6,406 research outputs found
Solar flux and its variations
Data are presented on the solar irradiance as derived from a number of sources. An attempt was made to bring these data onto a uniform scale. Summation of fluxes at all wavelengths yields a figure of 1357.826 for the solar constant. Estimates are made of the solar flux variations due to flares, active regions (slowly varying component), 27-day period, and the 11-yr cycle. Solar activity does not produce a significant variation in the value of the solar constant. Variations in the X-ray and EUV portions of the solar flux may be several orders of magnitude during solar activity, especially at times of major flares. It is established that these short wavelength flux enhancements cause significant changes in the terrestrial ionosphere
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Alternative causal inference methods in population health research: Evaluating tradeoffs and triangulating evidence.
Population health researchers from different fields often address similar substantive questions but rely on different study designs, reflecting their home disciplines. This is especially true in studies involving causal inference, for which semantic and substantive differences inhibit interdisciplinary dialogue and collaboration. In this paper, we group nonrandomized study designs into two categories: those that use confounder-control (such as regression adjustment or propensity score matching) and those that rely on an instrument (such as instrumental variables, regression discontinuity, or differences-in-differences approaches). Using the Shadish, Cook, and Campbell framework for evaluating threats to validity, we contrast the assumptions, strengths, and limitations of these two approaches and illustrate differences with examples from the literature on education and health. Across disciplines, all methods to test a hypothesized causal relationship involve unverifiable assumptions, and rarely is there clear justification for exclusive reliance on one method. Each method entails trade-offs between statistical power, internal validity, measurement quality, and generalizability. The choice between confounder-control and instrument-based methods should be guided by these tradeoffs and consideration of the most important limitations of previous work in the area. Our goals are to foster common understanding of the methods available for causal inference in population health research and the tradeoffs between them; to encourage researchers to objectively evaluate what can be learned from methods outside one's home discipline; and to facilitate the selection of methods that best answer the investigator's scientific questions
Two-Flavor Staggered Fermion Thermodynamics at N_t = 12
We present results of an ongoing study of the nature of the high temperature
crossover in QCD with two light fermion flavors. These results are obtained
with the conventional staggered fermion action at the smallest lattice spacing
to date---approximately 0.1 fm. Of particular interest are a study of the
temperature of the crossover a determination of the induced baryon charge and
baryon susceptibility, the scalar susceptibility, and the chiral order
parameter, used to test models of critical behavior associated with chiral
symmetry restoration. From our new data and published results for N_t = 4, 6,
and 8, we determine the QCD magnetic equation of state from the chiral order
parameter using O(4) and mean field critical exponents and compare it with the
corresponding equation of state obtained from an O(4) spin model and mean field
theory. We also present a scaling analysis of the Polyakov loop, suggesting a
temperature dependent ``constituent quark free energy.''Comment: LaTeX 25 pages, 15 Postscript figure
NMR Chemical Shifts of Trace Impurities: Common Laboratory Solvents, Organics, and Gases in Deuterated Solvents Relevant to the Organometallic Chemist
Tables of ^1H and ^(13)C NMR chemical shifts have been compiled for common organic compounds often used as reagents or found as products or contaminants in deuterated organic solvents. Building upon the work of Gottlieb, Kotlyar, and Nudelman in the Journal of Organic Chemistry, signals for common impurities are now reported in additional NMR solvents (tetrahydrofuran-d_8, toluene-d_8, dichloromethane-d_2, chlorobenzene-d_5, and 2,2,2-trifluoroethanol-d_3) which are frequently used in organometallic laboratories. Chemical shifts for other organics which are often used as reagents or internal standards or are found as products in organometallic chemistry are also reported for all the listed solvents
PO and PN in the wind of the oxygen-rich AGB star IK Tau
Phosphorus-bearing compounds have only been studied in the circumstellar
environments (CSEs) of the asymptotic giant branch (AGB) star IRC +10216 and
the protoplanetary nebula CRL 2688, both C-rich objects, and the O-rich red
supergiant VY CMa. The current chemical models cannot reproduce the high
abundances of PO and PN derived from observations of VY CMa. No observations
have been reported of phosphorus in the CSEs of O-rich AGB stars. We aim to set
observational constraints on the phosphorous chemistry in the CSEs of O-rich
AGB stars, by focussing on the Mira-type variable star IK Tau. Using the IRAM
30m telescope and the Submillimeter Array (SMA), we observed four rotational
transitions of PN (J=2-1,3-2,6-5,7-6) and four of PO
(J=5/2-3/2,7/2-5/2,13/2-11/2,15/2-13/2). The IRAM 30m observations were
dedicated line observations, while the SMA data come from an unbiased spectral
survey in the frequency range 279-355 GHz.
We present the first detections of PN and PO in an O-rich AGB star and
estimate abundances X(PN/H2) of about 3x10^-7 and X(PO/H2) in the range
0.5-6.0x10^-7. This is several orders of magnitude higher than what is found
for the C-rich AGB star IRC +10216. The diameter (<=0.7") of the PN and PO
emission distributions measured in the interferometric data corresponds to a
maximum radial extent of about 40 stellar radii. The abundances and the spatial
occurrence of the molecules are in very good agreement with the results
reported for VY CMa. We did not detect PS or PH3 in the survey. We suggest that
PN and PO are the main carriers of phosphorus in the gas phase, with abundances
possibly up to several 10^-7. The current chemical models cannot account for
this, underlining the strong need for updated chemical models that include
phosphorous compounds.Comment: Accepted for publication in Astronomy & Astrophysics, 10 pages, 8
figure
Detection of HC11N in the Cold Dust Cloud TMC-1
Two consecutive rotational transitions of the long cyanopolyyne HC11N,
J=39-38, and J=38-37, have been detected in the cold dust cloud TMC-1 at the
frequencies expected from recent laboratory measurements by Travers et al.
(1996), and at about the expected intensities. The astronomical lines have a
mean radial velocity of 5.8(1) km/s, in good agreement with the shorter
cyanopolyynes HC7N and HC9N observed in this very sharp-lined source [5.82(5)
and 5.83(5) km/s, respectively]. The column density of HC11N is calculated to
be 2.8x10^(11) cm^(-2). The abundance of the cyanopolyynes decreases smoothly
with length to HC11N, the decrement from one to the next being about 6 for the
longer carbon chains.Comment: plain tex 10 pages plus 3 ps fig file
Thermodynamics of lattice QCD with two light quark flavours on a 16^3 x 8 lattice II
We have extended our earlier simulations of the high temperature behaviour of
lattice QCD with two light flavours of staggered quarks on a
lattice to lower quark mass (m_q=0.00625). The transition from hadronic matter
to a quark-gluon plasma is observed at corresponding to a
temperature of MeV. We present measurements of observables
which probe the nature of the quark-gluon plasma and serve to distinguish it
from hadronic matter. Although the transition is quite abrupt, we have seen no
indications that it is first order.Comment: 23 pages, RevteX, 6 encapsulated postscript figure
The Phase Diagram of High Temperature QCD with Three Flavors of Improved Staggered Quarks
We report on progress in our study of high temperature QCD with three flavors
of improved staggered quarks. Simulations are being carried out with three
degenerate quarks with masses less than or equal to the strange quark mass,
, and with degenerate up and down quarks with masses in the range , and the strange quark mass fixed near its physical
value. For the quark masses studied to date we find rapid crossovers, which
sharpen as the quark mass is reduced, rather than bona fide phase transitions.Comment: Lattice 2003 (Nonzero temperature and density
Light hadrons with improved staggered quarks: approaching the continuum limit
We have extended our program of QCD simulations with an improved
Kogut-Susskind quark action to a smaller lattice spacing, approximately 0.09
fm. Also, the simulations with a approximately 0.12 fm have been extended to
smaller quark masses. In this paper we describe the new simulations and
computations of the static quark potential and light hadron spectrum. These
results give information about the remaining dependences on the lattice
spacing. We examine the dependence of computed quantities on the spatial size
of the lattice, on the numerical precision in the computations, and on the step
size used in the numerical integrations. We examine the effects of
autocorrelations in "simulation time" on the potential and spectrum. We see
effects of decays, or coupling to two-meson states, in the 0++, 1+, and 0-
meson propagators, and we make a preliminary mass computation for a radially
excited 0- meson.Comment: 43 pages, 16 figure
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