429,585 research outputs found
A survey of low-velocity collisional features in Saturn's F ring
No full textSmall (~50km scale), irregular features seen in Cassini images to be emanating from Saturn's F ring have been termed mini-jets by Attree et al. (2012). One particular mini-jet was tracked over half an orbital period, revealing its evolution with time and suggesting a collision with a local moonlet as its origin. In addition to these data we present here a much more detailed analysis of the full catalogue of over 800 F ring mini-jets, examining their distribution, morphology and lifetimes in order to place constraints on the underlying moonlet population. We find mini-jets randomly located in longitude around the ring, with little correlation to the moon Prometheus, and randomly distributed in time, over the full Cassini tour to date. They have a tendency to cluster together, forming complicated `multiple' structures, and have typical lifetimes of ~1d. Repeated observations of some features show significant evolution, including the creation of new mini-jets, implying repeated collisions by the same object. This suggests a population of ≲ 1 km radius objects with some internal strength and orbits spread over ±100 km in semi-major axis relative to the F ring but with the majority within 20 km. These objects likely formed in the ring under, and were subsequently scattered onto differing orbits by, the perturbing action of Prometheus. This reinforces the idea of the F ring as a region with a complex balance between collisions, disruption and accretion.Supplementary information available at http://www.maths.qmul.ac.uk/~attree/mini-jets
Full waveform tomographic images of the peak ring at the Chicxulub impact crater
No full text17/06/013 PDF version attached, OK to pub.Peak rings are a feature of large impact craters on the terrestrial planets and are generally believed to be formed from deeply buried rocks that are uplifted during crater formation. The precise lithology and kinematics of peak ring formation, however, remains unclear. Previous work has revealed a suite of bright inward-dipping reflectors beneath the peak ring at the Chicxulub impact crater and that the peak ring was formed from rocks with a relatively low seismic velocity. New 2D full-waveform tomographic velocity images show that the uppermost lithology of the peak ring is formed from a thin (~100-200 m thick) layer of low-velocity (~3000-3200 m/s) rocks. This low-velocity layer is most likely to be composed of highly porous, allogenic impact breccias. Our models also show that the change in velocity between lithologies within and outside the peak ring is more abrupt than previously realized and occurs close to the location of the dipping reflectors. Across the peak ring, velocity appears to correlate well with predicted shock pressures from a dynamic model of crater formation, where the rocks that form the peak ring originate from uplifted basement that has been subjected to high shock pressures (10-50 GPa), and lie above downthrown sedimentary rocks that have been subjected to shock pressures of < 5 GPa. These observations suggest that low-velocities within the peak ring may be related to shock effects and that the dipping reflectors underneath the peak ring might represent the boundary between highly-shocked basement and weakly-shocked sediments
Looking at the Wood, Seeing the Trees and More: Australia-New Zealand Tree-Ring Science Conference, January 2025
No full textThe Australia-New Zealand Tree-Ring Conference was held January 21–23, 2025, at Waipapa Taumata Rau/University of Auckland, in Aotearoa/New Zealand. It was intended to provide an opportunity for the Australian and New Zealand dendrochronological researchers to meet, present current research, and discuss the challenges and opportunities in working with Southern Hemisphere tree species, but it was open to others outside of Australasia, including some keynote speakers. The meeting brought together many researchers from within and outside academia for the first time since the pandemic, and in addition to providing a look at current interesting and ongoing dendrochronology projects, it promoted camaraderie for this regional tree-ring community
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Dendrochronological Modeling of the Effects of Climatic Change on Tree-Ring Width Chronologies from the Chaco Canyon Area, Southwestern United States
No full textHypotheses about the causes of the growth and decline of the Chacoan regional interaction system in the southwestern United States between A.D. 900 and 1200 are evaluated against tree-ring evidence and the results of an empirical model (PRECON) that computes the statistical relationships between climate and ring-width indices during the 20th century and applies the results to hypothesized precipitation or temperature changes. The statistical responses of 23 indexed conifer ring-width chronologies from New Mexico and Colorado to variations in monthly temperature and precipitation were calculated. Simulated decreases in prior autumn-winter precipitation markedly reduced ring widths, while decreased current summer precipitation was less effective, sometimes reducing ring width or having little effect. Decreased prior winter temperature slightly reduced ring width, while decreased growing season temperature usually increased or did not effect ring widths. Evaluated in terms of these results, the Chaco Canyon area tree-ring record (1) indicates that favorable climatic conditions in the 10th, 11th, and early 12th centuries fostered the growth of the Chacoan system, (2) shows that dry autumn-winter and summer conditions in the middle 1100s contributed to the downfall of the system, (3) does not support the proposition that centuries-long climatic fluctuations evident in southwestern Colorado affected Chaco Canyon, (4) does not support the idea of shifts from summer-to winter-dominant precipitation regimes, and (5) contributes little to assessing the role of anthropogenic environmental change in the collapse of the Chacoan system.This item is part of the Tree-Ring Research (formerly Tree-Ring Bulletin) archive. It was digitized from a physical copy provided by the Laboratory of Tree-Ring research at The University of Arizona. For more information about this peer-reviewed scholarly journal, please email the Editor of Tree-Ring Research at [email protected]
Exploratory temperature and precipitation reconstructions from the Qinling Mountains, north-central China
No full textFebruary-April (FMA) temperature at Foping (1879-1989) and July-August (JA) precipitation at Xian (1895–1988) have been reconstructed using total ring width (TRW) and maximum latewood density (MXD) from trees in the Qinling Mountains, at the northern limit of the East Asian monsoon, in central China. The Xian JA precipitation reconstruction, albeit short, represents the first well-replicated, crossdated dendroclimatic reconstruction of summer monsoon precipitation for this region. Reconstructed Xian precipitation shows significant positive relationships with historical evidence from the region. The key feature of the precipitation reconstruction is prolonged summer drought during the late 1920s and early 1930s. The Foping reconstruction displays warmer-than-average FMA temperatures during this time period. These exploratory reconstructions, along with a previous reconstruction from Huashan, demonstrate the complexity of attempting dendroclimatic reconstructions from this region. Our results indicate that further attempts to locate long-lived conifers from here can result in an extended well-calibrated and verified reconstruction of summer monsoon precipitation.This item is part of the Tree-Ring Research (formerly Tree-Ring Bulletin) archive. It was digitized from a physical copy provided by the Laboratory of Tree-Ring Research at The University of Arizona. For more information about this peer-reviewed scholarly journal, please email the Editor of Tree-Ring Research at [email protected]
Tree-ring-derived precipitation records from inner Mongolia；China；since a.d. 1627
No full textTwo Chinese pine (Pinus tabulaeformis) tree-ring width chronologies up to 375 years long were used to reconstruct rainfall from February to early July for the Wu Dangzhao region and from February to mid-July for the La Madong region, Inner Mongolia, China. The predictor variables account for 44.3% and 42.7% of the variance in precipitation, respectively. Both historical records and two other tree-ring based precipitation reconstructions from the environmentally sensitive zone (the northern Helan Mountain range and Baiyinaobao) confirm our results. After applying a 10-year moving average, the trends of four tree-ring based precipitation reconstructions vary synchronously. Periods with below-normal precipitation occurred during the 1720s–1730s, 1740s–1750s, 1790s, early 1810s, late 1830s–1860s, 1880s–1910s, late 1920s–1930s and after the late 1960s–early 1970s. Periods with above-normal precipitation occurred in the 1760s to early 1770s, 1820s to early 1830s, 1870s–1880s, early 1920s, 1940s to early 1960s, and 1990s. The late 1920s period was the most severe drought over a broad area in north China in the last 375 years. In contrast, the wettest period was in the late 1990s.This item is part of the Tree-Ring Research (formerly Tree-Ring Bulletin) archive. For more information about this peer-reviewed scholarly journal, please email the Editor of Tree-Ring Research at [email protected]
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Tree-Ring Dating Through Pattern-Matching of Stable-Carbon Isotope Time Series
No full textThis item is part of the Tree-Ring Research (formerly Tree-Ring Bulletin) archive. It was digitized from a physical copy provided by the Laboratory of Tree-Ring research at The University of Arizona. For more information about this peer-reviewed scholarly journal, please email the Editor of Tree-Ring Research at [email protected]
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Southwest Archaeological Tree-Ring Dating
No full textFinal Report on NSF grant BNS-8504241 / 1 August 1985 - 31 July 1990 / Submitted to National Science Foundation Archaeometry ProgramNational Science Foundation Archaeometry Program / grant BNS-8504241.This item is part of the Natural History Reports collection. It was digitized from a physical copy provided by the Laboratory of Tree-Ring Research at The University of Arizona. For more information about items in this collection, please contact the Lab's Curator, (520) 621-1608 or see http://ltrr.arizona.edu/collection
Synchronization of a WDM Packet-Switched Slotted Ring
No full textIn this paper, we present two different strategies of
slot synchronization in wavelength-division-multiplexing (WDM)
packet-switched slotted-ring networks. Emphasis is given to the
architecture behind the WDM Optical Network Demonstrator
over Rings (WONDER) project, which is based on tunable
transmitters and fixed receivers. The WONDER experimental
prototype is currently being developed at the laboratories
of Politecnico di Torino. In the former strategy, a slotsynchronization
signal is transmitted by the master station on a
dedicated control wavelength; in the latter, slave nodes achieve
slot synchronization aligning on data packets that are received
from the master. The performance of both synchronization strategies,
particularly in terms of packet-collision probability, was
evaluated by simulation. The technique based on transmitting a
timing signal on a dedicated control wavelength achieves better
performance, although it is more expensive due to the need for an
additional wavelength. However, the technique based on aligning
data packets that are received from the master, despite attaining
lower timing stability, still deserves further study, particularly
if limiting the number of wavelengths and receivers is a major
requirement. Some experimental results, which were measured on
the WONDER prototype, are also shown. Measurement results,
together with theoretical findings, demonstrate the good synchronization
performance of the prototype
Wake structure and kinematics in the vortex ring state
No full textHigh-resolution computational simulations of the vortical wake of a rotor operating both near to and within the vortex ring state have been conducted using Brown's vorticity transport model. The nonlinear vortex kinematics of the wake is exposed using three-dimensional visualizations of the simulated flow field. To reveal the vortex dynamics that underpin the highly unsteady flow within the vortex ring state, a rotor with just one blade was modeled. This blade was decoupled aerodynamically from the surrounding velocity field so that it acted merely as a source of trailed vorticity. The investigation identified a significant change in the dominant dynamics of the wake as it swapped fromthe tubular form that is characteristic of hover or very lowspeed descent into the toroidal geometry of the vortex ring state. Initial vortex 'pairing' leads to rotation of vortex filaments away from their original attitude. This phenomenon plays an important role in regulating the downwash that the rotor can produce and thus in precipitating the onset of the vortex ring state. The considerable and persistent coherence of the vortical structure of the wake when in the vortex ring state is revealed, despite these disturbances, as are themechanisms that lead to both small-scale and large-scale wake breakdown events. Simulations show the balance between the vortex pairing and short-wave instability modes to be different in the vortex ring state at high descent speed, where the wake lies above the rotor, compared to in the vortex ring state at low descent speed when the wake lies predominantly below the rotor. This yields subtle differences to the kinematics and structure of the wake in the two cases
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