123 results found
Carnielli G, Galand M, Leblanc F, et al., 2019, First 3D test particle model of Ganymede's ionosphere, Icarus, Vol: 330, Pages: 42-59, ISSN: 0019-1035
We present the first three-dimensional multi-species ionospheric model for Ganymede, based on a test particle Monte Carlo approach. Inputs include the electromagnetic field configuration around the moon from the magnetospheric models developed by Leclercq et al. (2016) and by Jia et al. (2009), and the number density, bulk velocity and temperature distributions of the neutral exosphere simulated by Leblanc et al. (2017). According to our simulations, O2+ is the most abundant ion species, followed by O+, H2+ and H2O+. For O+ and O2+, the majority of ions produced impact the moon's surface, while for the other species the majority escapes Ganymede's magnetosphere. For all ion species, the escape occurs either in the direction of corotation of the Jovian plasma or through the Alfvén wings.To validate our model, the output of our simulations, performed under the Galileo G2 flyby conditions, are compared to the observations. These include the electron density derived by the plasma wave instrument (PWS), the ion energy spectrogram measured by the plasma analyzer (PLS) and the associated plasma moments (Frank et al., 1997a).On the one hand, the electron density found by our model is consistently underestimated throughout the flyby, being at least one order of magnitude lower compared to observations. We argue that the prime reason for this discrepancy comes from the exospheric density, which may be underestimated. On the other hand, we find a remarkably good agreement between the modeled ion energy spectrogram and that recorded by PLS, providing a validation of the test particle model. Finally, we compare the modeled plasma moments along the G2 flyby with those analyzed by Frank et al. (1997a). The data seems to be more consistent with an ionosphere dominated by O2+ instead of H+ or O+, as suggested previously in the literature. This supports our finding that O2+ is the dominant ion species close to the surface.
Vigren E, Edberg NJT, Eriksson A, et al., 2019, The Evolution of the Electron Number Density in the Coma of Comet 67P at the Location of Rosetta from 2015 November through 2016 March, ASTROPHYSICAL JOURNAL, Vol: 881, ISSN: 0004-637X
Deca J, Henri P, Divin A, et al., 2019, Building a weakly outgassing comet from a generalized Ohm’s law, Physical Review Letters, Vol: 123, Pages: 055101-1-055101-7, ISSN: 0031-9007
When a weakly outgassing comet is sufficiently close to the Sun, the formation of an ionized coma results in solar wind mass loading and magnetic field draping around its nucleus. Using a 3D fully kinetic approach, we distill the components of a generalized Ohm’s law and the effective electron equation of state directly from the self-consistently simulated electron dynamics and identify the driving physics in the various regions of the cometary plasma environment. Using the example of space plasmas, in particular multispecies cometary plasmas, we show how the description for the complex kinetic electron dynamics can be simplified through a simple effective closure, and identify where an isotropic single-electron fluid Ohm’s law approximation can be used, and where it fails.
Götz C, Gunell H, Volwerk M, et al., 2019, Cometary plasma science -- A white paper in response to the voyage 2050call by the European space agency, Publisher: arXiv
Comets hold the key to the understanding of our solar system, its formationand its evolution, and to the fundamental plasma processes at work both in itand beyond it. A comet nucleus emits gas as it is heated by the sunlight. Thegas forms the coma, where it is ionised, becomes a plasma and eventuallyinteracts with the solar wind. Besides these neutral and ionised gases, thecoma also contains dust grains, released from the comet nucleus. As a cometaryatmosphere develops when the comet travels through the solar system,large-scale structures, such as the plasma boundaries, develop and disappear,while at planets such large-scale structures are only accessible in their fullygrown, quasi-steady state. In situ measurements at comets enable us to learnboth how such large-scale structures are formed or reformed and how small-scaleprocesses in the plasma affect the formation and properties of these largescale structures. Furthermore, a comet goes through a wide range of parameterregimes during its life cycle, where either collisional processes, involvingneutrals and charged particles, or collisionless processes are at play, andmight even compete in complicated transitional regimes. Thus a comet presents aunique opportunity to study this parameter space, from an asteroid-like to aMars- and Venus-like interaction. Fast flybys of comets have made many newdiscoveries, setting the stage for a multi-spacecraft mission to accompany acomet on its journey through the solar system. This white paper reviews thepresent-day knowledge of cometary plasmas, discusses the many questions thatremain unanswered, and outlines a multi-spacecraft ESA mission to accompany acomet that will answer these questions by combining both multi-spacecraftobservations and a rendezvous mission, and at the same time advance ourunderstanding of fundamental plasma physics and its role in planetary systems.
Bockelée-Morvan D, Filacchione G, Altwegg K, et al., 2019, AMBITION -- Comet nucleus cryogenic sample return (white paper for ESA's voyage 2050 programme), Publisher: arXiv
This white paper proposes that AMBITION, a Comet Nucleus Sample Returnmission, be a cornerstone of ESA's Voyage 2050 programme. We summarise some ofthe most important questions still open in cometary science after the successesof the Rosetta mission, many of which require sample analysis using techniquesthat are only possible in laboratories on Earth. We then summarisemeasurements, instrumentation and mission scenarios that can address thesequestions, with a recommendation that ESA select an ambitious cryogenic samplereturn mission. Rendezvous missions to Main Belt comets and Centaurs arecompelling cases for M-class missions, expanding our knowledge by exploring newclasses of comets. AMBITION would engage a wide community, drawing expertisefrom a vast range of disciplines within planetary science and astrophysics.With AMBITION, Europe will continue its leadership in the exploration of themost primitive Solar System bodies.
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