Numerical simulations of radiation and heating from non-thermal electrons in solar flares

This thesis investigates heating and thermal and non-thermal X-ray emission from magnetic loops in active regions of the solar atmosphere using numerical simulations. The simulations also allow investigation of Type III radio emission. In our model we vary a number of physical parameters such as magnetic field configuration and density models, investigating the effect they have on emission and loop heating as a result of the propagation of a beam of fast electrons moving through the ambient coronal plasma. Chapter 1 presents an overview of the Sun and the magnetic processes at work in the solar atmosphere. It also contains a summary of observations and current work corresponding to the phenomena discussed in later chapters, as well as current theories of particle acceleration and transport in an active region loop. Chapter 2 describes the theory behind, and implementation of, the numerical simulations used, and initial tests of the accuracy of the simulations by comparing results with analytical results for simplified models. The simulations are built on a core which models the evolution of the electron distribution function through stochastic processes. We derive the Fokker-Plank equation from which we obtain the expressions describing the progress along a magnetic field of an electron undergoing Coulomb collisions with particles of a background plasma. We describe the field and density models used, and consider the effects of gradient and curvature drifts on particles. In Chapter 3 we present results showing the non-thermal X-ray emission from magnetic loops with various density models and field configurations. We show results from a straightforward field model with no curvature as described in MacKinnon & Brown (1990), and then results from a more complex (and more realistic) X-point field model as described in Priest & Forbes (2000). These results illustrate the significant effects the field model and density of the background coronal plasma have on the loop emission, both in intensity and position (i.e. at which part of the loop the emission originates from). We also investigate the correlation between loop footpoint size and X-ray intensity, theoretically verifying work done by Schmahl et al. (2006) in which they present observations showing that X-ray intensity increases with footpoint size. In Chapter 4 we present results showing the evolution of the loop temperature profile over time. As the fast electrons collide with the particles of the ambient background plasma they lose energy, which is transferred to the plasma, increasing it's temperature. We include in these calculations the effects of radiative and conductive cooling of the loop, but we do not consider chromospheric evaporation (whereby heated plasma from the photosphere rises into the loop at the footpoints as a result of bombardment by the beam of fast electrons) or other bulk plasma effects. This would require a combination of stochastic and hydrodynamic simulations, which we do not cover in this work. Again, we show the effects of changing density and field models on the temperature profile. In Chapter 5 we investigate the thermal X-ray emission from the particles of the background plasma in the heated loop. We then combine the thermal and non-thermal emission to produce X-ray spectra from photon energies 6 - 100 keV, similar to those observed by satellites such as the Reuven-Ramaty High Energy Solar Spectroscopic Imager (RHESSI), thus verifying that our simulations successfully model some of the processes present in active regions. We also consider the limitations of our simulations and models and discuss what parameters and changes would produce results close to observational data. Chapter 6 is separate from the preceding chapters and is a brief study of the production of Reverse-Drift Type III radio bursts in a loop, specifically the position in the loop at which the condition for their development originates, given various plasma densities and particle injection profiles. In a beam of injected electrons, the faster (higher energy) electrons propagate along the loop more quickly than the slower particles, causing an instability to develop in the beam distribution. This instability leads to the growth of Langmuir waves, which in turn result in emission at radio wavelengths. We show the development of the condition leading to this emission from a loop as a function of time and position, with various field and density models and particle injection profiles. Chapter 7 summarises the main body of work in this thesis and discusses possible further development of these methods in investigating the physical processes and parameters at work in active region magnetic loops

The effects of pathogens on club cell investment in fathead minnows, Pimephales promelas

Fish skin is a metabolically active tissue that responds quickly to stressors and is the first line of defence against physical damage. Club cells, characteristic components of Ostariophysian fish skin, release their contents into the surrounding water upon rupture (e.g. during predation). These chemical cues act as public information of predation risk. Despite the assumption that club cells evolved under the selective force of predation, studies demonstrated that predation has no effect on club cell investment. Rather, club cell production is stimulated by skin penetrating pathogens and parasites. The experiments in this thesis investigate the responses of fish skin to manipulated pathogen risk. In the first experiment, fathead minnows (Pimephales promelas) were exposed to varying infective risk from two pathogen species that differ in pathogenicity, Saprolegnia ferax and S. parasitica. Although there was no difference in club cell density between fish exposed to the two Saprolegnia species, fish exposed to high concentrations of the pathogens had smaller club cells than those exposed to low concentrations. These results are the first to demonstrate a pathogen effect on the size of club cells. The second experiment investigated whether the physical presence of the pathogen was necessary for an alteration in epidermal parameters or whether Saprolegnia parastitica conditioned water was the only stimulus necessary to evoke a change. Results indicated a lack of treatment effect on club cell density, club cell size or epidermal thickness. The third experiment investigated the timing of club cell changes following a pathogen challenge. Although fish exposed to the Saprolegnia ferax treatment had higher club cell density than fish exposed to the control, there was no difference in club cell density between fish sacrificed on day 3, 6, 9 or 12. A portion of the test population for the third experiment was infected with black spot disease. When analyzed separately, trematode infected fish had smaller club cells than those that were uninfected. In light of inconsistent epidermal responses to pathogen challenges, and comparison with other studies, assessment of environmental stressors and population differences that may affect experimental outcomes and potentially interact with infectious agents is advised

Angiotensin II is Required to Induce Exaggerated Salt Sensitivity in Dahl Rats Exposed to Maternal Separation

We previously reported that maternal separation, rat model of early life stress, enhances pressor responses to acute and chronic stressors. The aims of this study were to determine whether Dahl salt-sensitive (DS) rats subjected to maternal separation (MatSep-DS) as compared to normally reared DS (Ctl-DS) rats show exaggerated blood pressure responses to acute behavioral stressors, such as restraint stress or air jet stress (AJS), or, hypertensive stimuli including chronic high-salt diet (4% NaCl) and angiotensin II (AngII) infusion (200 ng/Kg/min) during 1 week. MatSep was performed in male DS rats for 3 h/day from postnatal days 2-14. At 8 weeks of age, rats were implanted with telemetry transmitters and allowed to recover. Mean arterial pressure (MAP) was not different between MatSep-DS and Ctl-DS rats at baseline (120 ± 2 mmHg vs. 118 ± 1 mmHg, n = 4-8). Blood pressure responses during AJS and restraint stress were not different between MatSep-DS and Ctl-DS at 3 min. However, blood pressure recovery from AJS was significantly impaired in MatSep-DS rats compared to Ctl-DS rats (P \u3c 0.05). 3-h stress-induced similar responses in MatSep and Ctl-DS rats. Chronic blood pressure responses to AngII infusion in rats fed a high-salt diet displayed enhanced MAP in MatSep-DS when compared with Ctl-DS rats (167 ± 5 mmHg vs. 152 ± 2 mmHg, pinteraction \u3c0.05). However, MAP increased similarly in both groups in response to AngII infusion or high-salt diet separately. Renal parameters such as proteinuria, urine flow rate, and urine electrolytes were not different between groups in response to each treatment. In summary, salt sensitivity induces exaggerated blood pressor responses only in presence of AngII due to early life stress

Control theory for distributed parameter systems

In this thesis problems in control theory for distributed parameter systems are studied, using a semigroup approach. Firstly the control problem is formulated for systems z = Az + Bu on a Banach space Z, when the control operator B is unbounded on Z. The semigroup Tt generated by the system operator A is required to be smoothing so that the resultant operator TtB is bounded by an LÞfunction. The finite and infinite time regulator, and the tracking problems for such systems are then solved. By constructing an iterative sequence of sub-optimal controls it is shown that for the regulator problems the optimal control is feedback, and for the tracking problem is feedback plus open-loop. It is further shown that the feedback operator, time independent in the infinite time case, is the unique solution to an Integral Riccatl equation which is differentiable. The differential equation has unique solution also, where we make additional assumptions on the system operators in the infinite time case. The open-loop control of the tracking problem is also shown to be associated with the unique solutions to integral and differential equations. Arising out of the solution to the infinite time regulator problem, the stabilizability result, exact null controllability implies stabillzability, is also proved for these systems. The results obtained are then compared with those of other authors and applications given. Observer theory for distributed parameter systems described by semigroups is then considered. Conditions are found, in terms of the system operators, for an observer to be an asymptotic state estimator under feedback and general control action. The increase in cost due to using an observer as feedback in the regulator is studied and found to be dependent on the Initial state of the system, in general unknown

Studying media events in the European social surveys across research designs, countries, time, issues, and outcomes

This is the author accepted manuscript. The final version is available from Palgrave via the DOI in this record.Scholars often study isolated media effects using one method at one time point in one country. We seek to generalise the research in this area by examining hundreds of press-worthy events across dozens of countries at various points in time with an array of techniques and outcome measures. In particular, we merge a database containing thousands of events with five waves of the European Social Survey to conduct analyses across countries and individuals as well as within countries and for specific respondents. The results suggest that there is an impressive degree of heterogeneity when it comes to how citizens react to political developments. Some events generate significant opinion changes when groups of individuals who are 'treated' are compared with 'control' cases. However, other events produce modest or even null findings with methods that employ different counterfactuals. Thus, findings of both strong and weak media effects that scholars have uncovered over the years could be a function of methodological choices as well as context-specific factors such as institutional arrangements, media systems, eras, or event characteristics. Data limitations also make some research designs possible while they preclude others. We conclude with advice for others who wish to study political events in this manner as well as discussion of media effects, broadly construed

Manipulated vs. measured: Using an experimental benchmark to investigate the performance of self-reported media exposure

This is the author accepted manuscript. The final version is available from the publisher via the DOI in this record.Media exposure is one of the most important concepts in the social sciences, and yet scholars have struggled with how to operationalize it for decades. Some researchers have focused on the effects of variously worded self-report measures. Others advocate the use of aggregate and/or behavioral data that does not rely on a person’s ability to accurately recall exposure. The present study introduces the prototype of an experimental design that can be used to improve measures of exposure. In particular, we show how an experimental benchmark can be employed to (1) compare actual (i.e., manipulated) and self-reported values of news exposure; (2) assess how closely the self-reported measures approximates the performance of “true” exposure in an empirical application, and (3) leverage the experimental benchmark to investigate whether a variation in question wording improves the accuracy of self-reported exposure measures

Resetting predator baselines in coral reef ecosystems

What did coral reef ecosystems look like before human impacts became pervasive? Early efforts to reconstruct baselines resulted in the controversial suggestion that pristine coral reefs have inverted trophic pyramids, with disproportionally large top predator biomass. The validity of the coral reef inverted trophic pyramid has been questioned, but until now, was not resolved empirically. We use data from an eight-year tag-recapture program with spatially explicit, capture-recapture models to re-examine the population size and density of a key top predator at Palmyra atoll, the same location that inspired the idea of inverted trophic biomass pyramids in coral reef ecosystems. Given that animal movement is suspected to have significantly biased early biomass estimates of highly mobile top predators, we focused our reassessment on the most mobile and most abundant predator at Palmyra, the grey reef shark (Carcharhinus amblyrhynchos). We estimated a density of 21.3 (95% CI 17.8, 24.7) grey reef sharks/km2, which is an order of magnitude lower than the estimates that suggested an inverted trophic pyramid. Our results indicate that the trophic structure of an unexploited reef fish community is not inverted, and that even healthy top predator populations may be considerably smaller, and more precarious, than previously thought

A model of tuberculosis clustering in low incidence countries reveals more transmission in the United Kingdom than the Netherlands between 2010 and 2015

Tuberculosis (TB) remains a public health threat in low TB incidence countries, through a combination of reactivated disease and onward transmission. Using surveillance data from the United Kingdom (UK) and the Netherlands (NL), we demonstrate a simple and predictable relationship between the probability of observing a cluster and its size (the number of cases with a single genotype). We demonstrate that the full range of observed cluster sizes can be described using a modified branching process model with the individual reproduction number following a Poisson lognormal distribution. We estimate that, on average, between 2010 and 2015, a TB case generated 0.41 (95% CrI 0.30,0.60) secondary cases in the UK, and 0.24 (0.14,0.48) secondary cases in the NL. A majority of cases did not generate any secondary cases. Recent transmission accounted for 39% (26%,60%) of UK cases and 23%(13%,37%) of NL cases. We predict that reducing UK transmission rates to those observed in the NL would result in 538(266,818) fewer cases annually in the UK. In conclusion, while TB in low incidence countries is strongly associated with reactivated infections, we demonstrate that recent transmission remains sufficient to warrant policies aimed at limiting local TB spread

Endothelin.

The endothelins comprise three structurally similar 21-amino acid peptides. Endothelin-1 and -2 activate two G-protein coupled receptors, ETA and ETB, with equal affinity, whereas endothelin-3 has a lower affinity for the ETA subtype. Genes encoding the peptides are present only among vertebrates. The ligand-receptor signaling pathway is a vertebrate innovation and may reflect the evolution of endothelin-1 as the most potent vasoconstrictor in the human cardiovascular system with remarkably long lasting action. Highly selective peptide ETA and ETB antagonists and ETB agonists together with radiolabeled analogs have accurately delineated endothelin pharmacology in humans and animal models, although surprisingly no ETA agonist has been discovered. ET antagonists (bosentan, ambrisentan) have revolutionized the treatment of pulmonary arterial hypertension, with the next generation of antagonists exhibiting improved efficacy (macitentan). Clinical trials continue to explore new applications, particularly in renal failure and for reducing proteinuria in diabetic nephropathy. Translational studies suggest a potential benefit of ETB agonists in chemotherapy and neuroprotection. However, demonstrating clinical efficacy of combined inhibitors of the endothelin converting enzyme and neutral endopeptidase has proved elusive. Over 28 genetic modifications have been made to the ET system in mice through global or cell-specific knockouts, knock ins, or alterations in gene expression of endothelin ligands or their target receptors. These studies have identified key roles for the endothelin isoforms and new therapeutic targets in development, fluid-electrolyte homeostasis, and cardiovascular and neuronal function. For the future, novel pharmacological strategies are emerging via small molecule epigenetic modulators, biologicals such as ETB monoclonal antibodies and the potential of signaling pathway biased agonists and antagonists.We (APD, JJM) thank the British Heart Foundation (PS/02/001, PG/05/127/19872, FS/12/64/130001), Wellcome Trust Programme in Metabolic and Cardiovascular Disease 096822/Z/11/Z NIHR Cambridge Biomedical Research Centre and the Pulmonary Hypertension Association UK; Wellcome Biomedical Resources Grant 099156/Z/12/Z for support for IUPHAR/BPS Guide to PHARMACOLOGY (CS). We acknowledge National Heart, Lung, and Blood Institute Grants P01 HL95499 (D.E.K., K.A.H., D.M.P., J.S.P.), P01 HL69999 (D.M.P., J.S.P.), U01HL117684 (D.M.P.).This is the final version of the article. It first appeared from the American Society for Pharmacology and Experimental Therapeutics via https://doi.org/10.1124/pr.115.01183