2,178 research outputs found
On electrostatic screening of ions in astrophysical plasmas
There has been some controversy over the expression for the so-called
`interaction energy' due to screening of charged particles in a plasma. Even in
the relatively simple case of weak screening, first discussed in the context of
astrophysical plasmas by Salpeter (1954), there is disagreement. In particular,
Shaviv and Shaviv (1996) have claimed recently that by not considering
explicitly in his calculation the complete screening cloud, Salpeter obtained a
result for the interaction energy between two nuclei separated by a distance
which in the limit is only 2/3 the correct value. It appears that
this claim has arisen from a fundamental misconception concerning the dynamics
of the interaction. We rectify this misconception, and show that Salpeter's
formula is indeed correct.Comment: 17 pages, no figures, AAS Latex, to appear in The Astrophysical
Journa
Effect of rotation of cowpea (Vigna unguiculata) with fonio (Digitaria exilis) and millet (Pennisetum glaucum) on Macrophomina phaseolina densities and cowpea yield
Macrophomina phaseolina, the causal agent of charcoal rot, causes great damage to cowpea in the Sahel. One of the few options to manage the disease is by cropping nonhosts that may reduce the soil inoculum below a damage threshold level. To test this, fonio (Digitaria exilis) and millet (Pennisetum glaucum) were cropped continuously for 3 years in plots with a natural infestation of 24-53 microsclerotia g(-1) soil at the onset of the experiment. Next, a susceptible cowpea variety was grown to quantify disease incidence and severity on these soils. Fonio and millet both reduced microsclerotial densities in soils from the first year onwards. Reductions under fonio (81% after the 2(nd) year; 86% after the 3(rd) year) were significantly stronger than under millet (56 and 66% for the 2(nd) and 3(rd) year respectively). Fonio was not infected by M. phaseolina, while the root systems of millet had low densities of microsclerotia. Cowpea yielded significantly more hay and pods after 3 years of fonio than of millet. Cowpea yields and disease incidence (dead plants) could be explained well by pre-planting microsclerotial densities. We conclude that rotation of cowpea with a gramineous crop may lead to a relatively fast decline of inoculum density. In the case of a high inoculum density, fonio can be grown for three years to reduce M. phaseolina densities in soi
Informants in Organizational Marketing Research
Organizational research frequently involves seeking judgmental data from multiple informants within organizations. Researchers are often faced with determining how many informants to survey, who those informants should be and (if more than one) how best to aggregate responses when disagreement exists between those responses. Using both recall and forecasting data from a laboratory study involving the MARKSTRAT simulation, we show that when there are multiple respondents who disagree, responses aggregated using confidence-based or competence-based weights outperform those with data-based weights, which in turn provide significant gains in estimation accuracy over simply averaging respondent reports. We then illustrate how these results can be used to determine the best number of respondents for a market research task as well as to provide an effective screening mechanism when seeking a single, best informant.screening;marketing research;aggregation;organizational research;survey research
The Impact of Channel Function Performance on Buyer-Seller Relationships in Marketing Channels
Distributors, across sectors and countries, are faced by the threat of disintermediation. In many industries, horizontal consolidation and advances in information technology have made it easier for manufacturers to bypass distributors and do business directly with consumers. Distributors have responded to this threat or other destructive acts in a number of different ways that can be represented through Hirschman's (1970) Exit-Voice-Loyalty framework. One additional response that distributors frequently adopt is developing countervailing power through dependence-balancing actions. These actions are designed to strengthen bonds with customers and often manifest themselves in the provision of improved channel services to customers. Does this strategy work? We seek to address this in our paper. Specifically, we examine the nature and magnitude of the direct and interactive effects of (a) the performance of marketing functions and services by a distributor and (b) the dependence structure of its relationship with its customers on different dimensions of relationship quality - satisfaction, trust, commitment and conflict. Of particular interest to us is the effect of functional performance on relationship quality in situations characterized by high relative dependence of the distributor on the customer - this closely approximates the situation that many distributors, faced by the threat of disintermediation, find themselves in. Hypotheses from our model are tested using data collected from the paint industry in the Netherlands and Belgium.buyer-seller relationships;channel management;channel services;relationship marketing;empirical
Impact of tangled magnetic fields on AGN-blown bubbles
There is growing consensus that feedback from AGN is the main mechanism
responsible for stopping cooling flows in clusters of galaxies. AGN are known
to inflate buoyant bubbles that supply mechanical power to the intracluster gas
(ICM). High Reynolds number hydrodynamical simulations show that such bubbles
get entirely disrupted within 100 Myr, as they rise in cluster atmospheres,
which is contrary to observations. This artificial mixing has consequences for
models trying to quantify the amount of heating and star formation in cool core
clusters of galaxies. It has been suggested that magnetic fields can stabilize
bubbles against disruption. We perform MHD simulations of fossil bubbles in the
presence of tangled magnetic fields using the high order PENCIL code. We focus
on the physically-motivated case where thermal pressure dominates over magnetic
pressure and consider randomly oriented fields with and without maximum
helicity and a case where large scale external fields drape the bubble.We find
that helicity has some stabilizing effect. However, unless the coherence length
of magnetic fields exceeds the bubble size, the bubbles are quickly shredded.
As observations of Hydra A suggest that lengthscale of magnetic fields may be
smaller then typical bubble size, this may suggest that other mechanisms, such
as viscosity, may be responsible for stabilizing the bubbles. However, since
Faraday rotation observations of radio lobes do not constrain large scale ICM
fields well if they are aligned with the bubble surface, the draping case may
be a viable alternative solution to the problem. A generic feature found in our
simulations is the formation of magnetic wakes where fields are ordered and
amplified. We suggest that this effect could prevent evaporation by thermal
conduction of cold Halpha filaments observed in the Perseus cluster.Comment: accepted for publication in MNRAS, (downgraded resolution figures,
color printing recommended
Unveiling radio halos in galaxy clusters in the LOFAR era
Giant radio halos are mega-parsec scale synchrotron sources detected in a
fraction of massive and merging galaxy clusters. Radio halos provide one of the
most important pieces of evidence for non-thermal components in large scale
structure. Statistics of their properties can be used to discriminate among
various models for their origin. Therefore, theoretical predictions of the
occurrence of radio halos are important as several new radio telescopes are
about to begin to survey the sky at low frequencies with unprecedented
sensitivity. In this paper we carry out Monte Carlo simulations to model the
formation and evolution of radio halos in a cosmological framework. We extend
previous works on the statistical properties of radio halos in the context of
the turbulent re-acceleration model. First we compute the fraction of galaxy
clusters that show radio halos and derive the luminosity function of radio
halos. Then, we derive differential and integrated number count distributions
of radio halos at low radio frequencies with the main goal to explore the
potential of the upcoming LOFAR surveys. By restricting to the case of clusters
at redshifts <0.6, we find that the planned LOFAR all sky survey at 120 MHz is
expected to detect about 350 giant radio halos. About half of these halos have
spectral indices larger than 1.9 and substantially brighten at lower
frequencies. If detected they will allow for a confirmation that turbulence
accelerates the emitting particles. We expect that also commissioning surveys,
such as MSSS, have the potential to detect about 60 radio halos in clusters of
the ROSAT Brightest Cluster Sample and its extension (eBCS). These surveys will
allow us to constrain how the rate of formation of radio halos in these
clusters depends on cluster mass.Comment: 12 pages, 12 figures, accepted for publication in Astronomy and
Astrophysic
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