17,357 research outputs found
Pioneer Venus Orbiter Ultraviolet Spectrometer: Operations and Data Analysis
The Ultraviolet Spectrometer investigation on the Pioneer Venus Orbiter mission was extremely successful. The instrument was designed, built and tested at CU/LASP and delivered on time and within budget. The spacecraft and its instruments were required to operate for 243 days in Venus orbit. OUVS operated successfully for a further 13 years with only minor problems. The major scientific results listed above that deal with Venus were all unexpected and significant discoveries. The Comet Halley observations came about because of a favorable alignment of Halley, the Sun, and Venus, and were an important contribution to the international study of this comet. The scientific results of the OUVS investigation are to be found in the 41 papers listed in section 4 below. OUVS data provided material for 6 PhD and one MS dissertations, listed in section 5 below
Local-time asymmetries in the Venus thermosphere
Our current understanding of the global structure and dynamics of the Venus thermosphere is embodied in models such as the Venus Thermospheric General Circulation Model (VTGCM) and empirical composition models such as VIRA and VTS3. We have completed an analysis of ultraviolet images of Venus at 130 nm acquired by the Pioneer Venus Orbiter Ultraviolet Spectrometer (PVOUVS). We have examined 97 images spanning the 10-year period between 1980 and 1990, and have developed a technique for global radiative transfer modeling with which we create synthetic models of each image analyzed. We have developed a hypothesis for understanding the persistent local-time asymmetry observed as a signature of vertically propagating internal gravity waves interacting with the thermospheric SS-AS circulation. This hypothesis is presented
Variability of the X-ray P Cygni Line Profiles from Circinus X-1 Near Zero Phase
The luminous X-ray binary Circinus X-1 has been observed twice near zero
orbital phase using the High-Energy Transmission Grating Spectrometer (HETGS)
onboard Chandra. The source was in a high-flux state during a flare for the
first observation, and it was in a low-flux state during a dip for the second.
Spectra from both flux states show clear P Cygni lines, predominantly from
H-like and He-like ion species. These indicate the presence of a high-velocity
outflow from the Cir X-1 system which we interpret as an equatorial
accretion-disk wind, and from the blueshifted resonance absorption lines we
determine outflow velocities of 200 - 1900 km/s with no clear velocity
differences between the two flux states. The line strengths and profiles,
however, are strongly variable both between the two observations as well as
within the individual observations. We characterize this variability and
suggest that it is due to both changes in the amount of absorbing material
along the line of sight as well as changes in the ionization level of the wind.
We also refine constraints on the accretion-disk wind model using improved
plasma diagnostics such as the He-like Mg XI triplet, and we consider the
possibility that the X-ray absorption features seen from superluminal jet
sources can generally be explained via high-velocity outflows.Comment: 12 pages, 8 figures, accepted by ApJ (Main
Twistors, special relativity, conformal symmetry and minimal coupling - a review
An approach to special relativistic dynamics using the language of spinors
and twistors is presented. Exploiting the natural conformally invariant
symplectic structure of the twistor space, a model is constructed which
describes a relativistic massive, spinning and charged particle, minimally
coupled to an external electro-magnetic field. On the two-twistor phase space
the relativistic Hamiltonian dynamics is generated by a Poincare scalar
function obtained from the classical limit (appropriately defined by us) of the
second order, to an external electro-magnetic field minimally coupled, Dirac
operator. In the so defined relativistic classical limit there are no Grassman
variables. Besides, the arising equation that describes dynamics of the
relativistic spin differs significantly from the so called Thomas Bergman
Michel Telegdi equation.Comment: 39 pages, no figures, few erronous statements (not affecting anything
else in the papper) on page 23 delete
Holonomy in the Schwarzschild-Droste Geometry
Parallel transport of vectors in curved spacetimes generally results in a
deficit angle between the directions of the initial and final vectors. We
examine such holonomy in the Schwarzschild-Droste geometry and find a number of
interesting features that are not widely known. For example, parallel transport
around circular orbits results in a quantized band structure of holonomy
invariance. We also examine radial holonomy and extend the analysis to spinors
and to the Reissner-Nordstr\"om metric, where we find qualitatively different
behavior for the extremal () case. Our calculations provide a toolbox
that will hopefully be useful in the investigation of quantum parallel
transport in Hilbert-fibered spacetimes.Comment: 18 Latex pages, 3 figures. Second replacement. This version as
published in CQG with some misprints correcte
Calculation of quantum discord for qubit-qudit or N qubits
Quantum discord, a kind of quantum correlation, is defined as the difference
between quantum mutual information and classical correlation in a bipartite
system. It has been discussed so far for small systems with only a few
independent parameters. We extend here to a much broader class of states when
the second party is of arbitrary dimension d, so long as the first, measured,
party is a qubit. We present two formulae to calculate quantum discord, the
first relating to the original entropic definition and the second to a recently
proposed geometric distance measure which leads to an analytical formulation.
The tracing over the qubit in the entropic calculation is reduced to a very
simple prescription. And, when the d-dimensional system is a so-called X state,
the density matrix having non-zero elements only along the diagonal and
anti-diagonal so as to appear visually like the letter X, the entropic
calculation can be carried out analytically. Such states of the full bipartite
qubit-qudit system may be named "extended X states", whose density matrix is
built of four block matrices, each visually appearing as an X. The optimization
involved in the entropic calculation is generally over two parameters, reducing
to one for many cases, and avoided altogether for an overwhelmingly large set
of density matrices as our numerical investigations demonstrate. Our results
also apply to states of a N-qubit system, where "extended X states" consist of
(2^(N+2) - 1) states, larger in number than the (2^(N+1) - 1) of X states of N
qubits. While these are still smaller than the total number (2^(2N) - 1) of
states of N qubits, the number of parameters involved is nevertheless large. In
the case of N = 2, they encompass the entire 15-dimensional parameter space,
that is, the extended X states for N = 2 represent the full qubit-qubit system.Comment: 6 pages, 1 figur
Flow-driven transition and associated velocity profiles in a nematic liquid-crystal cell
S. A. Jewell, S. L. Cornford, F. Yang, P. S. Cann, and J. R. Sambles, Physical Review E, Vol. 80, article 041706 (2009) "Copyright © 2009 by the American Physical Society."The alignment properties and distribution of flow speed during Poiseuille flow through a microchannel of a nematic liquid crystal in a cell with homeotropic surface alignment has been measured using a combination of conoscopy, fluorescence confocal polarizing microscopy, and time-lapse imaging. Two topologically distinct director profiles, with associated fluid velocity fields, are found to exist with the preferred state dictated by the volumetric flow rate of the liquid crystal. The results show excellent agreement with model data produced using the Ericksen-Leslie nematodynamics theory
Hidden but Ubiquitous: The Pre-Rift Continental Mantle in the Red Sea Region
Volcanism in the western part of the Arabian plate resulted in one of the largest alkali basalt provinces in the world, where lava fields with sub-alkaline to alkaline affinity are scattered from Syria and the Dead Sea Transform Zone through western Saudi Arabia to Yemen. After the Afar plume emplacement (∼30 Ma), volcanism took place in Yemen and progressively propagated northward due to Red Sea rifting-related lithospheric thinning (initiated ∼27–25 Ma). Few lava fields were emplaced during the Mesozoic, with the oldest 200 Ma volcanic activity recorded in northern Israel. We report results from volcanic pipes in the Marthoum area, east of Harrat Uwayrid, where over a hundred pipes occupy a stratigraphic level in the early Ordovician Saq sandstones. Most of them are circular or elliptical features marked by craters aligned along NW-SE fractures in the sandstone resulting from phreatomagmatic explosions that occurred when rising magma columns came in contact with the water table in the porous sandstone host. These lavas have Sr-Pb-Nd-Hf isotopic compositions far from the Cenozoic Arabian alkaline volcanism field, being considerably more enriched in Nd-Hf and Pb isotopes than any other Arabian Plate lava ever reported. New K-Ar dating constrains their age from Late Cretaceous to Early Eocene, thus anticipating the Afar plume emplacement and the Red Sea rift. Basalt geochemistry indicates that these volcanic eruptions formed from low-degree partial melting of an enriched lithospheric mantle source triggered by local variations in the asthenosphere-lithosphere boundary. This mantle source has a composition similar to the HIMU-like enriched isotopic component reported in the East African Rift and considered to represent the lowermost lithospheric mantle of the Nubian Shield. The generated melt, mixed in different proportions with melt derived from a depleted asthenosphere, produces the HIMU-like character throughout the Cenozoic Arabian alkaline volcanism. Although apparently hidden, this enriched lithospheric component is therefore ubiquitous and widespread in the cratonic roots of the African and Arabian subcontinental mantle
Voltage rectification by a SQUID ratchet
We argue that the phase across an asymmetric dc SQUID threaded by a magnetic
flux can experience an effective ratchet (periodic and asymmetric) potential.
Under an external ac current, a rocking ratchet mechanism operates whereby one
sign of the time derivative of the phase is favored. We show that there exists
a range of parameters in which a fixed sign (and, in a narrower range, even a
fixed value) of the average voltage across the ring occurs, regardless of the
sign of the external current dc component.Comment: 4 pages, 4 EPS figures, uses psfig.sty. Revised version, to appear in
Physical Review Letters (26 August 1996
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