16,291 research outputs found
Results of soil moisture flights during April 1974
The results presented here are derived from measurements made during the April 5 and 6, 1974 flights of the NASA P-3A aircraft over the Phoenix, Arizona agricultural test site. The purpose of the mission was to study the use of microwave techniques for the remote sensing of soil moisture. These results include infrared (10-to 12 micrometers) 2.8-cm and 21-cm brightness temperatures for approximately 90 bare fields. These brightness temperatures are compared with surface measurements of the soil moisture made at the time of the overflights. These data indicate that the combination of the sum and difference of the vertically and the horizontally polarized brightness temperatures yield information on both the soil moisture and surface roughness conditions
Energy-momentum conservation in pre-metric electrodynamics with magnetic charges
A necessary and sufficient condition for energy-momentum conservation is
proved within a topological, pre-metric approach to classical electrodynamics
including magnetic as well as electric charges. The extended Lorentz force,
consisting of mutual actions by F=(E, B) on the electric current and G=(H, D)
on the magnetic current, can be derived from an energy-momentum "potential" if
and only if the constitutive relation G=G(F) satisfies a certain vanishing
condition. The electric-magnetic reciprocity introduced by Hehl and Obukhov is
seen to define a complex structure on the tensor product of 2-form pairs (F,G)
which is independent of but consistent with the Hodge star operator defined by
any Lorentzian metric. Contrary to a recent claim in the literature, it does
not define a complex structure on the space of 2-forms itself.Comment: 8 pages, 1 fugur
Computing Inferences for Large-Scale Continuous-Time Markov Chains by Combining Lumping with Imprecision
If the state space of a homogeneous continuous-time Markov chain is too
large, making inferences - here limited to determining marginal or limit
expectations - becomes computationally infeasible. Fortunately, the state space
of such a chain is usually too detailed for the inferences we are interested
in, in the sense that a less detailed - smaller - state space suffices to
unambiguously formalise the inference. However, in general this so-called
lumped state space inhibits computing exact inferences because the
corresponding dynamics are unknown and/or intractable to obtain. We address
this issue by considering an imprecise continuous-time Markov chain. In this
way, we are able to provide guaranteed lower and upper bounds for the
inferences of interest, without suffering from the curse of dimensionality.Comment: 9th International Conference on Soft Methods in Probability and
Statistics (SMPS 2018
The Propagation of Magneto-Centrifugally Launched Jets: I
We present simulations of the propagation of magnetized jets. This work
differs from previous studies in that the cross-sectional distributions of the
jets's state variables are derived from analytical models for
magneto-centrifugal launching. The source is a magnetized rotator whose
properties are specfied as boundary conditions. The jets in these simulations
are considerably more complex than the ``top-hat''constant density etc.
profiles used in previous work. We find that density and magnetic field
stratification (with radius) in the jet leads to new behavior including the
separation of an inner jet core from a low density collar. We find this {\it
jet within a jet} structure, along with the magnetic stresses, leads to
propagation behaviors not observed in previous simulation studies. Our
methodology allows us to compare MHD jets from different types of sources whose
properties could ultimately be derived from the behavior of the propagating
jets.Comment: 42 pages, accepted by the Ap
A study of blood contamination of Siqveland matrix bands
AIMS To use a sensitive forensic test to measure blood contamination of used Siqveland matrix bands following routine cleaning and sterilisation procedures in general dental practice. MATERIALS AND METHODS: Sixteen general dental practices in the West of Scotland participated. Details of instrument cleaning procedures were recorded for each practice. A total of 133 Siqveland matrix bands were recovered following cleaning and sterilisation and were examined for residual blood contamination by the Kastle-Meyer test, a well-recognised forensic technique. RESULTS: Ultrasonic baths were used for the cleaning of 62 (47%) bands and retainers and the remainder (53%) were hand scrubbed prior to autoclaving. Overall, 21% of the matrix bands and 19% of the retainers gave a positive Kastle-Meyer test, indicative of residual blood contamination, following cleaning and sterilisation. In relation to cleaning method, 34% of hand-scrubbed bands and 32% of hand-scrubbed retainers were positive for residual blood by the Kastle-Meyer test compared with 6% and 3% respectively of ultrasonically cleaned bands and retainers (P less than 0.001). CONCLUSIONS: If Siqveland matrix bands are re-processed in the assembled state, then adequate pre-sterilisation cleaning cannot be achieved reliably. Ultrasonic baths are significantly more effective than hand cleaning for these items of equipment
Structure and Stability of Keplerian MHD Jets
MHD jet equilibria that depend on source properties are obtained using a
simplified model for stationary, axisymmetric and rotating magnetized outflows.
The present rotation laws are more complex than previously considered and
include a Keplerian disc. The ensuing jets have a dense, current-carrying
central core surrounded by an outer collar with a return current. The
intermediate part of the jet is almost current-free and is magnetically
dominated. Most of the momentum is located around the axis in the dense core
and this region is likely to dominate the dynamics of the jet. We address the
linear stability and the non-linear development of instabilities for our models
using both analytical and 2.5-D numerical simulation's. The instabilities seen
in the simulations develop with a wavelength and growth time that are well
matched by the stability analysis. The modes explored in this work may provide
a natural explanation for knots observed in astrophysical jets.Comment: 35 pages, accepted by the Ap
Electron exchange model potential: Application to positronium-helium scattering
The formulation of a suitable nonlocal model potential for electron exchange
is presented, checked with electron-hydrogen and electron-helium scattering,
and applied to the study of elastic and inelastic scattering and ionization of
ortho positronium (Ps) by helium. The elastic scattering and the
excitations of Ps are investigated using a three-Ps-state close-coupling
approximation. The higher () excitations and ionization of Ps atom are
treated in the framework of Born approximation with present exchange.
Calculations are reported of phase shifts, and elastic, Ps-excitation, and
total cross sections. The present target elastic total cross section agrees
well with experimental results at thermal to medium energies.Comment: 16 latex pages, 7 postscript figure
High-frequency monitoring of nitrogen and phosphorus response in three rural catchments to the end of the 2011–2012 drought in England
This paper uses high-frequency bankside measurements from three catchments selected as part of the UK government-funded Demonstration Test Catchments (DTC) project. We compare the hydrological and hydrochemical patterns during the water year 2011–2012 from the Wylye tributary of the River Avon with mixed land use, the Blackwater tributary of the River Wensum with arable land use and the Newby Beck tributary of the River Eden with grassland land use. The beginning of the hydrological year was unusually dry and all three catchments were in states of drought. A sudden change to a wet summer occurred in April 2012 when a heavy rainfall event affected all three catchments. The year-long time series and the individual storm responses captured by in situ nutrient measurements of nitrate and phosphorus (total phosphorus and total reactive phosphorus) concentrations at each site reveal different pollutant sources and pathways operating in each catchment. Large storm-induced nutrient transfers of nitrogen and or phosphorus to each stream were recorded at all three sites during the late April rainfall event. Hysteresis loops suggested transport-limited delivery of nitrate in the Blackwater and of total phosphorus in the Wylye and Newby Beck, which was thought to be exacerbated by the dry antecedent conditions prior to the storm. The high rate of nutrient transport in each system highlights the scale of the challenges faced by environmental managers when designing mitigation measures to reduce the flux of nutrients to rivers from diffuse agricultural sources. It also highlights the scale of the challenge in adapting to future extreme weather events under a changing climate
The Effects of Clumps in Explaining X-ray Emission Lines from Hot Stars
It is now well established that stellar winds of hot stars are fragmentary
and that the X-ray emission from stellar winds has a strong contribution from
shocks in winds. Chandra high spectral resolution observations of line profiles
of O and B stars have shown numerous properties that had not been expected.
Here we suggest explanations by considering the X-rays as arising from bow
shocks that occur where the stellar wind impacts on spherical clumps in the
winds. We use an accurate and stable numerical hydrodynamical code to obtain
steady-state physical conditions for the temperature and density structure in a
bow shock. We use these solutions plus analytic approximations to interpret
some major X-ray features: the simple power-law distribution of the observed
emission measure derived from many hot star X-ray spectra and the wide range of
ionization stages that appear to be present in X-ray sources throughout the
winds. Also associated with the adiabatic cooling of the gas around a clump is
a significant transverse velocity for the hot plasma flow around the clumps,
and this can help to understand anomalies associated with observed line widths,
and the differences in widths seen in stars with high and low mass-loss rates.
The differences between bow shocks and the planar shocks that are often used
for hot stars are discussed. We introduce an ``on the shock'' (OTSh)
approximation that is useful for interpreting the X-rays and the consequences
of clumps in hot star winds and elsewhere in astronomy.Comment: to appear in the Astrophysical Journa
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