873 research outputs found
Grid-Free 2D Plasma Simulations of the Complex Interaction Between the Solar Wind and Small, Near-Earth Asteroids
We present results from a new grid-free 2D plasma simulation code applied to a small, unmagnetized body immersed in the streaming solar wind plasma. The body was purposely modeled as an irregular shape in order to examine photoemission and solar wind plasma flow in high detail on the dayside, night-side, terminator and surface-depressed 'pocket' regions. Our objective is to examine the overall morphology of the various plasma interaction regions that form around a small body like a small near-Earth asteroid (NEA). We find that the object obstructs the solar wind flow and creates a trailing wake region downstream, which involves the interplay between surface charging and ambipolar plasma expansion. Photoemission is modeled as a steady outflow of electrons from illuminated portions of the surface, and under direct illumination the surface forms a non-monotonic or ''double-sheath'' electric potential upstream of the body, which is important for understanding trajectories and equilibria of lofted dust grains in the presence of a complex asteroid geometry. The largest electric fields are found at the terminators, where ambipolar plasma expansion in the body-sized night-side wake merges seamlessly with the thin photoelectric sheath on the dayside. The pocket regions are found to be especially complex, with nearby sunlit regions of positive potential electrically connected to unlit negative potentials and forming adjacent natural electric dipoles. For objects near the surface, we find electrical dissipation times (through collection of local environmental solar wind currents) that vary over at least 5 orders of magnitude: from 39 Micro(s) inside the near-surface photoelectron cloud under direct sunlight to less than 1 s inside the particle-depleted night-side wake and shadowed pocket region
Particle-In-Cell Simulations of the Solar Wind Interaction with Lunar Crustal Magnetic Anomalies: Magnetic Cusp Regions
As the solar wind is incident upon the lunar surface, it will occasionally encounter lunar crustal remanent magnetic fields. These magnetic fields are small-scale, highly non-dipolar, have strengths up to hundreds of nanotesla, and typically interact with the solar wind in a kinetic fashion. Simulations, theoretical analyses, and spacecraft observations have shown that crustal fields can reflect solar wind protons via a combination of magnetic and electrostatic reflection; however, analyses of surface properties have suggested that protons may still access the lunar surface in the cusp regions of crustal magnetic fields. In this first report from a planned series of studies, we use a 1 1/2-dimensional, electrostatic particle-in-cell code to model the self-consistent interaction between the solar wind, the cusp regions of lunar crustal remanent magnetic fields, and the lunar surface. We describe the self-consistent electrostatic environment within crustal cusp regions and discuss the implications of this work for the role that crustal fields may play regulating space weathering of the lunar surface via proton bombardment
A Comparison of ARTEMIS Observations and Particle-in-cell Modeling of the Lunar Photoelectron Sheath in the Terrestrial Magnetotail
As an airless body in space with no global magnetic field, the Moon is exposed to both solar ultraviolet radiation and ambient plasmas. Photoemission from solar UV radiation and collection of ambient plasma are typically opposing charging currents and simple charging current balance predicts that the lunar dayside surface should charge positively; however, the two ARTEMIS probes have observed energydependent loss cones and high-energy, surface-originating electron beams above the dayside lunar surface for extended periods in the magnetosphere, which are indicative of negative surface potentials. In this paper, we compare observations by the ARTEMIS P1 spacecraft with a one dimensional particle-in-cell simulation and show that the energy-dependent loss cones and electron beams are due to the presence of stable, non-monotonic, negative potentials above the lunar surface. The simulations also show that while the magnitude of the non-monotonic potential is mainly driven by the incoming electron temperature, the incoming ion temperature can alter this magnitude, especially for periods in the plasma sheet when the ion temperature is more than twenty times the electron temperature. Finally, we note several other plasma phenomena associated with these non-monotonic potentials, such as broadband electrostatic noise and electron cyclotron harmonic emissions, and offer possible generation mechanisms for these phenomena
Discrete structure of ultrathin dielectric films and their surface optical properties
The boundary problem of linear classical optics about the interaction of
electromagnetic radiation with a thin dielectric film has been solved under
explicit consideration of its discrete structure. The main attention has been
paid to the investigation of the near-zone optical response of dielectrics. The
laws of reflection and refraction for discrete structures in the case of a
regular atomic distribution are studied and the structure of evanescent
harmonics induced by an external plane wave near the surface is investigated in
details. It is shown by means of analytical and numerical calculations that due
to the existence of the evanescent harmonics the laws of reflection and
refraction at the distances from the surface less than two interatomic
distances are principally different from the Fresnel laws. From the practical
point of view the results of this work might be useful for the near-field
optical microscopy of ultrahigh resolution.Comment: 25 pages, 16 figures, LaTeX2.09, to be published in Phys.Rev.
Compression Behaviour of Porous Dust Agglomerates
The early planetesimal growth proceeds through a sequence of sticking
collisions of dust agglomerates. Very uncertain is still the relative velocity
regime in which growth rather than destruction can take place. The outcome of a
collision depends on the bulk properties of the porous dust agglomerates.
Continuum models of dust agglomerates require a set of material parameters that
are often difficult to obtain from laboratory experiments. Here, we aim at
determining those parameters from ab-initio molecular dynamics simulations. Our
goal is to improveon the existing model that describe the interaction of
individual monomers. We use a molecular dynamics approach featuring a detailed
micro-physical model of the interaction of spherical grains. The model includes
normal forces, rolling, twisting and sliding between the dust grains. We
present a new treatment of wall-particle interaction that allows us to perform
customized simulations that directly correspond to laboratory experiments. We
find that the existing interaction model by Dominik & Tielens leads to a too
soft compressive strength behavior for uni and omni-directional compression.
Upon making the rolling and sliding coefficients stiffer we find excellent
agreement in both cases. Additionally, we find that the compressive strength
curve depends on the velocity with which the sample is compressed. The modified
interaction strengths between two individual dust grains will lead to a
different behaviour of the whole dust agglomerate. This will influences the
sticking probabilities and hence the growth of planetesimals. The new parameter
set might possibly lead to an enhanced sticking as more energy can be stored in
the system before breakup.Comment: 11 pages, 14 figures, accepted for publication in A&
Lunar Pickup Ions Observed by ARTEMIS: Spatial and Temporal Distribution and Constraints on Species and Source Locations
ARTEMIS observes pickup ions around the Moon, at distances of up to 20,000 km from the surface. The observed ions form a plume with a narrow spatial and angular extent, generally seen in a single energy/angle bin of the ESA instrument. Though ARTEMIS has no mass resolution capability, we can utilize the analytically describable characteristics of pickup ion trajectories to constrain the possible ion masses that can reach the spacecraft at the observation location in the correct energy/angle bin. We find that most of the observations are consistent with a mass range of approx. 20-45 amu, with a smaller fraction consistent with higher masses, and very few consistent with masses below 15 amu. With the assumption that the highest fluxes of pickup ions come from near the surface, the observations favor mass ranges of approx. 20-24 and approx. 36-40 amu. Although many of the observations have properties consistent with a surface or near-surface release of ions, some do not, suggesting that at least some of the observed ions have an exospheric source. Of all the proposed sources for ions and neutrals about the Moon, the pickup ion flux measured by ARTEMIS correlates best with the solar wind proton flux, indicating that sputtering plays a key role in either directly producing ions from the surface, or producing neutrals that subsequently become ionized
Cosmological solutions, p-branes and the Wheeler-DeWitt equation
The low energy effective actions which arise from string theory or M-theory
are considered in the cosmological context, where the graviton, dilaton and
antisymmetric tensor field strengths depend only on time. We show that previous
results can be extended to include cosmological solutions that are related to
the E_N Toda equations. The solutions of the Wheeler-DeWitt equation in
minisuperspace are obtained for some of the simpler cosmological models by
introducing intertwining operators that generate canonical transformations
which map the theories into free theories. We study the cosmological properties
of these solutions, and also briefly discuss generalised Brans-Dicke models in
our framework. The cosmological models are closely related to p-brane solitons,
which we discuss in the context of the E_N Toda equations. We give the explicit
solutions for extremal multi-charge (D-3)-branes in the truncated system
described by the D_4 =O(4,4) Toda equations.Comment: 11 pages (2-column), Revte
Китайські джерела щодо центральноазійського виміру політики КНР у галузі регіональній безпеки в постбіполярний період
We propose a method for detecting dyadic interactions: fine-grained, coordinated interactions between two people. Our model is capable of recognizing interactions such as a hand shake or a high five, and locating them in time and space. At the core of our method is a pictorial structures model that additionally takes into account the fine-grained movements around the joints of interest during the interaction. Compared to a bag-of-words approach, our method not only allows us to detect the specific type of actions more accurately, but it also provides the specific location of the interaction. The model is trained with both video data and body joint estimates obtained from Kinect. During testing, only video data is required. To demonstrate the efficacy of our approach, we introduce the ShakeFive dataset that consists of videos and Kinect data of hand shake and high five interactions. On this dataset, we obtain a mean average precision of 49.56%, outperforming a bag-of-words approach by 23.32%. We further demonstrate that the model can be learned from just a few interactions
Electron and hole transmission through superconductor - normal metal interfaces
We have investigated the transmission of electrons and holes through
interfaces between superconducting aluminum (Tc = 1.2 K) and various normal
non-magnetic metals (copper, gold, palladium, platinum, and silver) using
Andreev-reflection spectroscopy at T = 0.1 K. We analyzed the point contacts
with the modified BTK theory that includes Dynes' lifetime as a fitting
parameter G in addition to superconducting energy gap 2D and normal reflection
described by Z. For contact areas from 1 nm^2 to 10000 nm^2 the BTK Z parameter
was 0.5, corresponding to transmission coefficients of about 80 %, independent
of the normal metal. The very small variation of Z indicates that the
interfaces have a negligible dielectric tunneling barrier. Fermi surface
mismatch does not account for the observed transmission coefficient.Comment: 9 pages, 4 figures, submitted to Proceedings of the 19th
International Conference on Magnetism ICM2012 (Busan 2012
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