164 research outputs found
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Eclipse-induced wind changes over the British Isles on the 20 March 2015
The British Isles benefits from dense meteorological observation networks, enabling insights into the still-unresolved effects of solar eclipse events on the near-surface wind field. The near-surface effects of the solar eclipse of 20 March 2015 are derived through comparison of output from the Met Office's operational weather forecast model (which is ignorant of the eclipse) with data from two meteorological networks: the Met Office's land surface station (MIDAS) network and a roadside measurement network operated by Vaisala. Synoptic-evolution relative calculations reveal the cooling and increase in relative humidity almost universally attributed to eclipse events. In addition, a slackening of wind speeds by up to about 2 knots in already weak winds and backing in wind direction of about 20 degrees under clear skies across middle England are attributed to the eclipse event. The slackening of wind speed is consistent with the previously reported boundary layer stabilisation during eclipse events. Wind direction changes have previously been attributed to a large-scale `eclipse-induced cold-cored cyclone', mountain slope flows, and changes in the strength of sea breezes. A new explanation is proposed here by analogy with nocturnal wind changes at sunset and shown to predict direction changes consistent with those observed
Exact microscopic theory of electromagnetic heat transfer between a dielectric sphere and plate
Near-field electromagnetic heat transfer holds great potential for the
advancement of nanotechnology. Whereas far-field electromagnetic heat transfer
is constrained by Planck's blackbody limit, the increased density of states in
the near-field enhances heat transfer rates by orders of magnitude relative to
the conventional limit. Such enhancement opens new possibilities in numerous
applications, including thermal-photo-voltaics, nano-patterning, and imaging.
The advancement in this area, however, has been hampered by the lack of
rigorous theoretical treatment, especially for geometries that are of direct
experimental relevance. Here we introduce an efficient computational strategy,
and present the first rigorous calculation of electromagnetic heat transfer in
a sphere-plate geometry, the only geometry where transfer rate beyond blackbody
limit has been quantitatively probed at room temperature. Our approach results
in a definitive picture unifying various approximations previously used to
treat this problem, and provides new physical insights for designing
experiments aiming to explore enhanced thermal transfer.Comment: 1 page title 8 page content 1 page references 2 page figure captions
4 page figure
Minerva Cuevas: Disidencia
Bilingual catalogue for the exhibition Minerva Cuevas: Disidencia presented at Baruch College\u27s Mishkin Gallery
Laser frequency comb techniques for precise astronomical spectroscopy
Precise astronomical spectroscopic analyses routinely assume that individual
pixels in charge-coupled devices (CCDs) have uniform sensitivity to photons.
Intra-pixel sensitivity (IPS) variations may already cause small systematic
errors in, for example, studies of extra-solar planets via stellar radial
velocities and cosmological variability in fundamental constants via quasar
spectroscopy, but future experiments requiring velocity precisions approaching
~1 cm/s will be more strongly affected. Laser frequency combs have been shown
to provide highly precise wavelength calibration for astronomical
spectrographs, but here we show that they can also be used to measure IPS
variations in astronomical CCDs in situ. We successfully tested a laser
frequency comb system on the Ultra-High Resolution Facility spectrograph at the
Anglo-Australian Telescope. By modelling the 2-dimensional comb signal recorded
in a single CCD exposure, we find that the average IPS deviates by <8 per cent
if it is assumed to vary symmetrically about the pixel centre. We also
demonstrate that series of comb exposures with absolutely known offsets between
them can yield tighter constraints on symmetric IPS variations from ~100
pixels. We discuss measurement of asymmetric IPS variations and absolute
wavelength calibration of astronomical spectrographs and CCDs using frequency
combs.Comment: 11 pages, 7 figures. Accepted for publication in MNRA
Thermonuclear Kinetics in Astrophysics
Over the billions of years since the Big Bang, the lives, deaths and
afterlives of stars have enriched the Universe in the heavy elements that make
up so much of ourselves and our world. This review summarizes the methods used
to evolve these nuclear abundances within astrophysical simulations. These
methods fall into 2 categories; evolution via rate equations and via
equilibria. Because the rate equations in nucleosynthetic applications involve
a wide range of timescales, implicit methods have proven mandatory, leading to
the need to solve matrix equations. Efforts to improve the performance of such
rate equation methods are focused on efficient solution of these matrix
equations, in particular by making best use of the sparseness of these
matrices, and finding methods that require less frequent matrix solutions.
Recent work to produce hybrid schemes which use local equilibria to reduce the
computational cost of the rate equations is also discussed. Such schemes offer
significant improvements in the speed of reaction networks and are accurate
under circumstances where calculations which assume complete equilibrium fail.Comment: 27 pages, 2 figures, a review for a special issue of Nuclear Physics
Array-conditioned deconvolution of multiple component teleseismic recordings
We investigate the applicability of an array-conditioned deconvolution technique, developed for analyzing borehole seismic exploration data, to teleseismic receiver functions and data preprocessing steps for scattered wavefield imaging. This multichannel deconvolution technique constructs an approximate inverse filter to the estimated source signature by solving an overdetermined set of deconvolution equations, using an array of receivers detecting a common source. We find that this technique improves the efficiency and automation of receiverfunction calculation and data preprocessing workflow. We apply this technique to synthetic experiments and to teleseismic data recorded in a dense array in northern Canada. Our results show that this optimal deconvolution automatically determines and subsequently attenuates the noise from data, enhancing P-to-S converted phases in seismograms with various noise levels. In this context, the array-conditioned deconvolution presents a new, effective and automatic means for processing large amounts of array data, as it does not require any ad-hoc regularization; the regularization is achieved naturally by using the noise present in the array itself
Compilation and R-matrix analysis of Big Bang nuclear reaction rates
We use the R-matrix theory to fit low-energy data on nuclear reactions
involved in Big Bang nucleosynthesis. A special attention is paid to the rate
uncertainties which are evaluated on statistical grounds. We provide S factors
and reaction rates in tabular and graphical formats.Comment: 40 pages, accepted for publication at ADNDT, web site at
http://pntpm3.ulb.ac.be/bigban
Modules for Experiments in Stellar Astrophysics (MESA)
Stellar physics and evolution calculations enable a broad range of research
in astrophysics. Modules for Experiments in Stellar Astrophysics (MESA) is a
suite of open source libraries for a wide range of applications in
computational stellar astrophysics. A newly designed 1-D stellar evolution
module, MESA star, combines many of the numerical and physics modules for
simulations of a wide range of stellar evolution scenarios ranging from
very-low mass to massive stars, including advanced evolutionary phases. MESA
star solves the fully coupled structure and composition equations
simultaneously. It uses adaptive mesh refinement and sophisticated timestep
controls, and supports shared memory parallelism based on OpenMP. Independently
usable modules provide equation of state, opacity, nuclear reaction rates, and
atmosphere boundary conditions. Each module is constructed as a separate
Fortran 95 library with its own public interface. Examples include comparisons
to other codes and show evolutionary tracks of very low mass stars, brown
dwarfs, and gas giant planets; the complete evolution of a 1 Msun star from the
pre-main sequence to a cooling white dwarf; the Solar sound speed profile; the
evolution of intermediate mass stars through the thermal pulses on the He-shell
burning AGB phase; the interior structure of slowly pulsating B Stars and Beta
Cepheids; evolutionary tracks of massive stars from the pre-main sequence to
the onset of core collapse; stars undergoing Roche lobe overflow; and accretion
onto a neutron star. Instructions for downloading and installing MESA can be
found on the project web site (http://mesa.sourceforge.net/).Comment: 110 pages, 39 figures; submitted to ApJS; visit the MESA website at
http://mesa.sourceforge.ne
Cepheid calibration of type Ia Supernovae and the Hubble constant
We investigate how a different calibration of the Cepheid Period-Luminosity
(PL) relation taking into account the metallicity corrections, affects the
absolute magnitude calibration of Supernovae (SNe) Ia and, in turn, the
determination of the Hubble constant H_0. We exploit SN Ia light curves from
literature and previously unpublished data, to build the M_B -- Delta m_{15}(B)
relation and we calibrate the zero point by means of 9 type Ia SNe with Cepheid
measured distances. This relation was then used to build the Hubble diagram and
in turn to derive H_0. In the attempt to correct for the host galaxy
extinction, we found that the data seems to suggest a value for the total to
selective absorption ratio, R_B=3.5, which is smaller than the standard value
for our own Galaxy R_B=4.315. Depending on different metallicity corrections
for the Cepheids P-L relation, values of R_B and SN sample selection criteria,
we found that the values of the Hubble constant H_0 is in the range 68--74 km
s^{-1} Mpc^{-1}, with associated uncertainties of the order of 10%. Unpublished
photometry is also presented for 18 SNe of our sample (1991S, 1991T, 1992A,
1992K, 1993H, 1993L, 1994D, 1994M, 1994ae, 1995D, 1995ac, 1995bd, 1996bo,
1997bp, 1997br, 1999aa, 1999dk, 2000cx), which are the results of a long
standing effort for supernova monitoring at ESO - La Silla and Asiago
Observatories.Comment: 19 pages, 9 figures, accepted for publication in MNRA
Condensational symbols in British press coverage of Boko Haram
This study of British press coverage of Boko Haram, a militant group in Nigeria, concentrates on condensational symbols in news reports of one of its major acts of terrorism, the bombing of the United Nations House in Abuja, the country’s capital city, in August 2011. The study examines the visibility of Boko Haram in British newspapers before and after the attack. It identifies the condensational symbols that dominated the coverage and how these provided a particular trajectory that could have shaped newspaper readers’ understanding of the event. The study argues that the symbolic terms that journalists used in their reports were not only easily identifiable but were specifically chosen to simplify a complex story for audiences that were perhaps uninformed about the group and its activities. The terms also reflect the repertoire of news frames that journalists mine to reconstruct reality for their audiences
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