∼25 m s−1) while 100 μm radius particles were detected by the GIADA dust instrument ∼1 h later at a speed of 6 m s−1. The slowest were individual mm to cm sized grains observed by the OSIRIS cameras. Although the outburst originated just outside the FOV of the instruments, the source region and the magnitude of the outburst could be determined.

JOURNAL ARTICLE

Vigren E, Altwegg K, Edberg NJT, Eriksson AI, Galand M, Henri P, Johansson F, Odelstad E, Tzou C-Y, Valliéres Xet al., 2016, MODEL-OBSERVATION COMPARISONS OF ELECTRON NUMBER DENSITIES IN THE COMA OF 67P/CHURYUMOV–GERASIMENKO DURING 2015 JANUARY, Astronomical Journal, Vol: 152, ISSN: 1538-3881

During 2015 January 9–11, at a heliocentric distance of ~2.58–2.57 au, the ESA Rosetta spacecraft resided at a cometocentric distance of ~28 km from the nucleus of comet 67P/Churyumov–Gerasimenko, sweeping the terminator at northern latitudes of 43°N–58°N. Measurements by the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis/Comet Pressure Sensor (ROSINA/COPS) provided neutral number densities. We have computed modeled electron number densities using the neutral number densities as input into a Field Free Chemistry Free model, assuming H2O dominance and ion-electron pair formation by photoionization only. A good agreement (typically within 25%) is found between the modeled electron number densities and those observed from measurements by the Mutual Impedance Probe (RPC/MIP) and the Langmuir Probe (RPC/LAP), both being subsystems of the Rosetta Plasma Consortium. This indicates that ions along the nucleus-spacecraft line were strongly coupled to the neutrals, moving radially outward with about the same speed. Such a statement, we propose, can be further tested by observations of H3O+/H2O+ number density ratios and associated comparisons with model results.

JOURNAL ARTICLE

Vigren E, Galand M, Wellbrock A, Coates AJ, Cui J, Edberg NJT, Lavvas P, Sagnieres L, Snowden D, Vuitton V, Wahlund J-Eet al., 2016, Suprathermal electrons in Titan's sunlit ionosphere: model-observation comparisons, Astrophysical Journal, Vol: 826, ISSN: 1538-4357

JOURNAL ARTICLE

Mandt KE, Eriksson A, Edberg NJT, Koenders C, Broiles T, Fuselier SA, Henri P, Nemeth Z, Alho M, Biver N, Beth A, Burch J, Carr CM, Chae K, Coates AJ, Cupido E, Galand M, Glassmeier K-H, Goetz C, Goldstein R, Hansen KC, Haiducek J, Kallio E, Lebreton J-P, Luspay-Kuti A, Mokashi P, Nilsson H, Opitz A, Richter I, Samara M, Szego K, Tzou C-Y, Volwerk M, Simon Wedlund C, Stenberg Wieser Get al., 2016, RPC observation of the development and evolution of plasma interaction boundaries at 67P/ChuryumovGerasimenko, Monthly Notices of the Royal Astronomical Society, Vol: 462, Pages: S9-S22, ISSN: 1365-2966

One of the primary objectives of the Rosetta Plasma Consortium, a suite of five plasma instruments on-board the Rosetta spacecraft, is to observe the formation and evolution of plasma interaction regions at the comet 67P/Churyumov-Gerasimenko (67P/CG). Observations made between 2015 April and 2016 February show that solar wind–cometary plasma interaction boundaries and regions formed around 2015 mid-April and lasted through early 2016 January. At least two regions were observed, separated by an ion-neutral collisionopause boundary. The inner region was located on the nucleus side of the boundary and was characterized by low-energy water-group ions, reduced magnetic field pileup and enhanced electron densities. The outer region was located outside of the boundary and was characterized by reduced electron densities, water-group ions that are accelerated to energies above 100 eV and enhanced magnetic field pileup compared to the inner region. The boundary discussed here is outside of the diamagnetic cavity and shows characteristics similar to observations made on-board the Giotto spacecraft in the ion pileup region at 1P/Halley. We find that the boundary is likely to be related to ion-neutral collisions and that its location is influenced by variability in the neutral density and the solar wind dynamic pressure.

JOURNAL ARTICLE

Moore L, Stallard T, Galand MIF, 2016, Upper atmospheres of the giant planets, Heliophysics: Active Stars, their Astrospheres, and Impacts on Planetary Environments, Editors: Schrijver, Bagenal, Sojka, Publisher: Cambridge University Press, Pages: 175-200, ISBN: 9781107090477

BOOK CHAPTER

Chadney JM, Galand M, Koskinen TT, Miller S, Sanz-Forcada J, Unruh YC, Yelle RVet al., 2016, EUV-driven ionospheres and electron transport on extrasolar giant planets orbiting active stars, Astronomy & Astrophysics, Vol: 587, ISSN: 1432-0746

The composition and structure of the upper atmospheres of Extrasolar GiantPlanets (EGPs) are affected by the high-energy spectrum of their host starsfrom soft X-rays to EUV. This emission depends on the activity level of thestar, which is primarily determined by its age. We focus upon EGPs orbiting K-and M-dwarf stars of different ages. XUV spectra for these stars areconstructed using a coronal model. These spectra are used to drive both athermospheric model and an ionospheric model, providing densities of neutraland ion species. Ionisation is included through photo-ionisation andelectron-impact processes. We find that EGP ionospheres at all orbitaldistances considered and around all stars selected are dominated by thelong-lived H$^+$ ion. In addition, planets with upper atmospheres where H$_2$is not substantially dissociated have a layer in which H$_3^+$ is the major ionat the base of the ionosphere. For fast-rotating planets, densities ofshort-lived H$_3^+$ undergo significant diurnal variations, with the maximumvalue being driven by the stellar X-ray flux. In contrast, densities oflonger-lived H$^+$ show very little day/night variability and the magnitude isdriven by the level of stellar EUV flux. The H$_3^+$ peak in EGPs with upperatmospheres where H$_2$ is dissociated under strong stellar illumination ispushed to altitudes below the homopause, where this ion is likely to bedestroyed through reactions with heavy species. The inclusion of secondaryionisation processes produces significantly enhanced ion and electron densitiesat altitudes below the main EUV ionisation peak, as compared to models that donot include electron-impact ionisation. We estimate infrared emissions fromH$_3^+$, and while, in an H/H$_2$/He atmosphere, these are larger from planetsorbiting close to more active stars, they still appear too low to be detectedwith current observatories.

JOURNAL ARTICLE

Raghuram S, Bhardwaj A, Galand M, 2016, Prediction of forbidden ultraviolet and visible emissions in comet 67P/Churyumov-Gerasimenko, Astrophysical Journal, Vol: 818, ISSN: 1538-4357

Remote observation of spectroscopic emissions is a potential tool for theidentification and quantification of various species in comets. CO Cameron band(to trace \cod) and atomic oxygen emissions (to trace H$_2$O and/or CO$_2$, CO)have been used to probe neutral composition in the cometary coma. Using acoupled-chemistry emission model, various excitation processes controlling COCameron band and different atomic oxygen and atomic carbon have been modelledin comet 67P-Churyumov-Gerasimenko at 1.29~AU (perihelion) and at 3~AUheliocentric distances, which is being explored by ESA's Rosetta mission. Theintensities of CO Cameron band, atomic oxygen and atomic carbon emission linesas a function of projected distance are calculated for different CO and CO$_2$volume mixing ratios relative to water. Contributions of different excitationprocesses controlling these emissions are quantified. We assess how CO$_2$and/or CO volume mixing ratios with respect to H$_2$O can be derived based onthe observed intensities of CO Cameron band, atomic oxygen, and atomic carbonemission lines.The results presented in this work serve as base linecalculations to understand the behaviour of low out-gassing cometary coma andcompare them with the higher gas production rate cases (e.g. comet Halley).Quantitative analysis of different excitation processes governing thespectroscopic emissions is essential to study the chemistry of inner coma andto derive neutral gas composition.

JOURNAL ARTICLE

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