1,436 research outputs found
A 10-year record of Arctic summer sea ice freeboard from CryoSat-2
Satellite observations of pan-Arctic sea ice thickness have so far been constrained to winter months. For radar altimeters, conventional methods cannot differentiate leads from meltwater ponds that accumulate at the ice surface in summer months, which is a critical step in the ice thickness calculation. Here, we use over 350 optical and synthetic aperture radar (SAR) images from the summer months to train a 1D convolution neural network for separating CryoSat-2 radar altimeter returns from sea ice floes and leads with an accuracy >80%. This enables us to generate the first pan-Arctic measurements of sea ice radar freeboard for MayâSeptember between 2011 and 2020. Results indicate that the freeboard distributions in May and September compare closely to those from a conventional âwinterâ processor in April and October, respectively. The freeboards capture expected patterns of sea ice melt over the Arctic summer, matching well to ice draft observations from the Beaufort Gyre Exploration Program (BGEP) moorings. However, compared to airborne laser scanner freeboards from Operation IceBridge and airborne EM ice thickness surveys from the Alfred Wegener Institute (AWI) IceBird program, CryoSat-2 freeboards are underestimated by 0.02â0.2 m, and ice thickness is underestimated by 0.28â1.0 m, with the largest differences being over thicker multi-year sea ice. To create the first pan-Arctic summer sea ice thickness dataset we must address primary sources of uncertainty in the conversion from radar freeboard to ice thickness
An Inversion Method for Measuring Beta in Large Redshift Surveys
A precision method for determining the value of Beta= Omega_m^{0.6}/b, where
b is the galaxy bias parameter, is presented. In contrast to other existing
techniques that focus on estimating this quantity by measuring distortions in
the redshift space galaxy-galaxy correlation function or power spectrum, this
method removes the distortions by reconstructing the real space density field
and determining the value of Beta that results in a symmetric signal. To remove
the distortions, the method modifies the amplitudes of a Fourier plane-wave
expansion of the survey data parameterized by Beta. This technique is not
dependent on the small-angle/plane-parallel approximation and can make full use
of large redshift survey data. It has been tested using simulations with four
different cosmologies and returns the value of Beta to +/- 0.031, over a factor
of two improvement over existing techniques.Comment: 16 pages including 6 figures Submitted to The Astrophysical Journa
The Clustering of Colour Selected Galaxies
We present measurements of the angular correlation function of galaxies
selected from a B_J=23.5 multicolour survey of two 5 degree by 5 degree fields
located at high galactic latitudes. The galaxy catalogue of approximately
400,000 galaxies is comparable in size to catalogues used to determine the
galaxy correlation function at low-redshift. Measurements of the z=0.4
correlation function at large angular scales show no evidence for a break from
a power law though our results are not inconsistent with a break at >15 Mpc.
Despite the large fields-of-view, there are large discrepancies between the
measurements of the correlation function in each field, possibly due to dwarf
galaxies within z=0.11 clusters near the South Galactic Pole.
Colour selection is used to study the clustering of galaxies z=0 to z=0.4.
The galaxy correlation function is found to strongly depend on colour with red
galaxies more strongly clustered than blue galaxies by a factor of 5 at small
scales. The slope of the correlation function is also found to vary with colour
with gamma=1.8 for red galaxies while gamma=1.5 for blue galaxies. The
clustering of red galaxies is consistently strong over the entire magnitude
range studied though there are large variations between the two fields. The
clustering of blue galaxies is extremely weak over the observed magnitude range
with clustering consistent with r_0=2 Mpc. This is weaker than the clustering
of late-type galaxies in the local Universe and suggests galaxy clustering is
more strongly correlated with colour than morphology. This may also be the
first detection of a substantial low redshift galaxy population with clustering
properties similar to faint blue galaxies.Comment: Accepted for publication in MNRAS. 13 pages, 20 figure
A 120-Mpc Periodicity in the Three-Dimensional Distribution of Galaxy Superclusters
Using a new compilation of available data on galaxy clusters and
superclusters we present evidence for a quasiregular three-dimensional network
of rich superclusters and voids, with the regions of high density separated by
about 120 Mpc. We calculate the power spectrum for clusters of galaxies; it has
a peak on the wavelength equal to the step of the network; the excess in the
amplitude of the spectrum over that of the cold dark matter model is by a
factor of 1.4. The probability that the spectrum can be formed within the
framework of the standard cosmogony is very small. If the cluster distribution
reflects the distribution of all matter (luminous and dark), then there must
exists some hithero unknown process that produces regular structure on large
scales.Comment: Tex, 6 pages, 2 PostScript figures embedded, accepted by Nature on
November 19, 199
Optimal measurement of visual motion across spatial and temporal scales
Sensory systems use limited resources to mediate the perception of a great
variety of objects and events. Here a normative framework is presented for
exploring how the problem of efficient allocation of resources can be solved in
visual perception. Starting with a basic property of every measurement,
captured by Gabor's uncertainty relation about the location and frequency
content of signals, prescriptions are developed for optimal allocation of
sensors for reliable perception of visual motion. This study reveals that a
large-scale characteristic of human vision (the spatiotemporal contrast
sensitivity function) is similar to the optimal prescription, and it suggests
that some previously puzzling phenomena of visual sensitivity, adaptation, and
perceptual organization have simple principled explanations.Comment: 28 pages, 10 figures, 2 appendices; in press in Favorskaya MN and
Jain LC (Eds), Computer Vision in Advanced Control Systems using Conventional
and Intelligent Paradigms, Intelligent Systems Reference Library,
Springer-Verlag, Berli
The Canada-France deep fields survey-I: 100,000 galaxies, 1 deg^2: a precise measurement of \omega(\theta) to IAB~25
(abridged) Using the UH8K mosaic camera, we have measured the angular
correlation function \omega(\theta) for 100,000 galaxies over four widely
separated fields totalling ~1\deg^2 and reaching IAB~25.5. With this sample we
investigate the dependence of \omega(\theta) at 1', A_\omega(1'), on sample
median IAB magnitude in the range 19.5<I(AB-med)<24. Our results show that
A_\omega(1') decreases monotonically to IAB~25. At bright magnitudes,
\omega(\theta) is consistent with a power-law of slope \delta = -0.8 for
0.2'<\theta<3.0' but at fainter magnitudes we find \delta ~ -0.6. At the
3\sigma level, our observations are still consistent with \delta=-0.8.
Furthermore, in the magnitude ranges 18.5<IAB<24.0 and 18.5<IAB<23.0 we find
galaxies with 2.6<(V-I)AB<2.9 have A_\omega(1')'s which are ~10x higher than
field values. We demonstrate that our model redshift distributions for the
faint galaxy population are in good agreement with current spectroscopic
observations. Using these predictions, we find that for low-omega cosmologies
and assuming r_0=4.3/h Mpc, in the range 19.5<I(AB-med)<22, the growth of
galaxy clustering is \epsilon~0. However, at 22<I(AB-med)<24.0, our
observations are consistent with \epsilon>1. Models with \epsilon~0 cannot
simultaneously match both bright and faint measurements of A_\omega(1`). We
show how this result is a natural consequence of the ``bias-free'' nature of
the \epsilon formalism and is consistent with the field galaxy population in
the range 22.0<IAB<24.0 being dominated by galaxies of low intrinsic
luminosity.Comment: 20 pages, 21 figures, requires natbib.sty, accepted for publication
in Astronomy and Astrophysic
Cumulants as non-Gaussian qualifiers
We discuss the requirements of good statistics for quantifying
non-Gaussianity in the Cosmic Microwave Background. The importance of
rotational invariance and statistical independence is stressed, but we show
that these are sometimes incompatible. It is shown that the first of these
requirements prefers a real space (or wavelet) formulation, whereas the latter
favours quantities defined in Fourier space. Bearing this in mind we decide to
be eclectic and define two new sets of statistics to quantify the level of
non-Gaussianity. Both sets make use of the concept of cumulants of a
distribution. However, one set is defined in real space, with reference to the
wavelet transform, whereas the other is defined in Fourier space. We derive a
series of properties concerning these statistics for a Gaussian random field
and show how one can relate these quantities to the higher order moments of
temperature maps. Although our frameworks lead to an infinite hierarchy of
quantities we show how cosmic variance and experimental constraints give a
natural truncation of this hierarchy. We then focus on the real space
statistics and analyse the non-Gaussian signal generated by points sources
obscured by large scale Gaussian fluctuations. We conclude by discussing the
practical implementations of these techniques
Genetic Studies of Sulfadiazine-resistant and Methionine-requiring \u3cem\u3eNeisseria\u3c/em\u3e Isolated From Clinical Material
Deoxyribonucleate (DNA) preparations were extracted from Neisseria meningitidis (four isolates from spinal fluid and blood) and N. gonorrhoeae strains, all of which were resistant to sulfadiazine upon primary isolation. These DNA preparations, together with others from in vitro mutants of N. meningitidis and N. perflava, were examined in transformation tests by using as recipient a drug-susceptible strain of N. meningitidis (Ne 15 Sul-s Met+) which was able to grow in a methionine-free defined medium. The sulfadiazine resistance typical of each donor was introduced into the uniform constitution of this recipient. Production of p-aminobenzoic acid was not significantly altered thereby. Transformants elicited by DNA from the N. meningitidis clinical isolates were resistant to at least 200 ÎŒg of sulfadiazine/ml, and did not show a requirement for methionine (Sul-r Met+). DNA from six strains of N. gonorrhoeae, which were isolated during the period of therapeutic use of sulfonamides, conveyed lower degrees of resistance and, invariably, a concurrent methionine requirement (Sul-r/Metâ). The requirement of these transformants, and that of in vitro mutants selected on sulfadiazine-agar, was satisfied by methionine, but not by vitamin B12, homocysteine, cystathionine, homoserine, or cysteine. Sul-r Met+ and Sul-r/Metâ loci could coexist in the same genome, but were segregated during transformation. On the other hand, the dual Sul-r/Metâ properties were not separated by recombination, but were eliminated together. DNA from various Sul-r/Metâ clones tested against recipients having nonidentical Sul-r/Metâ mutant sites yielded Sul-s Met+ transformants. The met locus involved is genetically complex, and will be a valuable tool for studies of genetic fine structure of members of Neisseria, and of genetic homology between species
Hypophosphatasia: Enzyme Replacement Therapy (Asfotase Alfa) Decreases TNSALP Substrate Accumulation And Improves Functional Outcomes In Affected Adolescents And Adults
Enzyme Replacement Therapy In Different Clinical Phenotypes Of Hypophosphatasia â A Presentation Of Two Cases
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