55,034 research outputs found
Impact experimentation and the microgravity environment: An overview
Impact is an ubiguitous physical process in the solar system. It occurs on all solid bodies and operates over a spectrum of scales. Understanding impact phenomena is therefore paramount in constraining and underpinning a large number of research efforts into fundamental problems in planetary geology. Gravity is an important parameter in impact processes. The advantages of microgravity experimentation are discussed
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
Variations in mid-ocean ridge CO2 emissions driven by glacial cycles
The geological record shows links between glacial cycles and volcanic
productivity, both subaerially and at mid-ocean ridges. Sea-level-driven
pressure changes could also affect chemical properties of mid-ocean ridge
volcanism. We consider how changing sea-level could alter the CO2 emissions
rate from mid-ocean ridges, on both the segment and global scale. We develop a
simplified transport model for a highly incompatible element through a
homogenous mantle; variations in the melt concentration the emission rate of
the element are created by changes in the depth of first silicate melting. The
model predicts an average global mid-ocean ridge CO2 emissions-rate of 53
Mt/yr, in line with other estimates. We show that falling sea level would cause
an increase in ridge CO2 emissions with a lag of about 100 kyrs after the
causative sea level change. The lag and amplitude of the response are sensitive
to mantle permeability and plate spreading rate. For a reconstructed sea-level
time series of the past million years, we predict variations of up to 12% (7
Mt/yr) in global mid-ocean ridge CO2 emissions. The magnitude and timing of the
predicted variations in CO2 emissions suggests a potential role for ridge
carbon emissions in glacial cycles
Baryon Magnetic Moments in Alternate 1/N_c Expansions
Recent work shows not only the necessity of a 1/N_c expansion to explain the
observed mass spectrum of the lightest baryons, but also that at least two
distinct large N_c expansions, in which quarks transform under either the color
fundamental or the two-index antisymmetric representation of SU(N_c), work
comparably well. Here we show that the baryon magnetic moments do not support
this ambivalence; they strongly prefer the color-fundamental 1/N_c expansion,
providing experimental evidence that nature decisively distinguishes among
1/N_c expansions for this observable.Comment: 18 pages, ReVTe
Rank-based estimation for all-pass time series models
An autoregressive-moving average model in which all roots of the
autoregressive polynomial are reciprocals of roots of the moving average
polynomial and vice versa is called an all-pass time series model. All-pass
models are useful for identifying and modeling noncausal and noninvertible
autoregressive-moving average processes. We establish asymptotic normality and
consistency for rank-based estimators of all-pass model parameters. The
estimators are obtained by minimizing the rank-based residual dispersion
function given by Jaeckel [Ann. Math. Statist. 43 (1972) 1449--1458]. These
estimators can have the same asymptotic efficiency as maximum likelihood
estimators and are robust. The behavior of the estimators for finite samples is
studied via simulation and rank estimation is used in the deconvolution of a
simulated water gun seismogram.Comment: Published at http://dx.doi.org/10.1214/009053606000001316 in the
Annals of Statistics (http://www.imstat.org/aos/) by the Institute of
Mathematical Statistics (http://www.imstat.org
U.S. Beer Flows & the Impact of NAFTA
After World War II and up until the 1980’s, the liberalization of trade was realized on a multilateral basis. World trade grew at twice the pace of GDP growth (Krueger, 1999). However, starting in the mid 1980’s, preferential trading arrangements (PTAs) increased in numbers. Perhaps the most influential PTA ever to be signed could be the North America Free Trade Agreement, or simply NAFTA, which came into effect January 1, 1994. The agreement established a free-trade area between its member countries- US, Canada and Mexico- in which all tariffs would be phased out between them, but each country would maintain its separate national barriers against the rest of the world. A lot of attention has been paid to the impact of NAFTA on the welfare of its member countries and on the rest of the world. This paper will focus on the impact of the agreement on the US’s beer trade flows by analyzing annual import and export data using several methods. To our knowledge there is no precedent for such research. Section II provides a brief review of the conclusions and methodology of existing works on NAFTA trade issues, as well as some important aspects of the agreement. Section III provides an overview of the world beer industry, and the NAFTA member countries beer markets. Section IV provides in great detail the methodology that we will employ. The focus of Section V is to explain the results obtained. Section VI provides conclusions and implications for further research on this subject. References and other sources can be found in Section VII.beer trade on US market, NAFTA
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