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Papers by Tatjana Zivkovic Hansen

Annales Geophysicae

We investigate energy fluxes and small, kilometre-scale Birkeland currents in the magnetospheric ... more We investigate energy fluxes and small, kilometre-scale Birkeland currents in the magnetospheric cusp at a 1–3 Earth radii altitude and in the ionosphere using satellites when they were, according to the Tsyganenko model, in magnetic conjunction within 50–60 km and up to 15 min apart. We use Cluster and CHAMP satellites, and study three conjunction events that occurred in 2008 and 2009, when the Cluster spacecraft were crossing the cusps at only a few Earth radii altitude. Our goal is to understand better the influence of processes in the magnetospheric cusp on the upper thermosphere and its upwelling which was usually observed by the CHAMP satellite passing the cusp. Three studied events occurred under relatively quiet and steady magnetospheric and ionospheric conditions, which explains why observed thermospheric density enhancements were rather low. Our findings point out that for each studied event soft electron precipitation influences thermospheric density enhancements in a way...

Journal of Geophysical Research: Atmospheres, 2013

We apply a test for low-dimensional, deterministic dynamics to the Niño 3 time series for the El ... more We apply a test for low-dimensional, deterministic dynamics to the Niño 3 time series for the El Niño Southern Oscillation (ENSO). The test is negative, indicating that the dynamics is high-dimensional/stochastic. However, application of stochastic forcing to a time-delay equation for equatorial-wave dynamics can reproduce this stochastic dynamics and other important aspects of ENSO. Without such stochastic forcing this model yields low-dimensional, deterministic dynamics, hence these results emphasize the importance of the stochastic nature of the atmosphere-ocean interaction in low-dimensional models of ENSO.

We attempt to quantify how the solar wind and the magnetosphere of the Earth self-organize during... more We attempt to quantify how the solar wind and the magnetosphere of the Earth self-organize during geomagnetic storms and substorms. Particularly, we analyze the auroral electrojet (AE) and polar cap (PC) indices as proxies of magnetospheric dynamics, while the product of Bz component of the interplanetary magnetic field and Solar wind's flow speed is treated as a proxy for the East-West electric field and for the solar wind energy input to the magnetosphere. We implement methods suitable for the analysis of non-stationary and short time series with underlying nonlinear dynamics, such as recurrence plot, empirical mode decomposition and wavelet analysis. Our results show that the solar wind electric field becomes more organized and predictable prior to magnetic storms.The magnetosphere exhibits organization similar to the solar wind during substorms. This result is consistent with recent findings of similar multifractal spectra of the Bz and AE signal on substorm time scales. Dur...

We investigate kilometer-scale field-aligned currents that were detected both in the magnetospher... more We investigate kilometer-scale field-aligned currents that were detected both in the magnetospheric cusp at a few Earth radii altitude and in the ionosphere by satellites that were, according to the Tsyganenko model, within a few tens of kilometers and minutes on the same magnetic field line. Also thermosphere up-welling that often accompanies the dayside field-aligned currents in the inner cusp was seen. We used Cluster and CHAMP satellites, and searched for conjunctions during the whole year of 2008, as then the Cluster spacecrafts were mostly at mid-altitudes when crossing the cusps. We focus on two case studies from this period. Evidence is presented that sheets of small scale field-aligned current continue through the low altitude cusp and ionosphere. The ionospheric current densities are not particularly strong, a few micro A/m2 at about 340 km, and several tens of nA/m2 at about 20000 km, implying that these currents might be relatively common events, but are hard to detect d...

We analyze the Auroral Electrojet (AE) index data for periods of Solar minimum and maximum, with ... more We analyze the Auroral Electrojet (AE) index data for periods of Solar minimum and maximum, with respect to predictability and intermittency. Neural networks are employed to the AE- data for different intermittent as well as smooth intervals, and it is shown that the intermittent bursts can be predicted from the smooth sequences of the the AE-data, whereas the opposite does not hold true. This implies that the smooth dynamics in the data could cause intermittent bursts, but that it is difficult to find a causal link between the bursty dynamics and the subsequent smooth signal. It is, however, possible to establish a causal link between the smooth signals before and after the intermittent burst. We also compute the multifractal singularity spectrum as additional evidence for the existence of intermittency, and show that this spectrum is independent of the Solar activity.

$Research paper thumbnail of On the connection between the multifractality and the predictability from the auroral index time series$

We analyze the Auroral Electrojet (AE) index data from the period of Solar minimum and maximum, w... more We analyze the Auroral Electrojet (AE) index data from the period of Solar minimum and maximum, with respect to their predictability and intermittency. Neural networks are employed to predict the behavior of the AE- data for the different intermittent intervals, as well as recurrence plots where these intervals are visualized. We also compute the multifractal singularity spectrum as additional evidence for the existence of intermittency, and show that this spectrum is independent of the Solar activity.

In a toroidal plasma confined by a purely toroidal magnetic field the plasma transport is governe... more 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 potential and electron density fluctuations measured on stationary Langmuir probes essentially reflect the spatial poloidal structure of the turbulent field (Taylor hypothesis). On these time scales the signals reveals an intermittent character exposed via analysis of probability density functions and computation of multifractal dimension spectra in different regimes of time scales. This intermittency is associated with the shape and distribution of pronounced spikes in the signal. On time scales much longer than the rotation period there are strong global fluctuations in the plasma potent...

ABSTRACT It has recently been shown that fluctuations of global quantities (let us name it X) in ... more ABSTRACT It has recently been shown that fluctuations of global quantities (let us name it X) in certain avalanching and turbulent systems can be described by stochastic differential equations (SDEs) driven by a colored noise term with a diffusion coefficient depending on X. The equation also contains a deterministic drift term, which keeps X within certain limits. This SDE has been determined for the Bak-Tang-Wiesenfeld sandpile model in [1], and for the Zhang-sandpile and a simulated two-dimensional (2D) Navier-Stokes turbulence in [2] (see the poster by M. Rypdal in this session), and may provide criteria for distinguishing between sandpile avalanching and 2D turbulence from observational time-series data. In this contribution we employ this modeling technique to observational time-series data which are believed to reflect large-scale properties of the solar wind or the magnetosphere-ionosphere system. However, time-series analysis of quantities like the interplanetary magnetic field, disturbed storm-time index, and auroral electrojet index , reveal a multifractal structure. Thus, stochastic modeling of these signals requires that we replace the colored noise with a multifractal source term in the SDE. We demonstrate how this kind of analysis and modeling can be employed to characterize the dynamics of different elements in the Sun-Earth interaction. [1] M. Rypdal and K. Rypdal, arXiv: 0710.4010 [2] M. Rypdal and K. Rypdal, arXiv: 0807.3416

Physical Review E, 2008

In a toroidal plasma confined by a purely toroidal magnetic field with a weak vertical field supe... more In a toroidal plasma confined by a purely toroidal magnetic field with a weak vertical field superimposed a system of convection cells are generated spontaneously, interacting with a background electron density gradient. The dynamics of this interaction is low-dimensional, chaotic, and consistent with solutions of the Lorenz equations in the diffusionless limit.

Journal of Geophysical Research, 2011

The storm index SYM-H, the solar wind velocity v, and interplanetary magnetic field B z show no s... more The storm index SYM-H, the solar wind velocity v, and interplanetary magnetic field B z 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 change in determinism during magnetic storms for the solar wind parameters, there are significant enhancement of the predictability and exponents measuring persistence. Thus, magnetic storms are typically preceded by an increase in the persistence of the solar wind dynamics, and this increase is also present in the magnetospheric response to the solar wind.

Journal of Geophysical Research, 2012

The change in degree of organization of the magnetosphere during substorms is investigated by ana... more 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 flow speed. 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. This conclusion is based on analysis of five hundred substorms in the period from 2000 to 2002. A minimal dynamic-stochastic model of the driven magnetosphere that reproduces many statistical features of substorm indices is discussed.

The European Physical Journal Special Topics, 2008

We analyze probe data obtained from a toroidal magnetized plasma configuration suitable for studi... more 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 this could originate from either lowdimensional chaos, periodicity or quasi-periodicity. Therefore, we apply recurrence plot analysis as well as estimation of the largest Lyapunov exponent. These analyses provide evidence of low-dimensional chaos, in agreement with theoretical predictions.