133,780 research outputs found
Advertising and the evolution of market structure in the US car industry
This paper focuses on a single simple stylized fact which stands out from the post-war history of the US car industry, namely that industry concentration fell just at the same time as industry advertising expenditures rose sharply. Since both events were almost certainly caused by the entry and market penetration of (largely) foreign owned car producers, this stylized fact raises interesting questions about whether – and if so, how – advertising affects entry. We use a model of consumer switching behaviour to help interpret the facts. The model predicts a simple linear association between market and advertising shares (which we observe fairly clearly at two different levels of aggregation in the data), and provides the basis for arguing that advertising can facilitate entry, but only for finite periods of time
The local dayside reconnection rate for oblique interplanetary magnetic fields
We present an analysis of local properties of magnetic reconnection at the
dayside magnetopause for various interplanetary magnetic field (IMF)
orientations in global magnetospheric simulations. This has heretofore not been
practical because it is difficult to locate where reconnection occurs for
oblique IMF, but new techniques make this possible. The approach is to identify
magnetic separators, the curves separating four regions of differing magnetic
topology, which map the reconnection X-line. The electric field parallel to the
X-line is the local reconnection rate. We compare results to a simple model of
local two-dimensional asymmetric reconnection. To do so, we find the plasma
parameters that locally drive reconnection in the magnetosheath and
magnetosphere in planes perpendicular to the X-line at a large number of points
along the X-line. The global magnetohydrodynamic simulations are from the
three-dimensional Block-Adaptive, Tree Solarwind Roe-type Upwind Scheme
(BATS-R-US) code with a uniform resistivity, although the techniques described
here are extensible to any global magnetospheric simulation model. We find that
the predicted local reconnection rates scale well with the measured values for
all simulations, being nearly exact for due southward IMF. However, the
absolute predictions differ by an undetermined constant of proportionality,
whose magnitude increases as the IMF clock angle changes from southward to
northward. We also show similar scaling agreement in a simulation with oblique
southward IMF and a dipole tilt. The present results will be an important
component of a full understanding of the local and global properties of dayside
reconnection.Comment: 12 pages, 7 figures, 1 table, Submitted to Journal Geophysical
Research Space Physics February 12, 2016; Revised April 28, 201
Recognition and reconstruction of coherent energy with application to deep seismic reflection data
Reflections in deep seismic reflection data tend to be
visible on only a limited number of traces in a common
midpoint gather. To prevent stack degeneration,
any noncoherent reflection energy has to be removed.
In this paper, a standard classification technique in
remote sensing is presented to enhance data quality. It
consists of a recognition technique to detect and extract
coherent energy in both common shot gathers and fi-
nal stacks. This technique uses the statistics of a picked
seismic phase to obtain the likelihood distribution of its
presence. Multiplication of this likelihood distribution
with the original data results in a “cleaned up” section.
Application of the technique to data from a deep seismic
reflection experiment enhanced the visibility of all
reflectors considerably.
Because the recognition technique cannot produce an
estimate of “missing” data, it is extended with a reconstruction
method. Two methods are proposed: application
of semblance weighted local slant stacks after recognition,
and direct recognition in the linear tau-p domain.
In both cases, the power of the stacking process to increase the signal-to-noise ratio is combined with the direct selection of only specific seismic phases. The joint
application of recognition and reconstruction resulted in
data images which showed reflectors more clearly than
application of a single technique
The evolution of conifolds
We simulate the gravitational dynamics of the conifold geometries (resolved
and deformed) involved in the description of certain compact spacetimes. As the
cycles of the conifold collapse towards a singular geometry we find that a
horizon develops, shielding the external spacetime from the curvature
singularity of the newly formed black hole. The structure of the black hole is
examined for a range of initial conditions, and we find a candidate black-hole
solution for the final state of the collapse.Comment: 22 pages, 6 figure
Solitons, Links and Knots
Using numerical simulations of the full nonlinear equations of motion we
investigate topological solitons of a modified O(3) sigma model in three space
dimensions, in which the solitons are stabilized by the Hopf charge. We find
that for solitons up to charge five the solutions have the structure of closed
strings, which become increasingly twisted as the charge increases. However,
for higher charge the solutions are more exotic and comprise linked loops and
knots. We discuss the structure and formation of these solitons and demonstrate
that the key property responsible for producing such a rich variety of solitons
is that of string reconnection.Comment: 24 pages plus 14 figures in GIF forma
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