3,826 research outputs found
Determining electron column density fluctuations in a dominant scattering region using pulsar scintillation
Density fluctuations in the ionised interstellar medium have a profound
effect on radio pulsar observations, through angular scattering, intensity
scintillations, and small changes in time delays from dispersion. Here we show
that it is possible to recover the variations in dispersive delays that
originate from a dominant scattering region using measurements of the dynamic
spectrum of intensity scintillations, provided that the pulsar velocity and
scattering region location are known. We provide a theoretical framework for
the technique, which involves estimating the phase gradient from the dynamic
spectra and integrating that gradient to obtain phase variations. It can be
used to search for "extreme scattering events" (ESEs) in pulsars for which
precision dispersion delay measurements are not otherwise possible, or to
separate true dispersion variations from apparent variability caused by
frequency-dependent pulse shape changes. We demonstrate that it works in
practice by recovering an ESE in PSR J16037202, which is known from
precision dispersion delay measurements from pulsar timing. For this pulsar, we
find that the phase gradients also track the long-term variations in electron
column density observed by pulsar timing, indicating that the column density
variations and the scattering are dominated by the same thin scattering screen.
We identify a sudden increase in the scintillation strength and magnitude of
phase gradients over days in 2010, indicating a compact structure. A
decrease in the electron density in 2012 was associated with persistent phase
gradients and preceded a period of decreased scintillation strength and an
absence of scintillation arcs.Comment: 9 pages, 7 figures, Accepted for publication in MNRA
A new B-dot probe-based diagnostic for amplitude, polarization, and wavenumber measurements of ion cyclotron range-of frequency fields on ASDEX Upgrade
A new B-dot probe-based diagnostic has been installed on an ASDEX Upgrade tokamak to characterize ion cyclotron range-of frequency (ICRF) wave generation and interaction with magnetized plasma. The diagnostic consists of a field-aligned array of B-dot probes, oriented to measure fast and slow ICRF wave fields and their field-aligned wavenumber (k(//)) spectrum on the low field side of ASDEX Upgrade. A thorough description of the diagnostic and the supporting electronics is provided. In order to compare the measured dominant wavenumber of the local ICRF fields with the expected spectrum of the launched ICRF waves, in-air near-field measurements were performed on the newly installed 3-strap ICRF antenna to reconstruct the dominant launched toroidal wavenumbers (k(tor)). Measurements during a strap current phasing scan in tokamak discharges reveal an upshift in k(//) as strap phasing is moved away from the dipole configuration. This result is the opposite of the k(tor) trend expected from in-air near-field measurements; however, the near-field based reconstruction routine does not account for the effect of induced radiofrequency (RF) currents in the passive antenna structures. The measured exponential increase in the local ICRF wave field amplitude is in agreement with the upshifted k(//), as strap phasing moves away from the dipole configuration. An examination of discharges heated with two ICRF antennas simultaneously reveals the existence of beat waves at 1 kHz, as expected from the difference of the two antennas' operating frequencies. Beats are observed on both the fast and the slow wave probes suggesting that the two waves are coupled outside the active antennas. Although the new diagnostic shows consistent trends between the amplitude and the phase measurements in response to changes applied by the ICRF antennas, the disagreement with the in-air near-field measurements remains. An electromagnetic model is currently under development to address this issue. (C) 2015 AIP Publishing LLC
What’s next? COVID-19 as a planetary inflection point for people, places, policy, and research
Optimal Pacing for Running 400 m and 800 m Track Races
Physicists seeking to understand complex biological systems often find it
rewarding to create simple "toy models" that reproduce system behavior. Here a
toy model is used to understand a puzzling phenomenon from the sport of track
and field. Races are almost always won, and records set, in 400 m and 800 m
running events by people who run the first half of the race faster than the
second half, which is not true of shorter races, nor of longer. There is
general agreement that performance in the 400 m and 800 m is limited somehow by
the amount of anaerobic metabolism that can be tolerated in the working muscles
in the legs. A toy model of anaerobic metabolism is presented, from which an
optimal pacing strategy is analytically calculated via the Euler-Lagrange
equation. This optimal strategy is then modified to account for the fact that
the runner starts the race from rest; this modification is shown to result in
the best possible outcome by use of an elementary variational technique that
supplements what is found in undergraduate textbooks. The toy model reproduces
the pacing strategies of elite 400 m and 800 m runners better than existing
models do. The toy model also gives some insight into training strategies that
improve performance.Comment: 14 pages, 4 figures, submitted to the American Journal of Physic
Severe Takayasu's arteritis of the pulmonary arteries: Report of a case with successful surgical treatment
Although involvement of the aorta and its large branches is more common, Takayasu's arteritis involving the pulmonary arteries is well recognized. This report describes an adolescent girl with an uncommonly severe form ofTakayasu's arteritis involving the pulmonary arteries. A successful surgical treatment is presented
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Identifying children with anxiety disorders using brief versions of the Spence Children's Anxiety Scale for children, parents and teachers
Anxiety disorders are among the most prevalent mental health disorders experienced by children and are associated with significant negative outcomes. Only a minority of affected children, however, access professional help, and a failure to identify children with anxiety disorders presents a key barrier to treatment access. Existing child anxiety questionnaire measures are long and time consuming to complete, limiting their potential for widespread use as identification tools in community settings. We developed a brief questionnaire for parents, children, and teachers using items from the Spence Children’s Anxiety Scale (SCAS) and evaluated the new measure’s psychometric properties, capacity to discriminate between a community (n = 361) and clinic-referred sample (n = 338) of children aged 7–11, and identified optimal cut-off scores for accurate identification of preadolescent children experiencing clinically significant levels of anxiety. The findings provided support for the reliability and validity of 8-item versions of the SCAS, with the brief questionnaire scores displaying comparable internal consistency, agreement among reporters, and convergent/divergent validity to the full-length SCAS scores. The brief SCAS scores also discriminated between the community and clinic-referred samples and identified children in the clinic-referred sample with a moderate-to-good level of accuracy and acceptable sensitivity and specificity. Combining reporters improved sensitivity, but at the expense of specificity, and findings suggested parent report should be prioritized. This new brief questionnaire has potential for use in community settings as a tool to improve identification of children who are experiencing clinically significant levels of anxiety and warrant further assessment and potential support
IoT Expunge: Implementing Verifiable Retention of IoT Data
The growing deployment of Internet of Things (IoT) systems aims to ease the
daily life of end-users by providing several value-added services. However, IoT
systems may capture and store sensitive, personal data about individuals in the
cloud, thereby jeopardizing user-privacy. Emerging legislation, such as
California's CalOPPA and GDPR in Europe, support strong privacy laws to protect
an individual's data in the cloud. One such law relates to strict enforcement
of data retention policies. This paper proposes a framework, entitled IoT
Expunge that allows sensor data providers to store the data in cloud platforms
that will ensure enforcement of retention policies. Additionally, the cloud
provider produces verifiable proofs of its adherence to the retention policies.
Experimental results on a real-world smart building testbed show that IoT
Expunge imposes minimal overheads to the user to verify the data against data
retention policies.Comment: This paper has been accepted in 10th ACM Conference on Data and
Application Security and Privacy (CODASPY), 202
Spatial Reconstructions and Comparisons of Historic Snow Avalanche Frequency and Extent Using Tree Rings in Glacier National Park, Montana, U.S.A
Unscreened water-diversion pipes pose an entrainment risk to the threatened green sturgeon, Acipenser medirostris.
Over 3,300 unscreened agricultural water diversion pipes line the levees and riverbanks of the Sacramento River (California) watershed, where the threatened Southern Distinct Population Segment of green sturgeon, Acipenser medirostris, spawn. The number of sturgeon drawn into (entrained) and killed by these pipes is greatly unknown. We examined avoidance behaviors and entrainment susceptibility of juvenile green sturgeon (35±0.6 cm mean fork length) to entrainment in a large (>500-kl) outdoor flume with a 0.46-m-diameter water-diversion pipe. Fish entrainment was generally high (range: 26-61%), likely due to a lack of avoidance behavior prior to entering inescapable inflow conditions. We estimated that up to 52% of green sturgeon could be entrained after passing within 1.5 m of an active water-diversion pipe three times. These data suggest that green sturgeon are vulnerable to unscreened water-diversion pipes, and that additional research is needed to determine the potential impacts of entrainment mortality on declining sturgeon populations. Data under various hydraulic conditions also suggest that entrainment-related mortality could be decreased by extracting water at lower diversion rates over longer periods of time, balancing agricultural needs with green sturgeon conservation
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