6,196 research outputs found
Preliminary results of measurements of sq currents and the equatorial electrojet near peru
Measurement of electric current, magnetic field, and electron density in ionosphere using Nike-Apache sounding rocket
Early results from ISEE-A electric field measurements
In the solar wind and in middle latitude regions of the magnetosphere, spacecraft sheath fields obscure the ambient field under low plasma flux conditions such that valid measurements are confined to periods of moderately intense flux. Initial results show: (1) that the DC electric field is enhanced by roughly a factor of two in a narrow region at the front, increasing B, edge of the bow shock, (2) that scale lengths for large changes in E at the subsolar magnetopause are considerably shorter than scale lengths associated with the magnetic structure of the magnetopause, and (3) that the transverse distribution of B-aligned E-fields between the outer magnetosphere and ionospheric levels must be highly complex to account for the random turbulent appearance of the magnetospheric fields and the lack of corresponding time-space variations at ionospheric levels. Spike-like, non-oscillatory, fields lasting less than 0.2 seconds are occasionally seen at the bow shock and at the magnetopause and also intermittently appear in magnetosheath and plasma sheet regions under highly variable field conditions
Prisoner's Dilemma cellular automata revisited: evolution of cooperation under environmental pressure
We propose an extension of the evolutionary Prisoner's Dilemma cellular
automata, introduced by Nowak and May \cite{nm92}, in which the pressure of the
environment is taken into account. This is implemented by requiring that
individuals need to collect a minimum score , representing
indispensable resources (nutrients, energy, money, etc.) to prosper in this
environment. So the agents, instead of evolving just by adopting the behaviour
of the most successful neighbour (who got ), also take into account if
is above or below the threshold . If an
individual has a probability of adopting the opposite behaviour from the one
used by its most successful neighbour. This modification allows the evolution
of cooperation for payoffs for which defection was the rule (as it happens, for
example, when the sucker's payoff is much worse than the punishment for mutual
defection). We also analyse a more sophisticated version of this model in which
the selective rule is supplemented with a "win-stay, lose-shift" criterion. The
cluster structure is analyzed and, for this more complex version we found
power-law scaling for a restricted region in the parameter space.Comment: 15 pages, 8 figures; added figures and revised tex
Optimization of laser-plasma injector via beam loading effects using ionization-induced injection
Simulations of ionization induced injection in a laser driven plasma
wakefield show that high-quality electron injectors in the 50-200 MeV range can
be achieved in a gas cell with a tailored density profile. Using the PIC code
Warp with parameters close to existing experimental conditions, we show that
the concentration of in a hydrogen plasma with a tailored
density profile is an efficient parameter to tune electron beam properties
through the control of the interplay between beam loading effects and varying
accelerating field in the density profile. For a given laser plasma
configuration, with moderate normalized laser amplitude, and maximum
electron plasma density, , the
optimum concentration results in a robust configuration to generate electrons
at 150~MeV with a rms energy spread of 4\% and a spectral charge density of
1.8~pC/MeV.Comment: 13 pages, 10 figure
A solvable model of the evolutionary loop
A model for the evolution of a finite population in a rugged fitness
landscape is introduced and solved. The population is trapped in an
evolutionary loop, alternating periods of stasis to periods in which it
performs adaptive walks. The dependence of the average rarity of the population
(a quantity related to the fitness of the most adapted individual) and of the
duration of stases on population size and mutation rate is calculated.Comment: 6 pages, EuroLaTeX, 1 figur
Robust ecological pattern formation induced by demographic noise
We demonstrate that demographic noise can induce persistent spatial pattern
formation and temporal oscillations in the Levin-Segel predator-prey model for
plankton-herbivore population dynamics. Although the model exhibits a Turing
instability in mean field theory, demographic noise greatly enlarges the region
of parameter space where pattern formation occurs. To distinguish between
patterns generated by fluctuations and those present at the mean field level in
real ecosystems, we calculate the power spectrum in the noise-driven case and
predict the presence of fat tails not present in the mean field case. These
results may account for the prevalence of large-scale ecological patterns,
beyond that expected from traditional non-stochastic approaches.Comment: Revised version. Supporting simulation at:
http://guava.physics.uiuc.edu/~tom/Netlogo
Naming Game on Adaptive Weighted Networks
We examine a naming game on an adaptive weighted network. A weight of
connection for a given pair of agents depends on their communication success
rate and determines the probability with which the agents communicate. In some
cases, depending on the parameters of the model, the preference toward
successfully communicating agents is basically negligible and the model behaves
similarly to the naming game on a complete graph. In particular, it quickly
reaches a single-language state, albeit some details of the dynamics are
different from the complete-graph version. In some other cases, the preference
toward successfully communicating agents becomes much more relevant and the
model gets trapped in a multi-language regime. In this case gradual coarsening
and extinction of languages lead to the emergence of a dominant language,
albeit with some other languages still being present. A comparison of
distribution of languages in our model and in the human population is
discussed.Comment: 22 pages, accepted in Artificial Lif
Defensive alliances in spatial models of cyclical population interactions
As a generalization of the 3-strategy Rock-Scissors-Paper game dynamics in
space, cyclical interaction models of six mutating species are studied on a
square lattice, in which each species is supposed to have two dominant, two
subordinated and a neutral interacting partner. Depending on their interaction
topologies, these systems can be classified into four (isomorphic) groups
exhibiting significantly different behaviors as a function of mutation rate. On
three out of four cases three (or four) species form defensive alliances which
maintain themselves in a self-organizing polydomain structure via cyclic
invasions. Varying the mutation rate this mechanism results in an ordering
phenomenon analogous to that of magnetic Ising model.Comment: 4 pages, 3 figure
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