Solar magnetic fields, as well as temperature changes, introduce pressure deviations that play a significant role in modulating the resonant frequencies of p-mode oscillations. Those pressure deviations occurring in the atmosphere or sub-surface of the Sun can explain the frequency shifts observed on the timescale of the solar activity cycle. A separate study of the contribution of internal magnetic layers can clarify the relative importance of surface effects. Results from helioseismology provide realistic constraints for choosing parameters suitable to represent the magnetic layers buried in the solar interior and available for modelling, i.e. at the base of the convection zone and in the sunspots' anchoring zone. Diagnostics of the internal magnetic layers are obtained through a schematic model in which the Sun is plane-stratified. The influence of a buried magnetic field on p-modes is explored, and the nature of various waves and instabilities that can arise on such a buried magnetic field is assessed. By treating the effects of internal magnetic layers, this thesis contributes to the building of a bridge between theories and observations. On the one hand, the theoretical analysis is explored carefully in the course of its formulation, which generates new hypotheses that were not obvious so far. On the other hand, observations help to understand which explanations of the solar cycle frequency shifts may apply.

Observations of AB Doradus, a nearby, rapidly-rotating K0 dwarf are analysed, and the surface magnetic field is shown to be approximated by a potential field. Evolving the surface magnetic field according to diffusion and the observed differential rotation still yields good correlation between the calculated and observed radial field after 30 days, contradictory to the results of Barnes et al. (1998), leading to the conclusion that there is an additional cause for the evolution of the magnetic field. The chromospheric magnetic field is modelled as a potential field with a source surface. Using the stability criteria g.B = 0 and B.V(g.B) < 0, places where prominences can be stable are investigated. For agreement with the results of Donati et al. (2000)- that prominences form preferentially near the equatorial plane and at and beyond corotation- it is necessary to add a quasidipolar field of maximum strength ~20G.

Magnetic field permeates the solar atmosphere and plays a crucial role in the dynamics, energetics and structures observed. In particular, magnetic flux tubes provide the structure for coronal loops that extend from the solar surface into the corona. In this thesis, we present 3D numerical simulations examining the heating produced by reconnection between flux tubes driven by rotational footpoint motions. The basic model consists of two, initially aligned, flux tubes that are forced to interact by rotational driving velocities on the flux concentrations on the boundaries. A single, twisted current layer is created in the centre of the domain and strong, localised heating is produced. We extend this model by altering the number, distribution and strength of the sources, while maintaining the same total magnetic flux on the boundaries. The dynamical evolution and the resultant magnitude, distribution and timing of the heating events are examined for the different flux distributions. In all cases, the magnetic field is stressed by the boundary motions and a current grows within the domain. A comparison of cases with two and four sources shows that there are more locations of current concentrations, but with reduced maximum current density values, for the four source case. This produces weaker reconnection and less efficient heating. In addition, for the case with two sources, we also consider the effect of splitting up one of the sources into many smaller flux fragments. The evolution and heating are shown to be very similar to the two source case. The impact of increasing the strength of the background field between the flux tubes is also examined and we find that it delays and increases the strength of the heating, although by how much depends on the distribution of the flux sources.

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Please read the abstract in the 00front part of this document / Dissertation (M Eng (Metallurgical Engineering))--University of Pretoria, 2007. / Materials Science and Metallurgical Engineering / unrestricted

The South African railway company makes use of a train wheel detection system to monitor the trains present on a particular track, noting their lengths, positions and speeds. Interference due to distorted traction currents cause havoc with this system rendering the information gathered unreliable. To combat this interference two paths are available to reduce the detection systems susceptibility. These paths include the addition of shielding between the railway track and the wheel detectors, which form the functional entities of the train wheel detection system, and the installation of a cable running parallel to the railway track with connections to it some distance before and after the position of the wheel detector. To verify these paths, high frequency experiments were performed in the lab as well as FEM simulations. To perform the high frequency experiments a source capable of producing the high frequency current needed was designed and constructed, along with a well shielded measurement system to enable the mapping of the flux density within the region occupied by and surrounding the wheel detector. The results of both the experimental measurements and simulations yielded that the interfering magnetic field could indeed be reduced through the use of the two available paths, when they are both applied separately and in combination. To obtain the greater reduction in the interfering field within the area occupied by and surrounding the wheel detector the paths should be used in combination. Therefore through the use of a shield that is constructed from a magnetic material and the installation of a parallel cable the train wheel detection system can be made more robust. / Prof. W.A Cronjé Prof. I.W. Hofsajer

In electronic flat bands the electron kinetic energy is quenched and dominated by interaction and correlated states can emerge. These many-body collective modes are not only interesting enigmas to solve, but may also lead to real-life applications. This thesis studies correlated states in graphene, a tunable system that can be programmed by ex- ternal parameters such as electric field. Two types of graphene flat bands are examined. One, highly degenerate and discreet Landau levels created by external magnetic field. Two, moirè flat bands created by relative crystalline twist between graphene layers. Correlated states are studied with transport measurements. The results were measured in dual-gated graphite/Boron nitride encapsulated graphene heterostructures with very low disorder. The high quality of the heterostructure is showcased by ballistic electron optics including nega- tive refraction across a gate-defined pn junction. In the first type of flat band — a partially filled Landau level — the competition of electrons solid states and fractional quantum Hall liquid manifests as reentrant quantum Hall effect, with a valley and spin hierarchy unique to graphene. Alternatively, in the flat bands arising from moiré superlattices, we explore two tuning knobs of correlated states. In twisted bilayer graphene, the band width are tuned by changing interlayer hybridization via pressure. The resulting superconducting and correlated insulator states can be restored outside of a narrow range of twist angles near 1.1 degrees. New fermi surfaces also form at commensurate fillings of the flat band with reduced degeneracy. In twisted monolayer-bilayer graphene, we find extraordinary level of control and tunability because of the low symmetry. With perpendicular electric field, the system can alternate among correlated metallic and insulating states, as well as topological magnetic states. The magnetization direction can be switched purely with electrostatic doping at zero magnetic field.

We consider the question of the magnetic field configuration of central compact objects (CCOs), specifically if their observed spectra allow uniform surface temperatures and carbon atmospheres. Although it is theoretically plausible that young hot neutron stars will deplete their hydrogen and helium atmospheres through diffusive nuclear burning, we find that there is no strong observational evidence to suggest that any particular CCO has a uniform temperature carbon atmosphere. In fact, they all may have small hot spots, similar to what we have measured on the surface of RX J0822−4300, and what has been observed in the cases of two other CCOs, 1E 1207.4−5209 and PSR J1852+0040. We find it is likely that most CCOs have small magnetic inclination angles. We also study the magnetic field configurations of two particular young neutron stars through general relativistic modeling of the X-ray light curves produced by their thermal surface emission. In particular, we have analyzed over a decade of XMM-Newton observations of the central compact object RX J0822−4300 and also the transient magnetar XTEJ1810−197. We show that the CCO RX J0822−4300 has two heated regions with very dif-ferent sizes and temperatures, and we measure a significant deviation angle from a purelyantipodal geometry. This measurement can inform theoretical models of the strength and geometry of the crustal magnetic fields that conduct heat to toward these hot spots. We measure the size, temperature, angular emission pattern and viewing geometry toward the heated surface regions of the magnetar XTE J1810−197 in the years following its 2003 outburst. We demonstrate that, after the size and the temperature of the heated region shrank from what was measured in the initial outburst, the magnetar eventually entered a steady state with the hot spot luminosity powered by magnetic field decay. We find that the magnitude of the flux from the whole surface of XTE J1810−197, combined with several distance estimates, indicates that the mass of XTE J1810−197 must be significantly larger than the canonical 1.4 solar mass neutron star.

Yes / A workshop was recently held at Nagoya University (31 October – 02 November 2017), sponsored by the Center for International Collaborative Research, at the Institute for Space-Earth Environmental Research, Nagoya University, Japan, to quantitatively compare the performance of today’s operational solar flare forecasting facilities. Building upon Paper I of this series (Barnes et al. 2016), in Paper II (Leka et al. 2019) we described the participating methods for this latest comparison effort, the evaluation methodology, and presented quantitative comparisons. In this paper we focus on the behavior and performance of the methods when evaluated in the context of broad implementation differences. Acknowledging the short testing interval available and the small number of methods available, we do find that forecast performance: 1) appears to improve by including persistence or prior flare activity, region evolution, and a human “forecaster in the loop”; 2) is hurt by restricting data to disk-center observations; 3) may benefit from long-term statistics, but mostly when then combined with modern data sources and statistical approaches. These trends are arguably weak and must be viewed with numerous caveats, as discussed both here and in Paper II. Following this present work, we present in Paper IV a novel analysis method to evaluate temporal patterns of forecasting errors of both types (i.e., misses and false alarms; Park et al. 2019). Hence, most importantly, with this series of papers we demonstrate the techniques for facilitating comparisons in the interest of establishing performance-positive methodologies.

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