26,633 research outputs found
The global characteristics of atmosphere emissions in the lower thermosphere and their aeronomic implications
The green line of atomic oxygen and the Herzberg bands of molecular oxygen as observed from the OGO-4 airglow photometer are discussed in terms of their spatial and temporal distributions and their relation to the atomic oxygen content in the lower thermosphere. Daily maps of the distribution of emissions show considerable structure (cells, patches, and bands) with appreciable daily changes. When data are averaged over periods of several days in length, the resulting patterns have occasional tendencies to follow geomagnetic parallels. The Seasonal variations are characterized by maxima in both the Northern and Southern Hemispheres in October, with the Northern Hemisphere having substantially higher emission rates. Formulae are derived relating the vertical column emission rates of the green line and the Herzberg bands to the atomic oxygen peak density. Global averages for the time period for these data (August 1967 to January 1968), when converted to maximum atomic oxygen densities near 95 km, have a range of 2.0 x 10 to the 11th power/cu cm 2.7 x 10 to the 11th power/cu cm
Menâs and womenâs migration in coastal Ghana
This article uses life history calendar (LHC) data from coastal Ghana and event history statistical methods to examine inter-regional migration for men and women, focusing on four specific migration types: rural-urban, rural-rural, urban-urban, and urban-rural. Our analysis is unique because it examines how key determinants of migrationĂąâŹâincluding education, employment, marital status, and childbearingĂąâŹâdiffer by sex for these four types of migration. We find that women are significantly less mobile than men overall, but that more educated women are more likely to move (particularly to urban areas) than their male counterparts. Moreover, employment in the prior year is less of a deterrent to migration among women. While childbearing has a negative effect on migration, this impact is surprisingly stronger for men than for women, perhaps because womenâs search for assistance in childcare promotes migration. Meanwhile, being married or in union appears to have little effect on migration probabilities for either men or women. These results demonstrate the benefits of a LHC approach and suggest that migration research should further examine menâs and womenâs mobility as it relates to both human capital and household and family dynamics, particularly in developing settings.event history analysis, Ghana, life history, migration, Sub-Saharan Africa, urbanization
Propagators weakly associated to a family of Hamiltonians and the adiabatic theorem for the Landau Hamiltonian with a time-dependent Aharonov-Bohm flux
We study the dynamics of a quantum particle moving in a plane under the
influence of a constant magnetic field and driven by a slowly time-dependent
singular flux tube through a puncture. The known adiabatic results do not cover
these models as the Hamiltonian has time dependent domain. We give a meaning to
the propagator and prove an adiabatic theorem. To this end we introduce and
develop the new notion of a propagator weakly associated to a time-dependent
Hamiltonian.Comment: Title and Abstract changed, will appear in Journal of Mathematical
Physic
Quantum Inverse Square Interaction
Hamiltonians with inverse square interaction potential occur in the study of
a variety of physical systems and exhibit a rich mathematical structure. In
this talk we briefly mention some of the applications of such Hamiltonians and
then analyze the case of the N-body rational Calogero model as an example. This
model has recently been shown to admit novel solutions, whose properties are
discussed.Comment: Talk presented at the conference "Space-time and Fundamental
Interactions: Quantum Aspects" in honour of Prof. A.P.Balachandran's 65th
birthday, Vietri sul Mare, Italy, 26 - 31 May, 2003, Latex file, 9 pages.
Some references added in the replaced versio
Resonances Width in Crossed Electric and Magnetic Fields
We study the spectral properties of a charged particle confined to a
two-dimensional plane and submitted to homogeneous magnetic and electric fields
and an impurity potential. We use the method of complex translations to prove
that the life-times of resonances induced by the presence of electric field are
at least Gaussian long as the electric field tends to zero.Comment: 3 figure
Multiple classical limits in relativistic and nonrelativistic quantum mechanics
The existence of a classical limit describing interacting particles in a
second-quantized theory of identical particles with bosonic symmetry is proved.
This limit exists in addition to a previously established classical limit with
a classical field behavior, showing that the limit of the theory
is not unique. An analogous result is valid for a free massive scalar field:
two distinct classical limits are proved to exist, describing a system of
particles or a classical field. The introduction of local operators in order to
represent kinematical properties of interest is shown to break the permutation
symmetry under some localizability conditions, allowing the study of individual
particle properties.Comment: 13 page
A VLSI design for a systolic Viterbi decoder
A systolic Viterbi decoder for convolutional codes is developed. This decoder uses the trace-back method to reduce the amount of data needed to be stored in registers. It is shown that this new algorithm requires a smaller chip size and achieves a faster decoding time than other existing methods
Evolutionary Timescale of the DAV G117-B15A: The Most Stable Optical Clock Known
We observe G117-B15A, the most precise optical clock known, to measure the
rate of change of the main pulsation period of this blue-edge DAV white dwarf.
Even though the obtained value is only within 1 sigma, Pdot = (2.3 +/- 1.4) x
10^{-15} s/s, it is already constraining the evolutionary timescale of this
cooling white dwarf star.Comment: Accepted for publication in ApJ
Edge Currents for Quantum Hall Systems, I. One-Edge, Unbounded Geometries
Devices exhibiting the integer quantum Hall effect can be modeled by
one-electron Schroedinger operators describing the planar motion of an electron
in a perpendicular, constant magnetic field, and under the influence of an
electrostatic potential. The electron motion is confined to unbounded subsets
of the plane by confining potential barriers. The edges of the confining
potential barrier create edge currents. In this, the first of two papers, we
prove explicit lower bounds on the edge currents associated with one-edge,
unbounded geometries formed by various confining potentials. This work extends
some known results that we review. The edge currents are carried by states with
energy localized between any two Landau levels. These one-edge geometries
describe the electron confined to certain unbounded regions in the plane
obtained by deforming half-plane regions. We prove that the currents are stable
under various potential perturbations, provided the perturbations are suitably
small relative to the magnetic field strength, including perturbations by
random potentials. For these cases of one-edge geometries, the existence of,
and the estimates on, the edge currents imply that the corresponding
Hamiltonian has intervals of absolutely continuous spectrum. In the second
paper of this series, we consider the edge currents associated with two-edge
geometries describing bounded, cylinder-like regions, and unbounded,
strip-like, regions.Comment: 68 page
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