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Evidence of low-dimensional chaos in magnetized plasma turbulence
(Working paper; Arbeidsnotat, 2008-03-10)
We analyze probe data obtained from a toroidal magnetized plasma configuration suitable for studies of low-frequency gradient-driven instabilities. These instabilities give rise to field-aligned convection rolls analogous to Rayleigh-Benard cells in neutral fluids, and may theoretically develop similar routes to chaos. When using mean-field dimension analysis, we observe low dimensionality, but ...
Modeling temporal fluctuations in avalanching system
(Working paper; Arbeidsnotat, 2008-07-22)
We demonstrate how to model the toppling activity in avalanching systems by stochastic differential equations (SDEs). The theory is developed as a generalization of the classical mean field approach to sandpile dynamics by formulating it as a generalization of Itoh’s SDE. This equation contains a fractional Gaussian noise term representing the branching of an avalanche into small active clusters, ...
Stretched-exponential decay functions from a self-consistent model of dielectric relaxation
(Working paper; Arbeidsnotat, 2007-07-22)
There are many materials whose dielectric properties are described by a stretched exponential, the so-called Kohlrausch-Williams-Watts (KWW) relaxation function. Its physical origin and statistical-mechanical foundation have been a matter of debate in the literature. In this paper we suggest a model of dielectric relaxation, which naturally leads to a stretched exponential decay function. Some ...
E-pile model of self-organized criticality
(Working paper; Arbeidsnotat, 2007-11-28)
The concept of percolation is combined with a self-consistent treatment of the interaction between the dynamics on a lattice and the external drive. Such a treatment can provide a mechanism by which the system evolves to criticality without fine tuning, thus offering a route to self-organized criticality (SOC) which in many cases is more natural than the weak random drive combined with ...
Scale-free vortex cascade emerging from random forcing in a strongly coupled system
(Working paper; Arbeidsnotat, 2008-03-04)
The notions of self-organised criticality (SOC) and turbulence are traditionally considered to be applicable to disjoint classes of phenomena. Nevertheless, scale-free burst statistics is a feature shared by turbulent as well as self-organised critical dynamics. It has also been suggested that another shared feature is universal non-gaussian probability density functions (PDFs) of global fluctuations. ...
A stochastic theory for temporal fluctuations in self-organized critical systems
(Working paper; Arbeidsnotat, 2008-07-22)
A stochastic theory for the toppling activity in sandpile models is developed, based on a simple mean-field assumption about the toppling process. The theory describes the process as an anti-persistent Gaussian walk, where the diffusion coefficient is proportional to the activity. It is formulated as a generalization of the Itˆo stochastic differential equation with an anti-persistent fractional ...
Burst statistics of fluctuations in a simple magnetized torus configuration
(Working paper; Arbeidsnotat, 2009-04-19)
In a toroidal plasma confined by a purely toroidal magnetic field the plasma transport is governed by electrostatic turbulence driven by the flute interchange instability on the low-field side of the torus cross section. In this paper we revisit experimental data obtained from the Blaamann torus at the University of Tromso. On time-scales shorter than the poloidal rotation time, the time series of ...
Klimakrisen og økonomisk politikk
(Chronicle; Kronikk, 2015-11-14)
Low-dimensionality and predictability of solar wind and global magnetosphere during magnetic storms
(Journal article; Tidsskriftartikkel; Peer reviewed, 2011)
The storm index SYM-H, the solar wind velocity v, and interplanetary magnetic field Bz show no signatures of low-dimensional dynamics in quiet periods, but tests for determinism in the time series indicate that SYM-H exhibits a significant low-dimensional component during storm time, suggesting that self-organization takes place during magnetic storms. Even though our analysis yields no discernible ...
Organization of the magnetosphere during substorms
(Journal article; Tidsskriftartikkel; Peer reviewed, 2012)
The change in degree of organization of the magnetosphere during substorms is investigated by analyzing various geomagnetic indices, as well as interplanetary magnetic field z-component and solar wind velocity x-component. We conclude that the magnetosphere self-organizes globally during substorms, but neither the magnetosphere nor the solar wind become more predictable in the course of a substorm. ...