Fine Dust Grains Research Articles

Electrostatic charging of permanently shadowed craters on the Moon

ABSTRACT An open question of the electrostatic charge development on the lunar surface in the electron-rich region within the permanently shadowed craters (PSCs) is addressed. We propose that the fine dust grains on the crater surface may act as efficient field emission centres generating electrons via quantum field tunnelling. This return current may be sufficient to establish a steady-state dynamical equilibrium for the surface-plasma system. This leads to the crater surface attaining a finite electric potential. Our analysis illustrates that the PSC having ∼100 nm dust, covering 1 per cent of the surface area within the electron-rich region, may acquire a negative potential of few hundred volts in the steady-state condition.

Electron density modification in ionospheric E layer by inserting fine dust particles

In this paper, we have developed the kinetics of E-region ionospheric plasma comprising of fine dust grains and shown that the electron density in E-layer can purposely be reduced/enhanced up to desired level by inserting fine dust particles of appropriate physical/material properties; this may certainly be promising for preferred rf-signal processing through these layers. The analytical formulation is based on average charge theory and includes the number and energy balance of the plasma constituents along with charge balance over dust particles. The effect of varying number density, work function, and photo-efficiency of dust particles on ionospheric plasma density at different altitude in E-layer has been critically examined and presented graphically.

The electromagnetic pickup of submicron-sized dust above Enceladus’s northern hemisphere

As the saturnian magnetoplasma sweeps past Enceladus, it experiences both a decrease in electron content and sharp slowdown in the northern hemisphere region within ∼5 Enceladus Radii (Re). This slowdown is observed by Cassini in regions not obviously associated with the southern directed plume-originating ions. We suggest herein that the decrease in northern hemisphere electron content and plasma slowdown could both be related to the presence of fine dust grains that are being accelerated by the Lorentz force created within the saturnian magnetic field system.

RESOLVING THE CIRCUMSTELLAR DISK OF HL TAURI AT MILLIMETER WAVELENGTHS

We present results of high-resolution imaging toward HL Tau by the Combined Array for Research in Millimeter-wave Astronomy (CARMA). We have obtained 1.3 and 2.7 mm dust continua with an angular resolution down to 0.13 arc second. Through model fitting to the two wavelength data simultaneously in Bayesian inference using a flared viscous accretion disk model, we estimate the physical properties of HL Tau, such as density distribution, dust opacity spectral index, disk mass, disk size, inclination angle, position angle, and disk thickness. HL Tau has a circumstellar disk mass of 0.13 solar mass, a characteristic radius of 79 AU, an inclination of 40 degree, and a position angle of 136 degree. Although a thin disk model is preferred by our two wavelength data, a thick disk model is needed to explain the high mid- and far-infrared emission of the HL Tau spectral energy distribution. This could imply large dust grains settled down on the mid plane with fine dust grains mixed with gas. The HL Tau disk is likely gravitationally unstable and can be fragmented between 50 and 100 AU of radius. However, we did not detect dust thermal continuum supporting the protoplanet candidate claimed by a previous study using observations of the Very Large Array at 1.3 cm.

Lunar dust charging by photoelectric emissions

The lunar surface is covered with a thick layer of sub-micron/micron size dust grains formed by meteoritic impact over billions of years. The fine dust grains are levitated and transported on the lunar surface, as indicated by the transient dust clouds observed over the lunar horizon during the Apollo 17 mission. Theoretical models suggest that the dust grains on the lunar surface are charged by the solar ultraviolet (UV) radiation as well as the solar wind. Even without any physical activity, the dust grains are levitated by electrostatic fields and transported away from the surface in the near vacuum environment of the Moon. The current dust charging and levitation models, however, do not fully explain the observed phenomena. Since the abundance of dust on the Moon's surface with its observed adhesive characteristics has the potential of severe impact on human habitat and operations and lifetime of a variety of equipment, it is necessary to investigate the charging properties and the lunar dust phenomena in order to develop appropriate mitigating strategies. Photoelectric emission induced by the solar UV radiation with photon energies higher than the work function (WF) of the grain materials is recognized to be the dominant process for charging of the lunar dust, and requires measurements of the photoelectric yields to determine the charging and equilibrium potentials of individual dust grains. In this paper, we present the first laboratory measurements of the photoelectric efficiencies and yields of individual sub-micron/micron size dust grains selected from sample returns of Apollo 17 and Luna-24 missions as well as similar size dust grains from the JSC-1 simulants. The measurements were made on a laboratory facility based on an electrodynamic balance that permits a variety of experiments to be conducted on individual sub-micron/micron size dust grains in simulated space environments. The photoelectric emission measurements indicate grain size dependence with the yield increasing by an order of magnitude for grains of sub-micron to several micron size radii, at which it reaches asymptotic values. The yield for large size grains is found to be more than an order of magnitude higher than the bulk measurements on lunar fines reported in the literature.

Numerical Simulation of Dust Lifting within Dust Devils—Simulation of an Intense Vortex

Abstract Based on an advanced dust devil–scale large-eddy simulation (LES) model, the atmosphere flow of a modeled dust devil in a quasi–steady state was first simulated to illustrate the characteristics of the gas phase field in the mature stage, including the prediction of the lower pressure and higher temperature in the vortex core. The dust-lifting physics is examined in two aspects. Through the experimental data analysis, it is verified again that the horizontal swirling wind can only make solid particles saltate along the ground surface. Based on a Lagrangian reference frame, the tracks of dust grains with different density (material) and diameter are calculated to show the effect of dust particles entrained by the vertical swirling wind field. The movement of solid particles depends on the interactions between the aloft dust particles and the airflow field of dust devils, in which the drag and the centrifugal force component on the horizontal plane are the key force components. There is the trend of the fine dust grains rising along the inner helical tracks while the large dust grains are lifting along the outer helical tracks and then descending beyond the corner region, resulting in the impact between different-sized dust grains in the swirling atmospheric flow. This trend will make the dust stratification, developing a top small-sized grain domain and a bottom large-sized grain domain in dust devils.

Blowing in the wind: I. Velocities of chondrule-sized particles in a turbulent protoplanetary nebula

Small but macroscopic particles—chondrules, higher temperature mineral inclusions, metal grains, and their like—dominate the fabric of primitive meteorites. The properties of these constituents, and their relationship to the fine dust grains which surround them, suggest that they led an extended existence in a gaseous protoplanetary nebula prior to their incorporation into their parent primitive bodies. In this paper we explore in some detail the velocities acquired by such particles in a turbulent nebula. We treat velocities in inertial space (relevant to diffusion), velocities relative to the gas and entrained microscopic dust (relevant to accretion of dust rims), and velocities relative to each other (relevant to collisions). We extend previous work by presenting explicit, closed-form solutions for the magnitude and size dependence of these velocities in this important particle size regime, and we compare these expressions with new numerical calculations. The magnitude and size dependence of these velocities have immediate applications to chondrule and CAI rimming by fine dust and to their diffusion in the nebula, which we explore separately.

Some recent developments in gravitoelectrodynamics of charged dust in cosmic environments

The motion of charged circumplanetary dust particles under the combined influence of gravitational and electrodynamic forces of comparable magnitude (“gravito-electrodynamics”) was first discussed by Mendis and Axford (1974) over 25 years ago. This area got a major boost in the early eighties with the Voyager spacecraft observation of peculiar dust features in Saturn's rings (e.g. the so called “spokes”) which could not be explained by gravitational forces alone but could be easily explained by the inclusion of electrodynamic forces on the fine dust grains which were necessarily electrically charged. Since then the progress in the field has been steady with applications also to the dust tails of comets. More recently the role of gravito-electrodynamics in the solar system was underscored by the Ulysses and Galileo observation of periodic, collimated beams of high speed grains emanating from Jupiter. In this brief review I will discuss these as well as the possible role of electrical charging on the transport of dust (Al 2O 3 spherules) injected in large quantities into the terrestrial magnetosphere during solid rocket propellant burns. I will conclude with some comments on the role of gravitoelectrodynamic effects on the fine dust forming a circum-solar ring around 4 solar radii.

Observations of dusty plasmas with magnetized dust grains

We report a newly observed phenomenon in a dusty plasma device of the\\mbox{Q-machine} type. At low plasma densities the time required by theplasma to return to its no-dust conditions, after the dust dispenser is turnedoff, can be as long as many tens of seconds or longer. A tentativeinterpretation of this observation in terms of magnetized dust grains isadvanced. It appears that an important loss mechanism of fine dust grains isby ion drag along the magnetic field lines. The effect of ion drag is somewhatcounteracted by the -µ∇B force present when the magnetic field has amirror geometry.

Gyrophase drifts and the orbital evolution of dust at Jupiter's gossamer ring

Jupiter's “gossamer” ring, which apparently consists largely of fine dust grains, and extends outward from the main ring to the vicinity of Thebe at ∼3.11 R J, has a significant peak at the synchronous radius 2.24 R J. The ring's discoverers (Showalter, Burns, Cuzzi and Pollack) believe that large bodies distributed in the vicinity of synchronous radius act as sources for this dust, which then drifts radially away from the synchronous radius, due to plasma drag. Here we will show that the so-called gyrophase drift may exceed the plasma drag drift and may move small charged grains either toward or away from synchronous radius. In the presence of a radial gradient in plasma temperature, the grain gyrophase drifts toward the higher temperature, so if the plasma is hotter at synchronous radius, the gyrophase drift could overcome the plasma drag drift and concentrate the dust at synchronous radius, with Amalthea, for example, as a source. Gyrophase drift will also take place if there is a radial gradient in the relative concentrations of different plasma ion species, or because of plasma-grain velocity variation due to the grain's cycloidal motion through the plasma. In order to popularize the gyrophase drifts as ones which must be taken into account in any study of the fine grain dynamics in a magnetosphere, we evaluate some of them for grains launched at the local Kepler velocity. We also display numerical orbit integrations for comparison with the analytic adiabatic theory. The Poynting-Robertson drift is evaluated and found to be miniscule compared to the plasma drag or gyrophase drifts. The inner edge of the Io plasma torus is the location of a very steep gradient in plasma temperature which may, by means of the gyrophase drift, act to keep fine dust from Io's volcanos from penetrating to the inner magnetosphere. However, there is diffusive motion superposed on the steady gyrophase drift. For sufficiently large dust grains this diffusion will overcome the excluding effect of gyrophase drift and permit these grains to reach the inner magnetosphere.