131 results found
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.
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