487 research outputs found
Experimental study on the velocity limits of magnetized rotating plasmas
An experimental study on the physical limits of the rotation velocity of magnetized plasmas is presented. Experiments are performed in the Maryland Centrifugal Experiment (MCX) [R. F. Ellis , Phys. Plasmas 12, 055704 (2005)], a mirror magnetic field plasma rotating azimuthally. The externally applied parameters that control the plasma characteristics-applied voltage, external magnetic field, and fill pressure-are scanned across the entire available range of values. It is found that the plasma rotation velocity does not exceed the Alfven velocity, in agreement with the equilibrium requirements of magnetically confined plasmas. Measured rotation velocities are also lower than the critical ionization velocity in hydrogen, but a strict limit was not observable within MCX parametric capabilities. (c) 2008 American Institute of Physics
Refuge-Deptford mortuary complex
179 p. : ill., maps ; 26 cm.AMNH Library's copies lack t.p.Includes bibliographical references (p. 173-179).Mortuary archaeology on the Georgia coast / David Hurst Thomas and Clark Spencer Larsen -- Archaeological techniques and procedures / David Hurst Thomas and Clark Spencer Larsen -- The Cunningham Mound group / David Hurst Thomas, Clark Spencer Larsen, and Ann Marie Lunsford -- Seaside Mound group / David Hurst Thomas, Clark Spencer Larsen, and Ann Marie Lunsford -- Ceramics / Chester B. DePratter -- Comparisons / David Hurst Thomas -- Implications and some speculations / David Hurst Thomas -- Appendix. Seasonal growth variations in the shells of recent and prehistoric specimens of Mercenaria mercenaria from St. Catherines Island, Georgia / George R. Clark II."Data and implications from nine burial mounds excavated by crews of the American Museum of Natural History on St. Catherines Island, Georgia, are given in this monograph. As a group, these nine sites define a surprisingly homogeneous pattern and these data comprise virtually all that is known regarding the mortuary customs practiced along the Georgia and Florida coast between ca. 1500 B.C. to A.D. 600. The sites are rather unspectacular sand mounds, which exhibit a remarkably consistent stratigraphic patterning. The skeletal materials recovered are poorly preserved, except in the rare case when shell was added to the interment. The radiocarbon evidence, consisting of 29 determinations from the nine mounds, strongly suggests a marked periodicity in the use of these sites. In all cases, the areas seem to have been visited for centuries - in some cases millenia - prior to the actual construction of the mortuary mound. Several graves were placed into this pre-mound surface, and mound-building activities occurred at all sites sometime during the first 600 years A.D. A number of intrusive burials were added shortly thereafter, and then ceremonial activities virtually ceased at all sites. Most cultural materials found inside the mounds were accidental inclusions which resulted from pre-mound sacred (and probably secular) activities. Very few deliberate grave offerings were found. The biocultural evidence suggests that the Refuge-Deptford population enjoyed good health, although they suffered from an extreme rate of dental attrition. Although the data are limited, these sites seem to reflect an egalitarian sociopolitical organization operative during Refuge-Deptford times. The relatively high proportion of bundle burials seems to suggest that the annual round of these peoples exploited both island and mainland resources. We offer a number of cautious speculations regarding the nature of ritual and symbolism during the Refuge and Deptford phases. These suggestions are a variety of hypotheses that require testing with data from both mortuary and habitation sites of this period"--P. 5
Pluto's global surface composition through pixel-by-pixel Hapke modeling of New Horizons Ralph/LEISA data
On July 14th 2015, NASA's New Horizons mission gave us an unprecedented
detailed view of the Pluto system. The complex compositional diversity of
Pluto's encounter hemisphere was revealed by the Ralph/LEISA infrared
spectrometer on board of New Horizons. We present compositional maps of Pluto
defining the spatial distribution of the abundance and textural properties of
the volatiles methane and nitrogen ices and non-volatiles water ice and tholin.
These results are obtained by applying a pixel-by-pixel Hapke radiative
transfer model to the LEISA scans. Our analysis focuses mainly on the large
scale latitudinal variations of methane and nitrogen ices and aims at setting
observational constraints to volatile transport models. Specifically, we find
three latitudinal bands: the first, enriched in methane, extends from the pole
to 55deg N, the second dominated by nitrogen, continues south to 35deg N, and
the third, composed again mainly of methane, reaches 20deg N. We demonstrate
that the distribution of volatiles across these surface units can be explained
by differences in insolation over the past few decades. The latitudinal pattern
is broken by Sputnik Planitia, a large reservoir of volatiles, with nitrogen
playing the most important role. The physical properties of methane and
nitrogen in this region are suggestive of the presence of a cold trap or
possible volatile stratification. Furthermore our modeling results point to a
possible sublimation transport of nitrogen from the northwest edge of Sputnik
Planitia toward the south.Comment: 43 pages, 7 figures; accepted for publication in Icaru
The Green Bank North Celestial Cap Pulsar Survey. IV: Four New Timing Solutions
We present timing solutions for four pulsars discovered in the Green Bank
Northern Celestial Cap (GBNCC) survey. All four pulsars are isolated with spin
periods between 0.26s and 1.84s. PSR J00382501 has a 0.26s
period and a period derivative of ,
which is unusually low for isolated pulsars with similar periods. This low
period derivative may be simply an extreme value for an isolated pulsar or it
could indicate an unusual evolution path for PSR J00382501, such as a
disrupted recycled pulsar (DRP) from a binary system or an orphaned central
compact object (CCO). Correcting the observed spin-down rate for the Shklovskii
effect suggests that this pulsar may have an unusually low space velocity,
which is consistent with expectations for DRPs. There is no X-ray emission
detected from PSR J00382501 in an archival swift observation, which suggests
that it is not a young orphaned CCO. The high dispersion measure of PSR
J1949+3426 suggests a distance of 12.3kpc. This distance indicates that PSR
J1949+3426 is among the most distant 7% of Galactic field pulsars, and is one
of the most luminous pulsars.Comment: 7 pages, 5 figure
The Green Bank Northern Celestial Cap Pulsar Survey - I: Survey Description, Data Analysis, and Initial Results
We describe an ongoing search for pulsars and dispersed pulses of radio
emission, such as those from rotating radio transients (RRATs) and fast radio
bursts (FRBs), at 350 MHz using the Green Bank Telescope. With the Green Bank
Ultimate Pulsar Processing Instrument, we record 100 MHz of bandwidth divided
into 4,096 channels every 81.92 . This survey will cover the entire sky
visible to the Green Bank Telescope (, or 82% of the sky)
and outside of the Galactic Plane will be sensitive enough to detect slow
pulsars and low dispersion measure (30 ) millisecond
pulsars (MSPs) with a 0.08 duty cycle down to 1.1 mJy. For pulsars with a
spectral index of 1.6, we will be 2.5 times more sensitive than previous and
ongoing surveys over much of our survey region. Here we describe the survey,
the data analysis pipeline, initial discovery parameters for 62 pulsars, and
timing solutions for 5 new pulsars. PSR J02145222 is an MSP in a long-period
(512 days) orbit and has an optical counterpart identified in archival data.
PSR J06365129 is an MSP in a very short-period (96 minutes) orbit with a
very low mass companion (8 ). PSR J06455158 is an isolated MSP
with a timing residual RMS of 500 ns and has been added to pulsar timing array
experiments. PSR J14347257 is an isolated, intermediate-period pulsar that
has been partially recycled. PSR J18164510 is an eclipsing MSP in a
short-period orbit (8.7 hours) and may have recently completed its spin-up
phase.Comment: 18 pages, 10 figures, 5 tables, accepted by Ap
The Green Bank Northern Celestial Cap Pulsar Survey II: The Discovery and Timing of Ten Pulsars
We present timing solutions for ten pulsars discovered in 350 MHz searches
with the Green Bank Telescope. Nine of these were discovered in the Green Bank
Northern Celestial Cap survey and one was discovered by students in the Pulsar
Search Collaboratory program in analysis of drift-scan data. Following
discovery and confirmation with the Green Bank Telescope, timing has yielded
phase-connected solutions with high precision measurements of rotational and
astrometric parameters. Eight of the pulsars are slow and isolated, including
PSR J09302301, a pulsar with nulling fraction lower limit of 30\% and
nulling timescale of seconds to minutes. This pulsar also shows evidence of
mode changing. The remaining two pulsars have undergone recycling, accreting
material from binary companions, resulting in higher spin frequencies. PSR
J05572948 is an isolated, 44 \rm{ms} pulsar that has been partially recycled
and is likely a former member of a binary system which was disrupted by a
second supernova. The paucity of such so-called `disrupted binary pulsars'
(DRPs) compared to double neutron star (DNS) binaries can be used to test
current evolutionary scenarios, especially the kicks imparted on the neutron
stars in the second supernova. There is some evidence that DRPs have larger
space velocities, which could explain their small numbers. PSR J1806+2819 is a
15 \rm{ms} pulsar in a 44 day orbit with a low mass white dwarf companion. We
did not detect the companion in archival optical data, indicating that it must
be older than 1200 Myr.Comment: 9 pages, 5 figure
Elusive Ethylene Detected in Saturns Northern Storm Region
The massive eruption at 40 deg. N (planetographic latitude) on Saturn in 2010 December has produced significant and lasting effects in the northern hemisphere on temperature and species abundances. The northern storm region was observed on many occasions in 2011 by Cassini's Composite Infrared Spectrometer (CIRS). In 2011 May, temperatures in the stratosphere greater than 200 K were derived from CIRS spectra in the regions referred to as "beacons" (warm regions in the stratosphere). Ethylene has been detected in the beacon region in Saturn's northern storm region using CIRS. Ground-based observations using the high-resolution spectrometer Celeste on the McMath-Pierce Telescope on 2011 May 15 were used to confirm the detection and improve the altitude resolution in the retrieved profile. The derived ethylene profile from the CIRS data gives a C2H4 mole fraction of 5.9 +/- 4.5 x 10(exp -7) at 0.5 mbar, and from Celeste data it gives 2.7 +/- 0.45 x 10(exp -6) at 0.1 mbar. This is two orders of magnitude higher than the amount measured in the ultraviolet at other latitudes prior to the storm. It is also much higher than predicted by photochemical models, indicating that perhaps another production mechanism is required or a loss mechanism is being inhibited
Inflight Radiometric Calibration of New Horizons' Multispectral Visible Imaging Camera (MVIC)
We discuss two semi-independent calibration techniques used to determine the
in-flight radiometric calibration for the New Horizons' Multi-spectral Visible
Imaging Camera (MVIC). The first calibration technique compares the observed
stellar flux to modeled values. The difference between the two provides a
calibration factor that allows the observed flux to be adjusted to the expected
levels for all observations, for each detector. The second calibration
technique is a channel-wise relative radiometric calibration for MVIC's blue,
near-infrared and methane color channels using observations of Charon and
scaling from the red channel stellar calibration. Both calibration techniques
produce very similar results (better than 7% agreement), providing strong
validation for the techniques used. Since the stellar calibration can be
performed without a color target in the field of view and covers all of MVIC's
detectors, this calibration was used to provide the radiometric keywords
delivered by the New Horizons project to the Planetary Data System (PDS). These
keywords allow each observation to be converted from counts to physical units;
a description of how these keywords were generated is included. Finally,
mitigation techniques adopted for the gain drift observed in the near-infrared
detector and one of the panchromatic framing cameras is also discussed
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