9,355 research outputs found
A Unified Computational Model for Solar and Stellar Flares
We present a unified computational framework which can be used to describe
impulsive flares on the Sun and on dMe stars. The models assume that the flare
impulsive phase is caused by a beam of charged particles that is accelerated in
the corona and propagates downward depositing energy and momentum along the
way. This rapidly heats the lower stellar atmosphere causing it to explosively
expand and dramatically brighten. Our models consist of flux tubes that extend
from the sub-photosphere into the corona. We simulate how flare-accelerated
charged particles propagate down one-dimensional flux tubes and heat the
stellar atmosphere using the Fokker-Planck kinetic theory. Detailed radiative
transfer is included so that model predictions can be directly compared with
observations. The flux of flare-accelerated particles drives return currents
which additionally heat the stellar atmosphere. These effects are also included
in our models. We examine the impact of the flare-accelerated particle beams on
model solar and dMe stellar atmospheres and perform parameter studies varying
the injected particle energy spectra. We find the atmospheric response is
strongly dependent on the accelerated particle cutoff energy and spectral
index.Comment: Accepted for publication by the Astrophysical Journa
A New Multi-Resource cumulatives Constraint with Negative Heights
This paper presents a new cumulatives constraint which generalizes the original cumulative constraint in different ways. The two most important aspects consist in permitting multiple cumulative resources as well as negative heights for the resource consumption of the tasks. This allows modeling in an easy way new scheduling and planning problems. The introduction of negative heights has forced us to come up with new propagation algorithms and to revisit existing ones. The first propagation algorithm is derived from an idea called sweep which is extensively used in computational geometry; the second algorithm is based on a combination of sweep and constructive disjunction, while the last is a generalization of task intervals to this new context. A real-life timetabling problem originally motivated this constraint which was implemented within the SICStus finite domain solver and evaluated against different problem patterns
On the Reification of Global Constraints
We introduce a simple idea for deriving reified global constraints in a systematic way. It is based on
the observation that most global constraints can be reformulated as a conjunction of pure functional dependency
constraints together with a constraint that can be easily reified. We first show how the core constraints of the
Global Constraint Catalogue can be reified and we then identify several reification categories that apply to at
least 82% of the constraints in the Global Constraint Catalogue
Mobile travel services: A three-country study into the impact of local circumstances
In this paper we explore the difference in acceptance patterns of mobile services that are related to travelling in three countries: Finland, The Netherlands and New Zealand. The objective of this paper is to understand differences in the use of Mobile Travel Services in three countries that differ with regard to national travel patterns. This paper also contributes to the discussion of the relevance of the Technology Acceptance Model for mobile applications by focusing on the importance of context characteristics, such as the degree of mobility of the user, the social situation people are in, and their need for social interaction. Based on surveys in the three countries as executed in 2009, we use structural equation modelling to find differences in patterns. The paper concludes that context factors have an impact on the relation between the core concepts as used in TAM and DOI approach, and that t here is a clear need for closer research in the moderating effect of physical (e.g. mobile and fixed context) and social context, as well as the need for social interaction. Moreover it is clear that country specific characteristics play a role in the acceptance of mobile travel services. As we pointed out in many of our research projects before the acceptance and use of mobile services requires deep understanding from individual, context and technology related characteristics and their mutual interactions
Mobile travel services: the effect of moderating context factors
This article has two objectives: (1) to draw an international comparison regarding the acceptance of mobile travel services in three countries with different profiles when it comes to travelling and mobile telecommunications, and (2) to extend relevant literature on mobile applications, more specifically travel services, by including context-related concepts, taking moderating factors like location, mobility of users, physical, and social context into account. Based on surveys that were carried out in 2009, structural equation modelling is used to identify differences in patterns in the use of mobile travel services and in the role of context-related variables. the conclusion of this article is that context-related factors, that is, mobility and (physical and social) context, have an impact on the relationship between the core concepts of technology Acceptance model (TAM) and Diffusion of Innovation (DoI) research. many studies on the acceptance and use of mobile services indicate that a deep understanding is needed of individual, context-related, and technological characteristics and the way they interact. this is also highly relevant to the travel industry, which wants to utilize the opportunities provided by mobile technology
Modeling of Covalent Bonding in Solids by Inversion of Cohesive Energy Curves
We provide a systematic test of empirical theories of covalent bonding in
solids using an exact procedure to invert ab initio cohesive energy curves. By
considering multiple structures of the same material, it is possible for the
first time to test competing angular functions, expose inconsistencies in the
basic assumption of a cluster expansion, and extract general features of
covalent bonding. We test our methods on silicon, and provide the direct
evidence that the Tersoff-type bond order formalism correctly describes
coordination dependence. For bond-bending forces, we obtain skewed angular
functions that favor small angles, unlike existing models. As a
proof-of-principle demonstration, we derive a Si interatomic potential which
exhibits comparable accuracy to existing models.Comment: 4 pages revtex (twocolumn, psfig), 3 figures. Title and some wording
(but no content) changed since original submission on 24 April 199
Modeling Mg II h, k and Triplet Lines at Solar Flare Ribbons
Observations from the \textit{Interface Region Imaging Spectrograph}
(\textsl{IRIS}) often reveal significantly broadened and non-reversed profiles
of the Mg II h, k and triplet lines at flare ribbons. To understand the
formation of these optically thick Mg II lines, we perform plane parallel
radiative hydrodynamics modeling with the RADYN code, and then recalculate the
Mg II line profiles from RADYN atmosphere snapshots using the radiative
transfer code RH. We find that the current RH code significantly underestimates
the Mg II h \& k Stark widths. By implementing semi-classical perturbation
approximation results of quadratic Stark broadening from the STARK-B database
in the RH code, the Stark broadenings are found to be one order of magnitude
larger than those calculated from the current RH code. However, the improved
Stark widths are still too small, and another factor of 30 has to be multiplied
to reproduce the significantly broadened lines and adjacent continuum seen in
observations. Non-thermal electrons, magnetic fields, three-dimensional effects
or electron density effect may account for this factor. Without modifying the
RADYN atmosphere, we have also reproduced non-reversed Mg II h \& k profiles,
which appear when the electron beam energy flux is decreasing. These profiles
are formed at an electron density of
and a temperature of K, where the source function slightly
deviates from the Planck function. Our investigation also demonstrates that at
flare ribbons the triplet lines are formed in the upper chromosphere, close to
the formation heights of the h \& k lines
Large-scale exact diagonalizations reveal low-momentum scales of nuclei
Ab initio methods aim to solve the nuclear many-body problem with controlled
approximations. Virtually exact numerical solutions for realistic interactions
can only be obtained for certain special cases such as few-nucleon systems.
Here we extend the reach of exact diagonalization methods to handle model
spaces with dimension exceeding on a single compute node. This allows
us to perform no-core shell model (NCSM) calculations for 6Li in model spaces
up to and to reveal the 4He+d halo structure of this
nucleus. Still, the use of a finite harmonic-oscillator basis implies
truncations in both infrared (IR) and ultraviolet (UV) length scales. These
truncations impose finite-size corrections on observables computed in this
basis. We perform IR extrapolations of energies and radii computed in the NCSM
and with the coupled-cluster method at several fixed UV cutoffs. It is shown
that this strategy enables information gain also from data that is not fully UV
converged. IR extrapolations improve the accuracy of relevant bound-state
observables for a range of UV cutoffs, thus making them profitable tools. We
relate the momentum scale that governs the exponential IR convergence to the
threshold energy for the first open decay channel. Using large-scale NCSM
calculations we numerically verify this small-momentum scale of finite nuclei.Comment: Minor revisions.Accepted for publication in Physical Review
Pursuing More Sustainable Consumption by Analyzing Household Metabolism in European Countries and Cities
Bringing about more sustainable consumption patterns is an important challenge for society and science. In this article the concept of household metabolism is applied to analyzing consumption patterns and to identifying possibilities for the development of sustainable household consumption patterns. Household metabolism is determined in terms of total energy requirements, including both direct and indirect energy requirements, using a hybrid method. This method enables us to evaluate various determinants of the environmental load of consumption consistently at several levels—the national level, the local level, and the household level.
The average annual energy requirement of households varies considerably between the Netherlands, the United Kingdom, Norway, and Sweden, as well as within these countries. The average expenditure level per household explains a large part of the observed variations. Differences between these countries are also related to the efficiency of the production sectors and to the energy supply system. The consumption categories of food, transport, and recreation show the largest contributions to the environmental load. A comparison of consumer groups with different household characteristics shows remarkable differences in the division of spending over the consumption categories.
Thus, analyses of different types of households are important for providing a basis for options to induce decreases of the environmental load of household consumption. At the city level, options for change are provided by an analysis of the city infrastructure, which determines a large part of the direct energy use by households (for transport and heating). At the national level, energy efficiency in production and in electricity generation is an important trigger for decreasing household energy requirements.
- …