1,029 research outputs found
Regenerative properties of the linear hawkes process with unbounded memory
We prove regenerative properties for the linear Hawkes process under minimal
assumptions on the transfer function, which may have unbounded support. These
results are applicable to sliding window statistical estimators. We exploit
independence in the Poisson cluster point process decomposition, and the
regeneration times are not stopping times for the Hawkes process. The
regeneration time is interpreted as the renewal time at zero of a M/G/infinity
queue, which yields a formula for its Laplace transform. When the transfer
function admits some exponential moments, we stochastically dominate the
cluster length by exponential random variables with parameters expressed in
terms of these moments. This yields explicit bounds on the Laplace transform of
the regeneration time in terms of simple integrals or special functions
yielding an explicit negative upper-bound on its abscissa of convergence. These
regenerative results allow, e.g., to systematically derive long-time asymptotic
results in view of statistical applications. This is illustrated on a
concentration inequality previously obtained with coauthors
Cooperative surmounting of bottlenecks
The physics of activated escape of objects out of a metastable state plays a
key role in diverse scientific areas involving chemical kinetics, diffusion and
dislocation motion in solids, nucleation, electrical transport, motion of flux
lines superconductors, charge density waves, and transport processes of
macromolecules, to name but a few. The underlying activated processes present
the multidimensional extension of the Kramers problem of a single Brownian
particle. In comparison to the latter case, however, the dynamics ensuing from
the interactions of many coupled units can lead to intriguing novel phenomena
that are not present when only a single degree of freedom is involved. In this
review we report on a variety of such phenomena that are exhibited by systems
consisting of chains of interacting units in the presence of potential
barriers.
In the first part we consider recent developments in the case of a
deterministic dynamics driving cooperative escape processes of coupled
nonlinear units out of metastable states. The ability of chains of coupled
units to undergo spontaneous conformational transitions can lead to a
self-organised escape. The mechanism at work is that the energies of the units
become re-arranged, while keeping the total energy conserved, in forming
localised energy modes that in turn trigger the cooperative escape. We present
scenarios of significantly enhanced noise-free escape rates if compared to the
noise-assisted case.
The second part deals with the collective directed transport of systems of
interacting particles overcoming energetic barriers in periodic potential
landscapes. Escape processes in both time-homogeneous and time-dependent driven
systems are considered for the emergence of directed motion. It is shown that
ballistic channels immersed in the associated high-dimensional phase space are
the source for the directed long-range transport
Rossby and Drift Wave Turbulence and Zonal Flows: the Charney-Hasegawa-Mima model and its extensions
A detailed study of the Charney-Hasegawa-Mima model and its extensions is
presented. These simple nonlinear partial differential equations suggested for
both Rossby waves in the atmosphere and also drift waves in a
magnetically-confined plasma exhibit some remarkable and nontrivial properties,
which in their qualitative form survive in more realistic and complicated
models, and as such form a conceptual basis for understanding the turbulence
and zonal flow dynamics in real plasma and geophysical systems. Two idealised
scenarios of generation of zonal flows by small-scale turbulence are explored:
a modulational instability and turbulent cascades.
A detailed study of the generation of zonal flows by the modulational
instability reveals that the dynamics of this zonal flow generation mechanism
differ widely depending on the initial degree of nonlinearity. A numerical
proof is provided for the extra invariant in Rossby and drift wave turbulence
-zonostrophy and the invariant cascades are shown to be characterised by the
zonostrophy pushing the energy to the zonal scales.
A small scale instability forcing applied to the model demonstrates the
well-known drift wave - zonal flow feedback loop in which the turbulence which
initially leads to the zonal flow creation, is completely suppressed and the
zonal flows saturate. The turbulence spectrum is shown to diffuse in a manner
which has been mathematically predicted.
The insights gained from this simple model could provide a basis for
equivalent studies in more sophisticated plasma and geophysical fluid dynamics
models in an effort to fully understand the zonal flow generation, the
turbulent transport suppression and the zonal flow saturation processes in both
the plasma and geophysical contexts as well as other wave and turbulence
systems where order evolves from chaos.Comment: 64 pages, 33 figure
From a vortex gas to a vortex crystal in instability-driven two-dimensional turbulence
We study structure formation in two-dimensional turbulence driven by an
external force, interpolating between linear instability forcing and random
stirring, subject to nonlinear damping. Using extensive direct numerical
simulations, we uncover a rich parameter space featuring four distinct branches
of stationary solutions: large-scale vortices, hybrid states with embedded
shielded vortices (SVs) of either sign, and two states composed of many similar
SVs. Of the latter, the first is a dense vortex gas where all SVs have the same
sign and diffuse across the domain. The second is a hexagonal vortex crystal
forming from this gas when the instability is sufficiently weak. These
solutions coexist stably over a wide parameter range. The late-time evolution
of the system from small-amplitude initial conditions is nearly self-similar,
involving three phases: initial inverse cascade, random nucleation of SVs from
turbulence and, once a critical number of vortices is reached, a phase of
explosive nucleation of SVs, leading to a statistically stationary state. The
vortex gas is continued in the forcing parameter, revealing a sharp transition
towards the crystal state as the forcing strength decreases. This transition is
analysed in terms of the diffusion of individual vortices and tools from
statistical physics. The crystal can also decay via an inverse cascade
resulting from the breakdown of shielding or insufficient nonlinear damping
acting on SVs. Our study highlights the importance of the forcing details in
two-dimensional turbulence and reveals the presence of nontrivial SV states in
this system, specifically the emergence and melting of a vortex crystal
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