25 Years of Self-Organized Criticality: Solar and Astrophysics

Shortly after the seminal paper {\sl "Self-Organized Criticality: An explanation of 1/f noise"} by Bak, Tang, and Wiesenfeld (1987), the idea has been applied to solar physics, in {\sl "Avalanches and the Distribution of Solar Flares"} by Lu and Hamilton (1991). In the following years, an inspiring cross-fertilization from complexity theory to solar and astrophysics took place, where the SOC concept was initially applied to solar flares, stellar flares, and magnetospheric substorms, and later extended to the radiation belt, the heliosphere, lunar craters, the asteroid belt, the Saturn ring, pulsar glitches, soft X-ray repeaters, blazars, black-hole objects, cosmic rays, and boson clouds. The application of SOC concepts has been performed by numerical cellular automaton simulations, by analytical calculations of statistical (powerlaw-like) distributions based on physical scaling laws, and by observational tests of theoretically predicted size distributions and waiting time distributions. Attempts have been undertaken to import physical models into the numerical SOC toy models, such as the discretization of magneto-hydrodynamics (MHD) processes. The novel applications stimulated also vigorous debates about the discrimination between SOC models, SOC-like, and non-SOC processes, such as phase transitions, turbulence, random-walk diffusion, percolation, branching processes, network theory, chaos theory, fractality, multi-scale, and other complexity phenomena. We review SOC studies from the last 25 years and highlight new trends, open questions, and future challenges, as discussed during two recent ISSI workshops on this theme.Comment: 139 pages, 28 figures, Review based on ISSI workshops "Self-Organized Criticality and Turbulence" (2012, 2013, Bern, Switzerland

A new individual-based modelling framework for bacterial biofilm growth applied to cold plasma treatment

Biofilms are colonies of bacteria attached to the surface at a solid-fluid interface. Bacteria in biofilm produce exopolysaccharides (EPS) that form a gel-like matrix in which the bacteria are embedded. Biofilms have numerous consequences in industrial and medical settings, both positive (bioreactors, digestion) and negative (blocking, as corrosive damage of materials/devices, food contamination, clinical infection). The use of antibiotics or mechanical clearing can be effective at removing biofilms, but such treatments are not always effective or appropriate in all situations. Recently, non-thermal atmospheric plasma treatments have been proposed as an alternative (or complementary) form of treatment, that can target sites of infection with minimal damage to the surroundings (e.g. host cells in a clinical setting). These plasmas generate a multitude of chemical species, most of which are very short lived, that can infiltrate and diffuse into the biofilm killing the bacteria within. The aim of this thesis is to develop a multi-dimensional mathematical model to investigate the effect of a non- thermal plasma on biofilms in time and space and to identify key factors that determine effectiveness of the treatment. Most of the chemical products of cold plasmas are too short lived, or too reactive, to be effective in killing the biofilms, it is the longer live species, e.g. ozone, hydrogen peroxide, acid species, that penetrated the biofilm and do the most damage. However, the EPS in biofilms is an effective barrier against ozone and hydrogen peroxide. No published biofilm model combines multi-dimensional growth with a detailed description of EPS production, hence a new mathematical model is developed and applied to simulating plasma treatment. The thesis is split broadly into two parts. The first part presents a new biofilm model framework that simulates growth in response to any number of substrates (e.g. nutrient, oxygen). The model combines an Individual based model (IbM) description of bacteria (individuals or clusters) and substrates are described as a continuum. Novel features of the framework are the assumption that EPS forms a continuum over the domain and the explicit consideration of cellular energy (ATP). Simulations of this model demonstrate the contrast between biofilm grown with topical nutrient sources (forming irregular, bumpy biofilm) and basal nutrient source with topical oxygen such as biofilm grown on agar (forming regular spatially uniform biofilms). The former is in broad agreement with experiments whilst the latter, to our knowledge, has been the subject of very little experimental study. The second part extends the modelling framework to consider the effect of the plasma species. The simulations demonstrate that penetration is a key factor in their effectiveness, for which EPS plays a key role in preventing spread within and beyond the plasma treated zone. The simulations provide estimates of the timescale of equilibration of the main plasma species, predict the effect of combining these species and demonstrate how the constituents of the biofilm can change following treatment. A number of recommended suggestions for future theoretical and experimental study are discussed in the conclusions

Effects of Turbulent Flows and Superdiffusion on Reaction-Dffusion Systems

The basic question underlying the work presented in this thesis concerns the self-organization and pattern formation in inanimate media when a fluid flow is present. This thesis studies the active and passive transport in turbulent and chaotic fluid flows. Thereby the focus is mainly of experimental nature. Especial interest is placed on the experimental observation and description of new patterns emerging, when active media is subjected to a turbulent fluid flow. In particular the effect of intense mixing as can be achieved by highly chaotic or turbulent fluid flows is to be uncovered. The first goal is to characterize and explain the phenomenon of a global reactive wave in a similar experimental realization observed by Fernandez Garca et al. in 2008. One step towards this goal is the measurement of the mixing caused by the Faraday experiment. This experiment consists in the vertical forcing of a container filled with liquid. Once the velocity field had been characterized we aimed for a definition of suitable analysis methods in order to study the transport of active media on different time and length-scales. Especially for intermediate range Damkoehler numbers, i.e. where the ratio of the timescale of the fluid flow and those of the reaction timescale is similar has not been studied in an experimental system with an excitable chemical reaction before. The analysis tools applied to this experimental model system might also partly be valid for the characterization of other reaction-diffusion-advection processes as found in many natural and men-made systems, such as plankton blooms in the ocean, chemicals in the atmosphere or bioreactors. The understanding of the role of the interplay of the typical timescales of the reaction and advection processes are to be discovered. A simple model accounting partly for some of the observed characteristics, such as the local scale-free transport, is formulated. The interplay of diffusive and advective processes is further studied in detail for a numerical model flow imitating the gulf-stream current. The details of this interplay can also lead to superdiffusion and scale-free transport

Study of mitigation strategies of beam-induced background and Higgs boson couplings measurements at a muon collider.

Abstract in english: The goals of the scientific program of the Higgs boson physics at Future Colliders are: the improvement of the precision on the fermions and bosons coupling measurements, as deviation from the Standard Model could reveal New Physics, and measurement of Higgs boson self-couplings, that enable to determine the Higgs boson potential. The muon collider is a possible future machine in which these physics goals can be reached with enough precision. Indeed, at the high center of mass energies, in the regime of multi-TeV and with the luminosity conditions that are foreseen for such a machine, the single, double and triple Higgs bosons production rates will be high enough to meet the required precision and determine the Higgs potential. However, physics measurements at muon collider can be strongly affected by the huge amount of background that comes from the muon decays along the beam line. The purpose of this thesis is two fold. The first one is to study the properties of the beam-induced background and the proposed mitigation strategies that are necessary to reduce it, with full detector simulation, at a muon collider. In this environment the reconstruction of physical objects, like hadronic jets produced by the fragmentation of quarks is studied. The second one is the evaluation of the sensitivity on the Standard Model double Higgs production cross section measurement at 3 TeV center of mass energy. This analysis is the foundation for the evaluation of the sensitivity on the determination of the Higgs boson trilinear self-coupling at a muon collider. Abstract in italiano: Gli obiettivi del programma scientifico riguardante la fisica del bosone di Higgs ai collisori futuri sono: un aumento della precisione nelle misure degli accoppiamenti ai fermioni e ai bosoni, poiché deviazioni dal Modello Standard possono rivelare nuova fisica, e le misure degli auto-accoppiamenti del bosone di Higgs, che permettono di determinare il potenziale del bosone di Higgs. Il collisore di muoni è un possibile collisore futuro in cui tali obiettivi possono essere raggiunti con sufficiente precisione. Infatti ad energie del centro di massa, nel range del Multi-TeV, e alle condizioni di luminosità previste per tale collisore, i rate di produzione di singolo, doppio e triplo Higgs sono sufficienti per raggiungere le precisioni richieste e determinare il potenziale di Higgs. Tuttavia, le misure di fisica a un collisore di muoni possono essere affette dall’alto livello di fondo indotto dal fascio che deriva dal decadimento dei muoni lungo la linea di fascio. Questa tesi ha un duplice obiettivo: il primo è di studiare le proprietà del fondo indotto dal fascio e le strategie necessarie per ridurlo tramite una simulazione dell’intero detector del collisore di muoni. In tale contesto la ricostruzione di oggetti fisici come jets adronici prodotti dalla frammentazione dei quark sono studiati. Il secondo è la stima della sensitività sulla misura della sezione d’urto di produzione del doppio Higgs all’energia del centro di massa di 3 TeV. Questa analisi è la base per la stima della sensitività nella determinazione dell’ auto-accoppiamento trilineare del bosone di Higgs a un collisore di muon

Environmental controls, morphodynamic processes, and ecogeomorphic interactions of barchan to parabolic dune transformations

The transformation of barchans into parabolic dunes has been observed in various dune systems around the world. Precise details of how environmental controls influence the dune transformation and stabilisation mechanism, however, remain poorly understood. A ‘horns-anchoring’ mechanism and a ‘nebkhas-initiation’ mechanism have previously been proposed and selected environmental controls on the transformation have been explored by some modelling efforts, but the morphodynamic processes and eco-geomorphic interactions involved are unclear and comparison between different dune systems is challenging. This study extends a cellular automaton model, informed by empirical data from fieldwork and remote sensing, to fully explore how vegetation characteristics, boundary conditions, and wind regime influence the transformation process and the resulting dune morphologies. A ‘dynamic growth function’ is introduced for clump-like perennials to differentiate between growing and non-growing seasons and to simulate the development of young plants into mature plants over multiple years. Modelling results show that environmental parameters interact with each other in a complex manner to impact the transformation process. The study finds a fundamental power-law relation between a non-dimensional parameter group, so-called the ‘dune stabilising index’ (S⁎), and the normalised migration distance of the transforming dune, which can be used to reconstruct paleo-environmental conditions and monitor the impacts of changes in climate or land-use on a dune system. Four basic eco-geomorphic interaction zones are identified which bear different functionality in the barchan to parabolic dune transformation. The roles of different environmental controls in changing the eco-geomorphic interaction zones, transforming processes, and resulting dune morphologies are also clarified

Modeling the Evolution of Barrier Islands

Barrier islands form off the shore of many coastal areas and serve as the first line of defense, protecting littoral communities against storms. To study the effects that climate change has on barrier islands, we use a cellular model of wind erosion, surface dynamics, beach dynamics, marsh dynamics, and vegetation development. We will show the inhibition of movement when vegetation is present