665 research outputs found
Transport into the south polar vortex in early spring
Estimates of the mean circulation and diffusive transport of ozone and other species into the Antarctic polar vortex during the spring of 1987 are made using data from the Airborne Antarctic Ozone Experiment. Measurements of long-lived tracers of tropospheric origin remained relatively constant at the levels of the maximum rate of decline of ozone during September. At lower levels in the stratosphere some evidence exists to support intrusions of tropospheric or low latitude air. Given the distribution in latitude and height of these tracers measured from the ER-2 aircraft, it can be inferred that the Lagrangian or diabatic mean circulation was zero or downward over Antarctica during the period of the ozone decline. The observation of a decline in ozone therefore requires a photochemical sink for ozone. The magnitude of the required photochemical sink must be sufficient to offset the transport of ozone into the polar region and produce the observed decline. Quasi-isentropic mixing and downward motion are coupled and are difficult to estimate from a single tracer. The full suite of measured tracers and auxiliary information are brought together to provide an estimate of the rate at which air is cycled through the polar vortex during spring. Estimates of large scale transport of potential vorticity and ozone from previous years are generally consistent with the data from the airborne experiment in suggesting a relatively slow rate of mass flow through the polar vortex in the lower stratosphere during September
Thermally Activated Magnetization and Resistance Decay during Near Ambient Temperature Aging of Co Nanoflakes in a Confining Semi-metallic Environment
We report the observation of magnetic and resistive aging in a self assembled
nanoparticle system produced in a multilayer Co/Sb sandwich. The aging decays
are characterized by an initial slow decay followed by a more rapid decay in
both the magnetization and resistance. The decays are large accounting for
almost 70% of the magnetization and almost 40% of the resistance for samples
deposited at 35 . For samples deposited at 50 the magnetization
decay accounts for of the magnetization and 50% of the resistance.
During the more rapid part of the decay, the concavity of the slope of the
decay changes sign and this inflection point can be used to provide a
characteristic time. The characteristic time is strongly and systematically
temperature dependent, ranging from x at 400K to x at 320K in samples deposited at . Samples deposited at 50
displayed a 7-8 fold increase in the characteristic time (compared to the samples) for a given aging temperature, indicating that this timescale may
be tunable. Both the temperature scale and time scales are in potentially
useful regimes. Pre-Aging, Scanning Tunneling Microscopy (STM) reveals that the
Co forms in nanoscale flakes. During aging the nanoflakes melt and migrate into
each other in an anisotropic fashion forming elongated Co nanowires. This aging
behavior occurs within a confined environment of the enveloping Sb layers. The
relationship between the characteristic time and aging temperature fits an
Arrhenius law indicating activated dynamics
Trace gas measurements from whole air samples collected over the Antarctic continent
Whole air samples collected aboard the NASA DC-8 and ER-2 aircraft as part of the Airborne Antarctic Ozone Experiment (AAOE) were analyzed in a field laboratory set up at Punta Arenas, Chile, in August and September, 1987. Mixing ratios obtained from gas chromatographic analyses of these samples are presented for N2O, CFCl3, CFCl2, C2F3Cl3, CH3CCl3, CH4, and CO. Variations in the mixing ratios of these gases along the individual flight paths of the aircraft are used as tracers to indicate the history of air masses over and near the Antarctic continent
Discovery of an Optical Jet in the BL Lac Object 3C 371
We have detected an optical jet in the BL Lac object 3C 371 that coincides
with the radio jet in this object in the central few kpc. The most notable
feature is a bright optical knot 3 arcsec (4 kpc) from the nucleus that occurs
at the location where the jet apparently changes its direction by ~30 degrees.
The radio, near-infrared and optical observations of this knot are consistent
with a single power-law spectrum with a radio-optical spectral index alpha =
-0.81. One possible scenario for the observed turn is that the jet is
interacting with the material in the bridge connecting 3C 371 to nearby
galaxies and the pressure gradient is deflecting the jet significantly.Comment: 11 pages, LaTeX, 4 figures (1 eps, 3 gifs), accepted for publication
in ApJ Letter
Effect of high-pressure hot-water washing treatment on fruit quality, insects, and disease in apples and pears Part III. Use of silicone-based materials and mechanical methods to eliminate surface pests
Surface arthropods on pome fruits can cause export problems and disrupt commercial markets. Eliminating insects and mites on the packing line would be the last opportunity to provide for pest-free produce. In this study, an experimental packing line was used to evaluate techniques using different surfactant baths, pressurized water sprays, and styles of rotating brushes to remove field-collected and laboratory-reared grape mealybug, Pseudococcus maritimus (Ehrhorn) (Homoptera: Pseudococcidae), the diapausing two-spotted spider mite, Tetranychus urticae Koch (Acari: Tetranychidae) and the woolly apple aphid, Eriosoma lanigerum (Hausman) (Homoptera: Aphididae). The organosilicone Silwet L-77 was no more effective than a silicone-based food grade defoamer in aiding removal. Mechanical methods, such as the style of rotating brushes and pressurized sprays, were significantly effective in removing surface arthropods. No improvement in removal occurred when pressure was increased beyond 420 kPa. These techniques can be easily adapted to commercial facilities and will reduce the incidence of surface arthropods on marketed fresh fruits
Atmospheric implications of studies of Central American volcanic eruption clouds
During February 1978 a group of scientists from the National Center for Atmospheric Research, several colleges and universities, the U.S. Geological Survey, and NASA used a specially equipped Beech Queen Air aircraft to make 11 sampling flights in Guatemala through the eruption clouds from the volcanoes Pacaya, Fuego, and Santiguito. Measurements were made of SO42−, SO2, HCl, HF, and 11 cations that were in water-soluble form, on samples collected by a specially designed filter pack. Particle size distributions were obtained with a piezoelectric cascade impactor, and the particles were identified by energy dispersive X ray analysis. Evacuated canisters were flown to obtain samples for gas Chromatographic analysis. Some of the conclusions reached are that since most of the sulfur was found to be in the form of SO2, the H2SO4 droplets resulting from major explosive eruptions must largely result from the reaction of SO2 with OH, at the same time depleting the atmosphere of OH; the volume concentration ratio [SO2]/[HCl] always somewhat exceeded unity; and the amount of fine ash remaining in the stratosphere for long periods of time may depend on the crystallinity of the magma. Correlation spectrometry showed that each volcano was emitting 300–1500 metric tons of SO2 per day
The nature of the intranight variability of radio-quiet quasars
We select a sample of 10 radio-quiet quasars with confirmed intranight
optical variability and with available X-ray data. We compare the variability
properties and the broad band spectral constraints to the predictions of
intranight variability by three models: (i) irradiation of an accretion disk by
a variable X-ray flux (ii) an accretion disk instability (iii) the presence of
a weak blazar component. We concluded that the third model, e.g. the blazar
component model, is the most promising if we adopt a cannonball model for the
jet variable emission. In this case, the probability of detecting the
intranight variability is within 20-80%, depending on the ratio of the disk to
the jet optical luminosity. Variable X-ray irradiation mechanism is also
possible but only under additional requirement: either the source should have a
very narrow Hbeta line or occasional extremely strong flares should appear at
very large disk radii.Comment: MNRAS (in press
Constraining the black hole mass and accretion rate in the narrow-line Seyfert 1 RE J1034+396
We present a comprehensive study of the spectrum of the narrow-line Seyfert 1
galaxy RE J1034+396, summarizing the information obtained from the optical to
X-rays with observations from the William Herschel 4.2m Telescope (WHT), the
Hubble Space Telescope, the Extreme UltraViolet Explorer, ROSAT, ASCA and
BeppoSAX. The BeppoSAX spectra reveal a soft component which is
well-represented by two blackbodies with kT of about 60 eV and 160 eV,
mimicking that expected from a hot, optically-thick accretion disc around a
low-mass black hole. This is borne out by our modeling of the optical to X-ray
nuclear continuum, which constrains the physical parameters of a NLS1 for the
first time. The models demonstrate that RE J1034+396 is likely to be a system
with a nearly edge-on accretion disk (60 to 75 degrees from the disk axis),
accreting at nearly Eddington rates (0.3 to 0.7 L_edd) onto a low mass (about 2
million solar masses) black hole (abridged).Comment: ApJ accepte
The Pearson-Readhead Survey of Compact Extragalactic Radio Sources From Space. II. Analysis of Source Properties
We have performed a multi-dimensional correlation analysis on the observed
properties of a statistically complete core-selected sample of compact
radio-loud active galactic nuclei, based on data from the VLBI Space Observing
Programme (Paper I) and previously published studies. Our sample is drawn from
the well-studied Pearson-Readhead (PR) survey, and is ideally suited for
investigating the general effects of relativistic beaming in compact radio
sources. In addition to confirming many previously known correlations, we have
discovered several new trends that lend additional support to the beaming
model. These trends suggest that the most highly beamed sources in
core-selected samples tend to have a) high optical polarizations; b) large
pc/kpc-scale jet misalignments; c) prominent VLBI core components; d)
one-sided, core, or halo radio morphology on kiloparsec scales; e) narrow
emission line equivalent widths; and f) a strong tendency for intraday
variability at radio wavelengths. We have used higher resolution space and
ground-based VLBI maps to confirm the bi-modality of the jet misalignment
distribution for the PR survey, and find that the sources with aligned parsec-
and kiloparsec-scale jets generally have arcsecond-scale radio emission on both
sides of the core. The aligned sources also have broader emission line widths.
We find evidence that the BL Lacertae objects in the PR survey are all highly
beamed, and have very similar properties to the high-optically polarized
quasars, with the exception of smaller redshifts. A cluster analysis on our
data shows that after partialing out the effects of redshift, the luminosities
of our sample objects in various wave bands are generally well-correlated with
each other, but not with other source properties.Comment: 21 pages, 10 figures. Accepted for publication in the Astrophysical
Journal. Part I can be found at astro-ph/010227
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