166 research outputs found
A kinetic model for runaway electrons in the ionosphere
Electrodynamic models and measurements with satellites and incoherent scatter radars predict large field aligned current densities on one side of the auroral arcs. Different authors and different kinds of studies (experimental or modeling) agree that the current density can reach up to hundreds of &micro;A/m<sup>2</sup>. This large current density could be the cause of many phenomena such as tall red rays or triggering of unstable ion acoustic waves. In the present paper, we consider the issue of electrons moving through an ionospheric gas of positive ions and neutrals under the influence of a static electric field. We develop a kinetic model of collisions including electrons/electrons, electrons/ions and electrons/neutrals collisions. We use a Fokker-Planck approach to describe binary collisions between charged particles with a long-range interaction. We present the essential elements of this collision operator: the Langevin equation for electrons/ions and electrons/electrons collisions and the Monte-Carlo and null collision methods for electrons/neutrals collisions. A computational example is given illustrating the approach to equilibrium and the impact of the different terms (electrons/electrons and electrons/ions collisions on the one hand and electrons/neutrals collisions on the other hand). Then, a parallel electric field is applied in a new sample run. In this run, the electrons move in the z direction parallel to the electric field. The first results show that all the electron distribution functions are non-Maxwellian. Furthermore, runaway electrons can carry a significant part of the total current density, up to 20% of the total current density
Cavitating Langmuir Turbulence in the Terrestrial Aurora
Langmuir cavitons have been artificially produced in the earth's ionosphere,
but evidence of naturally-occurring cavitation has been elusive. By measuring
and modeling the spectra of electrostatic plasma modes, we show that natural
cavitating, or strong, Langmuir turbulence does occur in the ionosphere, via a
process in which a beam of auroral electrons drives Langmuir waves, which in
turn produce cascading Langmuir and ion-acoustic excitations and cavitating
Langmuir turbulence. The data presented here are the first direct evidence of
cavitating Langmuir turbulence occurring naturally in any space or
astrophysical plasma.Comment: 4 pages, 4 figures, published in PRL on 9 March 2012
http://link.aps.org/doi/10.1103/PhysRevLett.108.10500
Strigolactones: Plant Hormones with Promising Features
Almost 80 years after the discovery of the first plant hormone, auxin, a few years ago a new class of plant hormones, the strigolactones, was discovered. These molecules have unprecedented biological activity in a number of highly important biological processes in plants but also outside the plant in the rhizosphere, the layer of soil surrounding the roots of plants and teeming with life. The exploitation of this amazing biological activity is not without challenges: the synthesis of strigolactones is complicated and designing the desired activity a difficult task. This minireview describes the current state of knowledge about the strigolactones and how synthetic analogs can be developed that can potentially contribute to the development of a sustainable agriculture
Advanced Planning and Scheduling system: An overview of gaps and potential sample solutions
13 pagesInternational audienceThis paper intends to take up an overview of gaps existing in Advanced Planning Scheduling system (APS). Even if APS system provide a number of solution approaches for several practical problems (inventory and global costs reduction, customer service level increase, pertinent decision making), improvement ways still emerge. Firstly, we have led a functional analysis, to identify some gaps about supply chain management activities (financial aspect and Engineering To Order concept). Then, we made a literature review to identify works have been done in these areas. This review allowed us to outline a second kind of weakness liked to the functional range of some modules too (demand forecasting, proposition of lots sizing, reactivity). So, gaps and potential solutions proposed in the scientist literature, are presented in this paper. We conclude by underlining another type of problem, concerning the management needed to install correctly an APS system
Zakharov simulation study of spectral features of on-demand Langmuir turbulence in an inhomogeneous plasma
We have performed a simulation study of Langmuir turbulence in the Earth's
ionosphere by means of a Zakharov model with parameters relevant for the F
layer. The model includes dissipative terms to model collisions and Landau
damping of the electrons and ions, and a linear density profile, which models
the ionospheric plasma inhomogeneity whose length scale is of the order 10--100
km. The injection of energy into the system is modeled by a constant source
term in the Zakharov equation. Langmuir turbulence is excited ``on-demand'' in
controlled ionospheric modification experiments where the energy is provided by
an HF radio beam injected into the overhead ionospheric plasma. The ensuing
turbulence can be studied with radars and in the form of secondary radiation
recorded by ground-based receivers. We have analyzed spectral signatures of the
turbulence for different sets of parameters and different altitudes relative to
the turning point of the linear Langmuir mode where the Langmuir frequency
equals the local plasma frequency. By a parametric analysis, we have derived a
simple scaling law, which links the spectral width of the turbulent frequency
spectrum to the physical parameters in the ionosphere. The scaling law provides
a quantitative relation between the physical parameters (temperatures, electron
number density, ionospheric length scale, etc.) and the observed frequency
spectrum. This law may be useful for interpreting experimental results.Comment: 7 pages, 8 figure
Advanced Planning and Scheduling system: An overview of gaps and potential sample solutions
This paper intends to take up an overview of gaps existing in Advanced Planning Scheduling system (APS). Even if APS system provide a number of solution approaches for several practical problems (inventory and global costs reduction, customer service level increase, pertinent decision making), improvement ways still emerge. Firstly, we have led a functional analysis, to identify some gaps about supply chain management activities (financial aspect and Engineering To Order concept). Then, we made a literature review to identify works have been done in these areas. This review allowed us to outline a second kind of weakness liked to the functional range of some modules too (demand forecasting, proposition of lots sizing, reactivity). So, gaps and potential solutions proposed in the scientist literature, are presented in this paper. We conclude by underlining another type of problem, concerning the management needed to install correctly an APS system.Advanced Planning and Scheduling system, Supply Chain Management, APS gaps, functional analysis
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