English
norskBlar i forfatter "Chapman, Sandra"
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25 years of self-organized criticality: concepts and controversies
(Journal article; Tidsskriftartikkel; Peer reviewed, 2015-05-28)Introduced by the late Per Bak and his colleagues, self-organized criticality (SOC) has been one of the most stimulating concepts to come out of statistical mechanics and condensed matter theory in the last few decades, and has played a significant role in the development of complexity science. SOC, and more generally fractals and power laws, have attracted much comment, ranging from the very ... -
Charting the solar cycle
(Journal article; Tidsskriftartikkel; Peer reviewed, 2023-02-09)Sunspot records reveal that whilst the Sun has an approximately 11 year cycle of activity, no two cycles are of the same duration. Since this activity is a direct driver of space weather at Earth, this presents an operational challenge to quantifying space weather risk. We recently showed that the Hilbert transform of the sunspot record can be used to map the variable cycle length onto a regular ... -
Complex Systems Methods Characterizing Nonlinear Processes in the Near-Earth Electromagnetic Environment: Recent Advances and Open Challenges
(Journal article; Tidsskriftartikkel; Peer reviewed, 2023-07-12)Learning from successful applications of methods originating in statistical mechanics, complex systems science, or information theory in one scientific field (e.g., atmospheric physics or climatology) can provide important insights or conceptual ideas for other areas (e.g., space sciences) or even stimulate new research questions and approaches. For instance, quantification and attribution of ... -
Extreme Event Statistics in Dst, SYM-H, and SMR Geomagnetic Indices
(Journal article; Tidsskriftartikkel; Peer reviewed, 2023-03-20)Extreme space weather events are rare, and quantifying their likelihood is challenging, often relying on geomagnetic indices obtained from ground-based magnetometer observations that span multiple solar cycles. The Dst index ring-current monitor, derived from an hourly average over four low-latitude stations, is a benchmark for extreme space weather events, and has been extensively studied ... -
Extreme-value statistics from Lagrangian convex hull analysis for homogeneous turbulent Boussinesq convection and MHD convection
(Journal article; Tidsskriftartikkel; Peer reviewed, 2017-06-20)We investigate the utility of the convex hull of many Lagrangian tracers to analyze transport properties of turbulent flows with different anisotropy. In direct numerical simulations of statistically homogeneous and stationary Navier–Stokes turbulence, neutral fluid Boussinesq convection, and MHD Boussinesq convection a comparison with Lagrangian pair dispersion shows that convex hull statistics ... -
Global Dynamical Network of the Spatially Correlated Pc2 Wave Response for the 2015 St. Patrick's Day Storm
(Journal article; Tidsskriftartikkel; Peer reviewed, 2023-04-26)We show the global dynamics of spatial correlation of Pc2 wave activity can track the evolution of the 2015 St. Patrick's Day geomagnetic storm for an 8 hr time window around onset. The global spatially coherent response is tracked by forming a dynamical network from 1 s data using the full set of 100+ ground-based magnetometer stations collated by SuperMAG and Intermagnet. The pattern of spatial ... -
Limits to the quantification of local climate change
(Journal article; Tidsskriftartikkel; Peer reviewed, 2015-09-16)Abstract Wedemonstrate how the fundamental timescales of anthropogenic climate change limit the identification of societally relevant aspects of changes in precipitation.Weshow that it is nevertheless possible to extract, solely from observations, some confident quantified assessments of change at certain thresholds and locations. Maps of such changes, for a variety of hydrologically-relevant, ... -
Mapping climate change in European temperature distributions
(Journal article; Tidsskriftartikkel; Peer reviewed, 2013)Climate change poses challenges for decision makers across society, not just in preparing for the climate of the future but even when planning for the climate of the present day. When making climate sensitive decisions, policy makers and adaptation planners would benefit from information on local scales and for user-specific quantiles (e.g. the hottest/coldest 5% of days) and thresholds (e.g. ... -
Network analysis of geomagnetic substorms using the SuperMAG database of ground-based magnetometer stations
(Journal article; Tidsskriftartikkel; Peer reviewed, 2015-09-26)The overall morphology and dynamics of magnetospheric substorms is well established in terms of the observed qualitative auroral features seen in ground-based magnetometers. This paper focuses on the quantitative characterization of substorm dynamics captured by ground-based magnetometer stations. We present the first analysis of substorms using dynamical networks obtained from the full available ... -
A quantitative model for heat pulse propagation across large helical device plasmas
(Journal article; Tidsskriftartikkel; Peer reviewed, 2015-06-26)It is known that rapid edge cooling of magnetically confined plasmas can trigger heat pulses that propagate rapidly inward. These can result in large excursion, either positive or negative, in the electron temperature at the core. A set of particularly detailed measurements was obtained in Large Helical Device (LHD) plasmas [S. Inagaki et al., Plasma Phys. Controlled Fusion 52, 075002 (2010)], ... -
Robustness of predator-prey models for confinement regime transitions in fusion plasmas
(Journal article; Tidsskriftartikkel; Peer reviewed, 2013)Energy transport and confinement in tokamak fusion plasmas is usually determined by the coupled nonlinear interactions of small-scale drift turbulence and larger scale coherent nonlinear structures, such as zonal flows, together with free energy sources such as temperature gradients. Zero-dimensional models, designed to embody plausible physical narratives for these interactions, can help to ... -
Topology of turbulence within collisionless plasma reconnection
(Journal article; Tidsskriftartikkel; Peer reviewed, 2023-10-31)In near-collisionless plasmas, which are ubiquitous in astrophysics, entropy production relies on fully-nonlinear processes such as turbulence and reconnection, which lead to particle acceleration. Mechanisms for turbulent reconnection include multiple magnetic fux ropes interacting to generate thin current sheets which undergo reconnection, leading to mixing and magnetic merging and growth of ... -
Wavelet determination of magnetohydrodynamic-range power spectral exponents in solar wind turbulence seen by Parker Solar Probe
(Journal article; Tidsskriftartikkel; Peer reviewed, 2023-10-20)Context. The high Reynolds number solar wind flow provides a natural laboratory for the study of turbulence in situ. Parker Solar Probe samples the solar wind between 0.17 AU and 1 AU, providing an opportunity to study how turbulence evolves in the expanding solar wind.<p> <p>Aims. We aim to obtain estimates of the scaling exponents and scale breaks of the power spectra of magnetohydrodynamic ...
