6,419 research outputs found
The question–response system of Danish
This paper provides an overview of the question–response system of Danish, based on a collection of 350 questions (and responses) collected from video recordings of naturally occurring face-to-face interactions between native speakers of Danish. The paper identifies the lexico-grammatical options for formulating questions, the range of social actions that can be implemented through questions and the relationship between questions and responses. It further describes features where Danish questions differ from a range of other languages in terms of, for instance, distribution and the relationship between question format and social action. For instance, Danish has a high frequency of interrogatively formatted questions and questions that are negatively formulated, when compared to languages that have the same grammatical options. In terms of action, Danish shows a higher number of questions that are used for making suggestions, offers and requests and does not use repetition as a way of answering a question as often as other languages
A Weakly nonlinear theory for spiral density waves excited by accretion disc turbulence
We develop an analytic theory to describe spiral density waves propagating in
a shearing disc in the weakly nonlinear regime. Such waves are generically
found to be excited in simulations of turbulent accretion disks, in particular
if said turbulence arises from the magneto-rotational instability (MRI). We
derive a modified Burgers equation governing their dynamics, which includes the
effects of nonlinear steepening, dispersion, and a bulk viscosity to support
shocks. We solve this equation approximately to obtain nonlinear sawtooth
solutions that are asymptotically valid at late times. In this limit, the
presence of shocks is found to cause the wave amplitude to decrease with time
as 1/t^2. The validity of the analytic description is confirmed by direct
numerical solution of the full nonlinear equations of motion. The asymptotic
forms of the wave profiles of the state variables are also found to occur in
MRI simulations indicating that dissipation due to shocks plays a significant
role apart from any effects arising from direct coupling to the turbulence
Large-Scale Magnetic-Field Generation by Randomly Forced Shearing Waves
A rigorous theory for the generation of a large-scale magnetic field by
random non-helically forced motions of a conducting fluid combined with a
linear shear is presented in the analytically tractable limit of low Rm and
weak shear. The dynamo is kinematic and due to fluctuations in the net
(volume-averaged) electromotive force. This is a minimal proof-of-concept
quasilinear calculation aiming to put the shear dynamo, a new effect recently
found in numerical experiments, on a firm theoretical footing. Numerically
observed scalings of the wavenumber and growth rate of the fastest growing
mode, previously not understood, are derived analytically. The simplicity of
the model suggests that shear dynamo action may be a generic property of
sheared magnetohydrodynamic turbulence.Comment: Paper substantially rewritten, results changed (relative to v1).
Revised versio
Convection and the origin of Evershed flows in sunspot penumbrae
We discuss a numerical 3D radiation-MHD simulation of penumbral fine
structure in a small sunspot. This simulation shows the development of short
filamentary structures with horizontal flows, similar to observed Evershed
flows, and an inward propagation of these structures at a speed compatible with
observations. We conclude that the Evershed flow represents the horizontal flow
component of overturning convection in gaps with strongly reduced field
strength. The top of the flow is always directed outward--away from the umbra--
because of the broken symmetry due to the inclined magnetic field. Upflows
occur in the inner parts of the gaps and most of the gas turns over radially
(outwards and sideways), and descends back down again. The ascending, cooling
and overturning flow tends to bend magnetic field lines down, forcing a
weakening of the field that makes it easier for gas located in an adjacent
layer--further in--to initiate a similar sequence of motion, aided by lateral
heating, thus causing the inward propagation of the filament.Comment: 4 pages, 1 figure. Submitted to ApJ
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Vote buying or (Political) business (cycles) as usual?
We report robust evidence of a new short-run monetary election cycle: the monthly growth rate of the money supply (M1) around elections is higher than in other months in a sample of low- and middle-income countries. We hypothesize this is related to systemic vote buying. Consistent with this, we find no cycle in authoritarian countries and countries with strong political institutions and a pronounced cycle in elections where international election monitors reported vote buying or in close elections. Using survey data on daily consumer expenditures, we show that within-household consumption of food increases in the days before elections. </jats:p
Radiative transfer in decomposed domains
An efficient algorithm for calculating radiative transfer on massively
parallel computers using domain decomposition is presented. The integral
formulation of the transfer equation is used to divide the problem into a local
but compute-intensive part for calculating the intensity and optical depth
integrals, and a nonlocal part for communicating the intensity between adjacent
processors. The waiting time of idle processors during the nonlocal
communication part does not have a severe impact on the scaling. The wall clock
time thus scales nearly linearly with the inverse number of processors.Comment: 7 pages, 5 figures, 1 table; substantial improvements; recommended
for publication in A&
Distribuição espacial das necessidades hĂdricas das culturas do feijĂŁo, milho e soja na bacia do rio Tibaji, PR.
bitstream/item/89786/1/BOP-16.pd
Generation of Magnetic Field by Combined Action of Turbulence and Shear
The feasibility of a mean-field dynamo in nonhelical turbulence with
superimposed linear shear is studied numerically in elongated shearing boxes.
Exponential growth of magnetic field at scales much larger than the outer scale
of the turbulence is found. The charateristic scale of the field is l_B ~
S^{-1/2} and growth rate is gamma ~ S, where S is the shearing rate. This newly
discovered shear dynamo effect potentially represents a very generic mechanism
for generating large-scale magnetic fields in a broad class of astrophysical
systems with spatially coherent mean flows.Comment: 4 pages, 5 figures; replaced with revised version that matches the
published PR
Searches for Physics Beyond the Standard Model at Colliders
All experimental measurements of particle physics today are beautifully
described by the Standard Model. However, there are good reasons to believe
that new physics may be just around the corner at the TeV energy scale. This
energy range is currently probed by the Tevatron and HERA accelerators and
selected results of searches for physics beyond the Standard Model are
presented here. No signals for new physics have been found and limits are
placed on the allowed parameter space for a variety of different particles.Comment: Proceedings for 2007 Europhysics Conference on High Energy Physics,
Manchester, July 200
Self-Similar Magnetocentrifugal Disk Winds with Cylindrical Asymptotics
We construct a two-parameter family of models for self-collimated, radially
self-similar magnetized outflows from accretion disks. A flow at zero initial
poloidal speed leaves the surface of a rotating disk and is accelerated and
redirected toward the pole by helical magnetic fields threading the disk. At
large distances from the disk, the flow streamlines asymptote to wrap around
the surfaces of nested cylinders. In constrast to previous disk wind modeling,
we have explicitly implemented the cylindrical asymptotic boundary condition to
examine the consequences for flow dynamics. The solutions are characterized by
the logarithmic gradient of the magnetic field strength and the ratios between
the footpoint radius R_0 and asymptotic radius R_1 of streamlines; the Alfven
radius must be found as an eigenvalue. Cylindrical solutions require the
magnetic field to drop less steeply than 1/R. We find that the asymptotic
poloidal speed on any streamline is typically just a few tenths of the Kepler
speed at the corresponding disk footpoint. The asymptotic toroidal Alfven speed
is, however, a few times the footpoint Kepler speed. We discuss the
implications of the models for interpretations of observed optical jets and
molecular outflows from young stellar systems. We suggest that the difficulty
of achieving strong collimation in vector velocity simultaneously with a final
speed comparable to the disk rotation rate argues against isolated jets and in
favor of models with broader winds.Comment: 39 pages, Latex (uses AAS Latex macros), 6 eps figures, postscript
preprint with embedded figures available from
http://www.astro.umd.edu/~ostriker/professional/publications.html , to appear
in ApJ 9/1/9
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