Publications
166 results found
Hajra R, Henri P, Vallières X, et 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, Pages: 1-10, ISSN: 0004-6361
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.
Mendillo M, Narvaez C, Vogt MF, et 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.
Eriksson AI, Engelhardt IAD, Andre M, et 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
Heritier KL, Altwegg K, Balsiger H, et 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.
Nilsson H, Wieser GS, Behar E, et 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
Nilsson H, Wieser GS, Behar E, et 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.
Henri P, Vallières X, Hajra R, et 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.
Vigren E, André M, Edberg NJT, et 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.
Heritier KL, Henri P, Vallières X, et 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.
Beth A, Altwegg K, Balsiger H, et 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.
Vigren E, Altwegg K, Edberg NJT, et 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
Galand M, Unruh Y, 2017, Modelling the Upper Atmosphere of Gas-Giant Exoplanets Irradiated by Low-Mass Stars Supervisors' Foreword, MODELLING THE UPPER ATMOSPHERE OF GAS-GIANT EXOPLANETS IRRADIATED BY LOW-MASS STARS, Publisher: SPRINGER-VERLAG BERLIN, Pages: V-VI, ISBN: 978-3-319-63350-3
Galand M, Héritier KL, Odelstad E, et 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
Grün E, Agarwal J, Altobelli N, et 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.
Fuselier SA, Altwegg K, Balsiger H, et 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.
Vigren E, Altwegg K, Edberg NJT, et 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.
Vigren E, Galand M, Wellbrock A, et al., 2016, Suprathermal electrons in Titan's sunlit ionosphere: model-observation comparisons, Astrophysical Journal, Vol: 826, ISSN: 1538-4357
Mandt KE, Eriksson A, Edberg NJT, et 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.
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
Chadney JM, Galand M, Koskinen TT, et 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.
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.
Badman SV, Branduardi-Raymont G, Galand M, et al., 2016, Auroral Processes at the Giant Planets: Energy Deposition, Emission Mechanisms, Morphology and Spectra, MAGNETODISCS AND AURORAE OF GIANT PLANETS, Editors: Szego, Achilleos, Arridge, Badman, Delamere, Grodent, Kivelson, Louarn, Publisher: SPRINGER, Pages: 99-179, ISBN: 978-1-4939-3394-5
Tinetti G, Drossart P, Eccleston P, et al., 2015, The EChO science case, Experimental Astronomy, Vol: 40, Pages: 329-391, ISSN: 1572-9508
The discovery of almost two thousand exoplanets has revealed an unexpectedlydiverse planet population. We see gas giants in few-day orbits, whole multi-planet systemswithin the orbit of Mercury, and new populations of planets with masses between that of theEarth and Neptune—all unknown in the Solar System. Observations to date have shown thatour Solar System is certainly not representative of the general population of planets in ourMilky Way. The key science questions that urgently need addressing are therefore: What areexoplanets made of? Why are planets as they are? How do planetary systems work and whatcauses the exceptional diversity observed as compared to the Solar System? The EChO(Exoplanet Characterisation Observatory) space mission was conceived to take up thechallenge to explain this diversity in terms of formation, evolution, internal structure andplanet and atmospheric composition. This requires in-depth spectroscopic knowledge of theatmospheres of a large and well-defined planet sample for which precise physical, chemicaland dynamical information can be obtained. In order to fulfil this ambitious scientificprogram, EChO was designed as a dedicated survey mission for transit and eclipsespectroscopy capable of observing a large, diverse and well-defined planet sample withinits 4-year mission lifetime. The transit and eclipse spectroscopy method, whereby the signalfrom the star and planet are differentiated using knowledge of the planetary ephemerides,allows us to measure atmospheric signals from the planet at levels of at least 10−4 relative tothe star. This can only be achieved in conjunction with a carefully designed stable payloadand satellite platform. It is also necessary to provide broad instantaneous wavelengthcoverage to detect as many molecular species as possible, to probe the thermal structureof the planetary atmospheres and to correct for the contaminating effects of the stellarphotosphere. This requires wavelength coverage of at l
Fuselier SA, Altwegg K, Balsiger H, et al., 2015, ROSINA/DFMS and IES observations of 67P: Ion-neutral chemistry in the coma of a weakly outgassing comet, Astronomy & Astrophysics, Vol: 583, ISSN: 1432-0746
Context. The Rosetta encounter with comet 67P/Churyumov-Gerasimenko provides a unique opportunity for an in situ, up-closeinvestigation of ion-neutral chemistry in the coma of a weakly outgassing comet far from the Sun.Aims. Observations of primary and secondary ions and modeling are used to investigate the role of ion-neutral chemistry within thethin coma.Methods. Observations from late October through mid-December 2014 show the continuous presence of the solar wind 30 km fromthe comet nucleus. These and other observations indicate that there is no contact surface and the solar wind has direct access tothe nucleus. On several occasions during this time period, the Rosetta/ROSINA/Double Focusing Mass Spectrometer measured thelow-energy ion composition in the coma. Organic volatiles and water group ions and their breakup products (masses 14 through 19),CO+, and CO+2(masses 28 and 44) and other mass peaks (at masses 26, 27, and possibly 30) were observed. Secondary ions includeH3O+and HCO+(masses 19 and 29). These secondary ions indicate ion-neutral chemistry in the thin coma of the comet. A relativelysimple model is constructed to account for the low H3O+/H2O+and HCO+/CO+ratios observed in a water dominated coma. Resultsfrom this simple model are compared with results from models that include a more detailed chemical reaction network.Results. At low outgassing rates, predictions from the simple model agree with observations and with results from more complex modelsthat include much more chemistry. At higher outgassing rates, the ion-neutral chemistry is still limited and high HCO+/CO+ratiosare predicted and observed. However, at higher outgassing rates, the model predicts high H3O+/H2O+ratios and the observed ratiosare often low. These low ratios may be the result of the highly heterogeneous nature of the coma, where CO and CO2 number densitiescan exceed that of water.
Vigren E, Galand M, Eriksson AI, et al., 2015, ON THE ELECTRON-TO-NEUTRAL NUMBER DENSITY RATIO IN THE COMA OF COMET 67P/CHURYUMOV-GERASIMENKO: GUIDING EXPRESSION AND SOURCES FOR DEVIATIONS, ASTROPHYSICAL JOURNAL, Vol: 812, ISSN: 0004-637X
Lavvas P, Yelle RV, Heays AN, et al., 2015, N-2 state population in Titan's atmosphere, Icarus, Vol: 260, Pages: 29-59, ISSN: 1090-2643
We present a detailed model for the vibrational population of all non pre-dissociating excited electronic states of N2, as well as for the ground and ionic states, in Titan’s atmosphere. Our model includes the detailed energy deposition calculations presented in the past (Lavvas, P. et al. [2011]. Icarus 213(1), 233–251) as well as the more recent developments in the high resolution N2 photo-absorption cross sections that allow us to calculate photo-excitation rates for different vibrational levels of singlet nitrogen states, and provide information for their pre-dissociation yields. In addition, we consider the effect of collisions and chemical reactions in the population of the different states. Our results demonstrate that above 600 km altitude, collisional processes are efficient only for a small sub-set of the excited states limited to the A and W(ν = 0) triplet states, and to a smaller degree to the a′ singlet state. In addition, we find that a significant population of vibrationally excited ground state N2 survives in Titan’s upper atmosphere. Our calculations demonstrate that this hot N2 population can improve the agreement between models and observations for the emission of the View the MathML source state that is significantly affected by resonant scattering. Moreover we discuss the potential implications of the vibrationally excited population on the ionospheric densities.
Sagnieres LBM, Galand MIF, Cui J, et al., 2015, Influence of local ionization on ionospheric densities in Titan’s upper atmosphere, Journal of Geophysical Research: Space Physics, Vol: 120, Pages: 5899-5921, ISSN: 2169-9402
Titan has the most chemically complex ionosphere of the Solar System. The main sources of ions on the dayside are ionization by EUV solar radiation and on the nightside include ionization by precipitated electrons from Saturn's magnetosphere and transport of ions from the dayside, but many questions remain open. How well do models predict local ionization rates? How strongly do the ionization processes drive the ionospheric densities locally? To address these questions, we have carried out an analysis of ion densities from the Ion and Neutral Mass Spectrometer (INMS) from 16 close flybys of Titan's upper atmosphere. Using a simple chemical model applied to the INMS dataset, we have calculated the ion production rates and local ionization frequencies associated with primary ions inline image and inline image. We find that on the dayside the solar energy deposition model overestimates the INMS-derived inline image production rates by a factor of 2. On the nightside, however, the model driven by suprathermal electron intensities from the Cassini Plasma Spectrometer (CAPS) Electron Spectrometer (ELS) sometimes agrees, other times underestimates the INMS-derived inline image production rates by a factor of up to 2-3. We find that below 1200 km, all ion number densities correlate with the local ionization frequency, although the correlation is significantly stronger for short-lived ions than long-lived ions. Furthermore, we find that for a given N2 local ionization frequency inline image has higher densities on the day-side than on the nightside. We explain that this is due to inline image being more efficiently ionized by solar photons than by magnetospheric electrons for a given amount of N2 ionization.
Chadney, Galand M, Unruh YC, et al., 2015, XUV-driven mass loss from extrasolar giant planets orbiting active stars, Icarus, Vol: 250, Pages: 357-367, ISSN: 1090-2643
Upper atmospheres of Hot Jupiters are subject to extreme radiation conditions that can result in rapid atmospheric escape. The composition and structure of the upper atmospheres of these planets are affected by the high-energy spectrum of the host star. This emission depends on stellar type and age, which are thus important factors in understanding the behaviour of exoplanetary atmospheres. In this study, we focus on Extrasolar Giant Planets (EPGs) orbiting K and M dwarf stars. XUV spectra for three different stars – ∊ Eridani, AD Leonis and AU Microscopii – are constructed using a coronal model. Neutral density and temperature profiles in the upper atmosphere of hypothetical EGPs orbiting these stars are then obtained from a fluid model, incorporating atmospheric chemistry and taking atmospheric escape into account. We find that a simple scaling based solely on the host star’s X-ray emission gives large errors in mass loss rates from planetary atmospheres and so we have derived a new method to scale the EUV regions of the solar spectrum based upon stellar X-ray emission. This new method produces an outcome in terms of the planet’s neutral upper atmosphere very similar to that obtained using a detailed coronal model of the host star. Our results indicate that in planets subjected to radiation from active stars, the transition from Jeans escape to a regime of hydrodynamic escape at the top of the atmosphere occurs at larger orbital distances than for planets around low activity stars (such as the Sun).
Cui J, Galand M, Yelle RV, et al., 2015, Day-to-night transport in the Martian ionosphere: Implications from total electron content measurements, Journal of Geophysical Research: Space Physics, Vol: 120, Pages: 2333-2346, ISSN: 2169-9402
The nightside Martian ionosphere is thought to be contributed by day-to-night transport and electron precipitation, of which the former has not been well studied. In this work, we evaluate the role of day-to-night transport based on the total electron content (TEC) measurements made by the Mars Advanced Radar for Subsurface and Ionospheric Sounding on board Mars Express. This is accomplished by an examination of the variation of nightside TEC in the time domain rather than the traditional solar zenith angle domain. Our analyses here, being constrained to the Northern Hemisphere where the effects of crustal magnetic fields can be neglected, reveal that day-to-night transport serves as the dominant source for the nightside Martian ionosphere from terminator crossing up to time in darkness of ≈5.3 × 103 s, beyond which it is surpassed by electron precipitation. The observations are compared with predictions from a simplified time-dependent ionosphere model. We conclude that the solid body rotation of Mars is insufficient to account for the observed depletion of nightside TEC but the data could be reasonably reproduced by a zonal electron flow velocity of ≈1.9 km s−1.
Cui J, Galand M, Zhang SJ, et al., 2015, The electron thermal structure in the dayside Martian ionosphere implied by the MGS radio occultation data, Journal of Geophysical Research: Planets, Vol: 120, Pages: 278-286, ISSN: 2169-9100
We propose a revised Chapman model for the ionosphere of Mars by allowing for vertical variation of electron temperature. An approximate energy balance between solar EUV heating and CO2 collisional cooling is applied in the dayside Martian ionosphere, analogous to the method recently proposed by Withers et al. (2014). The essence of the model is to separate the contributions of the neutral and electron thermal structures to the apparent width of the main ionospheric layer. Application of the model to the electron density profiles from the Mars Global Surveyor (MGS) radio occultation measurements reveals a clear trend of elevated electron temperature with increasing solar zenith angle (SZA). It also reveals that the characteristic length scale for the change of electron temperature with altitude decreases with increasing SZA. These observations may imply enhanced topside heat influx near the terminator, presumably an outcome of the solar wind interactions with the Martian upper atmosphere. Our analysis also reveals a tentative asymmetry in electron temperature between the northern and southern hemispheres, consistent with the scenario of elevated electron temperature within minimagnetospheres.
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