906 research outputs found
Spacecraft charging and ion wake formation in the near-Sun environment
A three-dimensional (3-D), self-consistent code is employed to solve for the
static potential structure surrounding a spacecraft in a high photoelectron
environment. The numerical solutions show that, under certain conditions, a
spacecraft can take on a negative potential in spite of strong photoelectron
currents. The negative potential is due to an electrostatic barrier near the
surface of the spacecraft that can reflect a large fraction of the
photoelectron flux back to the spacecraft. This electrostatic barrier forms if
(1) the photoelectron density at the surface of the spacecraft greatly exceeds
the ambient plasma density, (2) the spacecraft size is significantly larger
than local Debye length of the photoelectrons, and (3) the thermal electron
energy is much larger than the characteristic energy of the escaping
photoelectrons. All of these conditions are present near the Sun. The numerical
solutions also show that the spacecraft's negative potential can be amplified
by an ion wake. The negative potential of the ion wake prevents secondary
electrons from escaping the part of spacecraft in contact with the wake. These
findings may be important for future spacecraft missions that go nearer to the
Sun, such as Solar Orbiter and Solar Probe Plus.Comment: 25 pages, 7 figures, accepted for publication in Physics of Plasma
The Wilkinson Microwave Anisotropy Probe (WMAP) Source Catalog
We present the list of point sources found in the WMAP 5-year maps. The
technique used in the first-year and three-year analysis now finds 390 point
sources, and the five-year source catalog is complete for regions of the sky
away from the galactic plane to a 2 Jy limit, with SNR > 4.7 in all bands in
the least covered parts of the sky. The noise at high frequencies is still
mainly radiometer noise, but at low frequencies the CMB anisotropy is the
largest uncertainty. A separate search of CMB-free V-W maps finds 99 sources of
which all but one can be identified with known radio sources. The sources seen
by WMAP are not strongly polarized. Many of the WMAP sources show significant
variability from year to year, with more than a 2:1 range between the minimum
and maximum fluxes.Comment: 31 pages Latex with 4 embedded figures. Version accepted by the ApJ
Supplement
Nine-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Cosmological Parameter Results
We present cosmological parameter constraints based on the final nine-year
WMAP data, in conjunction with additional cosmological data sets. The WMAP data
alone, and in combination, continue to be remarkably well fit by a
six-parameter LCDM model. When WMAP data are combined with measurements of the
high-l CMB anisotropy, the BAO scale, and the Hubble constant, the densities,
Omegabh2, Omegach2, and Omega_L, are each determined to a precision of ~1.5%.
The amplitude of the primordial spectrum is measured to within 3%, and there is
now evidence for a tilt in the primordial spectrum at the 5sigma level,
confirming the first detection of tilt based on the five-year WMAP data. At the
end of the WMAP mission, the nine-year data decrease the allowable volume of
the six-dimensional LCDM parameter space by a factor of 68,000 relative to
pre-WMAP measurements. We investigate a number of data combinations and show
that their LCDM parameter fits are consistent. New limits on deviations from
the six-parameter model are presented, for example: the fractional contribution
of tensor modes is limited to r<0.13 (95% CL); the spatial curvature parameter
is limited to -0.0027 (+0.0039/-0.0038); the summed mass of neutrinos is <0.44
eV (95% CL); and the number of relativistic species is found to be 3.84+/-0.40
when the full data are analyzed. The joint constraint on Neff and the
primordial helium abundance agrees with the prediction of standard Big Bang
nucleosynthesis. We compare recent PLANCK measurements of the
Sunyaev-Zel'dovich effect with our seven-year measurements, and show their
mutual agreement. Our analysis of the polarization pattern around temperature
extrema is updated. This confirms a fundamental prediction of the standard
cosmological model and provides a striking illustration of acoustic
oscillations and adiabatic initial conditions in the early universe.Comment: 32 pages, 12 figures, v3: Version accepted to Astrophysical Journal
Supplement Series. Includes improvements in response to referee and
community; corrected 3 entries in Table 10, (w0 & wa model). See the Legacy
Archive for Microwave Background Data Analysis (LAMBDA):
http://lambda.gsfc.nasa.gov/product/map/current/ for further detai
Seven-year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Galactic Foreground Emission
We present updated estimates of Galactic foreground emission using seven years of WMAP data. Using the power
spectrum of differences between multi-frequency template-cleaned maps, we find no evidence for foreground
contamination outside of the updated (KQ85y7) foreground mask.We place a 15μK upper bound on rms foreground
contamination in the cleaned maps used for cosmological analysis. Further, the cleaning process requires only three
power-law foregrounds outside of the mask. We find no evidence for polarized foregrounds beyond those from soft
(steep-spectrum) synchrotron and thermal dust emission; in particular we find no indication in the polarization data
of an extra “haze” of hard synchrotron emission from energetic electrons near the Galactic center. We provide an
updated map of the cosmic microwave background (CMB) using the internal linear combination method, updated
foreground masks, and updates to point source catalogs using two different techniques. With additional years of
data, we now detect 471 point sources using a five-band technique and 417 sources using a three-band CMB-free
technique. In total there are 62 newly detected point sources, a 12% increase over the five-year release. Also new
are tests of theMarkov chain Monte Carlo foreground fitting procedure against systematics in the time-stream data,
and tests against the observed beam asymmetry. Within a few degrees of the Galactic plane, the behavior in total
intensity of low-frequency foregrounds is complicated and not completely understood. WMAP data show a rapidly
steepening spectrum from 20 to 40 GHz, which may be due to emission from spinning dust grains, steepening
synchrotron, or other effects. Comparisons are made to a 1 deg 408 MHz map (Haslam et al.) and the 11 deg
ARCADE 2 data (Singal et al.).We find that spinning dust or steepening synchrotron models fit the combination of
WMAP and 408 MHz data equally well. ARCADE data appear inconsistent with the steepening synchrotron model
and consistent with the spinning dust model, though some discrepancies remain regarding the relative strength
of spinning dust emission. More high-resolution data in the 10–40 GHz range would shed much light on these
issues
Seven-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Are There Cosmic Microwave Background Anomalies?
(Abridged) A simple six-parameter LCDM model provides a successful fit to
WMAP data, both when the data are analyzed alone and in combination with other
cosmological data. Even so, it is appropriate to search for any hints of
deviations from the now standard model of cosmology, which includes inflation,
dark energy, dark matter, baryons, and neutrinos. The cosmological community
has subjected the WMAP data to extensive and varied analyses. While there is
widespread agreement as to the overall success of the six-parameter LCDM model,
various "anomalies" have been reported relative to that model. In this paper we
examine potential anomalies and present analyses and assessments of their
significance. In most cases we find that claimed anomalies depend on posterior
selection of some aspect or subset of the data. Compared with sky simulations
based on the best fit model, one can select for low probability features of the
WMAP data. Low probability features are expected, but it is not usually
straightforward to determine whether any particular low probability feature is
the result of the a posteriori selection or of non-standard cosmology. We
examine in detail the properties of the power spectrum with respect to the LCDM
model. We examine several potential or previously claimed anomalies in the sky
maps and power spectra, including cold spots, low quadrupole power,
quadropole-octupole alignment, hemispherical or dipole power asymmetry, and
quadrupole power asymmetry. We conclude that there is no compelling evidence
for deviations from the LCDM model, which is generally an acceptable
statistical fit to WMAP and other cosmological data.Comment: 19 pages, 17 figures, also available with higher-res figures on
http://lambda.gsfc.nasa.gov; accepted by ApJS; (v2) text as accepte
Carotid artery endarterectomy in patients with symptomatic non-stenotic carotid artery disease
Object We sought to determine the safety and efficacy in secondary stroke prevention of carotid endarterectomy (CEA) in patients with symptomatic non-stenotic carotid artery disease (SyNC). Methods This was a single-centre retrospective case series. All patients who underwent CEA for unilateral anterior circulation cerebrovascular events with ipsilateral <50% carotid stenosis from 2002 to 2020 were included. Imaging hallmarks including the degree of luminal stenosis and the presence of various vulnerable plaque characteristics (eg, intraplaque haemorrhage (IPH) on MR angiography, ulceration or low-density plaque on CT angiography) were assessed. The presence of vulnerable plaque characteristics was compared between arteries ipsilateral to the ischaemic event and contralateral arteries. The prevalence of perioperative/intraoperative complications, as well as recurrent ischaemic events at follow-up was determined. Results Thirty-two patients were included in the analysis, of which 25.0% were female. Carotid arteries ipsilateral to an ischaemic event had a significantly higher prevalence of IPH when compared with contralateral arteries (80.0% vs 0.0%; p<0.001). There were no intraoperative complications. One patient (3.1%) developed symptoms of transient ipsilateral ischaemia 1 day following CEA which resolved without treatment. In a median follow-up of 18.0 months (IQR 5.0-36.0), only one patient (3.1%) experienced a transient neurologic deficit with complete resolution (annualised rate of recurrent stroke after CEA of 1.5% for a total follow-up of 788 patient-months following CEA). All other patients (31/32, 96.9%) were free of recurrent ischaemic events. Conclusion CEA appears to be safe and well-tolerated in patients with SyNC. Additional studies with larger cohorts and longer follow-up intervals are needed in order to determine the role of CEA in this patient population
Life at high Deborah number
In many biological systems, microorganisms swim through complex polymeric
fluids, and usually deform the medium at a rate faster than the inverse fluid
relaxation time. We address the basic properties of such life at high Deborah
number analytically by considering the small-amplitude swimming of a body in an
arbitrary complex fluid. Using asymptotic analysis and differential geometry,
we show that for a given swimming gait, the time-averaged leading-order
swimming kinematics of the body can be expressed as an integral equation on the
solution to a series of simpler Newtonian problems. We then use our results to
demonstrate that Purcell's scallop theorem, which states that time-reversible
body motion cannot be used for locomotion in a Newtonian fluid, breaks down in
polymeric fluid environments
Five-Year Wilkinson Microwave Anisotropy Probe Observations: Source Catalog
We present the list of point sources found in the Wilkinson Microwave Anisotropy Probe (WMAP) five-year maps. The technique used in the first-year and three-year analyses now finds 390 point sources, and the five-year source catalog is complete for regions of the sky away from the Galactic plane to a 2 Jy limit, with SNR > 4.7 in all bands in the least covered parts of the sky. The noise at high frequencies is still mainly radiometer noise, but at low frequencies the cosmic microwave background (CMB) anisotropy is the largest uncertainty. A separate search of CMB-free V-W maps finds 99 sources of which all but one can be identified with known radio sources. The sources seen by WMAP are not strongly polarized. Many of the WMAP sources show significant variability from year to year, with more than a 2: 1 range between the minimum and maximum fluxes.NASA NNG05GE76G, NNX07AL75G S01, LTSA03-000-0090, ATPNNG04GK55G, ADP03-0000-092Astronom
Seven-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Power Spectra and WMAP-Derived Parameters
(Abridged) We present the angular power spectra derived from the 7-year maps
and discuss the cosmological conclusions that can be inferred from WMAP data
alone. The third acoustic peak in the TT spectrum is now well measured by WMAP.
In the context of a flat LambdaCDM model, this improvement allows us to place
tighter constraints on the matter density from WMAP data alone, and on the
epoch of matter-radiation equality, The temperature-polarization (TE) spectrum
is detected in the 7-year data with a significance of 20 sigma, compared to 13
sigma with the 5-year data. The low-l EE spectrum, a measure of the optical
depth due to reionization, is detected at 5.5 sigma significance when averaged
over l = 2-7. The BB spectrum, an important probe of gravitational waves from
inflation, remains consistent with zero. The upper limit on tensor modes from
polarization data alone is a factor of 2 lower with the 7-year data than it was
using the 5-year data (Komatsu et al. 2010). We test the parameter recovery
process for bias and find that the scalar spectral index, ns, is biased high,
but only by 0.09 sigma, while the remaining parameters are biased by < 0.15
sigma. The improvement in the third peak measurement leads to tighter lower
limits from WMAP on the number of relativistic degrees of freedom (e.g.,
neutrinos) in the early universe: Neff > 2.7 (95% CL). Also, using WMAP data
alone, the primordial helium mass fraction is found to be YHe = 0.28+0.14-0.15,
and with data from higher-resolution CMB experiments included, we now establish
the existence of pre-stellar helium at > 3 sigma (Komatsu et al. 2010).Comment: 22 pages, 14 figures, version accepted to Astrophysical Journal
Supplement Series, added high-l EE detection, consolidated parameter recovery
simulation
Five-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Angular Power Spectra
We present the temperature and polarization angular power spectra of the
cosmic microwave background (CMB) derived from the first 5 years of WMAP data.
The 5-year temperature (TT) spectrum is cosmic variance limited up to multipole
l=530, and individual l-modes have S/N>1 for l<920. The best fitting
six-parameter LambdaCDM model has a reduced chi^2 for l=33-1000 of
chi^2/nu=1.06, with a probability to exceed of 9.3%. There is now significantly
improved data near the third peak which leads to improved cosmological
constraints. The temperature-polarization correlation (TE) is seen with high
significance. After accounting for foreground emission, the low-l reionization
feature in the EE power spectrum is preferred by \Delta\chi^2=19.6 for optical
depth tau=0.089 by the EE data alone, and is now largely cosmic variance
limited for l=2-6. There is no evidence for cosmic signal in the BB, TB, or EB
spectra after accounting for foreground emission. We find that, when averaged
over l=2-6, l(l+1)C^{BB}_l/2\pi < 0.15 uK^2 (95% CL).Comment: 29 pages, 13 figures, accepted by ApJ
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