Evidence of low-dimensional chaos in magnetized plasma turbulence

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 low-dimensional 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

A parsimonious description and cross-country analysis of COVID-19 epidemic curve

In a given country, the cumulative death toll of the first wave of the COVID-19 epidemic follows a sigmoid curve as a function of time. In most cases, the curve is well described by the Gompertz function, which is characterized by two essential parameters, the initial growth rate and the decay rate as the first epidemic wave subsides. These parameters are determined by socioeconomic factors and the countermeasures to halt the epidemic. The Gompertz model implies that the total death toll depends exponentially, and hence very sensitively, on the ratio between these rates. The remarkably different epidemic curves for the first epidemic wave in Sweden and Norway and many other countries are classified and discussed in this framework, and their usefulness for the planning of mitigation strategies is discussed.Comment: 27 pages, 14 figure

Stretched exponential relaxation and ac universality in disordered dielectrics

This paper is concerned with the connection between the properties of dielectric relaxation and ac (alternating-current) conduction in disordered dielectrics. The discussion is divided between the classical linear-response theory and a self-consistent dynamical modeling. The key issues are, stretched exponential character of dielectric relaxation, power-law power spectral density, and anomalous dependence of ac conduction coefficient on frequency. We propose a self-consistent model of dielectric relaxation, in which the relaxations are described by a stretched exponential decay function. Mathematically, our study refers to the expanding area of fractional calculus and we propose a systematic derivation of the fractional relaxation and fractional diffusion equations from the property of ac universality.Comment: 8 pages, 2 figure

Statistical significance of rising and oscillatory trends in global ocean and land temperature in the past 160 years

Submitted manuscript version.Various interpretations of the notion of a trend in the context of global warming are discussed, contrasting the difference between viewing a trend as the deterministic response to an external forcing and viewing it as a slow variation which can be separated from the background spectral continuum of long-range persistent climate noise. The emphasis in this paper is on the latter notion, and a general scheme is presented for testing a multi-parameter trend model against a null hypothesis which models the observed climate record as an autocorrelated noise. The scheme is employed to the instrumental global sea-surface temperature record and the global land temperature record. A trend model comprising a linear plus an oscillatory trend with period of approximately 70 yr, and the statistical significance of the trends, are tested against three different null models: first-order autoregressive process, fractional Gaussian noise, and fractional Brownian motion. The parameters of the null models are estimated from the instrumental record, but are also checked to be consistent with a Northern Hemisphere temperature reconstruction prior to 1750 for which an anthropogenic trend is negligible. The linear trend in the period 1850–2010 AD is significant in all cases, but the oscillatory trend is insignificant for ocean data and barely significant for land data. However, by using the significance of the linear trend to constrain the null hypothesis, the oscillatory trend in the land record appears to be statistically significant. The results suggest that the global land record may be better suited for detection of the global warming signal than the ocean record

En analyse av vindkraftens og kraftutvekslingens betydning for effektkjøring av norske magasinkraftverk

Effektkjøring av magasinkraftverk er hurtige og store variasjoner i vannføring og produsert effekt. Spesielt kan store variasjoner innenfor et tidsrom på et døgn føre til tilsvarende variasjoner i vannstand i nedstrøm vassdrag og som kan føre til skader på livet i elva. Det er en gjengs oppfatning at et stort innslag av intermitterende, ikke-regulerbare kraftkilder som vindkraft, vil føre til hardere effektkjøring av den regulerbare vannkraften. Mange mener også at stor kapasitet for kraftutveksling med tilknytning til det nordiske og europeiske kraftmarkedet, kombinert med mye vindkraft, bidrar til økt effektkjøring

Spatial scaling of optical fluctuations during substorm-time aurora

International audienceA study of statistical features of auroras during substorm activity is presented, emphasizing characteristics which are commonly applied to turbulent flows. Data from all-sky television (TV) observations from the Barentsburg observatory (Svalbard) have been used. Features of the probability density function (PDF) of auroral fluctuations have been examined at different spatial scales. We find that the observed PDFs generally have a non-Gaussian, heavy-tailed shape. The generalized structure function (GSF) for the auroral luminosity fluctuations has been analyzed to determine the scaling properties of the higher (up to 6) order moments, and the evolution of the scaling indices during the actual substorm event has been determined. The scaling features obtained can be interpreted as signatures of turbulent motion of the magnetosphere-ionosphere plasma. Relations to previously obtained results of avalanche analysis of the same event, as well as possible implications for the validity of self-organized criticality models and turbulence models of the substorm activity, are discussed

Kausale design – Et gjennombrudd i samfunnsfagene eller keiserens nye klær?

Statistiske metoder som regresjonsanalyse og kausale design basert på naturlige eksperi-menter har etter hvert blitt hyllevare innen de fleste fag som behandler store datamengder,ikke minst innen epidemiologi, meteorologi og klimavitenskap.1 Et eksempel er prinsipalkomponentanalyse (PCA), som brukes til å finne karakteristiske mønstre i geofysiske felt.Mønstrene, eller prinsipalkomponentene, tolkes gjerne som uttrykk for karakteristiskedynamiske moder i de globale værsystemene. Disse tolkningene tillegges imidlertid vanlig-vis liten vekt hvis man ikke samtidig identifiserer de samme modene i dynamiske vær- ellerklimamodeller og forstår de viktigste fysiske mekanismene som ligger bak. Observasjonenekan være for dårlige eller sparsomme, eller systemet kan være så komplekst at prinsipal-komponentene ikke representerer noe som gir fysisk mening for oss.Menneskelige samfunn er langt mer komplekse enn klimasystemet og unndrar segmodellering basert på naturvitenskapelig reduksjonisme. Men både i samfunnsfagene og istudiene av komplekse naturlige systemer er det plass for enkle, konseptuelle dynamiskemodeller. Disse er imidlertid bare av verdi hvis man klarer å identifisere de viktigste dri-verne av endring, og filtrerer bort de uviktige. Kausale design kan være av stor verdi i denneseleksjonsprosessen, men det forutsetter at man ikke bruker dem primært for å «filtrereinn» mekanismer som man av en eller annen grunn tror er viktig, men at man heller brukerdem til å «filtrere ut». I motsatt fall kan bruk av vanskelig tilgjengelige kvantitative metoderlett bli et middel til å stilne protester fra dem som kan ha gode, innsiktsfulle motforestillin-ger til en gitt hypotese