960 research outputs found
The Spectrum and Variability of Circular Polarization in Sagittarius A* from 1.4 to 15 GHz
We report here multi-epoch, multi-frequency observations of the circular
polarization in Sagittarius A*, the compact radio source in the Galactic
Center. Data taken from the VLA archive indicate that the fractional circular
polarization at 4.8 GHz was -0.31% with an rms scatter of 0.13% from 1981 to
1998, in spite of a factor of 2 change in the total intensity. The sign
remained negative over the entire time range, indicating a stable magnetic
field polarity. In the Summer of 1999 we obtained 13 epochs of VLA A-array
observations at 1.4, 4.8, 8.4 and 15 GHz. In May, September and October of 1999
we obtained 11 epochs of Australia Telescope Compact Array observations at 4.8
and 8.5 GHz. In all three of the data sets, we find no evidence for linear
polarization greater than 0.1% in spite of strong circular polarization
detections. Both VLA and ATCA data sets support three conclusions regarding the
fractional circular polarization: the average spectrum is inverted with a
spectral index ~0.5 +/- 0.2; the degree of variability is roughly constant on
timescales of days to years; and, the degree of variability increases with
frequency. We also observed that the largest increase in fractional circular
polarization was coincident with the brightest flare in total intensity.
Significant variability in the total intensity and fractional circular
polarization on a timescale of 1 hour was observed during this flare,
indicating an upper limit to the size of 70 AU at 15 GHz. The fractional
circular polarization at 15 GHz reached -1.1% and the spectral index is
strongly inverted during this flare. We conclude that the spectrum has two
components that match the high and low frequency total intensity components.
(abridged)Comment: Accepted for publication in ApJ, 40 pages, 18 figure
Untwisting of a Strained Cholesteric Elastomer by Disclination Loop Nucleation
The application of a sufficiently strong strain perpendicular to the pitch
axis of a monodomain cholesteric elastomer unwinds the cholesteric helix.
Previous theoretical analyses of this transition ignored the effects of Frank
elasticity which we include here. We find that the strain needed to unwind the
helix is reduced because of the Frank penalty and the cholesteric state becomes
metastable above the transition. We consider in detail a previously proposed
mechanism by which the topologically stable helical texture is removed in the
metastable state, namely by the nucleation of twist disclination loops in the
plane perpendicular to the pitch axis. We present an approximate calculation of
the barrier energy for this nucleation process which neglects possible spatial
variation of the strain fields in the elastomer, as well as a more accurate
calculation based on a finite element modeling of the elastomer.Comment: 12 pages, 9 figure
Deep eddies in the Gulf of Mexico observed with floats
Author Posting. © American Meteorological Society, 2018. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 48 (2018): 2703-2719, doi:10.1175/JPO-D-17-0245.1.A new set of deep float trajectory data collected in the Gulf of Mexico from 2011 to 2015 at 1500- and 2500-m depths is analyzed to describe mesoscale processes, with particular attention paid to the western Gulf. Wavelet analysis is used to identify coherent eddies in the float trajectories, leading to a census of the basinwide coherent eddy population and statistics of the eddies’ kinematic properties. The eddy census reveals a new formation region for anticyclones off the Campeche Escarpment, located northwest of the Yucatan Peninsula. These eddies appear to form locally, with no apparent direct connection to the upper layer. Once formed, the eddies drift westward along the northern edge of the Sigsbee Abyssal Gyre, located in the southwestern Gulf of Mexico over the abyssal plain. The formation mechanism and upstream sources for the Campeche Escarpment eddies are explored: the observational data suggest that eddy formation may be linked to the collision of a Loop Current eddy with the western boundary of the Gulf. Specifically, the disintegration of a deep dipole traveling under the Loop Current eddy Kraken, caused by the interaction with the northwestern continental slope, may lead to the acceleration of the abyssal gyre and the boundary current in the Bay of Campeche region.The authors were supported by the
Department of the Interior, Bureau of Ocean Energy
Management (BOEM), Contract M10PC00112 to
Leidos, Inc., Raleigh, North Carolina.2019-05-0
Hydrography of the Gulf of Mexico using autonomous floats
Author Posting. © American Meteorological Society, 2018. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 48 (2018): 773-794, doi:10.1175/JPO-D-17-0205.1.Fourteen autonomous profiling floats, equipped with CTDs, were deployed in the deep eastern and western basins of the Gulf of Mexico over a four-year interval (July 2011–August 2015), producing a total of 706 casts. This is the first time since the early 1970s that there has been a comprehensive survey of water masses in the deep basins of the Gulf, with better vertical resolution than available from older ship-based surveys. Seven floats had 14-day cycles with parking depths of 1500 m, and the other half from the U.S. Argo program had varying cycle times. Maps of characteristic water masses, including Subtropical Underwater, Antarctic Intermediate Water (AAIW), and North Atlantic Deep Water, showed gradients from east to west, consistent with their sources being within the Loop Current (LC) and the Yucatan Channel waters. Altimeter SSH was used to characterize profiles being in LC or LC eddy water or in cold eddies. The two-layer nature of the deep Gulf shows isotherms being deeper in the warm anticyclonic LC and LC eddies and shallower in the cold cyclones. Mixed layer depths have an average seasonal signal that shows maximum depths (~60 m) in January and a minimum in June–July (~20 m). Basin-mean steric heights from 0–50-m dynamic heights and altimeter SSH show a seasonal range of ~12 cm, with significant interannual variability. The translation of LC eddies across the western basin produces a region of low homogeneous potential vorticity centered over the deepest part of the western basin.The authors were supported by
the Department of the Interior, Bureau of Ocean Energy
Management (BOEM), Contract M08PC20043 to
Leidos, Inc., Raleigh, North Carolina.2018-10-0
The Loop Current: Observations of deep eddies and topographic waves.
Author Posting. © American Meteorological Society, 2019. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 49(6), (2019):1463-1483, doi: 10.1175/JPO-D-18-0213.1.A set of float trajectories, deployed at 1500- and 2500-m depths throughout the deep Gulf of Mexico from 2011 to 2015, are analyzed for mesoscale processes under the Loop Current (LC). In the eastern basin, December 2012–June 2014 had >40 floats per month, which was of sufficient density to allow capturing detailed flow patterns of deep eddies and topographic Rossby waves (TRWs), while two LC eddies formed and separated. A northward advance of the LC front compresses the lower water column and generates an anticyclone. For an extended LC, baroclinic instability eddies (of both signs) develop under the southward-propagating large-scale meanders of the upper-layer jet, resulting in a transfer of eddy kinetic energy (EKE) to the lower layer. The increase in lower-layer EKE occurs only over a few months during meander activity and LC eddy detachment events, a relatively short interval compared with the LC intrusion cycle. Deep EKE of these eddies is dispersed to the west and northwest through radiating TRWs, of which examples were found to the west of the LC. Because of this radiation of EKE, the lower layer of the eastern basin becomes relatively quiescent, particularly in the northeastern basin, when the LC is retracted and a LC eddy has departed. A mean west-to-east, anticyclone–cyclone dipole flow under a mean LC was directly comparable to similar results from a previous moored LC array and also showed connections to an anticlockwise boundary current in the southeastern basin.The authors were supported by the Department of the Interior, Bureau of Ocean Energy Management (BOEM), Contract M08PC20043 to Leidos, Inc., Raleigh, NC. The authors also wish to acknowledge the enthusiastic support of Dr. Alexis Lugo-Fernández, the BOEM Contracting Officer’s Technical Representative, during the study into the Deep Circulation of the Gulf of Mexico, using Lagrangian Methods. Thanks go to the captains and crews of the R/V Pelican and B/O Justo Sierra, J. Malbrough (LUMCON), J. Singer (Leidos), J. Valdes (WHOI), B. Guest (WHOI), and the CANEK group (CICESE).2020-05-2
Observed deep cyclonic eddies around Southern Greenland
Author Posting. © American Meteorological Society, 2021. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 51(10), (2021): 3235–3252, https://doi.org/10.1175/JPO-D-20-0288.1.Recent mooring measurements from the Overturning in the Subpolar North Atlantic Program have revealed abundant cyclonic eddies at both sides of Cape Farewell, the southern tip of Greenland. In this study, we present further observational evidence, from both Eulerian and Lagrangian perspectives, of deep cyclonic eddies with intense rotation (ζ/f > 1) around southern Greenland and into the Labrador Sea. Most of the observed cyclones exhibit strongest rotation below the surface at 700–1000 dbar, where maximum azimuthal velocities are ~30 cm s−1 at radii of ~10 km, with rotational periods of 2–3 days. The cyclonic rotation can extend to the deep overflow water layer (below 1800 dbar), albeit with weaker azimuthal velocities (~10 cm s−1) and longer rotational periods of about one week. Within the middepth rotation cores, the cyclones are in near solid-body rotation and have the potential to trap and transport water. The first high-resolution hydrographic transect across such a cyclone indicates that it is characterized by a local (both vertically and horizontally) potential vorticity maximum in its middepth core and cold, fresh anomalies in the deep overflow water layer, suggesting its source as the Denmark Strait outflow. Additionally, the propagation and evolution of the cyclonic eddies are illustrated with deep Lagrangian floats, including their detachments from the boundary currents to the basin interior. Taken together, the combined Eulerian and Lagrangian observations have provided new insights on the boundary current variability and boundary–interior exchange over a geographically large scale near southern Greenland, calling for further investigations on the (sub)mesoscale dynamics in the region.OOI mooring data are based upon work supported by the National Science Foundation under Cooperative Agreement 1743430. S. Zou, A. Bower, and H. Furey gratefully acknowledge the support from the Physical Oceanography Program of the U.S. National Science Foundation Grant OCE-1756361. R.S. Pickart acknowledges support from National Science Foundation Grants OCE-1259618 and OCE-1756361. N. P. Holliday and L. Houpert were supported by NERC programs U.K. OSNAP (NE/K010875) and U.K. OSNAP-Decade (NE/T00858X/1)
Dominant circulation patterns of the deep Gulf of Mexico
Author Posting. © American Meteorological Society, 2018. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 48 (2018): 511-529, doi:10.1175/JPO-D-17-0140.1.The large-scale circulation of the bottom layer of the Gulf of Mexico is analyzed, with special attention to the historically least studied western basin. The analysis is based on 4 years of data collected by 158 subsurface floats parked at 1500 and 2500 m and is complemented with data collected by current meter moorings in the western basin during the same period. Three main circulation patterns stand out: a cyclonic boundary current, a cyclonic gyre in the abyssal plain, and the very high eddy kinetic energy observed in the eastern Gulf. The boundary current and the cyclonic gyre appear as distinct features, which interact in the western tip of the Yucatan shelf. The persistence and continuity of the boundary current is addressed. Although high variability is observed, the boundary flow serves as a pathway for water to travel around the western basin in approximately 2 years. An interesting discovery is the separation of the boundary current over the northwestern slope of the Yucatan shelf. The separation and retroflection of the along-slope current appears to be a persistent feature and is associated with anticyclonic eddies whose genesis mechanism remains to be understood. As the boundary flow separates, it feeds into the westward flow of the deep cyclonic gyre. The location of this gyre—named the Sigsbee Abyssal Gyre—coincides with closed geostrophic contours, so eddy–topography interaction via bottom form stresses may drive this mean flow. The contribution to the cyclonic vorticity of the gyre by modons traveling under Loop Current eddies is discussed.This work was supported by the
Bureau of Ocean Energy Management (BOEM) under
Contract M10PC00112 assigned to Leidos, Inc
Variable Radio Sources in the Galactic Plane
Using three epochs of VLA observations of the Galactic Plane in the first
quadrant taken ~15 years apart, we have conducted a search for a population of
variable Galactic radio emitters in the flux density range 1-100 mJy at 6 cm.
We find 39 variable sources in a total survey area of 23.2 sq deg. Correcting
for various selection effects and for the extragalactic variable population of
active galactic nuclei, we conclude there are ~1.6 Galactic sources per sq deg
which vary by more than 50% on a time scale of years (or shorter). We show that
these sources are much more highly variable than extragalactic objects; more
than 50% show variability by a factor >2 compared to <10% for extragalactic
objects in the same flux density range. We also show that the fraction of
variable sources increases toward the Galactic center (another indication that
this is a Galactic population), and that the spectral indices of many of these
sources are flat or inverted. A small number of the variables are coincident
with mid-IR sources and two are coincident with X-ray emitters, but most have
no known counterparts at other wavelengths. Intriguingly, one lies at the
center of a supernova remnant, while another appears to be a very compact
planetary nebula; several are likely to represent activity associated with star
formation regions. We discuss the possible source classes which could
contribute to the variable cohort and followup observations which could clarify
the nature of these sources.Comment: 11 pages, 7 figures; to be published in the Astronomical Journal;
data available on MAGPIS website at http://third.ucllnl.org/gps
Variable and Transient Radio Sources in the FIRST Survey
A comprehensive search for variable and transient radio sources has been
conducted using ~55,000 snapshot images of the FIRST survey. We present an
analysis leading to the discovery of 1,627 variable and transient objects down
to mJy levels over a wide range of timescales (few minutes to years).
Variations observed range from 20% to a factor of 25. Multi-wavelength matching
for counterparts reveals the diverse classes of objects exhibiting variability,
ranging from nearby stars and pulsars to galaxies and distant quasars.
Interestingly, more than half of the objects in the sample have either no
classified counterparts or no corresponding sources at any other wavelength and
require multi-wavelength follow-up observations. We discuss these classes of
variables and speculate on the identity of objects that lack multi-wavelength
counterparts.Comment: 63 pages, 18 encapsulated postscript figures (19 if individual
subfigures are counted), 3 tables. LaTeX style file fltpage.sty used.
Submitted, accepted and "in press" for publication in the Astrophysical
Journal. Full electronic version of table 1 (tab1.txt) has been uploaded and
can be obtained after extracting the zipped source fil
Resolving the inner jet structure of 1924-292 with the EVENT HORIZON TELESCOPE
We present the first 1.3 mm (230 GHz) very long baseline interferometry model
image of an AGN jet using closure phase techniques with a four-element array.
The model image of the quasar 1924-292 was obtained with four telescopes at
three observatories: the James Clerk Maxwell Telescope (JCMT) on Mauna Kea in
Hawaii, the Arizona Radio Observatory's Submillimeter Telescope (SMT) in
Arizona, and two telescopes of the Combined Array for Research in
Millimeterwave Astronomy (CARMA) in California in April 2009. With the greatly
improved resolution compared with previous observations and robust closure
phase measurement, the inner jet structure of 1924-292 was spatially resolved.
The inner jet extends to the northwest along a position angle of at
a distance of 0.38\,mas from the tentatively identified core, in agreement with
the inner jet structure inferred from lower frequencies, and making a position
angle difference of with respect to the cm-jet. The size of
the compact core is 0.15\,pc with a brightness temperature of
\,K. Compared with those measured at lower frequencies, the
low brightness temperature may argue in favor of the decelerating jet model or
particle-cascade models. The successful measurement of closure phase paves the
way for imaging and time resolving Sgr A* and nearby AGN with the Event Horizon
Telescope.Comment: 6 pages, 4 figures, accepted for publication in ApJ
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