3,408 research outputs found
Chemical ozone loss in the Arctic winter 1991–1992
Chemical ozone loss in winter 1991–1992 is recalculated based on observations of the HALOE satellite instrument, Version 19, ER-2 aircraft measurements and balloon data. HALOE satellite observations are shown to be reliable in the lower stratosphere below 400 K, at altitudes where the measurements are most likely disturbed by the enhanced sulfate aerosol loading, as a result of the Mt.~Pinatubo eruption in June 1991. Significant chemical ozone loss (13–17 DU) is observed below 380 K from Kiruna balloon observations and HALOE satellite data between December 1991 and March 1992. For the two winters after the Mt. Pinatubo eruption, HALOE satellite observations show a stronger extent of chemical ozone loss towards lower altitudes compared to other Arctic winters between 1991 and 2003. In spite of already occurring deactivation of chlorine in March 1992, MIPAS-B and LPMA balloon observations indicate that chlorine was still activated at lower altitudes, consistent with observed chemical ozone loss occurring between February and March and April. Large chemical ozone loss of more than 70 DU in the Arctic winter 1991–1992 as calculated in earlier studies is corroborated here
The onset and dynamics of avalanches in a rotating cylinder: From experimental data to a new geometric model
Particle image velocimetry has been applied to measure particle velocities on
the free surface of a bed of particles within a rotating cylinder during
avalanching. The particle velocities were used to examine the validity of
existing avalanche models and to propose an alternative model. The movement of
particles depends on their location on the surface of the bed: particles
located near the center of the bed travel the farthest, while the distance
travelled decreases at an increasing rate for particles located farther from
the center. The start of an avalanche can be determined to a single initiation
point, that can also be located on the bottom half of the bed; the avalanche
quickly propagates through the entire free surface, with 90% of the surface in
motion within 257 ms. The experimental insight is used to formulate a new
geometric model, in which three equal sized sections flow down the bed during
an avalanche. The predictions of the model are confirmed by experimental mixing
measurements
Excited OH+, H2O+, and H3O+ in NGC 4418 and Arp 220
We report on Herschel/PACS observations of absorption lines of OH+, H2O+ and
H3O+ in NGC 4418 and Arp 220. Excited lines of OH+ and H2O+ with E_lower of at
least 285 and \sim200 K, respectively, are detected in both sources, indicating
radiative pumping and location in the high radiation density environment of the
nuclear regions. Abundance ratios OH+/H2O+ of 1-2.5 are estimated in the nuclei
of both sources. The inferred OH+ column and abundance relative to H nuclei are
(0.5-1)x10^{16} cm-2 and \sim2x10^{-8}, respectively. Additionally, in Arp 220,
an extended low excitation component around the nuclear region is found to have
OH+/H2O+\sim5-10. H3O+ is detected in both sources with
N(H3O+)\sim(0.5-2)x10^{16} cm-2, and in Arp 220 the pure inversion, metastable
lines indicate a high rotational temperature of ~500 K, indicative of formation
pumping and/or hot gas. Simple chemical models favor an ionization sequence
dominated by H+ - O+ - OH+ - H2O+ - H3O+, and we also argue that the H+
production is most likely dominated by X-ray/cosmic ray ionization. The full
set of observations and models leads us to propose that the molecular ions
arise in a relatively low density (\gtrsim10^4 cm-3) interclump medium, in
which case the ionization rate per H nucleus (including secondary ionizations)
is zeta>10^{-13} s-1, a lower limit that is severalx10^2 times the highest rate
estimates for Galactic regions. In Arp 220, our lower limit for zeta is
compatible with estimates for the cosmic ray energy density inferred previously
from the supernova rate and synchrotron radio emission, and also with the
expected ionization rate produced by X-rays. In NGC 4418, we argue that X-ray
ionization due to an AGN is responsible for the molecular ion production.Comment: 24 pages, 13 figures. Accepted for publication in Astronomy &
Astrophysic
Semiclassical approach to discrete symmetries in quantum chaos
We use semiclassical methods to evaluate the spectral two-point correlation
function of quantum chaotic systems with discrete geometrical symmetries. The
energy spectra of these systems can be divided into subspectra that are
associated to irreducible representations of the corresponding symmetry group.
We show that for (spinless) time reversal invariant systems the statistics
inside these subspectra depend on the type of irreducible representation. For
real representations the spectral statistics agree with those of the Gaussian
Orthogonal Ensemble (GOE) of Random Matrix Theory (RMT), whereas complex
representations correspond to the Gaussian Unitary Ensemble (GUE). For systems
without time reversal invariance all subspectra show GUE statistics. There are
no correlations between non-degenerate subspectra. Our techniques generalize
recent developments in the semiclassical approach to quantum chaos allowing one
to obtain full agreement with the two-point correlation function predicted by
RMT, including oscillatory contributions.Comment: 26 pages, 8 Figure
Carrageenan Is a Potent Inhibitor of Papillomavirus Infection
Certain sexually transmitted human papillomavirus (HPV) types are causally associated with the development of cervical cancer. Our recent development of high-titer HPV pseudoviruses has made it possible to perform high-throughput in vitro screens to identify HPV infection inhibitors. Comparison of a variety of compounds revealed that carrageenan, a type of sulfated polysaccharide extracted from red algae, is an extremely potent infection inhibitor for a broad range of sexually transmitted HPVs. Although carrageenan can inhibit herpes simplex viruses and some strains of HIV in vitro, genital HPVs are about a thousand-fold more susceptible, with 50% inhibitory doses in the low ng/ml range. Carrageenan acts primarily by preventing the binding of HPV virions to cells. This finding is consistent with the fact that carrageenan resembles heparan sulfate, an HPV cell-attachment factor. However, carrageenan is three orders of magnitude more potent than heparin, a form of cell-free heparan sulfate that has been regarded as a highly effective model HPV inhibitor. Carrageenan can also block HPV infection through a second, postattachment heparan sulfate–independent effect. Carrageenan is in widespread commercial use as a thickener in a variety of cosmetic and food products, ranging from sexual lubricants to infant feeding formulas. Some of these products block HPV infectivity in vitro, even when diluted a million-fold. Clinical trials are needed to determine whether carrageenan-based products are effective as topical microbicides against genital HPVs
Cosmic-Ray Positrons: Are There Primary Sources?
Cosmic rays at the Earth include a secondary component originating in
collisions of primary particles with the diffuse interstellar gas. The
secondary cosmic rays are relatively rare but carry important information on
the Galactic propagation of the primary particles. The secondary component
includes a small fraction of antimatter particles, positrons and antiprotons.
In addition, positrons and antiprotons may also come from unusual sources and
possibly provide insight into new physics. For instance, the annihilation of
heavy supersymmetric dark matter particles within the Galactic halo could lead
to positrons or antiprotons with distinctive energy signatures. With the
High-Energy Antimatter Telescope (HEAT) balloon-borne instrument, we have
measured the abundances of positrons and electrons at energies between 1 and 50
GeV. The data suggest that indeed a small additional antimatter component may
be present that cannot be explained by a purely secondary production mechanism.
Here we describe the signature of the effect and discuss its possible origin.Comment: 15 pages, Latex, epsfig and aasms4 macros required, to appear in
Astroparticle Physics (1999
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