493 research outputs found
Periodicities in data observed during the minimum and the rising phase of solar cycle 23; years 1996 - 1999
Three types of observations: the daily values of the solar radio flux at 7
frequencies, the daily international sunspot number and the daily Stanford mean
solar magnetic field were processed in order to find all the periodicities
hidden in the data. Using a new approach to the radio data, two time series
were obtained for each frequency examined, one more sensitive to spot magnetic
fields, the other to large magnetic structures not connected with sunspots.
Power spectrum analysis of the data was carried out separately for the minimum
(540 days from 1 March 1996 to 22 August 1997) and for the rising phase (708
days from 23 August 1997 to 31 July 1999) of the solar cycle 23. The Scargle
periodograms obtained, normalized for the effect of autocorrelation, show the
majority of known periods and reveal a clear difference between the
periodicities found in the minimum and the rising phase. We determined the
rotation rate of the `active longitudes' in the rising phase as equal to 444.4
4 nHz (26\fd0 \pm 0\fd3). The results indicate that appropriate and
careful analysis of daily radio data at several frequencies allows the
investigation of solar periodicities generated in different layers of the solar
atmosphere by various phenomena related to the periodic emergence of diverse
magnetic structures.Comment: 15 pages, 8 figure
Exponential Mixing for a Stochastic PDE Driven by Degenerate Noise
We study stochastic partial differential equations of the reaction-diffusion
type. We show that, even if the forcing is very degenerate (i.e. has not full
rank), one has exponential convergence towards the invariant measure. The
convergence takes place in the topology induced by a weighted variation norm
and uses a kind of (uniform) Doeblin condition.Comment: 10 pages, 1 figur
Legal geographies of irregular migration : An outlook on immigration detention
In this article, I discuss legal geographies of irregular migration, drawing on a case study on immigration detention in Finland. Based on analysis of detention records, four different types of legal geographies are identified, relating to southânorth movement of thirdâcountry nationals inside Europe, criminalised Eastern European EU citizens, irregularity during the asylum process (in particular, related to the Dublin Regulation) and irregularly residing foreign nationals, including deportable longâterm residents. The analysis focuses on the relations between space, law and persons during detainees' irregular migration trajectories, paying attention to their varying entry routes, residence times, legal grounds for removal and detention and removal countries. I argue for the need for empirically contextualised analysis that addresses the complex relations between law and geography beyond a particular national context, in order to better understand the dynamics of irregular migration in all its variety.In this article, I discuss legal geographies of irregular migration, drawing on a case study on immigration detention in Finland. Based on analysis of detention records, four different types of legal geographies are identified, relating to southânorth movement of thirdâcountry nationals inside Europe, criminalised Eastern European EU citizens, irregularity during the asylum process (in particular, related to the Dublin Regulation) and irregularly residing foreign nationals, including deportable longâterm residents. The analysis focuses on the relations between space, law and persons during detainees' irregular migration trajectories, paying attention to their varying entry routes, residence times, legal grounds for removal and detention and removal countries. I argue for the need for empirically contextualised analysis that addresses the complex relations between law and geography beyond a particular national context, in order to better understand the dynamics of irregular migration in all its variety.Peer reviewe
Stochastic evolution equations driven by Liouville fractional Brownian motion
Let H be a Hilbert space and E a Banach space. We set up a theory of
stochastic integration of L(H,E)-valued functions with respect to H-cylindrical
Liouville fractional Brownian motions (fBm) with arbitrary Hurst parameter in
the interval (0,1). For Hurst parameters in (0,1/2) we show that a function
F:(0,T)\to L(H,E) is stochastically integrable with respect to an H-cylindrical
Liouville fBm if and only if it is stochastically integrable with respect to an
H-cylindrical fBm with the same Hurst parameter. As an application we show that
second-order parabolic SPDEs on bounded domains in \mathbb{R}^d, driven by
space-time noise which is white in space and Liouville fractional in time with
Hurst parameter in (d/4,1) admit mild solution which are H\"older continuous
both and space.Comment: To appear in Czech. Math.
Statistical properties of stochastic 2D Navier-Stokes equations from linear models
A new approach to the old-standing problem of the anomaly of the scaling
exponents of nonlinear models of turbulence has been proposed and tested
through numerical simulations. This is achieved by constructing, for any given
nonlinear model, a linear model of passive advection of an auxiliary field
whose anomalous scaling exponents are the same as the scaling exponents of the
nonlinear problem. In this paper, we investigate this conjecture for the 2D
Navier-Stokes equations driven by an additive noise. In order to check this
conjecture, we analyze the coupled system Navier-Stokes/linear advection system
in the unknowns . We introduce a parameter which gives a
system ; this system is studied for any
proving its well posedness and the uniqueness of its invariant measure
.
The key point is that for any the fields and
have the same scaling exponents, by assuming universality of the
scaling exponents to the force. In order to prove the same for the original
fields and , we investigate the limit as , proving that
weakly converges to , where is the only invariant
measure for the joint system for when .Comment: 23 pages; improved versio
On a Unique Ergodicity of Some Markov Processes
It is proved that the sufficient condition for the uniqueness of an invariant
measure for Markov processes with the strong asymptotic Feller property formulated
by Hairer and Mattingly (Ann Math 164(3):993â1032, 2006) entails the existence of
at most one invariant measure for e-processes as well. Some application to timehomogeneous
Markov processes associated with a nonlinear heat equation driven by
an impulsive noise is also given
Causes and evolution of winter polynyas north of Greenland
During the 42-year period (1979â2020) of satellite measurements, four major winter (DecemberâMarch) polynyas have been observed north of Greenland: one in December 1986 and three in the last decade, i.e., February of 2011, 2017, and 2018. The 2018 polynya was unparalleled in its magnitude and duration compared to the three previous events. Given the apparent recent increase in the occurrence of these extreme events, this study aims to examine their evolution and causality, in terms of forced versus natural variability. The limited weather station and remotely sensed sea ice data are analyzed combining with output from the fully coupled Regional Arctic System Model (RASM), including one hindcast and two ensemble simulations. We found that neither the accompanying anomalous warm surface air intrusion nor the ocean below had an impact (i.e., no significant ice melting) on the evolution of the observed winter open-water episodes in the region. Instead, the extreme atmospheric wind forcing resulted in greater sea ice deformation and transport offshore, accounting for the majority of sea ice loss in all four polynyas. Our analysis suggests that strong southerly winds (i.e., northward wind with speeds greater than 10âmâsâ1) blowing persistently over the study region for at least 2âd or more were required over the study region to mechanically redistribute some of the thickest Arctic sea ice out of the region and thus to create open-water areas (i.e., a latent heat polynya). To assess the role of internal variability versus external forcing of such events, we carried out and examined results from the two RASM ensembles dynamically downscaled with output from the Community Earth System Model (CESM) Decadal Prediction Large Ensemble (DPLE) simulations. Out of 100 winters in each of the two ensembles (initialized 30 years apart: one in December 1985 and another in December 2015), 17 and 16 winter polynyas were produced north of Greenland, respectively. The frequency of polynya occurrence had no apparent sensitivity to the initial sea ice thickness in the study area pointing to internal variability of atmospheric forcing as a dominant cause of winter polynyas north of Greenland. We assert that dynamical downscaling using a high-resolution regional climate model offers a robust tool for process-level examination in space and time, synthesis with limited observations, and probabilistic forecasts of Arctic events, such as the ones being investigated here and elsewhere.</p
Controllability and Qualitative properties of the solutions to SPDEs driven by boundary L\'evy noise
Let be the solution to the following stochastic evolution equation (1)
du(t,x)& = &A u(t,x) dt + B \sigma(u(t,x)) dL(t),\quad t>0; u(0,x) = x taking
values in an Hilbert space \HH, where is a \RR valued L\'evy process,
an infinitesimal generator of a strongly continuous semigroup,
\sigma:H\to \RR bounded from below and Lipschitz continuous, and B:\RR\to H
a possible unbounded operator. A typical example of such an equation is a
stochastic Partial differential equation with boundary L\'evy noise. Let
\CP=(\CP_t)_{t\ge 0} %{\CP_t:0\le t<\infty}T>0BAx\in H\CP_T^\star \delta_xH\HHLAB$ the solution of Equation [1] is
asymptotically strong Feller, respective, has a unique invariant measure. We
apply these results to the damped wave equation driven by L\'evy boundary
noise
Homogenized dynamics of stochastic partial differential equations with dynamical boundary conditions
A microscopic heterogeneous system under random influence is considered. The
randomness enters the system at physical boundary of small scale obstacles as
well as at the interior of the physical medium. This system is modeled by a
stochastic partial differential equation defined on a domain perforated with
small holes (obstacles or heterogeneities), together with random dynamical
boundary conditions on the boundaries of these small holes.
A homogenized macroscopic model for this microscopic heterogeneous stochastic
system is derived. This homogenized effective model is a new stochastic partial
differential equation defined on a unified domain without small holes, with
static boundary condition only. In fact, the random dynamical boundary
conditions are homogenized out, but the impact of random forces on the small
holes' boundaries is quantified as an extra stochastic term in the homogenized
stochastic partial differential equation. Moreover, the validity of the
homogenized model is justified by showing that the solutions of the microscopic
model converge to those of the effective macroscopic model in probability
distribution, as the size of small holes diminishes to zero.Comment: Communications in Mathematical Physics, to appear, 200
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