10,635 research outputs found
Testing of a spacecraft model in a combined environment simulator
A scale model of a satellite was tested in a large vacuum facility under electron bombardment and vacuum ultraviolet radiation to investigate the charging of dielectric materials on curved surfaces. The model was tested both stationary and rotating relative to the electron sources as well as grounded through one megohm and floating relative to the chamber. Surface potential measurements are presented and compared with the predictions of computer modelling of the stationary tests. Discharge activity observed during the stationary tests is discussed and signals from sensing devices located inside and outside of the model are presented
The Jets of AGN as giant co-axial cables
The currents carried by the jets of active galactic nuclei (AGNs) can be
probed using maps of the Faraday rotation measure (RM), since a jet current
will be accompanied by a toroidal magnetic (B) field, which will give rise to a
systematic change in the RM across the jet. The aim of this study is to
identify new AGNs displaying statistically significant transverse RM gradients
across their parsec-scale jets, and to look for overall patterns in the implied
directions for the toroidal B-field components and jet currents. We have
carried out new analyses of Faraday RM maps derived from previously published
8.1, 8.4, 12.1 and 15.3 GHz data obtained in 2006 on the NRAO VLBA. In a number
of important ways, our procedures were identical to those of the original
authors, but several other key aspects of the new imaging and analysis differ
from the original methods. Our new analysis has substantially increased the
number of AGNs known to display transverse RM gradients. The collected data on
parsec and kiloparsec scales indicate that the current typically flows inward
along the jet axis and outward in a more extended region surrounding the jet,
typical to the current structure of a coaxial cable, accompanied by a
self-consistent system of nested helical B fields, whose toroidal components
give rise to the observed transverse RM gradients. These new results make it
possible for the first time to conclusively demonstrate the existence of a
preferred direction for the toroidal B-field components - and therefore of the
currents - of AGN jets. Discerning the origin of this current-field system is
of cardinal importance for understanding the physical mechanisms leading to the
formation of the intrinsic jet B field, which likely plays an important role in
the propagation and collimation of the jets; one possibility is the action of a
"cosmic battery".Comment: 12 pages, 2 figures, 3 tables; accepted for publication in Astronomy
& Astrophysic
NASCAP modelling of environmental-charging-induced discharges in satellites
The charging and discharging characteristics of a typical geosynchronous satellite experiencing time-varying geomagnetic substorms, in sunlight, were studied utilizing the NASA Charging Analyzer Program (NASCAP). An electric field criteria of 150,000 volts/cm to initiate discharges and transfer of 67 percent of the stored charge was used based on ground test results. The substorm characteristics were arbitrarily chosen to evaluate effects of electron temperature and particle density (which is equivalent to current density). It was found that while there is a minimum electron temperature for discharges to occur, the rate of discharges is dependent on particle density and duration times of the encounter. Hence, it is important to define the temporal variations in the substorm environments
Airborne observations of methane in Comet Kohoutek
The experiment is described for airborne observations of Comet Kohoutek using an infrared tilting-filter photometer. Preliminary analysis of the data established an upper limit to the Comet's fluorescence radiation in methane lines at 3.3 microns
From classroom tutor to hypertext adviser: An evaluation
This paper describes a three‐year experiment to investigate the possibility of making economies by replacing practical laboratory sessions with courseware while attempting to ensure that the quality of the student learning experience did not suffer. Pathology labs are a central component of the first‐year medical undergraduate curriculum at Southampton. Activities in these labs had been carefully designed and they were supervised by lab demonstrators who were subject domain experts. The labs were successful in the eyes of both staff and students but were expensive to conduct, in terms of equipment and staffing. Year by year evaluation of the introduction of courseware revealed that there was no measurable difference in student performance as a result of introducing the courseware, but that students were unhappy about the loss of interaction with the demonstrators. The final outcome of this experiment was a courseware replacement for six labs which included a software online hypertext adviser. The contribution of this work is that it adds to the body of empirical evidence in support of the importance of maintaining dialogue with students when introducing courseware, and it presents an example of how this interaction might be achieved in software
Vortex spectrum in superfluid turbulence: interpretation of a recent experiment
We discuss a recent experiment in which the spectrum of the vortex line
density fluctuations has been measured in superfluid turbulence. The observed
frequency dependence of the spectrum, , disagrees with classical
vorticity spectra if, following the literature, the vortex line density is
interpreted as a measure of the vorticity or enstrophy. We argue that the
disagrement is solved if the vortex line density field is decomposed into a
polarised field (which carries most of the energy) and an isotropic field
(which is responsible for the spectrum).Comment: Submitted for publication
http://crtbt.grenoble.cnrs.fr/helio/GROUP/infa.html
http://www.mas.ncl.ac.uk/~ncfb
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Evapotranspiration Mapping for Forest Management in California's Sierra Nevada
We assessed the response of densely forested watersheds with little apparent annual water limitation to forest disturbance
and climate variability, by studying how past wildfires changed forest evapotranspiration, and what past evapotranspiration
patterns imply for the availability of subsurface water storage for drought resistance. We determined annual spatial patterns
of evapotranspiration using a top-down statistical model, correlating measured annual evapotranspiration from eddycovariance
towers across California with NDVI (Normalized Difference Vegetation Index) measured by satellite, and with
annual precipitation. The study area was the Yuba and American River watersheds, two densely forested watersheds in the
northern Sierra Nevada. Wildfires in the 1985-2015 period resulted in significant post-fire reductions in evapotranspiration
for at least 5 years, and in some cases for more than 20 years. The levels of biomass removed in medium-intensity fires (25-
75% basal area loss), similar to magnitudes expected from forest treatments for fuels reduction and forest health, reduced
evapotranspiration by as much 150-200 mm yr-1 for the first 5 years. Rates of recovery in post-wildfire evapotranspiration
confirm the need for follow-up forest treatments at intervals of 5-20 years to sustain lower evapotranspiration, depending
on local landscape attributes and interannual climate. Using the metric of cumulative precipitation minus evapotranspiration
(P-ET) during multi-year dry periods, we found that forests in the study area showed little evidence of moisture stress
during the 1985-2018 period of our analysis, owing to relatively small reliance on interannual subsurface water storage to
meet dry-year evapotranspiration needs of vegetation. However, more-severe or sustained drought periods will push some
lower-elevation forests in the area studied toward the cumulative P-ET thresholds previously associated with widespread
forest mortality in the southern Sierra Nevada
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Promoting positive communication environments: a service evaluation.
Purpose
The purpose of this paper is to present an evaluation of a programme of training and support provided to staff, which aimed to encourage supported communication environments for people with learning disabilities.
Design/methodology/approach
Training, monitoring and support for communication, specifically augmentative and alternative communication (AAC) strategies, was provided by speech and language therapy staff to two residential services over 46 weeks. Staff and service user communications were observed pre- and post-intervention.
Findings
In one provision there was an increase in service user initiations and the use of some AAC strategies by support staff. In the other provision there was no change in service user initiations and a decrease in the range of AAC strategies used. It appears that some forms for AAC remain challenging for staff to implement.
Originality/value
This evaluation explores ways of using specialist support services to improve communication environments for people with learning difficulties. Possible reasons for differences in the outcome of the intervention are discussed. Future research into the types of communication interactions experienced by people with learning disabilities across the range of communication styles may be useful so that support staff can be better helped to provide sustained and enriched communication environments
Quantum turbulence at finite temperature: the two-fluids cascade
To model isotropic homogeneous quantum turbulence in superfluid helium, we
have performed Direct Numerical Simulations (DNS) of two fluids (the normal
fluid and the superfluid) coupled by mutual friction. We have found evidence of
strong locking of superfluid and normal fluid along the turbulent cascade, from
the large scale structures where only one fluid is forced down to the vorticity
structures at small scales. We have determined the residual slip velocity
between the two fluids, and, for each fluid, the relative balance of inertial,
viscous and friction forces along the scales. Our calculations show that the
classical relation between energy injection and dissipation scale is not valid
in quantum turbulence, but we have been able to derive a temperature--dependent
superfluid analogous relation. Finally, we discuss our DNS results in terms of
the current understanding of quantum turbulence, including the value of the
effective kinematic viscosity
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