67P/C-G inner coma dust properties from 2.2 au inbound to 2.0 auoutbound to the Sun

GIADA (Grain Impact Analyzer and Dust Accumulator) on-board the Rosetta space probe is designed to measure the momentum, mass and speed of individual dust particles escaping the nucleus of comet 67P/Churyumov-Gerasimenko (hereafter 67P). From 2014 August to 2016 June, Rosetta escorted comet 67P during its journey around the Sun. Here, we focus on GIADA data taken between 2015 January and 2016 February which included 67P's perihelion passage. To better understand cometary activity and more specifically the presence of dust structures in cometary comae, we mapped the spatial distribution of dust density in 67P's coma. In this manner, we could track the evolution of high-density regions of coma dust and their connections with nucleus illumination conditions, namely tracking 67P's seasons. We also studied the link between dust particle speeds and their masses with respect to heliocentric distance, i.e. the level of cometary activity. This allowed us to derive a global and a local correlation of the dust particles' speed distribution with respect to the H2O production rate. © 2016 The Authors.Peer Reviewe

Origine et structure du champ magnétique permanent dans la magnétosphère et l'espace interplanétaire

La structure du champ magnétique dans l'espace interplanétaire est déterminée par l'expansion de la couronne solaire (vent solaire) ainsi que par l'état instantané du champ magnétique de la photosphère ; le présent exposé commence donc par une revue rapide des connaissances actuelles concernant le vent solaire. La seconde partie est consacrée au champ magnétosphérique. On expose à cette occasion le problème de l'interaction entre ce champ et le vent solaire, interaction qui détermine la forme de la cavité magnétosphérique et la configuration du champ dans les régions externes de la cavité. En conclusion, un aperçu est donné sur l'interaction du vent solaire avec la Lune

Multidimensional physicochemical models of the near-nucleus coma: Present achievements and requested future developments

The published multidimensional physicochemical simulations of the near-nucleus coma (hereafter “NNC”) are reviewed. The goals of these simulations are to understand the physical origin of the observed NNC structure(s), and to relate the latter to properties of the nucleus – an inference which is often done on the basis of subjective impressions only. The NNC simulations are mostly based on ad hoc benchmark model nuclei. However, a systematic program of simulation of the NNC of comet Halley exists, which uses the observed shape of its nucleus with or without filtering of topographic details. The main conclusions to be derived from the simulations of the NNC around idealized benchmark nuclei are that: (1) only fully comprehensive quantitative modelling of the whole NNC can reveal the origin of NNC structures: there does not exist any universal relation between one nucleus feature and the NNC structure adjacent to it, independent from its surrounding; (2) in particular, it is impossible to separate the effect of the nucleus heterogeneity on the NNC, from that of the surface topography, except by global quantitative modelling; (3) these two conclusions follow from the physical laws of gas flow and dust motion, hence are independent from the nucleus gas and dust production mechanism. Regarding comet P/Halley, it was possible, using the nucleus shape derived from the cameras, to show that the observed dust “filament” structures are the signatures of the nucleus shape, and are insensitive to whether the nucleus is assumed compositionally homogeneous or inhomogeneous. Application of similar models to the high quality nucleus and NNC observations made during the recent flybys of comets Borelly, Wild-2, and Tempel-I is badly needed. In the future, the nucleus-orbiting mission Rosetta should provide the ultimate benchmark to achieve our understanding of the NNC

Time-dependent, three-dimensional fluid model of the outer coma, with application to the comet Hale-Bopp gas spirals

We describe here the first tridimensional, time-dependent model of a gas coma, and its first application to observed rotationally induced gas coma structures. The present version of the model uses inviscid flow equations and, because of it, is applicable rigorously only to highly productive comets, such as comet C1995/O1 Hale-Bopp in the inner Solar System. Using parameters suitable for this comet, and an arbitrary aspherical nucleus shape and rotation mode, the model is capable to generate large-scale gas spirals of the kind observed in this comet, thus demonstrating that the existence of such spirals does not require surface heterogeneity (active/inactive areas). This result does not in itself exclude heterogeneity. The question whether, in the absence of direct nucleus observations, any reliable interpretation of such structures is possible is raised

3D Time-Dependent Model Of The Dusty-Gas Atmosphere Of Comet 67P/Churyumov-Gerasimenko: Recent Improvements.

International audienceUnder support from the French Space Agency (CNES), a 3D+t dusty-gas model of Comet 67P/Churyumov-Gerasimenko is being developed, to compute, from the first 2014 Rosetta orbital data, the aerodynamic forces exerted on the Rosetta orbiter and on the descent lander. We report the recently developed dust dynamics part of the code. The multi-species (presently H2O and CO) gas code is optimized in terms of computational speed owing to the use of two complementary methods: (a) 3D+t Direct Simulation Monte Carlo (DSMC) runs in the non-equilibrium regions adjacent to the surface and very distant from it, and (b) solutions of the Navier-Stokes equations in-between. The model is used presently using Lamy et al. (Space Sci. Rev., 2007, 128, 23) coarse information on 67P nucleus shape and rotation, and a range of possible gas production rates Q for the early Rosetta observations at rh 3 AU (Q 1026 - 1027 s-1). In the interim version, simplifying assumptions are made with respect to the physical processes of gas production and with respect to the dust grain structure (grain sphericity). The dust code is based on the 3D+t Dust Monte-Carlo (DMC) approach of Crifo et al. ( Icarus, 2005, 176, 192) which allows for three forces: nucleus gravity, aerodynamic force, and solar radiation pressure. The dynamics of grains of a large size range (submicron - millimeter) is described in detail, and three implications are discussed: (a) structure of the near-nucleus dust coma, (b) evolution of the nucleus surface, and (3) dust impacts on the lander

Navier–Stokes and direct Monte-Carlo simulations of the circumnuclear gas coma III. Spherical, inhomogeneous sources

We pursue our program of comparative simulations of the cometary gas coma by the two most advanced techniques available: (1) numerical solution of Navier–Stokes equations coupled to the Boltzman equation in the surface boundary layer, and (2) direct Monte-Carlo simulation. Here, we consider two different spherical but compositionally inhomogeneous nuclei, at three very different levels of gas production. The results show the same excellent agreement between the two methods in a domain adjacent to the surface as found precedingly, practically down to free-molecular conditions. A wealth of coma density patterns with non-intuitive structure is obtained. Some of these structures appear even under free-molecular effusion from the surface. The physical origin of all structures is discussed, and their evolution with changing gas production is studied. The computed comae are compared to those computed by various authors precedingly. Intercomparison of the present results demonstrates that differing inhomogeneity patterns may lead to similar structures in the gas coma. Comparison between these structures and those created by homogeneous, aspherical surfaces shows that it is not possible to guess from empirical rules which one of the two processes is responsible for the creation of a given structure. The implications for the interpretation of future high resolution images, or of future in situ mass spectrometric samplings of the near-nucleus gas coma are discussed

Monte-Carlo and multifluid modelling of the circumnuclear dust coma II. Aspherical-homogeneous, and spherical-inhomogeneous nuclei

International audienceWe use our newly developed Dust Monte-Carlo (DMC) simulation technique (Crifo et al., 2005a) to study the dynamics of dust grains in the vicinity of some of the benchmark aspherical, homogeneous cometary nuclei and of the benchmark spherical, inhomogeneous nuclei studied by us precedingly. We use the interim unrealistic simplifying assumptions of grain sphericity, negligible nucleus rotation rate, and negligible tidal force, but take accurately into account the nucleus gravitational force, gas coma aerodynamic force, and solar radiation pressure force, and consider the full mass range of ejectable spherical grains. The resulting complicated grain motions are described in detail, as well as the resulting complicated and often counter-intuitive dust coma structure. The results are used to answer several important questions: (1) When computing coma dust distributions, (a) is it acceptable to take into consideration only one or two of the above mentioned forces (as currently done)? (b) to which accuracy must these forces be known, in particular is it acceptable to represent the gravity of an aspherical nucleus by a spherically symmetric gravity? (c) how do the more efficient but less general Dust Multi-Fluid (DMF) computations compare with the DMC results? (2) Are there simple structural relationships between the dust coma of a nucleus at small heliocentric distance , and that of the same nucleus at large ? (3) Are there similarities between the gas coma structures and the associated dust coma structures? (4) Are there dust coma signatures revealing non-ambiguously a spherical nucleus inhomogeneity or an homogeneous nucleus asphericity? (5) What are the implications of the apparently quite general process of grain fall-backs for the evolution of the nucleus surface, and for the survival of a landed probe

The prediction of the gas environment of the PHILAE probe during its 2014 descent to the nucleus of the comet 67P

International audienceOne of the objectives of the ESA ”ROSETTA” mission to the comet 67P was to insert, in August 2014, an orbiter probe around the so-called nucleus of the comet, and to deposit the ”PHILAE” lander at the surface of the nucleus in November 2014. The selection of the landing site and the definition of the release point and initial descent velocity vector were made in the period August to October 2014 on the basis of simulations of the descent trajectory. This requested an assessment of the gravitational and aerodynamic forces on PHILAE. We here describe the so-called RZC model developed to predict the gas environment of 67P in November 2014 and compute the aerodynamic force. We first outline the unusual diffculties resulting from (1) the complexity of the nucleus surface on all scales, (2) the absence of direct measurements of the gas flux at the surface itself, (3) the time-dependence of the gas production induced by the fast nucleus rotation, (4) the need to perform the whole program within less than three months. Then we outline the physical approach adopted to overcome these diffculties, and describe the RZC model which included three differing tools: (1) a set of gasdynamic/gaskinetic codes to compute the vacuum outflow of a rarefied gas mixture from a highly aspherical rotating solid source; (2) an heuristic approach to deal with the solid/gas initial boundary conditions, and (3) an iterative procedure to derive the gas production parameters on the nucleus surface from the observational data acquired from the orbiter probe. The satisfactory operation of the RZC code in the weeks preceding the November 2014 PHILAE descent is shown, and the forecasted aerodynamic force during the PHILAE descent is compared to the gravitational force

First attempt at interpreting millimetric observations of CO in comet C/1995 O1 (Hale-Bopp) using 3D+t hydrodynamical coma simulations

International audienceMillimetre line observations of comet C/1995 O1 (Hale-Bopp) close to perihelion, completed using the IRAM Plateau de Bure Interferometer, have detected temporal variations in the CO J(2–1) 230 GHz line shape, and in the position of its maximum emission brightness within the field-of-view, whose heuristic analysis has suggested the presence in the coma of a slowly rotating spiral-shaped enhancement of the CO density. Aims. Here, we reanalyse these data using a physically consistent model of the coma. Methods. We consider a large, rotating, icy nucleus with an arbitrarily aspherical shape and an adhoc rotation mode, and compute its tridimensional, time-dependent (”3D+t”) mixed CO + H2O coma, using a previously developed tridimensional hydrodynamical code (HDC). The line emission of CO is then computed using a molecular excitation and radiation transfer code (ERC). In the present, pioneering phase, the HDC and ERC both contain crude, and not thoroughly mutually consistent approximations. Several alternative CO surface flux distributions are considered, and the resulting CO 230 GHz line spectra and brightness maps are compared with observations. Results. We find that when an uniform surface flux of CO is assumed, the spiral structures created by the nucleus asphericity in the CO coma are too faint to account for the observational data, whereas we confirm earlier conclusions based on a heuristic approach that the assumption of an area of suitable dimensions and localization with increased CO flux leads to results in agreement with a large subset of (but not all) the data. This suggests that the true CO coma production map may be more complex than the presently assumed rather simple-minded one. Refined and mutually consistent HDC and ERC are needed for a more satisfactory interpretation of the present and any similar future data

The spatial distribution of molecular species observed in comet Hale Bopp with the Plateau de Bure Interferometer

International audienc