18,677 research outputs found
Gravity, geoid and the oceanic lithosphere
Plate tectonics and its contribution to progress in studies of the Earth's gravitational field is discussed. In acquisition, the development of forced feedback accelerometers, satellite navigation, and satellite radar altimetry significantly improved the accuracy and coverage of gravity data over the oceans. In interpretation, gravity and geoid anomalies are used to determine information on the thermal and mechanical properties of the oceanic lithosphere and the forces that drive plate motions
Novelty and Collective Attention
The subject of collective attention is central to an information age where
millions of people are inundated with daily messages. It is thus of interest to
understand how attention to novel items propagates and eventually fades among
large populations. We have analyzed the dynamics of collective attention among
one million users of an interactive website -- \texttt{digg.com} -- devoted to
thousands of novel news stories. The observations can be described by a
dynamical model characterized by a single novelty factor. Our measurements
indicate that novelty within groups decays with a stretched-exponential law,
suggesting the existence of a natural time scale over which attention fades
Analysis and geological interpretation of gravity data from GEOS-3 altimeter
A number of detailed gravimetric geoids of portions of the world's oceans from marine gravity measurements were constructed. The geoids were constructed by computing 1 x 1 deg or 10 x 10 deg averages of free-air anomaly data and subtracting these values from currently used satellite derived Earth models. The resulting difference gravity anomalies are then integrated over a sphere using a simplified form of Stoke's equation to obtain a difference geoid. This difference geoid is added to the satellite derived model to obtain a 1 x 1 deg or 10 x 10 deg total gravimetric geoid. The geoid undulations are studied by comparison of the altimeter measurements with the morphology of the ocean floor. Utilizing a combination of altimetry data, gravity and seismic reflection data, geophysical models of the earth can be constructed
Spatial clusters of gonorrhoea in England with particular reference to the outcome of partner notification: 2012 and 2013
Background: This study explored spatial-temporal variation in diagnoses of gonorrhoea to identify and quantify endemic areas and clusters in relation to patient characteristics and outcomes of partner notification (PN) across England, UK. Methods: Endemic areas and clusters were identified using a two-stage analysis with Kulldorffâs scan statistics (SaTScan). Results Of 2,571,838 tests, 53,547 diagnoses were gonorrhoea positive (positivity = 2.08%). The proportion of diagnoses in heterosexual males was 1.5 times that in heterosexual females. Among index cases, men who have sex with men (MSM) were 8 times more likely to be diagnosed with gonorrhoea than heterosexual males (p<0.0001). After controlling for age, gender, ethnicity and deprivation rank, 4 endemic areas were identified including 11,047 diagnoses, 86% of which occurred in London. 33 clusters included 17,629 diagnoses (34% of total diagnoses in 2012 and 2013) and spanned 21 locations, some of which were dominated by heterosexually acquired infection, whilst others were MSM focused. Of the 53,547 diagnoses, 14.5% (7,775) were the result of PN. The proportion of patients who attended services as a result of PN varied from 0% to 61% within different age, gender and sexual orientation cohorts. A third of tests resulting from PN were positive for gonorrhoea. 25% of Local Authorities (n = 81, 95% CI: 20.2, 29.5) had a higher than expected proportion for female PN diagnoses as compared to 16% for males (n = 52, 95% CI: 12.0, 19.9). Conclusions: The English gonorrhoea epidemic is characterised by spatial-temporal variation. PN success varied between endemic areas and clusters. Greater emphasis should be placed on the role of PN in the control of gonorrhoea to reduce the risk of onward transmission, re-infection, and complications of infection
Contexts for questioning: Two zones of teaching and learning in undergraduate science
This is the author's accepted manuscript. The final published article is available from the link below. Copyright @ Springer Science+Business Media B.V. 2012.Higher education institutions are currently undertaking a challenging process in moving from teacher-orientated to student-focused approaches. Studentsâ ability to asking questions is fundamental to developing critical reasoning, and to the process of scientific enquiry itself. Our premise is that questioning competences should become a central focus of current reforms in higher education. This study, part of a broader naturalistic research project, aims at developing a theoretical framework for conceptualizing different contexts for questioning, illustrating the application of the proposed framework (contextual questioning zones) and reflecting about some of the dimensions of teaching and learning, for overcoming some of the challenges that higher education institutions are facing presently. The discussion of two âoppositeâ contexts of enquiry is based on qualitative data, gathered through close collaboration with four teachers of undergraduate biology at a Portuguese university. These teachers were observed during their âdaily activityâ during an academic year. Data was also gathered by interviewing these teachers and 8 selected students, at the end of the year, and used to sustain the argumentation. The paper concludes with some reflections and suggestions to promote authentic enquiry-based learning experiences.Portuguese FundaçaÌo para a CiĂȘncia e a Tecnologi
Neutron star glitches have a substantial minimum size
Glitches are sudden spin-up events that punctuate the steady spin down of
pulsars and are thought to be due to the presence of a superfluid component
within neutron stars. The precise glitch mechanism and its trigger, however,
remain unknown. The size of glitches is a key diagnostic for models of the
underlying physics. While the largest glitches have long been taken into
account by theoretical models, it has always been assumed that the minimum size
lay below the detectability limit of the measurements. In this paper we define
general glitch detectability limits and use them on 29 years of daily
observations of the Crab pulsar, carried out at Jodrell Bank Observatory. We
find that all glitches lie well above the detectability limits and by using an
automated method to search for small events we are able to uncover the full
glitch size distribution, with no biases. Contrary to the prediction of most
models, the distribution presents a rapid decrease of the number of glitches
below ~0.05 Hz. This substantial minimum size indicates that a glitch must
involve the motion of at least several billion superfluid vortices and provides
an extra observable which can greatly help the identification of the trigger
mechanism. Our study also shows that glitches are clearly separated from all
the other rotation irregularities. This supports the idea that the origin of
glitches is different to that of timing noise, which comprises the unmodelled
random fluctuations in the rotation rates of pulsars.Comment: 8 pages; 4 figures. Accepted for publication in MNRA
Null vectors, 3-point and 4-point functions in conformal field theory
We consider 3-point and 4-point correlation functions in a conformal field
theory with a W-algebra symmetry. Whereas in a theory with only Virasoro
symmetry the three point functions of descendants fields are uniquely
determined by the three point function of the corresponding primary fields this
is not the case for a theory with algebra symmetry. The generic 3-point
functions of W-descendant fields have a countable degree of arbitrariness. We
find, however, that if one of the fields belongs to a representation with null
states that this has implications for the 3-point functions. In particular if
one of the representations is doubly-degenerate then the 3-point function is
determined up to an overall constant. We extend our analysis to 4-point
functions and find that if two of the W-primary fields are doubly degenerate
then the intermediate channels are limited to a finite set and that the
corresponding chiral blocks are determined up to an overall constant. This
corresponds to the existence of a linear differential equation for the chiral
blocks with two completely degenerate fields as has been found in the work of
Bajnok~et~al.Comment: 10 pages, LaTeX 2.09, DAMTP-93-4
Type I X-ray bursts and burst oscillations in the accreting millisecond X-ray pulsar IGR J17511-3057
We report the discovery of burst oscillations at the spin frequency in ten
thermonuclear bursts from the accreting millisecond X-ray pulsar (AMXP) IGR
J17511-3057. The burst oscillation properties are, like those from the
persistent AMXPs SAX J1808.4-3658 and XTE J1814-338, anomalous compared to
burst oscillations from intermittent pulsars or non-pulsing LMXBs. Like SAX
J1808.4-3658 they show frequency drifts in the rising phase rather than the
tail. There is also evidence for harmonic content. Where IGR J17511-3057 is
unusual compared to the other two persistent pulsars is that oscillations are
not detected throughout all bursts. As accretion rate drops the bursts get
brighter and their rise/decay time scales become shorter, while the oscillation
amplitude falls below the detection threshold: first in the burst peak and then
also in the rise. None of the bursts from IGR J17511-3057 show evidence for
photospheric radius expansion (which might be expected to suppress oscillation
amplitude) which allow us to set an upper limit to the distance of 6.9 kpc. We
discuss the implications of our results for models of the burst oscillation
mechanism.Comment: 11 pages, 5 figures, MNRAS in pres
Evidence from satellite altimetry for small-scale convection in the mantle
Small scale convection can be defined as that part of the mantle circulation in which upwellings and downwellings can occur beneath the lithosphere within the interiors of plates, in contrast to the large scale flow associated with plate motions where upwellings and downwellings occur at ridges and trenches. The two scales of convection will interact so that the form of the small scale convection will depend on how it arises within the large scale flow. Observations based on GEOS-3 and SEASAT altimetry suggest that small scale convection occurs in at least two different ways
Structural and functional studies of the nicotinic acetylcholine receptor by solid-state NMR
Over the last seven years, solid-state NMR has been widely employed to study structural and functional aspects of the nicotinic acetylcholine receptor. These studies have provided detailed structural information relating to both the ligand binding site and the transmembrane domain of the receptor. Studies of the ligand binding domain have elucidated the nature and the orientation of the pharmacophores responsible for the binding of the agonist acetylcholine within the agonist binding site. Analyses of small transmembrane fragments derived from the nicotinic acetylcholine receptor have also revealed the secondary structure and the orientation of these transmembrane domains. These experiments have expanded our understanding of the channel's structural properties and are providing an insight into how they might be modulated by the surrounding lipid environment. In this article we review the advances in solid-state NMR applied to the nicotinic acetylcholine receptor and compare the results with recent electron diffraction and X-ray crystallographic studie
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