CHARGING OF DUST GRAINS IN COMETARY PLASMA ENVIRONMENTS

The equilibrium potential of fluffy dust grains in plasma environments is modeled numerically for comet P/Halley. It is found that the dust grain acquires its largest negative potential and probability of disruption at about 50, 000km from the comet where the mass spectrum measured by the spacecraft took a great change.     

河外星际尘埃辐射对微波背景辐射的影响

本文借助于观测到的大、小麦哲伦云的星际尘埃辐射，估计总星系内的“冷”星际尘埃热再辐射对宇宙微波背景辐射的影响．结果表明：总星系内的星际“冷”尘埃的热辐射所形成的背景辐射对微波背景辐射的扰动强烈地依赖于宇宙减速因子和“冷”尘埃量，在宇宙背景探测者（ＣＯＢＥ）的观测结果的限制下，无论宇宙减速因子取何种值，“冷”尘埃所占的比例都是非常少的，如果Ｏｓｔｒｉｋｅｒ所作的平均每个星系内由尘埃产生的蓝光光深τＢ＝０．５的假定是合理的，那么星际尘埃量随温度的分布是非常不均匀的。     

The temperature of nonspherical circumstellar dust grains

The temperatures of prolate and oblate spheroidal dust grains in the envelopes of stars of various spectral types are calculated. Homogeneous particles with aspect ratios a/b≤10 composed of amorphous carbon, iron, dirty ice, various silicates, and other materials are considered. The temperatures of spherical and spheroidal particles were found to vary similarly with particle size, distance to the star, and stellar temperature. The temperature ratio T _d(spheroid)/T _d(sphere) depends most strongly on the grain chemical composition and shape. Spheroidal grains are generally colder than spherical particles of the same volume; only iron spheroids can be slightly hotter than iron spheres. At a/b≈2, the temperature differences do not exceed 10%. If a/b≥4, the temperatures can differ by 30–40%. For a fixed dust mass in the medium, the fluxes at wavelengths λ≥100 are higher if the grains are nonspherical, which gives overestimated dust masses from millimeter observations. The effect of grain shape should also be taken into account when modeling Galactic-dust emission properties, which are calculated when searching for fluctuations of the cosmic microwave background radiation in its Wien wing.     

Hamiltonian Formulation and Perturbations for Dust Motion Around Cometary Nuclei

In this paper we analyze the dynamical behavior of large dust grains in the vicinity of a cometary nucleus. To this end we consider the gravitational field of the irregularly shaped body, as well as its electric and magnetic fields. Without considering the effect of gas friction and solar radiation, we find that there exist grains which are static relative to the cometary nucleus; the positions of these grains are the stable equilibria. There also exist grains in the stable periodic orbits close to the cometary nucleus. The grains in the stable equilibria or the stable periodic orbits won’t escape or impact on the surface of the cometary nucleus. The results are applicable for large charge dusts with small area-mass ratio which are near the cometary nucleus and far from the Solar. It is found that the resonant periodic orbit can be stable, and there exist stable non-resonant periodic orbits, stable resonant periodic orbits and unstable resonant periodic orbits in the potential field of cometary nuclei. The comet gravity force, solar gravity force, electric force, magnetic force, solar radiation pressure, as well as the gas drag force are all considered to analyze the order of magnitude of these forces acting on the grains with different parameters. Let the distance of the dust grain relative to the mass centre of the cometary nucleus, the charge and the mass of the dust grain vary, respectively, fix other parameters, we calculated the strengths of different forces. The motion of the dust grain depends on the area-mass ratio, the charge, and the distance relative to the comet’s mass center. For a large dust grain (> 1 mm) close to the cometary nucleus which has a small value of area-mass ratio, the comet gravity is the largest force acting on the dust grain. For a small dust grain (< 1 mm) close to the cometary nucleus with large value of area-mass ratio, both the solar radiation pressure and the comet gravity are two major forces. If the a small dust grain which is close to the cometary nucleus have the large value of charge, the magnetic force, the solar radiation pressure, and the electric force are all major forces. When the large dust grain is far away from the cometary nucleus, the solar gravity and solar radiation pressure are both major forces.     

Dynamical behaviors of size graded dust grains levitated in robust sheath in inhomogeneous plasmas

Our interest is to study the sheath formation in an inhomogeneous plasma coexisting with an interaction of weak ionization. Pseudopotential analysis has been employed to derive the coherent structures of sheath in plasma. It has shown that the ionization affects the growth of sheath in plasma and nature depends fully on plasma constituents as well. After getting a robust sheath, dynamical behaviors of a levitated dust grain into the robust sheath has been studied which, in fact, leads to find the variation of dust potential, dust sizes along with the net force generated on grains. Results are obtained numerical for some typical plasma parameters. It has demonstrated that the plasma constituent effects the clustering of dust grains in different region within the sheath as a result of which dust agglomeration forms nebulons: patches of dust cloud-like structures with changing fleece.     

The effect of grain size distribution on H2 formation rate in the interstellar medium

The formation of molecular hydrogen  (H₂)  in the interstellar medium takes place on the surfaces of dust grains. Hydrogen molecules play a role in gas-phase reactions that produce other molecules, some of which serve as coolants during gravitational collapse and star formation. Thus, the evaluation of the production rate of hydrogen molecules and its dependence on the physical conditions in the cloud are of great importance. Interstellar dust grains exhibit a broad size distribution in which the small grains capture most of the surface area. Recent studies have shown that the production efficiency strongly depends on the grain composition and temperature as well as on its size. In this paper, we present a formula that provides the total production rate of  H₂  per unit volume in the cloud, taking into account the grain composition and temperature as well as the grain size distribution. The formula agrees very well with the master equation results. It shows that for a physically relevant range of grain temperatures, the production rate of  H₂  is significantly enhanced due to their broad size distribution.     

Arbitrary Amplitude Dust-acoustic Solitary Structures in aThree-component Dusty Plasma

A rigorous theoretical investigation has been made of arbitrary amplitude dust-acoustic solitary structures in an unmagnetized three-component dusty plasma whose constituents are an inertial charged dust fluid and Boltzmann distributed ions and electrons. The pseudo-potential approach and the reductive perturbation technique are employed for this study. It is found from both weakly and highly nonlinear analyses that the dusty plasma model can support solitary waves only with negative potential but not with positive potential. The effects of equilibrium free electron density and its temperature on these solitary structures are discussed. The implications of these results to some astrophysical and space plasma systems, especially to planetary ring-systems and cometary tails, are briefly mentioned. This revised version was published online in July 2006 with corrections to the Cover Date.     

The charge distribution on interstellar grains—The effect of induced charge

A calculation of the equilibrium charge acquired by interstellar grains is given, which takes account of polarization charges that are induced in a grain by incident ions and electrons.Both metal and dielectric grains are considered and photoionization of the latter grains by UV radiation is taken into account where necessary. It is found that the inclusion of the polarization charges in the calculation is only important in gas clouds where the mean charge on a grain is low (<1e); that is, for HI regions and dense molecular clouds. In such clouds, the effect of the polarization charges is to increase the amount of negative charge acquired by a grain. A discussion is given concerning the validity of the classical electrostatic theory employed in the paper for small grains of radius 10^–6 cm, and some astrophysical consequences of the modification of the grain charge by polarization effects are considered.     

Effect of Flow on Resonant Absorption of Slow MHD Waves in Coronal Arcades

Resonant absorption of slow MHD waves is studied numerically by using the SGH method and is applied to a model of a coronal arcade in the presence of equilibrium plasma flows. The arcade is approximated by a 1D horizontal magnetic slab that is non-uniform along the vertical direction and which is surrounded by two homogeneous media. While propagating from the photosphere upwards into the corona, the magneto-acoustic waves can be resonantly absorbed in the inhomogeneous region of the arcade. Computational results show that the resonant absorption of the impinging waves strongly depends on the equilibrium model and on the characteristics of the driving wave. The results also indicate that the presence of an equilibrium plasma flow along the magnetic field of the arcade reduces the resonant absorption for the flow speed parameters considered.     

Lofted charged dust distribution above the Moon surface

We developed kinetic theory for the charging processes of small dust grains near the lunar surface due to interaction with the anisotropic solar wind plasma. Once charged, these dust grains, which are exposed to the electric field in the sheath region near the lunar surface, could loft and distribute around such heights off the surface where they reach equilibrium with the local gravitational force. Analytical solutions were derived for the charging time, grain floating potential, and grain charge, characterizing the charging processes of small dust grains in a two-component and in a multi-component solar wind plasma, and further highlighting the unique features presented by the high streaming plasma velocity. We have also formulated a novel kinetic theory of sheath formation around an absorbing planar surface immersed in the anisotropic solar wind plasma in the case of a negligible photoelectric effect and presented solutions for the sheath structure. In this study we combined the results from these analyses and provided estimates for the size distribution function of dust that is expected to be lofted in regions dominated by the solar wind plasma, such as near the terminator and in nearby shadowed craters. Corresponding to the two dominant streaming velocity peaks of 300 and 800 km/s, mean dust diameters of 500 and 350 nm, respectively, are expected to be found at equilibrium at heights of relevance to exploration operations, e.g., around 1.5 m height off the lunar surface. In shadowed craters near the terminator region, where isotropic plasma should be dominating, we estimate mean lofted dust diameter of 800 nm around the same 1.5 m height off the lunar surface. The generally applicable solutions could be used to readily calculate the expected lofted size distribution near the lunar surface as a function of plasma parameters, dust grain composition, and other parameters of interest.     

Modelling the circumnuclear coma of comets: objectives,methods and recent results

We investigate whether the modelling of the immediate vicinity of an active nucleus—currently unobservable—can, as the modelling of the outer, observable coma, be based on unrealistic simple assumptions such as those of nucleus and dust grains sphericity. We point out the inconsistency of models based on such assumptions, which, to manage compatibility with the observations, have to introduce additional assumptions that conflict with the previous ones, such as the existence of active areas of the nucleus. We argue that, while the outer coma models being phenomenological in nature, can perhaps tolerate such inconsistencies, the circumnuclear coma models must be predictive, having to obviate the lack of observational data, and therefore must exclude implausible and ad hoc assumptions, and advocate only well-understood physical processes and duly validated modelling methods. We describe the first steps of development of a predictive circumnuclear coma model, and present a set of results obtained with parameters fitted to comet P/Wirtanen, the target of the Rosetta mission, but of a quite general significance. Considering, first an inhomogeneous spherical nucleus with spherical dust grains, and then an aspherical homogeneous nucleus with spherical dust grains, we show that, in both cases (1) the surface temperature and initial gas parameters differ considerably from the Hertz–Knudsen values; (2) the near-surface gas and dust flows are not in general vertical, (3) the gas and dust density do not always monotonically decrease outwards, (4) the gas and dust velocity vary strongly from point to point, (5) shock structures are formed, which result in the formation of pseudo-jets of dust grains originating from various points of the surface. No simple method to distinguish between dust structures created by the surface inhomogeneity and by the surface orography is found. We show, for the first time, the deformation of the near-nucleus dust coma during a full rotation of an homogeneous, aspherical nucleus. We also show that identical active regions located at different points of an inhomogeneous spherical nucleus produce very different dust distributions, suggesting that the dust distribution is also strongly deformed during the rotation of such a nucleus. Finally, we consider, for the first time, a spherical homogeneous nucleus emitting aspherical dust grains. We show that, in such a case, the terminal grain velocity depends upon the shape, initial position, and even possibly upon the initial orientation of the grain at the surface, so that there cannot exist a precise relation between terminal velocity and dust grain mass. We conclude that, far from giving an approximate or average representation of the circumnuclear coma, the classical modelling approach yields in this region predictions that are in total conflict with the real behaviour of the gas and dust. As a most dramatic consequence, the use of this classical approach may have obscured completely the significance of the few direct and of the many indirect informations acquired hitherto on the nucleus activity.     

Observing the effects of the polarization force in strongly coupled dusty plasmas with suprathermal electrons

Our objective here is to investigate a strongly coupled dusty plasma system with the presence of polarization force (PF). This plasma consists of superthermal electrons, Maxwellian ions, and negatively charged dust grains. The nonlinear propagation of dust-acoustic (DA) waves in such dusty plasma system has been theoretically investigated by employing the reductive perturbation method. The Burgers’ and K-dV equations have been derived to and numerically analyzed. It has been found that the dust-acoustic shock and solitary waves exist associated with a negative potential only, and that the effect of the dust fluid temperature significantly modifies the basic properties (amplitude and width) of such nonlinear waves’ potential structures. We hope that the results of our present investigation should help us in understanding the localized electrostatic disturbances in space and laboratory strongly coupled dusty plasmas with superthermal electrons and polarization force.     

Some constraints on particles in Saturn's spokes

The consequences of taking into account some physical processes previously ignored in the context of Saturn's spokes were studies. Secondary emission, which can ensure large changes of potential following small environment variations, may provide a plausible mechanism for triggering spoke formation. Taking into account the grains' centrifugal disruption due to their spin, should modify the constraints on size previously derived on the basis of electrostatic disruption alone. Taking into account the dielectric polarization when calculating the electrostatic forces on grains resting on parent bodies gives ejection conditions more stringent than previously expected. Finally, a simplified evaluations shows that if secondary emission properties of ring particle are of the order of those of many other oxides, and if the plasma temperature is near one of their threshold temperatures, then a very small environmental variation can trigger the ejection of about 0.1- to 1-μm grains from about 1-mm to 10-cm parent bodies in a very short time scale.     

Microwave emission from aligned dust

Polarized microwave emission from dust is an important foreground that may contaminate polarized CMB studies unless carefully accounted for. We discuss potential difficulties associated with this foreground, namely, the existence of different grain populations with very different emission/polarization properties and variations of the polarization yield with grain temperature. In particular, we discuss observational evidence in favor of rotational emission from tiny PAH particles with dipole moments, i.e. “spinning dust”, and also consider magneto-dipole emission from strongly magnetized grains. We argue that in terms of polarization, the magneto-dipole emission may dominate even if its contribution to total emissivity is subdominant. Addressing polarized emission at frequencies larger than 100 GHz, we discuss the complications arising from the existence of dust components with different temperatures and possibly different alignment properties.     

K-dV and Burgers’ equations on DA waves with strongly coupled dusty plasma

A rigorous theoretical investigation has been made on the nonlinear structures, mainly, dust-acoustic (DA) solitary and shock waves propagating in a strongly coupled dusty plasma consisting of strongly coupled correlated positively and negatively charged inertial cold dust fluid, weakly correlated inertialess Maxwellian electron and ion fluids. The presence of arbitrary (negative and positive) charged dust grains in such a plasma system causes the presence opposite potentials (positive and negative) DA solitary and shock structures and significantly modify it’s basic features. The results obtained from this analysis can be employed in understanding the nature of plasma waves both in laboratory and space plasma system.     

Modeling the IR spectrum of the dust shell around the [WC] star: V605 AQL

Nebulae around low mass Wolf-Rayet [WC] stars often show a strong midand near infrared (MN-IR) emission which cannot be explained with asingle grain size and an equilibrium temperature model. Weinvestigated the effect of the grain size, radiation density, andtemperature fluctuations of small dust grains on the spectral energydistribution (SED) in the infrared.A model of ISOCAM and IRAS measurements of V605 Aql (Kimeswenger et al., 1998) has beencomputed using a gas-dust combination model including transiently heatedparticles. Our calculations show that the central knot in V605 Aql(Abell 58) is too compact or just on the lower boundary that temperature fluctuations really have to be considered in model calculations forthe infrared SED.     

Demonstration of crystalline forsterite grain formation due to coalescence growth of Mg and SiO smoke particles

Abstract— Experimental studies of coalescence between Mg grains and SiO grains in smoke reveal the direct production of crystalline forsterite grains. The present results also show that different materials can be produced by grain‐grain collisions, which have been considered one of the models of grain formation in the interstellar medium. The fundamentals of coalescence growth in smoke, which have been developed in our series of experiments, are presented in this paper. Mg₂Si polyhedral grains were obtained in a Mg grain‐rich atmosphere. Mg₂SiO₄ polyhedral grains were obtained in a SiO grain‐rich atmosphere. The IR spectra of the resultant grains showed the characteristics of crystalline forsterite.     

Thermal coagulation of charged grains in dense clouds

A treatment is given of the thermal coagulation of interstellar grains, taking their charge fully into account. The original calculations showed this charge to be negative unless photoionization dominated when it became positive. More recently, the effects of discreteness in the charge and of grain polarization have been considered and all these effects are included in the present work. The general conclusion is that the effect of charge is to decrease grain coagulation although a very small increase can occur for certain situations in the presence of photoionization.     

The potential of an electrostatically clean geostationary satellite and its use in plasma diagnostics

Electrostatic charging has given rise to problems on several geostationary spacecraft. This has led to a rigorous electrostatic cleanliness approach in the case of the scientific geostationary satellite GEOS in order to secure correct electric field and low energy plasma measurements. The present paper outlines the relevant charging mechanism, describes a new method for the determination of the equilibrium potential, and reports on actual potential measurements. The potentials observed are very closely related to the actual plasma conditions at the geostationary orbit. It is generally possible to use the potential measurements to characterize the particle population encountered by the spacecraft.Measurements carried out over a period of 4 years are presented by way of examples. A careful analysis shows that the chosen examples are representative and reflect the conditions observed on all other days of the mission. The results lead to the overall conclusion that the equilibrium potential of GEOS in sunlight is always moderately positive and only rarely exceeds + 10 V with respect to ambient space. At no instance in the sunlit portion of the orbit does the spacecraft assume a negative potential. We find that the observed moderate positive equilibrium potential generally is a function of cold plasma density. During the night and early morning part of the orbit we can, however, identify periods where the high energy particle population dictates the equilibrium potential. The electrostatic cleanliness design of GEOS avoids negative charging also under these conditions. In eclipse, a negative potential cannot be avoided but here the electrostatic cleanliness approach chosen for GEOS prevents any differential charging and avoids potentials of several thousand volts which have appeared on other satellites. The cost, in time and effort, of the precautions employed has clearly been justified. The specially developed techniques have since been used on other satellites and the lessons learned have also been applied successfully to operational spacecraft such as METEOSAT 2.