9,518 research outputs found
Saturation-Dependence of Dispersion in Porous Media
In this study, we develop a saturation-dependent treatment of dispersion in
porous media using concepts from critical path analysis, cluster statistics of
percolation, and fractal scaling of percolation clusters. We calculate spatial
solute distributions as a function of time and calculate arrival time
distributions as a function of system size. Our previous results correctly
predict the range of observed dispersivity values over ten orders of magnitude
in experimental length scale, but that theory contains no explicit dependence
on porosity or relative saturation. This omission complicates comparisons with
experimental results for dispersion, which are often conducted at saturation
less than 1. We now make specific comparisons of our predictions for the
arrival time distribution with experiments on a single column over a range of
saturations. This comparison suggests that the most important predictor of such
distributions as a function of saturation is not the value of the saturation
per se, but the applicability of either random or invasion percolation models,
depending on experimental conditions
Direct UV observations of the circumstellar envelope of alpha Orionis
Observations were made in the IUE LWP camera, low dispersion mode, with alpha Ori being offset various distances from the center of the Long Wavelength Large Aperture along its major axis. Signal was acquired at all offset positions and is comprised of unequal components of background/dark counts, telescope-scattered light, and scattered light emanating from the extended circumstellar shell. The star is known from optical and infrared observations to possess an extended, arc-minute sized, shell of cool material. Attempts to observe this shell with the IUE are described, although the deconvolution of the stellar signal from the telescope scattered light requires further calibration effort
Imaging and burst location with the EXIST high-energy telescope
The primary instrument of the proposed EXIST mission is a coded mask high
energy telescope (the HET), that must have a wide field of view and extremely
good sensitivity. It will be crucial to minimize systematic errors so that even
for very long total integration times the imaging performance is close to the
statistical photon limit. There is also a requirement to be able to reconstruct
images on-board in near real time in order to detect and localize gamma-ray
bursts. This must be done while the spacecraft is scanning the sky. The
scanning provides all-sky coverage and is key to reducing systematic errors.
The on-board computational problem is made even more challenging for EXIST by
the very large number of detector pixels. Numerous alternative designs for the
HET have been evaluated. The baseline concept adopted depends on a unique coded
mask with two spatial scales. Monte Carlo simulations and analytic analysis
techniques have been used to demonstrate the capabilities of the design and of
the proposed two-step burst localization procedure
Localization dynamics of fluids in random confinement
The dynamics of two-dimensional fluids confined within a random matrix of
obstacles is investigated using both colloidal model experiments and molecular
dynamics simulations. By varying fluid and matrix area fractions in the
experiment, we find delocalized tracer particle dynamics at small matrix area
fractions and localized motion of the tracers at high matrix area fractions. In
the delocalized region, the dynamics is subdiffusive at intermediate times, and
diffusive at long times, while in the localized regime, trapping in finite
pockets of the matrix is observed. These observations are found to agree with
the simulation of an ideal gas confined in a weakly correlated matrix. Our
results show that Lorentz gas systems with soft interactions are exhibiting a
smoothening of the critical dynamics and consequently a rounded
delocalization-to-localization transition.Comment: 5 pages, 3 figure
Simulating Ability: Representing Skills in Games
Throughout the history of games, representing the abilities of the various
agents acting on behalf of the players has been a central concern. With
increasingly sophisticated games emerging, these simulations have become more
realistic, but the underlying mechanisms are still, to a large extent, of an ad
hoc nature. This paper proposes using a logistic model from psychometrics as a
unified mechanism for task resolution in simulation-oriented games
The High Energy Telescope on EXIST
The Energetic X-ray Imaging Survey Telescope (EXIST) is a proposed next
generation multi-wavelength survey mission. The primary instrument is a High
Energy telescope (HET) that conducts the deepest survey for Gamma-ray Bursts
(GRBs), obscured-accreting and dormant Supermassive Black Holes and Transients
of all varieties for immediate followup studies by the two secondary
instruments: a Soft X-ray Imager (SXI) and an Optical/Infrared Telescope (IRT).
EXIST will explore the early Universe using high redshift GRBs as cosmic probes
and survey black holes on all scales. The HET is a coded aperture telescope
employing a large array of imaging CZT detectors (4.5 m^2, 0.6 mm pixel) and a
hybrid Tungsten mask. We review the current HET concept which follows an
intensive design revision by the HET imaging working group and the recent
engineering studies in the Instrument and Mission Design Lab at the Goddard
Space Flight Center. The HET will locate GRBs and transients quickly (<10-30
sec) and accurately (< 20") for rapid (< 1-3 min) onboard followup soft X-ray
and optical/IR (0.3-2.2 micron) imaging and spectroscopy. The broad energy band
(5-600 keV) and the wide field of view (~90 deg x 70 deg at 10% coding
fraction) are optimal for capturing GRBs, obscured AGNs and rare transients.
The continuous scan of the entire sky every 3 hours will establish a
finely-sampled long-term history of many X-ray sources, opening up new
possibilities for variability studies.Comment: 10 pages, 6 figures, 3 tables, SPIE conference proceedings (UV,
X-ray, and Gamma-Ray Space Instrumentation for Astronomy XVI, 7435-9
Very High Resolution Solar X-ray Imaging Using Diffractive Optics
This paper describes the development of X-ray diffractive optics for imaging
solar flares with better than 0.1 arcsec angular resolution. X-ray images with
this resolution of the \geq10 MK plasma in solar active regions and solar
flares would allow the cross-sectional area of magnetic loops to be resolved
and the coronal flare energy release region itself to be probed. The objective
of this work is to obtain X-ray images in the iron-line complex at 6.7 keV
observed during solar flares with an angular resolution as fine as 0.1 arcsec -
over an order of magnitude finer than is now possible. This line emission is
from highly ionized iron atoms, primarily Fe xxv, in the hottest flare plasma
at temperatures in excess of \approx10 MK. It provides information on the flare
morphology, the iron abundance, and the distribution of the hot plasma.
Studying how this plasma is heated to such high temperatures in such short
times during solar flares is of critical importance in understanding these
powerful transient events, one of the major objectives of solar physics. We
describe the design, fabrication, and testing of phase zone plate X-ray lenses
with focal lengths of \approx100 m at these energies that would be capable of
achieving these objectives. We show how such lenses could be included on a
two-spacecraft formation-flying mission with the lenses on the spacecraft
closest to the Sun and an X-ray imaging array on the second spacecraft in the
focal plane \approx100 m away. High resolution X-ray images could be obtained
when the two spacecraft are aligned with the region of interest on the Sun.
Requirements and constraints for the control of the two spacecraft are
discussed together with the overall feasibility of such a formation-flying
mission
Io: IUE observations of its atmosphere and the plasma torus
Two of the main components of the atmosphere of Io, neutral oxygen and sulfur, were detected with the IUE. Four observations yield brightnesses that are similar, regardless of whether the upstream or the downstream sides of the torus plasma flow around Io is observed. A simple model requires the emissions to be produced by the interaction of O and S columns in the exospheric range with 2 eV electrons. Cooling of the 5 eV torus electrons is required prior to their interaction with the atmosphere of Io. Inconsistencies in the characteristics of the spectra that cannot be accounted for in this model require further analysis with improved atomic data. The Io plasma torus was monitored with the IUE. The long-term stability of the warm torus is established. The observed brightnesses were analyzed using a model of the torus, and variations of less than 30 percent in the composition are observed, the quantitative results being model dependent
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