21,007 research outputs found
Multi-beam Energy Moments of Multibeam Particle Velocity Distributions
High resolution electron and ion velocity distributions, f(v), which consist
of N effectively disjoint beams, have been measured by NASA's Magnetospheric
Multi-Scale Mission (MMS) observatories and in reconnection simulations.
Commonly used standard velocity moments generally assume a single
mean-flow-velocity for the entire distribution, which can lead to
counterintuitive results for a multibeam f(v). An example is the (false)
standard thermal energy moment of a pair of equal and opposite cold particle
beams, which is nonzero even though each beam has zero thermal energy. By
contrast, a multibeam moment of two or more beams has no false thermal energy.
A multibeam moment is obtained by taking a standard moment of each beam and
then summing over beams. In this paper we will generalize these notions,
explore their consequences and apply them to an f(v) which is sum of
tri-Maxwellians. Both standard and multibeam energy moments have coherent and
incoherent forms. Examples of incoherent moments are the thermal energy
density, the pressure and the thermal energy flux (enthalpy flux plus heat
flux). Corresponding coherent moments are the bulk kinetic energy density, the
RAM pressure and the bulk kinetic energy flux. The false part of an incoherent
moment is defined as the difference between the standard incoherent moment and
the corresponding multibeam moment. The sum of a pair of corresponding coherent
and incoherent moments will be called the undecomposed moment. Undecomposed
moments are independent of whether the sum is standard or multibeam and
therefore have advantages when studying moments of measured f(v).Comment: 27 single-spaced pages. Three Figure
A nonstationary generalization of the Kerr congruence
Making use of the Kerr theorem for shear-free null congruences and of
Newman's representation for a virtual charge ``moving'' in complex space-time,
we obtain an axisymmetric time-dependent generalization of the Kerr congruence,
with a singular ring uniformly contracting to a point and expanding then to
infinity. Electromagnetic and complex eikonal field distributions are naturally
associated with the obtained congruence, with electric charge being
necesssarily unit (``elementary''). We conjecture that the corresponding
solution to the Einstein-Maxwell equations could describe the process of
continious transition of the naked ringlike singularitiy into a rotating black
hole and vice versa, under a particular current radius of the singular ring.Comment: 6 pages, twocolum
Volunteer studies replacing animal experiments in brain research - Report and recommendations of a Volunteers in Research and Testing workshop
Signatures of Secondary Collisionless Magnetic Reconnection Driven by Kink Instability of a Flux Rope
The kinetic features of secondary magnetic reconnection in a single flux rope
undergoing internal kink instability are studied by means of three-dimensional
Particle-in-Cell simulations. Several signatures of secondary magnetic
reconnection are identified in the plane perpendicular to the flux rope: a
quadrupolar electron and ion density structure and a bipolar Hall magnetic
field develop in proximity of the reconnection region. The most intense
electric fields form perpendicularly to the local magnetic field, and a
reconnection electric field is identified in the plane perpendicular to the
flux rope. An electron current develops along the reconnection line in the
opposite direction of the electron current supporting the flux rope magnetic
field structure. Along the reconnection line, several bipolar structures of the
electric field parallel to the magnetic field occur making the magnetic
reconnection region turbulent. The reported signatures of secondary magnetic
reconnection can help to localize magnetic reconnection events in space,
astrophysical and fusion plasmas
Graph Metrics for Temporal Networks
Temporal networks, i.e., networks in which the interactions among a set of
elementary units change over time, can be modelled in terms of time-varying
graphs, which are time-ordered sequences of graphs over a set of nodes. In such
graphs, the concepts of node adjacency and reachability crucially depend on the
exact temporal ordering of the links. Consequently, all the concepts and
metrics proposed and used for the characterisation of static complex networks
have to be redefined or appropriately extended to time-varying graphs, in order
to take into account the effects of time ordering on causality. In this chapter
we discuss how to represent temporal networks and we review the definitions of
walks, paths, connectedness and connected components valid for graphs in which
the links fluctuate over time. We then focus on temporal node-node distance,
and we discuss how to characterise link persistence and the temporal
small-world behaviour in this class of networks. Finally, we discuss the
extension of classic centrality measures, including closeness, betweenness and
spectral centrality, to the case of time-varying graphs, and we review the work
on temporal motifs analysis and the definition of modularity for temporal
graphs.Comment: 26 pages, 5 figures, Chapter in Temporal Networks (Petter Holme and
Jari Saram\"aki editors). Springer. Berlin, Heidelberg 201
Critical Behavior of an Ising System on the Sierpinski Carpet: A Short-Time Dynamics Study
The short-time dynamic evolution of an Ising model embedded in an infinitely
ramified fractal structure with noninteger Hausdorff dimension was studied
using Monte Carlo simulations. Completely ordered and disordered spin
configurations were used as initial states for the dynamic simulations. In both
cases, the evolution of the physical observables follows a power-law behavior.
Based on this fact, the complete set of critical exponents characteristic of a
second-order phase transition was evaluated. Also, the dynamic exponent of the critical initial increase in magnetization, as well as the critical
temperature, were computed. The exponent exhibits a weak dependence
on the initial (small) magnetization. On the other hand, the dynamic exponent
shows a systematic decrease when the segmentation step is increased, i.e.,
when the system size becomes larger. Our results suggest that the effective
noninteger dimension for the second-order phase transition is noticeably
smaller than the Hausdorff dimension. Even when the behavior of the
magnetization (in the case of the ordered initial state) and the
autocorrelation (in the case of the disordered initial state) with time are
very well fitted by power laws, the precision of our simulations allows us to
detect the presence of a soft oscillation of the same type in both magnitudes
that we attribute to the topological details of the generating cell at any
scale.Comment: 10 figures, 4 tables and 14 page
Topological Effects caused by the Fractal Substrate on the Nonequilibrium Critical Behavior of the Ising Magnet
The nonequilibrium critical dynamics of the Ising magnet on a fractal
substrate, namely the Sierpinski carpet with Hausdorff dimension =1.7925,
has been studied within the short-time regime by means of Monte Carlo
simulations. The evolution of the physical observables was followed at
criticality, after both annealing ordered spin configurations (ground state)
and quenching disordered initial configurations (high temperature state), for
three segmentation steps of the fractal. The topological effects become evident
from the emergence of a logarithmic periodic oscillation superimposed to a
power law in the decay of the magnetization and its logarithmic derivative and
also from the dependence of the critical exponents on the segmentation step.
These oscillations are discussed in the framework of the discrete scale
invariance of the substrate and carefully characterized in order to determine
the critical temperature of the second-order phase transition and the critical
exponents corresponding to the short-time regime. The exponent of the
initial increase in the magnetization was also obtained and the results suggest
that it would be almost independent of the fractal dimension of the susbstrate,
provided that is close enough to d=2.Comment: 9 figures, 3 tables, 10 page
Observations on computational methodologies for use in large-scale, gradient-based, multidisciplinary design incorporating advanced CFD codes
How a combination of various computational methodologies could reduce the enormous computational costs envisioned in using advanced CFD codes in gradient based optimized multidisciplinary design (MdD) procedures is briefly outlined. Implications of these MdD requirements upon advanced CFD codes are somewhat different than those imposed by a single discipline design. A means for satisfying these MdD requirements for gradient information is presented which appear to permit: (1) some leeway in the CFD solution algorithms which can be used; (2) an extension to 3-D problems; and (3) straightforward use of other computational methodologies. Many of these observations have previously been discussed as possibilities for doing parts of the problem more efficiently; the contribution here is observing how they fit together in a mutually beneficial way
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