1,579 research outputs found
Magnetospheric convection electric field dynamics and stormtime particle energization: Case study of the magnetic storm of 4 May 1998
It is shown that narrow channels of high electric field are an effective mechanism for injecting plasma into the inner magnetosphere. Analytical expressions for the electric field cannot produce these channels of intense plasma flow, and thus, result in less entry and adiabatic energization of the plasma sheet into near-Earth space. For the ions, omission of these channels leads to an underprediction of the strength of the stormtime ring current and therefore, an underestimation of the geoeffectiveness of the storm event. For the electrons, omission of these channels leads to the inability to create a seed population of 10-100 keV electrons deep in the inner magnetosphere. These electrons can eventually be accelerated into MeV radiation belt particles. To examine this, the 1-7 May 1998 magnetic storm is studied with a plasma transport model by using three different convection electric field models: Volland-Stern, Weimer, and AMIE. It is found that the AMIE model can produce particle fluxes that are several orders of magnitude higher in the <i>L</i> = 2 – 4 range of the inner magnetosphere, even for a similar total cross-tail potential difference. <br><br><b>Key words.</b> Space plasma physics (charged particle motion and acceleration) – Magnetospheric physics (electric fields, storms and substorms
Multiscaling to Standard Scaling Crossover in the Bray-Humayun Model for Phase Ordering Kinetics
The Bray-Humayun model for phase ordering dynamics is solved numerically in
one and two space dimensions with conserved and non conserved order parameter.
The scaling properties are analysed in detail finding the crossover from
multiscaling to standard scaling in the conserved case. Both in the
nonconserved case and in the conserved case when standard scaling holds the
novel feature of an exponential tail in the scaling function is found.Comment: 21 pages, 10 Postscript figure
Glassy dynamics near zero temperature
We numerically study finite-dimensional spin glasses at low and zero
temperature, finding evidences for (i) strong time/space heterogeneities, (ii)
spontaneous time scale separation and (iii) power law distributions of flipping
times. Using zero temperature dynamics we study blocking, clustering and
persistence phenomena
Community Structure in Congressional Cosponsorship Networks
We study the United States Congress by constructing networks between Members
of Congress based on the legislation that they cosponsor. Using the concept of
modularity, we identify the community structure of Congressmen, as connected
via sponsorship/cosponsorship of the same legislation, to investigate the
collaborative communities of legislators in both chambers of Congress. This
analysis yields an explicit and conceptually clear measure of political
polarization, demonstrating a sharp increase in partisan polarization which
preceded and then culminated in the 104th Congress (1995-1996), when
Republicans took control of both chambers. Although polarization has since
waned in the U.S. Senate, it remains at historically high levels in the House
of Representatives.Comment: 8 pages, 4 figures (some with multiple parts), to appear in Physica
A; additional background info and explanations added from last versio
Shift of percolation thresholds for epidemic spread between static and dynamic small-world networks
The aim of the study was to compare the epidemic spread on static and dynamic
small-world networks. The network was constructed as a 2-dimensional
Watts-Strogatz model (500x500 square lattice with additional shortcuts), and
the dynamics involved rewiring shortcuts in every time step of the epidemic
spread. The model of the epidemic is SIR with latency time of 3 time steps. The
behaviour of the epidemic was checked over the range of shortcut probability
per underlying bond 0-0.5. The quantity of interest was percolation threshold
for the epidemic spread, for which numerical results were checked against an
approximate analytical model. We find a significant lowering of percolation
thresholds for the dynamic network in the parameter range given. The result
shows that the behaviour of the epidemic on dynamic network is that of a static
small world with the number of shortcuts increased by 20.7 +/- 1.4%, while the
overall qualitative behaviour stays the same. We derive corrections to the
analytical model which account for the effect. For both dynamic and static
small-world we observe suppression of the average epidemic size dependence on
network size in comparison with finite-size scaling known for regular lattice.
We also study the effect of dynamics for several rewiring rates relative to
latency time of the disease.Comment: 13 pages, 6 figure
Fate of Zero-Temperature Ising Ferromagnets
We investigate the relaxation of homogeneous Ising ferromagnets on finite
lattices with zero-temperature spin-flip dynamics. On the square lattice, a
frozen two-stripe state is apparently reached approximately 1/4 of the time,
while the ground state is reached otherwise. The asymptotic relaxation is
characterized by two distinct time scales, with the longer stemming from the
influence of a long-lived diagonal stripe ``defect''. In greater than two
dimensions, the probability to reach the ground state rapidly vanishes as the
size increases and the system typically ends up wandering forever within an
iso-energy set of stochastically ``blinking'' metastable states.Comment: 4 pages in column format, 6 figure
Metastable States in Spin Glasses and Disordered Ferromagnets
We study analytically M-spin-flip stable states in disordered short-ranged
Ising models (spin glasses and ferromagnets) in all dimensions and for all M.
Our approach is primarily dynamical and is based on the convergence of a
zero-temperature dynamical process with flips of lattice animals up to size M
and starting from a deep quench, to a metastable limit. The results (rigorous
and nonrigorous, in infinite and finite volumes) concern many aspects of
metastable states: their numbers, basins of attraction, energy densities,
overlaps, remanent magnetizations and relations to thermodynamic states. For
example, we show that their overlap distribution is a delta-function at zero.
We also define a dynamics for M=infinity, which provides a potential tool for
investigating ground state structure.Comment: 34 pages (LaTeX); to appear in Physical Review
Persistence in higher dimensions : a finite size scaling study
We show that the persistence probability , in a coarsening system of
linear size at a time , has the finite size scaling form where is the persistence exponent and
is the coarsening exponent. The scaling function for
and is constant for large . The scaling form implies a fractal
distribution of persistent sites with power-law spatial correlations. We study
the scaling numerically for Glauber-Ising model at dimension to 4 and
extend the study to the diffusion problem. Our finite size scaling ansatz is
satisfied in all these cases providing a good estimate of the exponent
.Comment: 4 pages in RevTeX with 6 figures. To appear in Phys. Rev.
The three-dimensional random field Ising magnet: interfaces, scaling, and the nature of states
The nature of the zero temperature ordering transition in the 3D Gaussian
random field Ising magnet is studied numerically, aided by scaling analyses. In
the ferromagnetic phase the scaling of the roughness of the domain walls,
, is consistent with the theoretical prediction .
As the randomness is increased through the transition, the probability
distribution of the interfacial tension of domain walls scales as for a single
second order transition. At the critical point, the fractal dimensions of
domain walls and the fractal dimension of the outer surface of spin clusters
are investigated: there are at least two distinct physically important fractal
dimensions. These dimensions are argued to be related to combinations of the
energy scaling exponent, , which determines the violation of
hyperscaling, the correlation length exponent , and the magnetization
exponent . The value is derived from the
magnetization: this estimate is supported by the study of the spin cluster size
distribution at criticality. The variation of configurations in the interior of
a sample with boundary conditions is consistent with the hypothesis that there
is a single transition separating the disordered phase with one ground state
from the ordered phase with two ground states. The array of results are shown
to be consistent with a scaling picture and a geometric description of the
influence of boundary conditions on the spins. The details of the algorithm
used and its implementation are also described.Comment: 32 pp., 2 columns, 32 figure
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