Author name code: zakharov ADS astronomy entries on 2022-09-14 =author:"Zakharov, V.V." ------------------------------------------------------------------------ Title: Comets beyond 4 au: How pristine are Oort nuclei? Authors: Fulle, Marco; Lazzarin, M.; La Forgia, F.; Zakharov, V. V.; Bertini, I.; Mazzotta Epifani, E.; Ammannito, E.; Buzzoni, A.; Capria, M. T.; Carbognani, A.; Da Deppo, V.; Della Corte, V.; Fiscale, S.; Frattin, E.; Inno, L.; Migliorini, A.; Pernechele, C.; Rotundi, A.; Sindoni, G.; Tubiana, C.; Milani, G.; Aletti, A.; Bacci, P.; Baj, G.; Bellini, F.; Bryssinck, E.; Di Grazia, M.; Facchini, M.; Feraco, M.; Guido, E.; Ligustri, R.; Kugel, F.; Maestripieri, M.; Tirelli, D.; Valvasori, A.; Snodgrass, C.; Jones, G. H. Bibcode: 2022MNRAS.513.5377F Altcode: The ESA mission Comet Interceptor will target an Oort or interstellar comet during its first approach to the Sun. Meanwhile, the Vera Rubin LSST Survey will observe hundreds of active comets per month beyond 4 au from the Sun, where water vapour pressure is expected to be too low to eject dust. We discuss observations of dust tails at heliocentric distances larger than 4 au in order to retrieve the physical parameters driving cometary activity beyond Jupiter by means of a probabilistic tail model, which is consistent with the activity model defining the gas coma parameters due to the sublimation of carbon monoxide, molecular oxygen, methane, ethane, and carbon dioxide since the activity onset at 85 au from the Sun. We find that: (i) All the observed dust tails are consistent with the adopted activity model; (ii) The tail fits depend on three free parameters only, all correlated to the nucleus size; (iii) Tail fits are always improved by anisotropic dust ejection, suggesting activity of Oort nuclei dominated by seasons; (iv) Inbound seasons suggest cometary activity before the ejection of protocomets into the Oort cloud, as predicted by the activity model; (v) Oort nuclei larger than 1 km may be characterized by a fallout up to ≍100 m thick deposited during ≍60 yr inbound; (vi) On the other side, Oort nuclei smaller than 1 km may appear more pristine than Jupiter Family Comets when observed at 1 au from the Sun. Title: On the similarity of dust flows in the inner coma of comets Authors: Zakharov, V. V.; Rotundi, A.; Della Corte, V.; Fulle, M.; Ivanovski, S. L.; Rodionov, A. V.; Bykov, N. Y. Bibcode: 2021Icar..36414476Z Altcode: The atmosphere of a comet is formed by the sublimation, due to solar illumination, of its volatile component and the dust particles ejected from its nucleus and entrained by the gas flow. Contemporary dusty-gas coma models take into account numerous physical processes occurring in the coma and a complex geometry of the nucleus. For the description of the dusty-gas flow in the coma, such models introduce a large number of governing parameters characterizing physical properties and processes. The relative role of these processes is not easy to ascribe therefore a relevant inter-comparison of model results becomes difficult.

The present work introduces a set of universal, dimensionless parameters, which characterize the dust motion in the inner cometary coma. This approach allows one to: (i) reduce the number of parameters for analysis; (ii) reveal dust flows similarities; (iii) rescale the available numerical solutions. The present work demonstrates application of this approach to a realistic coma model.

Description of dust motion with dimensionless parameters allows us to make a parametric study for a broad range of conditions and to find simple analytic approximations (via a polynomial function) of the numerical results suitable for rough estimations of dust density in the coma. Title: Observational constraints to the dynamics of dust particles in the coma of comet 67P/Churyumov-Gerasimenko Authors: Frattin, E.; Bertini, I.; Ivanovski, S. L.; Marzari, F.; Fulle, M.; Zakharov, V. V.; Moreno, F.; Naletto, G.; Lazzarin, M.; Cambianica, P.; Cremonese, G.; Ferrari, S.; Ferri, F.; Güttler, C.; La Forgia, F.; Lucchetti, A.; Pajola, M.; Penasa, L.; Rotundi, A.; Sierks, H.; Tubiana, C. Bibcode: 2021MNRAS.504.4687F Altcode: 2021arXiv210410950F; 2021MNRAS.tmp.1157F In this work, we aim to characterize the dust motion in the inner coma of comet 67P/Churyumov-Gerasimenko to provide constraints for theoretical 3D coma models. The OSIRIS camera on-board the Rosetta mission was able for the first time to acquire images of single dust particles from inside the cometary coma, very close to the nucleus. We analyse a large number of particles, performing a significant statistic of their behaviour during the post-perihelion period, when the spacecraft covered distances from the nucleus ranging between 80 and 400 km. We describe the particle trajectories, investigating their orientation and finding highly radial motion with respect to the nucleus. Then, from the particle brightness profiles, we derive a particle rotational frequency of ν < 3.6 Hz, revealing that they are slow rotators and do not undergo fragmentation. We use scattering models to compare the observed spectral radiance of the particles with the simulated ones in order to estimate their size, finding values that range from millimetres up to centimetres. The statistics performed in this paper provide useful parameters to constrain the cometary coma dynamical models. Title: Practical relations for assessments of the gas coma parameters Authors: Zakharov, V. V.; Rodionov, A. V.; Fulle, M.; Ivanovski, S. L.; Bykov, N. Y.; Della Corte, V.; Rotundi, A. Bibcode: 2021Icar..35414091Z Altcode: To ensure the safety of a spacecraft and efficiency of the instrument operations it is indispensable to have simple (i.e. with minimal number of parameters and which does not require long time simulations) models for the assessments of the dusty-gas coma. A dusty-gas flow from cometary nucleus preserves common general structure regardless the features of a particular comet. Therefore, elementary models which account only for the main factors affecting the dusty-gas motion could be used for rough estimations of such characteristics and asymptotic behavior of dusty-gas motion.

In a wide range of conditions a presence of dust in the coma does not affect the gas motion. Therefore, it is possible to study the dynamics of a gas environment separately, without taking into account the dust. The present work is devoted to the gas environment of the comet. The main goal of the work is to develop a simplified model of gas environment which allows to estimate spatial distribution of gas parameters in the coma in the range of nucleus-centric distances from ten to thousand radius of the nucleus. Title: Experimental Phase Function and Degree of Linear Polarization Curves of Millimeter-sized Cosmic Dust Analogs Authors: Muñoz, O.; Moreno, F.; Gómez-Martín, J. C.; Vargas-Martín, F.; Guirado, D.; Ramos, J. L.; Bustamante, I.; Bertini, I.; Frattin, E.; Markannen, J.; Tubiana, C.; Fulle, M.; Güttler, C.; Sierks, H.; Rotundi, A.; Della Corte, V.; Ivanovski, S.; Zakharov, V. V.; Bockelée-Morvan, D.; Blum, J.; Merouane, S.; Levasseur-Regourd, A. C.; Kolokolova, L.; Jardiel, T.; Caballero, A. C. Bibcode: 2020ApJS..247...19M Altcode: We present laboratory measurements of the phase functions and degree of linear polarization (DLP) curves of a selection of millimeter-sized cosmic dust analog particles. The set includes particles with similar sizes but diverse internal structure (compact and porous) and absorbing properties. The measured phase functions are found to be in all cases very different from those of micron-sized particles. They show a monotonic decrease with increasing phase angle from the back- to the side-scattering region, reaching a minimum at large phase angles before a steep increase of the forward peak. This is in stark contrast to the phase functions of micron-sized particles, which are rather flat at low and intermediate phase angles. The maximum of the DLP for millimeter-sized compact particles is shifted toward larger phase angles (∼130°) compared to that of micron-sized particles (∼90°). Porosity plays an important role in the measured DLP curves: the maximum significantly decreases for increasing porosity as a result of multiple scattering within the particle. Large porous particles with highly absorbing inclusions can reproduce both the OSIRIS/Rosetta phase functions and ground-based DLP observations of comet 67P/Churyumov-Gerasimenko. Title: Models of Rosetta/OSIRIS 67P Dust Coma Phase Function Authors: Moreno, F.; Guirado, D.; Muñoz, O.; Bertini, I.; Tubiana, C.; Güttler, C.; Fulle, M.; Rotundi, A.; Della Corte, V.; Ivanovski, S. L.; Rinaldi, G.; Bockelée-Morvan, D.; Zakharov, V. V.; Agarwal, J.; Mottola, S.; Toth, I.; Frattin, E.; Lara, L. M.; Gutiérrez, P. J.; Lin, Z. Y.; Kolokolova, L.; Sierks, H.; Naletto, G.; Lamy, P. L.; Rodrigo, R.; Koschny, D.; Davidsson, B.; Barucci, M. A.; Bertaux, J. -L.; Bodewits, D.; Cremonese, G.; Da Deppo, V.; Debei, S.; De Cecco, M.; Deller, J.; Fornasier, S.; Ip, W. -H.; Keller, H. U.; Lazzarin, M.; López-Moreno, J. J.; Marzari, F.; Shi, X. Bibcode: 2018AJ....156..237M Altcode: 2018arXiv180910424M The phase function of the dust coma of comet 67P has been determined from Rosetta/OSIRIS images. This function shows a deep minimum at phase angles near 100°, and a strong backscattering enhancement. These two properties cannot be reproduced by regular models of cometary dust, most of them based on wavelength-sized and randomly oriented aggregate particles. We show, however, that an ensemble of oriented elongated particles of a wide variety of aspect ratios, with radii r ≳ 10 μm, and whose long axes are perpendicular to the direction of the solar radiation, are capable of reproducing the observed phase function. These particles must be absorbing, with an imaginary part of the refractive index of about 0.1 to match the expected geometric albedo, and with porosity in the 60%-70% range. Title: The near-nucleus gas coma of comet 67P/Churyumov-Gerasimenko prior to the descent of the surface lander PHILAE Authors: Zakharov, V. V.; Crifo, J. -F.; Rodionov, A. V.; Rubin, M.; Altwegg, K. Bibcode: 2018A&A...618A..71Z Altcode: Context. The European Space Agency (ESA) Rosetta mission was the most comprehensive study of a comet ever performed. In particular, the Rosetta orbiter, which carried many instruments for monitoring the evolution of the dusty gas emitted by the cometary nucleus, returned an enormous volume of observational data collected from the close vicinity of the nucleus of comet 67P/Churyumov-Gerasimenko.
Aims: Such data are expected to yield unique information on the physical processes of gas and dust emission, using current physical model fits to the data. We present such a model (the RZC model) and our procedure of adjustment of this model to the data.
Methods: The RZC model consists of two components: (1) a numerical three-dimensional time-dependent code solving the Eulerian/Navier-Stokes equations governing the gas outflow, and a direct simulation Monte Carlo (DSMC) gaskinetic code with the same objective; and (2) an iterative procedure to adjust the assumed model parameters to best-fit the observational data at all times.
Results: We demonstrate that our model is able to reproduce the overall features of the local neutral number density and composition measurements of Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) Comet Pressure Sensor (COPS) and Double Focusing Mass Spectrometer (DFMS) instruments in the period August 1-November 30, 2014. The results of numerical simulations show that illumination conditions on the nucleus are the main driver for the gas activity of the comet. We present the distribution of surface inhomogeneity best-fitted to the ROSINA COPS and DFMS in situ measurements. Title: Asymptotics for spherical particle motion in a spherically expanding flow Authors: Zakharov, V. V.; Ivanovski, S. L.; Crifo, J. -F.; Della Corte, V.; Rotundi, A.; Fulle, M. Bibcode: 2018Icar..312..121Z Altcode: In the context of an increasing number of complex multiparametric dust coma models it was found convenient to construct an elementary model with a minimum number of parameters selected to represent the key processes acting on the dust. The models outputs can be used as a reference evaluation of these processes with rough estimates of the resulting dust properties e.g. velocity.

The present work introduces three, universal, dimensionless parameters which characterize the dust motion in an expanding flow, and computes as a function of these parameters the dust terminal velocity, the time it takes to acquire it, and the distance at which it is acquired.

The motion of dust grains is presented as a system of dimensionless ordinary differential equations the solution of which depends upon the above mentioned three parameters. The numerical integration of this system was performed over a wide range of parameter space covering the whole range of physically possible conditions.

Precomputed results of dust terminal velocity, time and distance where it is reached are presented in dimensionless form. To obtain dimensional values for a particular case it is sufficient to perform algebraic operations. Title: The Near-Nucleus Dusty Gas Coma of Comet 67P Prior to the Descent of PHILAE Authors: Zakharov, V. V.; Marzari, F.; Crifo, J. -F.; Bertini, I.; Rodionov, A. V.; Fulle, M. Bibcode: 2017EPSC...11..683Z Altcode: We here describe the RZC model developed to predict the gas environment of the comet 67P (it was used for estimation of the aerodynamic forces on the Rosetta lander in November 2014) and the results of adjustment of this model to the observational data obtained by the Rosetta probe before landing. We outline the physical approach adopted to overcome the difficulties resulting from (1) the extreme complexity of the nucleus surface, (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. Also, we present the results of our attempts to fit the dust coma images obtained by the Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS). Title: Dynamics of aspherical dust grains in a cometary atmosphere: I. axially symmetric grains in a spherically symmetric atmosphere Authors: Ivanovski, S. L.; Zakharov, V. V.; Della Corte, V.; Crifo, J. -F.; Rotundi, A.; Fulle, M. Bibcode: 2017Icar..282..333I Altcode: In-situ measurements of individual dust grain parameters in the immediate vicinity of a cometary nucleus are being carried by the Rosetta spacecraft at comet 67P/Churyumov-Gerasimenko. For the interpretations of these observational data, a model of dust grain motion as realistic as possible is requested. In particular, the results of the Stardust mission and analysis of samples of interplanetary dust have shown that these particles are highly aspherical, which should be taken into account in any credible model. The aim of the present work is to study the dynamics of ellipsoidal shape particles with various aspect ratios introduced in a spherically symmetric expanding gas flow and to reveal the possible differences in dynamics between spherical and aspherical particles. Their translational and rotational motion under influence of the gravity and of the aerodynamic force and torque is numerically integrated in a wide range of physical parameters values including those of comet 67P/Churyumov-Gerasimenko. The main distinctions of the dynamics of spherical and ellipsoidal particles are discussed. The aerodynamic characteristics of the ellipsoidal particles, and examples of their translational and rotational motion in the postulated gas flow are presented. Title: The 2016 Feb 19 outburst of comet 67P/CG: an ESA Rosetta multi-instrument study Authors: Grün, E.; Agarwal, J.; Altobelli, N.; Altwegg, K.; Bentley, M. S.; Biver, N.; Della Corte, V.; Edberg, N.; Feldman, P. D.; Galand, M.; Geiger, B.; Götz, C.; Grieger, B.; Güttler, C.; Henri, P.; Hofstadter, M.; Horanyi, M.; Jehin, E.; Krüger, H.; Lee, S.; Mannel, T.; Morales, E.; Mousis, O.; Müller, M.; Opitom, C.; Rotundi, A.; Schmied, R.; Schmidt, F.; Sierks, H.; Snodgrass, C.; Soja, R. H.; Sommer, M.; Srama, R.; Tzou, C. -Y.; Vincent, J. -B.; Yanamandra-Fisher, P.; A'Hearn, M. F.; Erikson, A. I.; Barbieri, C.; Barucci, M. A.; Bertaux, J. -L.; Bertini, I.; Burch, J.; Colangeli, L.; Cremonese, G.; Da Deppo, V.; Davidsson, B.; Debei, S.; De Cecco, M.; Deller, J.; Feaga, L. M.; Ferrari, M.; Fornasier, S.; Fulle, M.; Gicquel, A.; Gillon, M.; Green, S. F.; Groussin, O.; Gutiérrez, P. J.; Hofmann, M.; Hviid, S. F.; Ip, W. -H.; Ivanovski, S.; Jorda, L.; Keller, H. U.; Knight, M. M.; Knollenberg, J.; Koschny, D.; Kramm, J. -R.; Kührt, E.; Küppers, M.; Lamy, P. L.; Lara, L. M.; Lazzarin, M.; Lòpez-Moreno, J. J.; Manfroid, J.; Epifani, E. Mazzotta; Marzari, F.; Naletto, G.; Oklay, N.; Palumbo, P.; Parker, J. Wm.; Rickman, H.; Rodrigo, R.; Rodrìguez, J.; Schindhelm, R.; Shi, X.; Sordini, R.; Steffl, A. J.; Stern, S. A.; Thomas, N.; Tubiana, C.; Weaver, H. A.; Weissman, P.; Zakharov, V. V.; Taylor, M. G. G. T. Bibcode: 2016MNRAS.462S.220G Altcode: 2016MNRAS.tmp.1212G 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. Title: 3D numerical simulations of radiative transfer in the cometary coma Authors: Zakharov, V. V.; Crifo, J. -F.; Bockelée-Morvan, D.; Biver, N. Bibcode: 2013EPSC....8..719Z Altcode: We report about the resent advances in the development of our 3D model of water excitation in cometary atmosphere in which radiation transfer is treated on the base of the accelerated Monte Carlo method. Title: 3D+t Mathematical Simulation of the Dusty-Gas Cometary Atmosphere (Application to the Comet 103P/ Hartley 2) Authors: Zakharov, V. V.; Crifo, J. -F.; Rodionov, A. V. Bibcode: 2012epsc.conf..678Z Altcode: 2012espc.conf..678Z We report the results of application of our 3D+t dustgas coma model to a model nucleus which shape and dimensions close to those of 103P/Hartley 2 nucleus and conditions at the moment of closest approach i.e. at 13:59:47.31 UTC on 4 November 2010 (1.064 AU from the Sun). We assume various cases of gas and dust production of the nucleus taking into account available observational data. Title: Dynamics of aspherical dust for the GIADA experiment in the coma of 67P/Churyumov- Gerasimenko: I. Comparison with the spherical approximation Authors: Ivanovski, S. L.; Zakharov, V. V.; Crifo, J. -F.; Della Corte, V.; Fulle, M.; Rotundi, A. Bibcode: 2012epsc.conf..592I Altcode: 2012espc.conf..592I We report the recent advances in developing of the model of aspherical dust grain dynamics [1] in the cometary atmosphere of 67P/Churyumov- Gerasimenko. The present model is aimed to support the scientific objectives of GIADA in-situ experiment [2] on board of the ESA ROSETTA probe and will provide predictions on the real dust cometary grains. The currently used 3D+t models [3, 4] assume sphericity of the grains but the dynamics of aspherical grains can be very different from spherical [5].At this stage we study grains moving under the influence of two forces: aerodynamic and gravitational and discuss the distinctions between the aspherical and the spherical approximations. Title: Investigation of dust and water ice in comet 9P/Tempel 1 from Spitzer observations of the Deep Impact event Authors: Gicquel, A.; Bockelée-Morvan, D.; Zakharov, V. V.; Kelley, M. S.; Woodward, C. E.; Wooden, D. H. Bibcode: 2012A&A...542A.119G Altcode: Context. The Spitzer spacecraft monitored the Deep Impact event on 2005 July 4 providing unique infrared spectrophotometric data that enabled exploration of comet 9P/Tempel 1's activity and coma properties prior to and after the collision of the impactor.
Aims: The time series of spectra take with the Spitzer Infrared Spectrograph (IRS) show fluorescence emission of the H2O ν2 band at 6.4 μm superimposed on the dust thermal continuum. These data provide constraints on the properties of the dust ejecta cloud (dust size distribution, velocity, and mass), as well as on the water component (origin and mass). Our goal is to determine the dust-to-ice ratio of the material ejected from the impact site.
Methods: The temporal evolution of the continuum was analyzed using a dust thermal model which considers amorphous carbon and intimate silicate-carbon mixtures. The water emission was extracted from the spectra and the water columns within the Spitzer extraction aperture were inferred using a fluorescence excitation model. Time-dependent models simulating the development of the ejecta cloud and the sublimation of icy grains were developed to interpret the temporal evolution of both dust and water emissions within the field of view.
Results: Both the color temperature of the grains in the ejecta cloud (375 ± 5 K) and their size distribution show that a large quantity of submicron grains were ejected by the impact. The velocity of the smallest grains is 230 m s-1, with a power index for the size dependence of 0.3-0.5, in agreement with gas loading. The total mass of dust is (0.7-1.2) × 105 kg for grain sizes 0.1-1 μm and (0.5-2.1) × 106 kg for sizes 0.1-100 μm. A sustained production of water is observed, which can be explained by the sublimation of pure ice grains with sizes less than 1 μm and comprising a mass of ice of (0.8-1.8) × 107 kg. The contribution of dirty ice grains to the ice budget is negligible. Assuming that water was essentially produced by icy grains present in the ejecta cloud, our measurements suggest a very high ice-to-dust ratio of about 10 in the excavated material, which greatly exceeds the gas-to-dust production rate ratio of ~0.5 measured for the background coma. Alternately, if a large amount of material felt back to the surface and sublimated, ice-to-dust ratios of 1-3 are not excluded. A better understanding of the cratering event on 9P/Tempel 1 is required to disentangle between the two hypotheses. Evidence for grain fragmentation in the ejecta cloud is found in the data. The pre-impact water production rate is measured to be (4.7 ± 0.7) × 1027 mol s-1. Title: Model of Aspherical Dust Dynamics for GIADA Experiment in the Coma of 67P/Churyumov-Gerasimenko: I. Comparison with the Spherical Approximation Authors: Ivanovski, S. L.; Zakharov, V. V.; Crifo, J. F.; Della Corte, V.; Rotundi, A.; Fulle, M. Bibcode: 2012LPICo1667.6444I Altcode: We report the recent advances in developing of the model of aspherical dust grain dynamics in the cometary atmosphere for GIADA experiment on a board of ROSETTA orbiter. We discuss its distinctions from the spherical grain model. Title: Mathematical Simulation of the 3D+t Dusty-Gas Cometary Atmosphere of Comet 103P/ Hartley 2 Authors: Zakharov, V. V.; Crifo, J. -F.; Rodionov, A. V. Bibcode: 2012LPICo1667.6401Z Altcode: We report the results of application of our 3D+t dust-gas coma model to a model nucleus which shape and dimensions close to those of 103P/Hartley 2 nucleus and conditions at the moment of closest approach in EPOXI mission. Title: Pure ice grains in the coma of 9P/Tempel 1 after Deep Impact Authors: Gicquel, A.; Bockelée-Morvan, D.; Zakharov, V. V.; Kelley, M. S.; Woodward, C. E.; Wooden, D. H. Bibcode: 2011epsc.conf..233G Altcode: 2011DPS....43..233G The Deep Impact (DI) spacecraft encountered comet 9P/Tempel 1 on July 4th, 2005. The spacecraft released an impactor that collided with the comet nucleus and excavated (possibly unprocessed) cometary material in a prominent ejecta plume. We report on the temporal evolution of water molecules observed with the IRS instrument on the Spitzer Space telescope after impact. We show that it can only be explained by the presence of subliming pure ice grains. The mass of water ice in subliming 0.1 - 1 μm grains is close to 2 x 106 kg. Title: A numerical study of the dusty-gas atmosphere of comet 67P/Churyumov-Gerasimenko Authors: Zakharov, V. V.; Rodionov, A. V.; Crifo, J. -F.; Fulle, M. Bibcode: 2011epsc.conf..126Z Altcode: 2011DPS....43..126Z We apply out 3D+t model to study the influence of H2O and CO production and its nonuniformity on the gas and dust dynamics in the vicinity of a rotating three-dimensional nucleus. The parameters of simulations correspond to the conditions of the rendezvous of the Rosetta probe with the comet 67P/Churyumov-Gerasimenko. We assume various models of gas and dust production of the nucleus taking into account available VLT observations. We present also the estimations of aerodynamic forces and dust contamination of the Rosetta orbiter and the lander. Title: Monte-Carlo and multifluid modelling of the circumnuclear dust coma II. Aspherical-homogeneous, and spherical-inhomogeneous nuclei Authors: Zakharov, V. V.; Rodionov, A. V.; Lukianov, G. A.; Crifo, J. F. Bibcode: 2009Icar..201..358Z Altcode: We use our newly developed Dust Monte-Carlo (DMC) simulation technique [Crifo, J.F., Lukianov, G.A., Rodionov, A.V., Zakharov, V.V., 2005. Icarus 176, 192-219] 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 r, and that of the same nucleus at large r? (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? Title: Navier Stokes and direct Monte-Carlo simulations of the circumnuclear gas coma. III. Spherical, inhomogeneous sources Authors: Zakharov, V. V.; Rodionov, A. V.; Lukyanov, G. A.; Crifo, J. F. Bibcode: 2008Icar..194..327Z Altcode: 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. Title: Monte-Carlo Modelling of the Circumnuclear Dust Coma: Benchmark Aspherical-Homogeneous and Spherical-Inhomogeneous Nuclei Authors: Zakharov, V. V.; Crifo, J. F.; Lukianov, G. A.; Rodionov, A. V. Bibcode: 2008LPICo1405.8148Z Altcode: No abstract at ADS Title: Numerical Simulations of Water Spectra Obtained with the Microwave Instrument for the Rosetta Orbiter (MIRO) from Comet 67P/Churyumov-Gerasimenko Authors: Zakharov, V. V.; Bockelée-Morvan, D.; Biver, N.; Crovisier, J.; Crifo, J. F.; Gulkis, S.; Rodionov, A. V. Bibcode: 2008LPICo1405.8144Z Altcode: No abstract at ADS Title: High-resolution CN spectroscopy of small-scale solar magnetic features Authors: Zakharov, V. V.; Gandorfer, A.; Solanki, S. K. Bibcode: 2007msfa.conf..161Z Altcode: High-resolution spectroscopic observations of small-scale magnetic elements in the solar photosphere were carried out in the spectral region 387.5388.4 nm with the 1-m Swedish Solar Telescope (SST). This part of the spectrum covers not only the violet CN band-head, but also contains some lines of the CH molecule. The analysis of the line-core intensity contrasts of the CN and CH lines in bright points (BPs) yielded that on average the BPs appear brighter, thus providing a higher rms contrast, in the CN than in the CH lines in the same spectral band. Title: High-Resolution CN Spectroscopy of Small-Scale Solar Magnetic Features Authors: Zakharov, V. V.; Gandorfer, A.; Solanki, S. K. Bibcode: 2006IAUJD...3E..87Z Altcode: High-resolution spectroscopic observations of the Sun have been carried out with the TRIPPEL spectrograph installed at the new 1-m Swedish Solar Telescope (SST) using realtime AO correction. A detailed spectroscopic analysis of individual photospheric bright points (BP) and faculae-like structures simultaneously in two spectral domains, i.e 387.588<λ<388.473 nm (violet CN band) and in a blue spectral band at 436.1<λ<436.9 nm, containing absorption lines of CH, obtained at the disc center and near the limb is presented. The estimated spatial resolution of the obtained spectra is around 0.25 arcsec while the spectral resolving power is around 130.000 in the first domain, and 76.000 in the second spectral region, respectivlely. The first spectral band covers absorption lines of both, CH and CN molecules, as well as many atomic lines. This enabled us to make a quantitative comparison of their absorption and Doppler shifts in the different photospheric features. The absorption lines of the CN molecule and many atoms are depressed in a BP's interior with respect to those in the quiet Sun. Our quantitative comparison of the relative line depression of CH lines with respect to CN lines showed that the latter have weaker absorption by a factor of 1.28 at the disc centre and 1.32 near the limb. The CN line-core intensity, at the disc centre, has higher BP contrast than the contrast in the CH line-core by a factor of 1.9, and the ratio of these contrasts is decreasing with increasing continuum intensity of the BPs. This trend is similar to that obtained from previous simultaneous G-band and violet CN-band imaging observations. Measurements of contrasts and rms contrasts of line-core, integrated and local continuum intensities are provided. Analysis of Doppler shifts and line broadening of an Fe I line at 387.777 nm revealed an increase of the FWHM in the BP's interior and in dark intergranular lanes and a decrease with increasing intensity of the granules. The first results of a direct comparison of observed CN spectra with those simulated in MHD models in different photospheric features is presented. Title: A new approach for modeling the dust dynamics in the near-nucleus coma Authors: Lukyanov, G. A.; Crifo, J. F.; Zakharov, V. V.; Rodionov, A. V. Bibcode: 2006AdSpR..38.1976L Altcode: We describe the first step of development of a most general code for the investigation of the dust dynamics inside the gas-dust interaction region surrounding an active cometary nucleus. The code is based on a Monte Carlo approach, which allows (a) consideration of realistic, i.e., extremely complicated assumptions with respect to the dust and nucleus size, shape and composition, (b) the derivation of the dust velocity distribution at each point, an information needed for the analysis of future in situ dust samplings, and (c) allowance for all the forces acting on the grains, i.e., aerodynamical, gravitational, radiative and inertial. In the present first step, the trivial (unrealistic) assumptions of nucleus and dust grain homogeneity and sphericity are still made, and only the nucleus gravity, the aerodynamic force and the radiation pressure force are considered. The code was tested assuming a Halley-like grain mass distribution extending from 10 -18 to 10 -2 g, and nucleus H 2O sublimation rates from 3 × 10 26 to 3 × 10 28 molecule/s. The results demonstrate that (1) even in this simple case, none of the preceding forces can be a priori neglected; (2) due to the combined action of these forces, a wealth of currently overlooked complex grain trajectories exist. These results are compared to the previously developed dust multi-fluid (DMF) method, thus evidencing the relative capabilities of the two methods. Title: Direct Monte Carlo and multifluid modeling of the circumnuclear dust coma. Spherical grain dynamics revisited Authors: Crifo, J. -F.; Loukianov, G. A.; Rodionov, A. V.; Zakharov, V. V. Bibcode: 2005Icar..176..192C Altcode: This paper describes the first computations of dust distributions in the vicinity of an active cometary nucleus, using a multidimensional Direct Simulation Monte Carlo Method (DSMC). The physical model is simplistic: spherical grains of a broad range of sizes are liberated by H 2O sublimation from a selection of nonrotating sunlit spherical nuclei, and submitted to the nucleus gravity, the gas drag, and the solar radiation pressure. The results are compared to those obtained by the previously described Dust Multi-Fluid Method (DMF) and demonstrate an excellent agreement in the regions where the DMF is usable. Most importantly, the DSMC allows the discovery of hitherto unsuspected dust coma properties in those cases which cannot be treated by the DMF. This leads to a thorough reconsideration of the properties of the near-nucleus dust dynamics. In particular, the results show that (1) none of the three forces considered here can be neglected a priori, in particular not the radiation pressure; (2) hitherto unsuspected new families of grain trajectories exist, for instance trajectories leading from the nightside surface to the dayside coma; (3) a wealth of balistic-like trajectories leading from one point of the surface to another point exist; on the dayside, such trajectories lead to the formation of "mini-volcanoes." The present model and results are discussed carefully. It is shown that (1) the neglected forces (inertia associated with a nucleus rotation, solar tidal force) are, in general, not negligible everywhere, and (2) when allowing for these additional forces, a time-dependent model will, in general, have to be used. The future steps of development of the model are outlined. Title: Comparison between Navier-Stokes and DSMC Simulations of the Rarefied Gas Flow from Model Cometary Nuclei Authors: Lukyanov, G. A.; Zakharov, V. V.; Rodionov, A. V.; Crifo, J. F. Bibcode: 2005AIPC..762..331L Altcode: We compare two fundamental ways of modeling the steady gas flows in the near-nuclear atmosphere (coma) of comets. Several heliocentric distances and several homogeneous non-rotating model nuclei - spherical and aspherical - were considered, in order to cover a wide range of boundary conditions at the surface of the nucleus and of gas flow rarefaction in the coma. Two methods were used for simulations: (1) the Direct Monte-Carlo Simulation (DSMC), (2) the so-called ``BE-NSE'' method, which self-consistently solves the Boltzmann equation (BE) in the immediate nonequilibrium vicinity of the nucleus surface, and the Navier-Stokes equations (NSE) in the downstream region. For the BE solution, a locally plane-parallel approximation is made, using Cercignani's (1981) algorithms. The two methods were found to agree over an unexpectedly large parameter range. To maximize the modeling efficiency, a hybrid approach combining the BE-NSE approach in the denser regions, with DSMC in the more rarefied regions is also described. Title: Physical Model of the coma of Comet 67P/Churyumov-Gerasimenko Authors: Crifo, J. F.; Lukyanov, G. A.; Zakharov, V. V.; Rodionov, A. V. Bibcode: 2004ASSL..311..119C Altcode: 2004nrt..conf..119C No abstract at ADS Title: Physical Model of the Coma of Comet 67P/Churyumov-Gerasimenko Authors: Crifo, J. -F.; Rodionov, A. V.; Bockelée-Morvan, D.; Lukyanov, G. A.; Zakharov, V. V. Bibcode: 2004cosp...35.1046C Altcode: 2004cosp.meet.1046C Several Rosetta mission instruments will perform totally new observations which require an extremely realistic physical modelling of the coma -- not just approximate orders of magnitude: for instance, the high resolution mm wave sounder MIRO, and the in-situ dust velocity analyzer GIADA. In addition, the deposition of the lander will require an accurate and predictive model of the gas distribution. We describe the design specifications and the present state of an ongoing program of three-dimensional, time-dependent, physically realistic simulation of the gas and dust coma of the comet. The model excludes unphysical simplifications: the nucleus can have any external shape, and assume any realistic rotation state. The gas is assumed to be produced either by surface sublimation, or by diffusion from the interior, or by both processes, and the dust is assumed to be detached from it by the aerodynamic force. Realistic dust shapes can be handled. To compute the gas and dust distribution, the model uses Fluid Equations and/or Direct Monte-Carlo Simulations. For want of space, we cannot show here time-dependent simulations (see paper by A.V. Rodionov et al. in session B1.2), but only present a few recent steady-state results illustrating the present capabilities of the model: (1) an example of DSMC simulation of the gas coma near 3 AU, revealing the extent of the fluid region; (2) the computed structure of the near-nucleus gas and dust coma if the nucleus has a shape similar to that considered possible by Lamy et al (2003); (3) an example of DSMC simulation of the dynamics of heavy grains (near to the lift-off limit), showing complex trajectories as well as fall-backs onto the surface. Title: Navier-Stokes and direct Monte Carlo simulations of the circumnuclear coma II. Homogeneous, aspherical sources Authors: Crifo, J. F.; Loukianov, G. A.; Rodionov, A. V.; Zakharov, V. V. Bibcode: 2003Icar..163..479C Altcode: The dayside near-nucleus comae formed by solar-driven sublimation from two different aspherical nuclei made of an homogeneous mixture of ice and dust are computed by (1) solving Navier-Stokes equations and (2) direct Monte Carlo simulations, for different nucleus sizes, heliocentric distances, and dust-to-ice mixing ratios. Excellent agreement between the two methods is found down to surprisingly low production rates; it is found that the limit of validity of the first method is not simply related to the coma rarefaction: a new dimensionless number is tentatively offered to characterize this limit. The present solutions show that the weak shocks always present in the fluid coma persist practically down to truly free-molecular conditions, excluding the observational discovery of a structureless coma. They also show that rarefied flow in the near-nucleus coma can have a quite complicated structure, in particular inside topographic depressions. As an example, coma recondensation on the sunlit flanks of a cavity was found to be possible. We compute, for the first time, a true collisionless coma and show that structures are still present in it but are confined to the immediate vicinity of the surface. Finally, we describe in detail the kinetic conditions in a rarefied water coma, i.e., the velocity distribution asymmetry and the rotational-translational nonequilibrium. The significance of the results for future missions to comets is outlined. Title: Comparison between Navier-Stokes and Direct Monte-Carlo Simulations of the Circumnuclear Coma. I. Homogeneous, Spherical Source Authors: Crifo, J. F.; Lukianov, G. A.; Rodionov, A. V.; Khanlarov, G. O.; Zakharov, V. V. Bibcode: 2002Icar..156..249C Altcode: The structure of the near-nucleus H 2O atmosphere formed by sublimation under solar heating of a hypothetical large dusty ice sphere is computed (1) by the direct simulation Monte-Carlo method (DSMC) and (2) by solving Navier-Stokes equations (NSE) combined with a locally plane-parallel solution of the collisional Boltzmann equation for the nonequilibrium near-surface Knudsen layer. For Hale-Bopp-like comets, perfect agreement is obtained between the two methods on the day and night sides. This excellent agreement is maintained on the near-nucleus dayside for less productive comets, even down to production rates as low as those expected for Comet P/Wirtanen near 3 AU. It provides a direct validation of the gasdynamic simulations performed in support of the ESA Rosetta mission lander descent optimization studies (Crifo et al. 2001) and also confirms the similarity between the dayside coma and underexpanded axially symmetric free jets pointed out in Crifo (1986). On the nightside, moderate to high discrepancies appear between the two solutions as the production rate decreases, revealing the limits of the NSE method. The limits of the present study are delineated, and directions for future investigations are indicated.