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.

We calculate the circularly polarized Stokes V(λ) profile for emission lines, formed in hot-star winds threaded with a weak split monopole magnetic field. Invoking the weak-field approximation, we find that the V(λ) profile has a characteristic shape with the ubiquitous sign reversal across line center, but also with a sign reveral in each wing. For the optically thin lines treated here, we also conclude that the V(λ) profile integrates to zero on each side of the line separately. The overall scale of V(λ)/I(λ) is set by the ratio of the field strength to the flow speed, B/v, characteristic of the line-forming region, and is of the order of 0.1% for a wind magnetic field B ≅ 100 G at depths where the windspeed is v ≅ 100 km s-1.

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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Yes / Solar flares produce radiation which can have an almost immediate effect on the near-Earth environ- ment, making it crucial to forecast flares in order to mitigate their negative effects. The number of published approaches to flare forecasting using photospheric magnetic field observations has prolifer- ated, with varying claims about how well each works. Because of the different analysis techniques and data sets used, it is essentially impossible to compare the results from the literature. This problem is exacerbated by the low event rates of large solar flares. The challenges of forecasting rare events have long been recognized in the meteorology community, but have yet to be fully acknowledged by the space weather community. During the interagency workshop on “all clear” forecasts held in Boulder, CO in 2009, the performance of a number of existing algorithms was compared on common data sets, specifically line-of-sight magnetic field and continuum intensity images from MDI, with consistent definitions of what constitutes an event. We demonstrate the importance of making such systematic comparisons, and of using standard verification statistics to determine what constitutes a good prediction scheme. When a comparison was made in this fashion, no one method clearly outperformed all others, which may in part be due to the strong correlations among the parameters used by different methods to characterize an active region. For M-class flares and above, the set of methods tends towards a weakly positive skill score (as measured with several distinct metrics), with no participating method proving substantially better than climatological forecasts. / This work is the outcome of many collaborative and cooperative efforts. The 2009 “Forecasting the All-Clear” Workshop in Boulder, CO was sponsored by NASA/Johnson Space Flight Center’s Space Radiation Analysis Group, the National Center for Atmospheric Research, and the NOAA/Space Weather Prediction Center, with additional travel support for participating scientists from NASA LWS TRT NNH09CE72C to NWRA. The authors thank the participants of that workshop, in particular Drs. Neal Zapp, Dan Fry, Doug Biesecker, for the informative discussions during those three crazy days, and NCAR’s Susan Baltuch and NWRA’s Janet Biggs for organizational prowess. Workshop preparation and analysis support was provided for GB, KDL by NASA LWS TRT NNH09CE72C, and NASA Heliophysics GI NNH12CG10C. PAH and DSB received funding from the European Space Agency PRODEX Programme, while DSB and MKG also received funding from the European Union’s Horizon 2020 research and in- novation programme under grant agreement No. 640216 (FLARECAST project). MKG also acknowledges research performed under the A-EFFort project and subsequent service implementation, supported under ESA Contract number 4000111994/14/D/MPR. YY was supported by the National Science Foundation under grants ATM 09-36665, ATM 07-16950, ATM-0745744 and by NASA under grants NNX0-7AH78G, NNXO-8AQ90G. YY owes his deepest gratitude to his advisers Prof. Frank Y. Shih, Prof. Haimin Wang and Prof. Ju Jing for long discussions, for reading previous drafts of his work and providing many valuable comments that improved the presentation and contents of this work. JMA was supported by NSF Career Grant AGS-1255024 and by a NMSU Vice President for Research Interdisciplinary Research Grant.

No / The acceleration of electrons and protons caused by a super-Dreicer electric field directed along the longitudinal component By of the magnetic field is investigated. The three-component magnetic field in a nonneutral current sheet occurring at the top of the reconnecting flaring loops on the charged particle trajectories and energies is considered. Particle trajectories in the reconnecting current sheet (RCS) and their energy spectra at the point of ejection from the RCS are simulated from the motion equation for different sheet thicknesses. A super-Dreicer electric field of the current sheet is found to accelerate particles to coherent energy spectra in a range of 10-100 keV for electrons and 100-400 keV for protons with energy slightly increasing with the sheet thickness. A longitudinal By component was found to define the gyration directions of particles with opposite charges toward the RCS midplane, i.e., the trajectory symmetry. For the ratio By/Bz < 10-6 the trajectories are fully symmetric, which results in particle ejection from an RCS as neutral beams. For the ratio By/Bz > 10-2 the trajectories completely lose their symmetry toward the RCS midplane, leading to the separation of particles with opposite charges into the opposite halves from an RCS midplane and the following ejection into different legs of the reconnecting loops. For the intermediate values of By/Bz the trajectories are partially symmetric toward the midplane, leading to electrons prevailing in one leg and protons in the other.