5,910 research outputs found
Weak positive cloud-to-ground flashes in Northeastern Colorado
The frequency distributions of the peak magnetic field associated with the first detected return stroke of positive and negative cloud-to-ground (CG) flashes were studied using lightning data from northeastern Colorado. These data were obtained during 1985 with a medium-to-high gain network of three direction finders (DF's). The median signal strength of positive flashes was almost two times that of the negatives for flashes within 300 km of the DF's, which have an inherent detection-threshold bias that tends to discriminate against weak signals. This bias increases with range, and affects the detection of positive and negative flashes in different ways, because of the differing character of their distributions. Positive flashes appear to have a large percentage of signals clustered around very weak values that are lost to the medium-to-high gain Colorado Detection System very quickly with increasing range. The resulting median for positive signals could thus appear to be much larger than the median for negative signals, which are more clustered around intermediate values. When only flashes very close to the DF's are considered, however, the two distributions have almost identical medians. The large percentage of weak positive signals detected close to the DF's has not been explored previously. They have been suggested to come from intracloud discharges and thus are improperly classified as CG flashes. Evidence in hand, points to their being real positive, albeit weak CG flashes. Whether or not they are real positive ground flashes, it is important to be aware of their presence in data from magnetic DF networks
A clumpy and anisotropic galaxy halo at z=1 from gravitational-arc tomography
Every star-forming galaxy has a halo of metal-enriched gas extending out to
at least 100 kpc, as revealed by the absorption lines this gas imprints on the
spectra of background quasars. However, quasars are sparse and typically probe
only one narrow pencil beam through the intervening galaxy. Close quasar pairs
and gravitationally lensed quasars have been used to circumvent this inherently
one-dimensional technique, but these objects are rare and the structure of the
circum-galactic medium remains poorly constrained. As a result, our
understanding of the physical processes that drive the re-cycling of baryons
across the lifetime of a galaxy is limited. Here we report integral-field
(tomographic) spectroscopy of an extended background source -a bright giant
gravitational arc. We can thus coherently map the spatial and kinematic
distribution of Mg II absorption -a standard tracer of enriched gas- in an
intervening galaxy system at redshift 0.98 (i.e., ~8 Gyr ago). Our
gravitational-arc tomography unveils a clumpy medium in which the
absorption-strength decreases with increasing impact parameter, in good
agreement with the statistics towards quasars; furthermore, we find strong
evidence that the gas is not distributed isotropically. Interestingly, we
detect little kinematic variation over a projected area of ~600 kpc squared,
with all line-of-sight velocities confined to within a few tens of km/s of each
other. These results suggest that the detected absorption originates from
entrained recycled material, rather than in a galactic outflow.Comment: Published online in Nature on 31 January 201
Large-Scale Correlations in the Lyman-alpha Forest at z = 3-4
We present a study of the spatial coherence of the intergalactic medium
toward two pairs of high-redshift quasars with moderate angular separations
observed with Keck/ESI, Q1422+2309A/Q1424+2255 (z_em = 3.63, theta = 39") and
Q1439-0034A/B (z_em = 4.25, theta = 33"). The crosscorrelation of transmitted
flux in the Lyman-alpha forest shows a 5-7 sigma peak at zero velocity lag for
both pairs. This strongly suggests that at least some of the absorbing
structures span the 230-300/h_70 proper kpc transverse separation between
sightlines. We also statistically examine the similarity between paired spectra
as a function of transmitted flux, a measure which may be useful for comparison
with numerical simulations. In investigating the dependence of the correlation
functions on spectral characteristics, we find that photon noise has little
impact for S/N >~ 10 per resolution element. However, the agreement between the
autocorrelation along the line sight and the crosscorrelation between
sightlines, a potential test of cosmological geometry, depends significantly on
instrumental resolution. Finally, we present an inventory of metal lines. These
include a a pair of strong C IV systems at z ~ 3.4 appearing only toward
Q1439B, and a Mg II + Fe II system present toward Q1439 A and B at z = 1.68.Comment: 33 pages, 13 figures, submitted to Ap
firestarâadvances in the prediction of functionally important residues
firestar is a server for predicting catalytic and ligand-binding residues in protein sequences. Here, we present the important developments since the first release of firestar. Previous versions of the server required human interpretation of the results; the server is now fully automatized. firestar has been implemented as a web service and can now be run in high-throughput mode. Prediction coverage has been greatly improved with the extension of the FireDB database and the addition of alignments generated by HHsearch. Ligands in FireDB are now classified for biological relevance. Many of the changes have been motivated by the critical assessment of techniques for protein structure prediction (CASP) ligand-binding prediction experiment, which provided us with a framework to test the performance of firestar. URL: http://firedb.bioinfo.cnio.es/Php/FireStar.php
Cape Sable Seaside Sparrow Habitat â Vegetation Monitoring: FY 2009 - Final Report
This document summarizes the activities that were accomplished in FY 2009 on the research project âCape Sable seaside sparrow habitat â Vegetation Monitoringâ, a collaborative effort among the US Army Corps of Engineers, Florida International University, and the US Geological Survey. The major activities in 2009 included field work, data analysis and presentations. The results of 2009 field work were presented at the 4th International Congress of Fire Ecology and Management, Savannah, GA from November 30 to Dec 5, 2009 and at the Cape Sable seaside sparrow (CSSS) Fire Meeting, held at the Krome Center, Homestead, FL on December 8, 2009. Field sampling was conducted between March 23 and June 3, 2009, during which we resurveyed 234 sites: 191 Census sites, 3 sites on Transect B, 7 sites on Transect D, and 33 sites on Transect F. The number of sites sampled in 2009 was higher than in any previous year, primarily because a large number of sites burned in Mustang Corner fire and three other wild fires in 2008 were included in 2009 sampling. At all sites surveyed in 2009, we recorded structural and compositional vegetation parameters following the methods used in previous years (2003-2008) and tagged shrubs and trees (woody plants \u3e 1 m) present in the 5 x 60 m plots. In addition, for the first time, we measured height of sawgrass (Cladium jamaicense) stubble in the compositional plots at the sites that were burned in 2008. Field data were entered by field crews, and were thoroughly checked by Jay Sah (Co-PI) to ensure that the data were complete, correct, and compliant with sampling methodologies. The data are stored under a project folder on a shared network drive maintained by the Southeast Environmental Research Center (SERC) at FIU. The shared network drive is backed up daily
Infrared catastrophe and tunneling into strongly correlated electron systems: Perturbative x-ray edge limit
The tunneling density of states exhibits anomalies (cusps, algebraic
suppressions, and pseudogaps) at the Fermi energy in a wide variety of
low-dimensional and strongly correlated electron systems. We argue that in many
cases these spectral anomalies are caused by an infrared catastrophe in the
screening response to the sudden introduction of a new electron into the system
during a tunneling event. A nonperturbative functional-integral method is
introduced to account for this effect, making use of methods developed for the
x-ray edge singularity problem. The formalism is applicable to lattice or
continuum models of any dimensionality, with or without translational
invariance. An approximate version of the technique is applied to the 1D
electron gas and the 2D Hall fluid, yielding qualitatively correct results.Comment: 6 page
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