131 results found
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.
Hajra R, Henri P, Myllys M, et 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.
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
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