2,409 research outputs found
Effects of sheep grazing exclusion on alpine tall tussock grassland
Substantial areas of alpine tall tussock grasslands are being retired from grazing as part of Crown
pastoral lease tenure review because of the perceived negative impact of grazing livestock. However, relatively
little is known about the effect of sheep exclusion on these grasslands. We analysed data from five grazing
exclosure plots over a 6-year period to examine the effect merino sheep have relative to hares and rabbits in
alpine tall tussock grasslands used for summer grazing. Unfortunately, because of snow damage to fences,
we were unable to detect any significant hare and rabbit effects. Over the time of this study, there was no
evidence for significant vegetation recovery after exclusion of sheep grazing. This may be because of other
grazing animals in the system, or the low stocking rates and non-random grazing behaviour of merino ewes.
There was, however, a significant increase in the cover of exotic herbs Pilosella officinarum and P. praealta
and a significant decrease in the cover of native tussocks Festuca novae-zelandiae and Poa colensoi regardless
of grazing exclusion. While this pattern has been previously documented in studies at lower elevations and
usually with a history of burning, our results demonstrate that alpine grasslands with no burning history can
also be invaded by Pilosella spp. over relatively short time frames. Replicated grazing exclosure trials such as
the one described in this paper are important for providing objective information on both long-term trends in
vegetation composition and the impacts of grazing animals in alpine grasslands as a basis for making informed
decisions on their future management
Radio Source Heating in the ICM: The Example of Cygnus A
One of the most promising solutions for the cooling flow problem involves
energy injection from the central AGN. However it is still not clear how
collimated jets can heat the ICM at large scale, and very little is known
concerning the effect of radio lobe expansion as they enter into pressure
equilibrium with the surrounding cluster gas. Cygnus A is one of the best
examples of a nearby powerful radio galaxy for which the synchrotron emitting
plasma and thermal emitting intra-cluster medium can be mapped in fine detail,
and previous observations have inferred possible shock structure at the
location of the cocoon. We use new XMM-Newton observations of Cygnus A, in
combination with deep Chandra observations, to measure the temperature of the
intra-cluster medium around the expanding radio cavities. We investigate how
inflation of the cavities may relate to shock heating of the intra-cluster gas,
and whether such a mechanism is sufficient to provide enough energy to offset
cooling to the extent observed.Comment: To appear in the Proceedings of "Heating vs. Cooling in Galaxies and
Clusters of Galaxies", August 2006, Garching (Germany), Eds. H. Boehringer,
G.W. Pratt, A. Finoguenov, P. Schuecker, Springer-Verlag series "ESO
Astrophysics Symposia", p.101, in press. 8 pages, 3 multiple figure
Probing Transport Theories via Two-Proton Source Imaging
Imaging technique is applied to two-proton correlation functions to extract
quantitative information about the space-time properties of the emitting source
and about the fraction of protons that can be attributed to fast emission
mechanisms. These new analysis techniques resolve important ambiguities that
bedeviled prior comparisons between measured correlation functions and those
calculated by transport theory. Quantitative comparisons to transport theory
are presented here. The results of the present analysis differ from those
reported previously for the same reaction systems. The shape of the two-proton
emitting sources are strongly sensitive to the details about the in-medium
nucleon-nucleon cross sections and their density dependence.Comment: 23 pages, 11 figures. Figures are in GIF format. If you need
postscript format, please contact: [email protected]
Automated Telemetric Irrigation Controller
An electronic, microprocessor-based controller was developed
and tested for automating surface irrigation systems. Communication
between the central controller and individual satellite field stations
is by tone telemetry over a single 3-conductor wire. The reliable Dual
Tone Multiple Frequency or Touch Tone system is the same as that used in
telephone communications. The system is designed to actuate momentarily
energized pilot valves commonly used in automated surface irrigation
systems. Because of its low power requirement, the control system can
be battery-powered. It is being field tested in three different, automated
surface systems
Spring-Operated Semi-automatic Irrigation Valves
TORSION spring operators for standard low pressure
butterfly type irrigation valves are described. These
are used with 24-h timers to semiautomate gated pipe
irrigation systems and are particularly well suited for use
with flow-thru single pipeline systems. They are presently
marketed in 150 mm (6 in.), 200 mm (8 in.) and 250 mm
(10 in.) diameter sizes
Multiresolution analysis of active region magnetic structure and its correlation with the Mt. Wilson classification and flaring activity
Two different multi-resolution analyses are used to decompose the structure
of active region magnetic flux into concentrations of different size scales.
Lines separating these opposite polarity regions of flux at each size scale are
found. These lines are used as a mask on a map of the magnetic field gradient
to sample the local gradient between opposite polarity regions of given scale
sizes. It is shown that the maximum, average and standard deviation of the
magnetic flux gradient for alpha, beta, beta-gamma and beta-gamma-delta active
regions increase in the order listed, and that the order is maintained over all
length-scales. This study demonstrates that, on average, the Mt. Wilson
classification encodes the notion of activity over all length-scales in the
active region, and not just those length-scales at which the strongest flux
gradients are found. Further, it is also shown that the average gradients in
the field, and the average length-scale at which they occur, also increase in
the same order. Finally, there are significant differences in the gradient
distribution, between flaring and non-flaring active regions, which are
maintained over all length-scales. It is also shown that the average gradient
content of active regions that have large flares (GOES class 'M' and above) is
larger than that for active regions containing flares of all flare sizes; this
difference is also maintained at all length-scales.Comment: Accepted for publication in Solar Physic
Lattice-switch Monte Carlo
We present a Monte Carlo method for the direct evaluation of the difference
between the free energies of two crystal structures. The method is built on a
lattice-switch transformation that maps a configuration of one structure onto a
candidate configuration of the other by `switching' one set of lattice vectors
for the other, while keeping the displacements with respect to the lattice
sites constant. The sampling of the displacement configurations is biased,
multicanonically, to favor paths leading to `gateway' arrangements for which
the Monte Carlo switch to the candidate configuration will be accepted. The
configurations of both structures can then be efficiently sampled in a single
process, and the difference between their free energies evaluated from their
measured probabilities. We explore and exploit the method in the context of
extensive studies of systems of hard spheres. We show that the efficiency of
the method is controlled by the extent to which the switch conserves correlated
microstructure. We also show how, microscopically, the procedure works: the
system finds gateway arrangements which fulfill the sampling bias
intelligently. We establish, with high precision, the differences between the
free energies of the two close packed structures (fcc and hcp) in both the
constant density and the constant pressure ensembles.Comment: 34 pages, 9 figures, RevTeX. To appear in Phys. Rev.
Stochastic String Motion Above and Below the World Sheet Horizon
We study the stochastic motion of a relativistic trailing string in black
hole AdS_5. The classical string solution develops a world-sheet horizon and we
determine the associated Hawking radiation spectrum. The emitted radiation
causes fluctuations on the string both above and below the world-sheet horizon.
In contrast to standard black hole physics, the fluctuations below the horizon
are causally connected with the boundary of AdS. We derive a bulk stochastic
equation of motion for the dual string and use the AdS/CFT correspondence to
determine the evolution a fast heavy quark in the strongly coupled
plasma. We find that the kinetic mass of the quark decreases by while the correlation time of world sheet
fluctuations increases by .Comment: 27 pages, 5 figures; v2 final version, small changes, references
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