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Lava flow morphology at an erupting andesitic stratovolcano: a satellite perspective on El Reventador, Ecuador
Lava flows pose a significant hazard to infrastructure and property located close to volcanoes, and understanding how flows advance is necessary to manage volcanic hazard during eruptions. Compared to low-silica basaltic flows, flows of andesite composition are infrequently erupted and so relatively few studies of their characteristics and behaviour exist. We use El Reventador, Ecuador as a target to investigate andesitic lava flow properties during a 4.5 year period of extrusive eruption between February 2012 and August 2016. We use satellite radar to map the dimensions of 43 lava flows and look at variations in their emplacement behaviour over time. We find that flows descend the north and south flanks of El Reventador, and were mostly emplaced during durations shorter than the satellite repeat interval of 24 days.Flows ranged in length from 0.3 to 1.7 km, and the length of these flows decreased over the observation period. We measure a decrease in flow volume with time that is correlated with a long-term exponential decrease in eruption rate, and propose that this behaviour is caused by temporary magma storage in the conduit acting as a melt capacitor between the magma reservoir and the surface. We use the dimensions of the flow levees and widths to estimate the flow yield strengths, which were of the order of 10-100 kPa. We observe that some flows were diverted by topographic obstacles, and compare measurements of decreased channel width and increased flow thickness at the obstacles with observations from laboratory experiments. Radar observations, such as those presented here, could be used to map and measure properties of evolving lava flow fields at other remote or difficult to monitor volcanoes
VLA 8.4-GHz monitoring observations of the CLASS gravitational lens B1933+503
The complex ten-component gravitational lens system B1933+503 has been
monitored with the VLA during the period February to June 1998 with a view to
measuring the time delay between the four compact components and hence to
determine the Hubble parameter. Here we present the results of an `A'
configuration 8.4-GHz monitoring campaign which consists of 37 epochs with an
average spacing of 2.8 days. The data have yielded light curves for the four
flat-spectrum radio components (components 1, 3, 4 and 6). We observe only
small flux density changes in the four flat-spectrum components which we do not
believe are predominantly intrinsic to the source. Therefore the variations do
not allow us to determine the independent time delays in this system. However,
the data do allow us to accurately determine the flux density ratios between
the four flat-spectrum components. These will prove important as modelling
constraints and could prove crucial in future monitoring observations should
these data show only a monotonic increase or decrease in the flux densities of
the flat-spectrum components.Comment: Accepted for publication in MNRAS. 5 pages, 2 included PostScript
figure
Time-delayed feedback control in astrodynamics
In this paper we present time-delayed feedback control (TDFC) for the purpose of autonomously driving trajectories of nonlinear systems into periodic orbits. As the generation of periodic orbits is a major component of many problems in astodynamics we propose this method as a useful tool in such applications. To motivate the use of this method we apply it to a number of well known problems in the astrodynamics literature. Firstly, TDFC is applied to control in the chaotic attitude motion of an asymmetric satellite in an elliptical orbit. Secondly, we apply TDFC to the problem of maintaining a spacecraft in a periodic orbit about a body with large ellipticity (such as an asteroid) and finally, we apply TDFC to eliminate the drift between two satellites in low Earth orbits to ensure their relative motion is bounded
A revised lens time delay for JVAS B0218+357 from a reanalysis of VLA monitoring data
We have reanalysed the 1996/1997 VLA monitoring data of the gravitational
lens system JVAS B0218+357 to produce improved total flux density and
polarization variability curves at 15, 8.4 and 5 GHz. This has been done using
improved calibration techniques, accurate subtraction of the emission from the
Einstein ring and careful correction of various systematic effects, especially
an offset in polarization position angle that is hour-angle dependent. The
variations in total and polarized flux density give the best constraints and we
determine a combined delay estimate of d (1). This is
consistent with the -ray value recently derived using the Fermi
Gamma-ray Space Telescope and thus we find no evidence for a positional shift
between the radio and -ray emitting regions. Combined with the
previously published lens model found using LensClean, the new delay gives a
value for the Hubble constant of km s Mpc
(1).Comment: 17 pages and 14 figures. Accepted for publication in MNRA
MERLIN/VLA imaging of the gravitational lens system B0218+357
Gravitational lenses offer the possibility of accurately determining the
Hubble parameter (H_0) over cosmological distances, and B0218+357 is one of the
most promising systems for an application of this technique. In particular this
system has an accurately measured time delay (10.5+/-0.4 d; Biggs et al. 1999)
and preliminary mass modelling has given a value for H_0 of 69 +13/-19
km/s/Mpc. The error on this estimate is now dominated by the uncertainty in the
mass modelling. As this system contains an Einstein ring it should be possible
to constrain the model better by imaging the ring at high resolution. To
achieve this we have combined data from MERLIN and the VLA at a frequency of 5
GHz. In particular MERLIN has been used in multi-frequency mode in order to
improve substantially the aperture coverage of the combined data set. The
resulting map is the best that has been made of the ring and contains many new
and interesting features. Efforts are currently underway to exploit the new
data for lensing constraints using the LensClean algorithm (Kochanek & Narayan
1992).Comment: Accepted for publication in MNRAS. 6 pages, 4 included PostScript
figure
Models for the lens and source of B0218+357 - A LensClean approach to determine H0
B0218+357 is one of the most promising systems to determine the Hubble
constant from gravitational lenses. Consisting of two bright resolved images
plus an Einstein ring, it provides better constraints for the mass model than
other systems. The main problem left until now was the poorly determined
position of the lensing galaxy.
After presenting results from classical lens modelling, we apply our improved
version of LensClean which utilizes the Einstein ring for lens modelling
purposes. The primary result using isothermal models is a well defined lens
position which allows the first reliable measurement of the Hubble constant
from this system. The result of H0=(78+-6) km/s/Mpc (2 sigma) is high compared
with other lenses but compatible with the HST key project and WMAP results. We
furthermore discuss effects of different radial mass profiles. The power-law
exponent of the potential is constrained by VLBI data to be beta=1.04+-0.02,
very close to isothermal. The effect on H0 is expected to be very small.
We also present a composite map (lensed and unlensed) which shows the rich
structure of B0218+357 on scales from milli-arcseconds to arcseconds. Finally
we use a comparison of observations at different frequencies to investigate the
question of possible weakening of one of the images by propagation effects
and/or source shifts with frequency. The data clearly favour the model of
significant extinction without noticeable source position shifts.
The technical details of our variant of the LensClean method are presented in
the accompanying Paper I.Comment: To appear in MNRAS together with Paper I (astro-ph/0312262). 18
Pages. Full resolution version available at
http://www.astro.physik.uni-potsdam.de/~olaf or on reques
Transmission Of Withering Syndrome In Black Abalone, Haliotis Cracherodii Leach
Withering syndrome (WS) has been associated with catastrophic declines in black abalone populations in southern and central California. In an effort to identify the etiological agent of WS and to characterize the progression of this disease, we initiated a transmission study in which abalone from Ano Nuevo Island, a location free of WS, shared aquaria with animals from Vandenberg Airforce Base, a location where WS is epizootic. The mean incubation period of WS (time to develop overt signs of the disease) was 245 days with a mean time to death after development of clinical signs of 42 days. Median time to death was 41 wk in the experimentally exposed Ano Nuevo Island abalone and 16 wk in the positive control Vandenberg abalone. Cumulative mortality was significantly different between the negative control (unexposed) Ano Nuevo Island abalone (25% mortality) and both the exposed Ano Nuevo Island abalone (85% mortality; P = 0.0001) and the positive control Vandenberg abalone (100% mortality; P = 0.0001). In addition, significant differences in prevalences of a recently described Rickettsiales-like procaryote (RLP), Candidatus Xenohaliotis californiensis, were observed between negative control animals (no RLPs) and those with WS (both the experimentally exposed Ano Nuevo Island and Vandenberg abalone were infected with RLPs; P \u3c 0.001). All abalone infected with the RLP had signs of WS, including decreased condition indices, foot muscle atrophy, and digestive gland degeneration (P \u3c 0.05). No correlation between intensity of RLP infection and degree of WS was observed (P \u3e 0.05), suggesting a complex relationship between the RLP and clinical disease in black abalone. Despite this, these data in conjunction with a lack of observation of any other significant pathogens in the abalone provides evidence that the RLP infecting abalone ( Candidatus Xenohaliotis californiensis ) is the etiological agent of WS
Some Exact Results on the Potts Model Partition Function in a Magnetic Field
We consider the Potts model in a magnetic field on an arbitrary graph .
Using a formula of F. Y. Wu for the partition function of this model as a
sum over spanning subgraphs of , we prove some properties of concerning
factorization, monotonicity, and zeros. A generalization of the Tutte
polynomial is presented that corresponds to this partition function. In this
context we formulate and discuss two weighted graph-coloring problems. We also
give a general structural result for for cyclic strip graphs.Comment: 5 pages, late
A Search for Sub-Millisecond Pulsars
We have conducted a search of 19 southern Galactic globular clusters for
sub-millisecond pulsars at 660 MHz with the Parkes 64-m radio telescope. To
minimize dispersion smearing we used the CPSR baseband recorder, which samples
the 20 MHz observing band at the Nyquist rate. By possessing a complete
description of the signal we could synthesize an optimal filterbank in
software, and in the case of globular clusters of known dispersion measure,
much of the dispersion could be removed using coherent techniques. This allowed
for very high time resolution (25.6 us in most cases), making our searches in
general sensitive to sub-millisecond pulsars with flux densities greater than
about 3 mJy at 50 cm. No new pulsars were discovered, placing important
constraints on the proportion of pulsars with very short spin periods in these
clusters.Comment: 8 pages, 3 figures, to appear in Ap
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