Magnetic fields are prevalent in a wide variety of low mass stellar systems and play an important role in their evolution. Yet the process through which these fields are generated is not well understood. To understand how such systems can generate strong field structures characterization of these fields is required. Radio emission traces the fields directly and the properties of this emission can be modeled leading to constraints on the field geometry and magnetic parameters. The new Karl Jansky Very Large Array (VLA) provides highly sensitive radio observations. My thesis involves combining VLA observations with the development of magnetospheric emission models in order to characterize the magnetic fields in two fully convective cool star systems: (1) Young Stellar Objects (YSOs); (2) Ultracool dwarf stars. I conducted multi epoch observations of DG Tau, a YSO with a highly active, collimated outflow. The radio emission observed from this source was found to be optically thick thermal emission with no indication of the magnetic activity observed in X-rays. I determined that the outflow is highly collimated very close to the central source, in agreement with jet launching models. Additionally, I constrained the mass loss of the ionized component of the jet and found that close to the central source the majority of mass is lost through this component. Using lower angular resolution observations, I detected shock formations in the extended jet of DG Tau and modeled their evolution with time. Taking full advantage of the upgraded bandwidth on the VLA, I made wideband observations of two UCDs, TVLM513-46 and 2M 0746+20. Combining these observations with previously published and archival VLA observations I was able to fully characterize the spectral and temporal properties of the radio emission. I found that the emission is dominated by a mildly polarized, non-thermal quiescent component with periodic strongly polarized flare emission. The spectral energy distribution and polarization of the quiescent emission is well modeled using gyrosynchrotron emission with a mean field B~100 G, mildly relativistic power-law electrons with a density ne~105-6 cm-3, and source size of R~2R*. We were able to model the pulsed emission by coherent electron cyclotron radiation from a small number of isolated loops of high magnetic field (2-3 kG) with scale heights~1.2-2.7 stellar radii. The loops are well-separated in magnetic longitude, and are not part of a single dipolar magnetosphere. The overall magnetic configuration of both stars appears to confirm recent suggestions that radio over-luminous UCD's have `weak field' non-axisymmetric topologies, but with isolated regions of high magnetic field.

Radio Astronomy, the study of distant objects in the radio spectrum, is set at defiance against all other users of the electromagnetic spectrum. Where traditional use would fill the electromagnetic spectrum with as much transmission and information as possible, Radio Astronomy would rather no man-made signals existed. Since that is not possible, they grumble and have to deal with unwanted transmissions impinging upon their instruments. I have demonstrated that subspace projection can remove these unwanted signals from Radio Astronomical data in post processing. I demonstrate it both on simulated data and on data taken from the Very Large Array radio telescope.In the process of implementing the algorithm, I show that the self power terms contain necessary information about the array element responses to RFI sources. While the autocorrelation are not used in the normal synthesis imaging process, my work shows that RFI mitigation using Subspace Projection performs better with the autocorrelations retained when computing projection matrices. Furthermore, I demonstrate that proper data collection allows a significant decrease in error under subspace projection. Potential enhancements to subspace projection are also briefly considered.

No description available.

The Brigham Young University (BYU) Radio Astronomy Systems group, in collaboration with the National Radio Astronomy Observatory (NRAO), the Center for Astrophysics at West Virginia University (WVU), and the Green Bank Observatory (GBO) have developed, and commissioned, a broadband real-time digital back end processing system for a 38-element phased array feed (PAF) with 150 MHz of instantaneous bandwidth. This system is capable of producing coarse and fine channel correlations, and implements a real-time beamformer that forms 7 simultaneous dual-polarized beams. This thesis outlines the hardware and software development for the digital back end and presents on-telescope commissioning results. This system has been measured to provide an unprecedented low Tsys/η noise level of 28 K and can perform maps of galactic hydrogen observations in a fraction of the time of a conventional single horn feed. The National Radio Astronomy Observatory (NRAO) has recently announced the concept and development of the next generation Very Large Array (ngVLA), a large interferometric array consisting of 300 radio telescopes and longest baseline (distance between a pair of antennas) of 300 km. Large interferometric arrays have been shown to attenuate radio frequency interference (RFI) because it is decorrelated as it propagates across long baselines. This is not always sufficient, especially with dense core array geometries and with the ever-increasing amount of strong RFI sources. Conventional RFI projection-based mitigation techniques have performed poorly on large interferometers because of covariance matrix estimation error due to decorrelation when identifying interference subspace parameters. This thesis presents an algorithm that overcomes the challenge of decorrelation by applying subspace projection via subarray processing (SP-SAP). Each subarray is designed to have a set of elements with high mutual correlation in the interferer for better estimation of subspace parameters. In simulation, compared to the former approach of applying subspace projection on the full array, SP-SAP improves mitigation of the RFI on the order of 9 dB. A signal of interest is shown then to be observable through the RFI in a full synthetic image.

Thesis (PhD)-- Stellenbosch University, 2013. / ENGLISH ABSTRACT: Superconducting electronics can make the Square Kilometre Array (SKA) a better instrument. The largest radio telescope in the world will consist of several arrays, the largest of which, consisting of more than 3000 dishes, will be situated primarily in South Africa. The ambitions of the SKA are grand and their realisation requires technology that does not exist today. Current plans see signals in the band of interest ampli ed, channelised, mixed down and then digitised. An all-digital frontend could simplify receiver structure and improve its performance. Semiconductor (analog-to-digital converters) ADCs continue to make great progress and will likely nd applications in the SKA, but superconductor ADCs bene t from higher clock speeds and quantum accurate quantisation. We propose a superconducting softwarede ned radio frontend. The key component of such a frontend is a superconducting ash ADC. We show that employing such an ADC, even a small- to moderately-sized one, will signi cantly improve the instantaneous bandwidth observable by the SKA, yet retain adequate signal-to-noise ratio so as to achieve a net improvement in sensitivity. This improvement could approach factor 2 when compared to conventional technologies (at least for continuum observations). We analyse key components of such an ADC analytically, numerically and experimentally and conclude that fabrication of such an ADC for SKA purposes is certainly possible and useful. Simultaneously, we address the power requirements of high-performance computing (HPC). HPC on a hitherto unprecedented scale is a necessity for processing the vast raw data output of the SKA. Utilising the ultra-low-energy switching events of superconducting switches (certain Josephson junctions), we develop rst demonstrators of the promising eSFQ logic family, achieving experimentally veri ed shift-registers and deserialisers with sub-aJ/bit energy requirements. We also propose and show by simulation how to expand the applicability of the eSFQ design concept to arbitrary (unclocked) gates. / AFRIKAANSE OPSOMMING: Supergeleier-elektronika kan 'n beter instrument maak van die \Square Kilometre Array" (SKA). Die wêreld se grootse radioteleskoop sal bestaan uit etlike skikkings, waarvan die grootste - met meer as 3 000 skottels - hoofsaaklik in Suid-Afrika gesetel sal wees. Die SKA is ambisieus en vereis tegnologie wat nog nie vandag bestaan nie. Volgens huidige planne sal seine in die band van belang versterk, gekanalisieer, afgemeng en dan versyfer word. 'n Heel-digitale kopstuk sal die ontvangerstruktuur kan vereenvoudig en sy prestasie kan verbeter. Halfgeleier analoog-na-digital omsetters (ADOs) verbeter voortdurend en sal waarskynlik toepassings in die SKA vind, maar supergeleier ADOs trek voordeel uit hoër klok spoed en kwantumakkurate kwantisering. Ons stel 'n supergeleier sagteware-gede nieerde radio kopstuk voor. Die sleutelkomponent van so 'n kopstuk is 'n supergeleier \ ash" ADO. Ons toon hoe die gebruik van so 'n ADO, selfs een van klein tot matige bisgrootte, die oombliklike bandwydte waarneembaar deur die SKA aansienlik sal verbeter en 'n voldoende sein-tot-ruis verhouding sal behou, en gevolglik 'n netto verbetering in sensitiwiteit sal bereik. Hierdie verbetering kan, vergeleke met konvensionele tegnologie, 'n faktor van 2 nader (ten minste vir kontinuum waarnemings). Ons analiseer belangrike komponente van so 'n ADO analities, numeries and eksperimenteel en lei af dat die vervaardiging van so 'n ADO vir SKA doeleindes beide moontlik en nuttig is. Terselfdertyd spreek ons die drywingsverkwisting van Hoë-verrigting rekenaars aan. Sulke rekenaars van 'n tot dusver ongekende skaal is 'n noodsaaklikheid vir die verwerking van die enorme rou data uitset van die SKA. Deur die gebruik van die ultra-lae-energie skakels van supergeleier skakelaars (sekere Josephson-vlakke), ontwikkel ons die eerste demonstratiewe hekke van die veelbelowende eSFQ logiese familie, en toon eksperimenteel bevestigte skuifregisters en deserieëliseerders met sub-aJ/bis energievereistes. Ons stel verder voor en wys met simulasies hoe om die toepaslikheid van die eSFQ ontwerpkonsep na arbitr^ere (ongeklokte) hekke uit te brei.

Thesis (PhD)--Stellenbosch University, 2015. / AFRIKAANSE OPSOMMING : Die vierkantkilometer-radioteleskoop SKA (―Square Kilometre Array‖) sal na verwagting die grootste teleskoop op aarde en die grootste wetenskapprojek in Afrika wees. Hierdie verhandeling konsentreer op hoe die SKA vanaf September 2011 tot Augustus 2012 in die Suid-Afrikaanse media uitgebeeld is. Dit handel oor die leemtes in die literatuur oor wetenskapkommunikasie, veral wat betref wetenskapkommunikasie in Afrikaverband. Deur uit massa- en wetenskapkommunikasieteorie te put, modelleer die studie kommunikasieprosesse en -uitsette met behulp van ‘n konseptuele raamwerk wat op die gedagte van die openbare sfeer berus. Die navorsing word in die besonder onderstut deur onderhoude met sleutelinformante en die ontleding van nuus- en sosiale media. Die wetenskapkommunikasie oor die SKA gedurende hierdie tydperk was stelselmatig gekenmerk deur ‘n hoë vlak van koördinasie tussen die aansporings, strategieë en strukture van alle hoofrolspelers oor die kommunikasiestelsel heen. Al die hoofrolspelers buite die media het die SKA op so ‘n manier in die openbare sfeer probeer uitbeeld dat dit openbare steun sou werf. Primêre hekwagterfunksies is aan die voorste SKA-organisasie- en openbaresektor-rolspelers toegewys, maar was verder afgewentel binne universiteite. Hekwagterfunksies in die media is hoofsaaklik deur nuuswaarde sowel as institusionele kultuur en organisatoriese kenmerke bepaal. Verhoudings tussen wetenskaplikes en joernaliste was oënskynlik oor die algemeen positief en gegrond op vertroue. Die beduidendste bron van inligting vir joernaliste was die SKA self, gevolg deur rolspelers in die openbare sektor. Tog is ander rolspelers, bepaald plaaslike belanghebbendes, gemarginaliseer. Die Afrikaanstalige media het veral onder plaaslike gemeenskappe ‘n belangrike rol gespeel en het meer dikwels as ander oor die SKA berig. Die sosiale media het die SKA merendeels op ‘n soortgelyke wyse as die nuusmedia hanteer, maar die struktuur van kommunikasie-uitsette is beïnvloed deur die kenmerkende eienskappe van sosiale media self, waaronder die virale verspreiding van boodskappe en hoër vlakke van vryewilsoptrede (―agency‖) deur individuele rolspelers. Tog was daar aanduidings van beduidende media-integrasie, in die sin dat groot Suid-Afrikaanse mediahuise die bron was van die meeste boodskappe in sowel die nuus- as sosiale media. / ENGLISH ABSTRACT : The Square Kilometre Array (SKA) radio telescope is set to become the largest telescope on Earth, and also the largest science project in Africa. This dissertation focuses on the manner in which the SKA was represented in the South African media from September 2011 to August 2012. This addresses gaps in the literature on science communication, particularly with respect to science communication in an African context. By drawing on mass communication theory and science communication theory, it models communication processes and outputs using a conceptual framework based on the notion of the public sphere. Empirically, the study is underpinned by key informant interviews and the analysis of news media and social media content. Systemically, the science communication of the SKA during this period was characterized by a high level of alignment of incentives, strategies, and structures across the main actors in the communication system. Main actors outside the media all aimed to position the SKA in the public sphere in a manner that would grow public support. Primary gatekeeping functions were positioned at the apex of the SKA organisation and public sector actors, but were more devolved within universities. Gatekeeping in the media was primarily driven by news value, as well as institutional culture and organisational attributes. Relationships between scientists and journalists were found to be generally positive and trust-based. The most significant source of information for journalists was the SKA itself, followed by public sector actors. However, other actors were marginalized, particularly local stakeholders. The Afrikaans language media played an important role, particularly among local communities, and published about the SKA relatively more often. The social media represented the SKA in a broadly similar manner to the news media, but the structure of communication outputs was influenced by the distinct attributes of the social media, including the viral propagation of messages and higher levels of agency by individual actors. However, there were indications of substantial media integration, in that major South African media corporations were the source of the majority of messages in both the news and social media. The tone of media outputs was largely neutral or positive, framing the SKA as a 'good news story‘. The overarching dominant frame for representing the SKA was the site allocation process, in which South Africa and Australia competed in a bid to host the telescope for the international SKA consortium. Science and technology aspects of the project constituted the second most common framing, within which the search for extra-terrestrial intelligence was the most commonly represented research question. Representations of technological aspects of the SKA were generally shallow, with most articles mentioning only one aspect of the project‘s technology. The SKA was also constructed as a symbol, representing an instance of African scientific and technological achievement, African participation in globalized science, and a refutation of Afro-pessimism. The impact of the SKA on the public imagination, and the public sphere, was thus clearly far wider and deeper than only its science and technology accomplishments and contributions.