text: DSpace Collection: http://hdl.handle.net/2381/396 2015-03-30T23:01:52Z Development of a Fabry-Perot Etalon spectrometer for high-resolution aerosol observations in the oxygen A-band http://hdl.handle.net/2381/31840 Title: Development of a Fabry-Perot Etalon spectrometer for high-resolution aerosol observations in the oxygen A-band Authors: Geddes, Alexander Gordon Abstract: Aerosols are an important factor of the Earth climatic system and they play a key role for air quality and public health. Observations of the Oxygen A-Band at 760 nm can provide information on the vertical distribution of aerosols from passive sensors, that can be of great interest for operational monitoring applications with high spatial coverage if the aerosol information is obtained with sufficient precision, accuracy and vertical resolution. To address this issue, retrieval simulations of the aerosol vertical profile retrieval from Oxygen A-Band observations by GOSAT, the upcoming OCO-2 and Sentinel 5-P mission and the proposed CarbonSat mission have been carried out. These simulations suggest that such observations will be useful in the measurement of uplifted aerosols such as volcanic plumes and forest fires. Retrievals from GOSAT were also undertaken that showed promising results in challenging conditions, warranting further study. The Fabry-Perot Etalon spectrometer provides a compact, robust and affordable alternative to grating or Fourier transform spectrometers. Such an instrument was designed and optimised for the measurement of aerosol in the Oxygen A-Band and its performance simulated in comparison to GOSAT, where it competed well despite its low cost. Initial calibrations of the instrument were undertaken and showed promising results in the instrument operation concept. Following on from this simulations were used to study the performance of groundbased observations showing that using and combining a range of viewing angles would contribute significantly to the vertical information content whereas polarisation proved not to be useful except possibly in extreme circumstances. 2015-03-10T15:09:44Z Metal Cluster Production by Condensation of Metal Vapour in Supersonic Expansions and by Aggregation in Droplets http://hdl.handle.net/2381/31834 Title: Metal Cluster Production by Condensation of Metal Vapour in Supersonic Expansions and by Aggregation in Droplets Authors: Akraiam, Atea I. Bubaker Abstract: Metal clusters have been produced by condensation of metal atoms on or inside small argon clusters and by collision of supersonic atomic argon beams with atomic metal vapours. A key parameter in both processes is the metal atom particle density. For iron, metal particle densities above a thermal open crucible-type evaporator were determined using quartz-micro-balance mass flux measurements, revealing a point-source-like dependence on the distance from the crucible. For silver atoms the particle densities were also determined using scattering from small argon clusters. Formation and soft-deposition of iron nanoparticles was first attempted using a supersonic beam of argon atoms that was blown into a vapour of iron atoms. Transmission electron microscopy showed the presence of iron nanoparticles whose size depended on the deposition time, showing that aggregation takes place after deposition. The deposition rates were of the order of 0.01 nm/s. In the second part of the study argon clusters containing on average 21 atoms were directed through vapours of xenon or silver atoms. Time-of-flight mass spectrometry showed the presence of xenon and silver clusters when the xenon or silver particle densities were increased. The xenon clusters contained up to four atoms whereas silver would only formdimers. The mass spectra also showed argon atoms attached to the xenon clusters, but not to the silver dimers, which was attributed to the high temperature of the silver dimers. 2015-03-10T14:18:48Z X-ray observations of Mercury http://hdl.handle.net/2381/31433 Title: X-ray observations of Mercury Authors: Lindsay, Simon Thomas Abstract: Data from MESSENGER and the GOES solar activity archive are used to deduce properties of Mercury and its X-ray environment in preparation for the forthcoming ESA/JAXA BepiColombo mission to Mercury. Data from MESSENGER's X-ray spectrometer operations at Mercury exhibit background X-ray enhancements which are identified as the results of astrophysical sources crossing the instrument field of view and of electron bombardment of the X-ray spectrometer causing fluorescence within the instrument. These electron enhancements are analysed and mapped, leading to a three-dimensional representation of low-energy electron populations in the Mercury environment, the distributions of which are associated with features of Mercury's magnetosphere, including the sunward magnetopause and magnetic equator. The population is characterised, producing estimates of low-energy electron number densities which are consistent with previous observations. Thus, the X-ray spectrometer functions as a proxy detector for electrons with energies below the threshold of dedicated electron detectors. Variations in seasonal X-ray background flux observed by the MESSENGER X-ray spectrometer are investigated, with a modulation visible with a periodicity of one Mercury year. Several telemetry parameters, including temperature, show corresponding variations. The origin of the variation in background flux is identified as the calculation in spacecraft telemetry relating instrument live time to instrument integration time, which varies on the same yearly period. The GOES solar activity archive is used to generate a model of solar X-ray flux levels, with the intention of providing an estimate of the levels of X-ray flux the BepiColombo mission will encounter, and subsequently the volume and rate of data that the MIXS instrument will produce. This model is used to predict the likelihood of the instrument experiencing data loss events by exceeding the data storage available, and how this likelihood varies with the level of solar flux and the onboard storage space that is allocated to the instrument. 2015-01-16T12:15:28Z Three dimensional simulations of rotating core collapse supernovae http://hdl.handle.net/2381/30693 Title: Three dimensional simulations of rotating core collapse supernovae Authors: Telford, Rosie Abstract: A core collapse supernova is the dramatic death of a massive star by core implosion and subsequent explosion. Massive stars are known to rotate appreciably, yet the vast majority of supernova simulations over the years have not included rotation or its effects.;It is thought that moderate stellar rotation could assist the supernova explosion by lowering the effective gravitational potential in the core. More rapid rotation could give rise to jets and/or bipolar explosions. At the most extreme rotation rates it is thought that gamma ray bursts (GRB) are produced. These bursts may be delayed or revived at late times as a result of the collapsing core becoming rotationally unstable and fragmenting.;In this thesis the effects of rotation on core collapse are studied. Sophisticated progenitor models with rotation rates of up to a significant fraction of Keplerian are used as the starting points for three dimensional simulations. The computational method of Smoothed Particle Hydrodynamics is used to follow the collapse until core "bounce", the point at which the collapse is halted.;It is shown that, before bounce, no instabilities occur even for the most rapid rotators. The maximum value obtained for the ratio of rotational to gravitational binding energy is around 0.13, just below the limit of 0.14 required for instability on a secular timescale. However, the more rapidly rotating models obtain interesting structures as they collapse. In these models the density distribution remains centrally peaked but is surrounded by a torus of centrifugally supported material, consistent with the collapsar model of GRB.;The gravitational wave signals emitted in collapse are also calculated. It is found that these are strongest for the slowly rotating models, in which the collapse is not slowed significantly. A supernova of this type in the Virgo galaxy cluster would be beyond the range of the current generation of gravitational wave detectors. 2014-12-15T10:41:24Z Rapid data classification via Kohonen self-organising maps http://hdl.handle.net/2381/30694 Title: Rapid data classification via Kohonen self-organising maps Authors: Brett, David Roger Abstract: I here present my version of the Kohonen Self-Organising Map (KSOM) as applied to the classification, or rather clustering, of astronomical data. The main body of this work is concerned with the grouping of period-folded stellar lightcurves and clustering based on the lightcurve shape alone. It has been found that the algorithm is an extremely stable grouping mechanism for data of low (3cr signal to noise) to good quality. With further analysis of the results it is possible to locate underpopulated samples of data that exist within the data. This can be successfully achieved for samples of 1%, or less, total population. Additionally the same algorithm has been applied to the extraction of planetary transit lightcurves from those of eclipsing binaries (chapter 5), and to the grouping of X-ray/optical data from the XMM-Subaru deep-field observations (chapter 6). In both cases the algorithm has shown itself to be quite capable of performing such tasks and as such I propose that it could become a very useful astronomical tool. In summary I also present a few ideas for further refinement of the results presented by the KSOM and how these may be used in future study. 2014-12-15T10:41:24Z Brown dwarfs in open clusters http://hdl.handle.net/2381/30691 Title: Brown dwarfs in open clusters Authors: Osborne, Samantha Louise Abstract: This thesis presents the results of the work undertaken during the course of this PhD.;Firstly, the results of a large, deep survey of 5.65 square degrees of the Taurus Dark Clouds are presented. Using a combination of three colour-magnitude diagrams, 165 brown dwarf candidates are identified from RIZ photometry. The presence of deep TiO absorption bands and weak Halpha emission in follow-up far-optical spectra revealed 3 borderline brown dwarfs, and the presence of gravity sensitive H 2O absorption features and weak NaI lines in follow-up infrared spectra confirmed 3 more brown dwarfs.;Secondly, the results of a detailed study of brown dwarfs in the Pleiades are presented. The proper motion for 48 brown dwarf candidates is calculated to determine their membership status, with 37 being confirmed as genuine members. Theoretical models indicate that these objects have masses in the range 0.087-0.031 M⊙ (+/-0.015 M⊙ ). The effects on the IMF of the cluster are then described. In addition, infrared spectra for 12 of the candidates are presented, with all but 1 of these showing the broad H2O absorption features typical of brown dwarfs.;Finally, the results of an extremely large scale survey of the Hyades are presented. Data from the USNO and 2MASS catalogues are analysed in terms of proper motion and photometry in order to compile a complete list of the very low mass and brown dwarf cluster members. Unfortunately, no brown dwarfs are found. The core radius of the cluster is calculated for objects of typical brown dwarf mass, and their escape velocity is found to be greater than that of the cluster. Therefore any substellar objects originally formed within the Hyades would have since been ejected. 2014-12-15T10:41:23Z Ultra low frequency waves in the magentosphere-ionosphere system : a joint space- and ground-based investigation http://hdl.handle.net/2381/30692 Title: Ultra low frequency waves in the magentosphere-ionosphere system : a joint space- and ground-based investigation Authors: Scoffield, Hannah Clare Abstract: This thesis examines the behaviour of large scale magnetohydrodynamic ultra low frequency (ULF) waves in the coupled magnetosphere-ionosphere system. Wave energy from solar wind driven disturbances at the magnetopause, is carried through the magnetosphere by compressional, fast mode waves, which couple to field guided Alfven mode waves on geomagnetic field lines. Field lines with eigenfrequencies corresponding to the driving frequency become resonant. Energy is dissipated in the ionosphere where partial reflection of the Alfven waves takes place, via Joule heating. The general aim of work presented in this thesis is to combine ground based measurements of the large scale structure of individual ULF waves with in-situ measurements of the small scale structure of the electric field, magnetic field and particle precipitation, made by the Fast Auroral SnapshoT (FAST) satellite. With a more specific aim to investigate the small scale structures which give rise to small regions of high current density, leading to parallel electric fields, particle acceleration and aurora. Details of the mechanisms which result in particle acceleration are not fully understood and are of considerable interest at present.;Four field line resonances (FLRs) with conjugate radar, magnetometer and FAST Satellite observations have been studied and compared. In each case a simple FLR model was created and scaled using the wave's spatial and temporal characteristics inferred from SuperDARN radar and ground magnetometer observations. The model field aligned current is compared with field aligned currents derived from the FAST energetic particle spectra and magnetic field measurements. In all four events downward currents appear to be carried, partially by upgoing electrons below the FAST energy detection threshold (5 eV), but also consist of a mixture of hotter downgoing magnetospheric electrons and upgoing ionospheric electrons of energies 30 eV -- 1 keV. In two of the events downgoing magnetospheric electrons with energies of a few keV, which are associated with upward field aligned currents of ~ 1 microA m --2, are observed. Strong intervals of upward current show that small-scale structuring of ~50 km has been imposed on the current carriers, which is thought to be associated with a mode conversion of an ideal magnetohydrodynamic (MHD) Alfven wave to an inertial Alfven wave. 2014-12-15T10:41:23Z An investigation into the accuracy of surface temperature retrievals from the AATSR http://hdl.handle.net/2381/30690 Title: An investigation into the accuracy of surface temperature retrievals from the AATSR Authors: Noyes, Elizabeth J. Abstract: This thesis investigates the accuracy of operational sea surface temperature (SST) and land surface temperature (LST), data from the Advanced Along-Track Scanning Radiometer (AATSR). The study includes some of the first in situ validation results for these data sets, and forms an integral part of the overall AATSR validation programme. In addition, a comprehensive sensitive study of the response of these retrievals to changes in atmospheric and surface conditions is also presented as in aid to interpreting validation results. AATSR SSTs recorded during 2003 have been validated over the Caribbean sea, using in situ observations of SST from the Marine-Atmosphere Emitted Radiance Interferometer (M-AERI). This validation experiment, which extends over a full year, is the most extensive yet performed for any of the ATSR instruments (ATSR-1, ATSR-2 and AATSR) using in situ SSTs derived from radiometric measurements. These data also provide a first opportunity to study seasonal biases using observations of this type. The results demonstrate that clear-sky SSTs obtained using the unique dual-viewing geometry of the AATSR agree with the in situ data to within 0.3 K. Nadir-only retrievals performed using three channels also provide SSTs that approach this accuracy, but the split-window retrievals are found to be warm-biased by ∼0.6 K. When tropospheric dust aerosol is present, the accuracy of the retrieved SSTs is reduced, with the nadir-view SSTs becoming cold biased and the dual-view SSTs, warm biased. For the first time, the potential for validation of LST over heterogeneous land sites is also explored. Two methodologies are presented for upscaling point in situ LSTs to the 1-km spatial scale of the AATSR, together with initial validation results over a field site in Morocco, which suggest that the AATSR LSTs are warm-biased by at least 0.6 K over this site. The findings of this study suggest that validation over heterogeneous sites is possible for situations where the variation of LST is characterised by several in situ point observations of LST.. 2014-12-15T10:41:23Z Measurements of HCFC-22 in the upper troposphere and lower stratosphere from the MIPAS-E instrument http://hdl.handle.net/2381/30689 Title: Measurements of HCFC-22 in the upper troposphere and lower stratosphere from the MIPAS-E instrument Authors: Moore, David Philip Abstract: In this thesis, the potential for retrieval of HCFC-22 volume mixing ratios in the upper troposphere and lower stratosphere (UTLS) from a spaceborne infrared limb-sounding spectrometer is assessed. Hydrochlorofluorocarbon-22 (HCFC-22) is an important trace constituent in the atmosphere through its role as a greenhouse gas and its potential influence on stratospheric ozone chemistry. Current estimates of its vertical distribution are limited to infrequent and sparse balloon and aircraft campaigns, giving poor spatial coverage. Atmospheric limb-emission spectra measured by the Michelson Interferometer for Passive Atmospheric Sounding onboard the ENVISAT satellite (MIPAS-E) have been used to successfully detect and retrieve HCFC-22 volume mixing ratios (vmrs) between 9 and 15 km. Global and seasonal means are presented from 2003. This is achieved using the Optimal Estimation Retrieval Algorithm (OPERA) developed as part of this thesis and designed specifically to invert measurements from MIPAS-E data to trace gas concentrations. The scheme performs a joint retrieval of the target gas and total extinction from clouds or aerosols. High extinctions are observed globally at 9 km. In January 2003, global mean HCFC-22 vmr data at 9 km agreed to within 8% of the measured mean surface value. Mean agreement for all seasons in 2003 was 15%. Indications are that mean HCFC-22 data at 9 km are accurate to better than 10%. Vertical profiles were distinguished between 9 and 15 km. OPERA CFC-12 retrievals were used as a consistency check on the likely accuracy of HCFC-22 data. These data were validated against independent measurements. Agreement with HAGAR measurements in spring 2003 was better than 7%. The results of this thesis demonstrate that HCFC-22 (and CFC-12) can be detected and retrieved from MIPAS-E data in the UTLS based on current concentrations. In addition, they show the potential to obtain other vertical profiles of particle extinction on a global basis throughout the UTLS. 2014-12-15T10:41:22Z Gamma-ray bursts and their environments http://hdl.handle.net/2381/30687 Title: Gamma-ray bursts and their environments Authors: Levan, Andrew J. Abstract: Gamma-ray Bursts (GRBs) are the most energetic explosive events known in the universe and manifest themselves as a brief flash of high energy (X-ray and gamma-ray) photons lasting for a few seconds. Subsequent to the prompt gamma-ray flash, longer lasting afterglow emission is seen at X-ray, optical and radio wavelengths. These afterglows have allowed the precise positioning of GRBs on the sky, redshift measurements and the identification of host galaxies. They have also demonstrated that GRBs with durations of > 2s are relativistically beamed jetted outflows associated with the collapse of massive stars in a type Ic supernova.;In this thesis I present observations of several GRB afterglows and a large survey of GRB host galaxies. The goal of the research is to better understand the connection between GRBs, stellar collapse and star formation. The work presented here demonstrates that GRBs are a diverse population, showing that the supernovae associated with GRBs can span an order of magnitude in absolute luminosity, and are sometimes invisible, even in deep searches.;This work has also located the first good example of a GRB which is heavily optically extinguished due to the presence of dust in its host galaxy, demonstrating that dust extinction can account for a fraction of the "dark bursts", which are invisible at optical wavelengths.;Finally, the connection of GRBs to star formation at large is addressed via a comparison of the properties of the host galaxies of GRBs and core collapse supernovae. I show that GRBs are much more concentrated on their host galaxy light than supernovae and, furthermore, have host galaxies significantly smaller and less luminous that those of SN. Jointly these results indicate that GRBs may originate in low metallicity environments, and come from the most massive stars. This result may have important consequence for the use of GRBs as probes of high redshift star formation. 2014-12-15T10:41:21Z Magnetism of Fe and Co nanostructured materials http://hdl.handle.net/2381/30688 Title: Magnetism of Fe and Co nanostructured materials Authors: Louch, Sharon Abstract: The main purpose of this work was to produce a material with a saturation magnetisation greater than 2.45 T, the highest known value, which occurs in the alloy Fe65Co35.;In situ XMCD measurements carried out on size selected Fe nanoclusters deposited onto HOPG substrates revealed an enhancement of the orbital and spin moments above the bulk values for these systems. The measurements also show that there is a further increase in the moments when the Fe clusters are coated with Co. These experiments took place on beamline ID12B at the ESRF in Grenoble.;Following on from the XMCD results, samples were made consisting of Fe nanoparticles embedded in a Co matrix, and also, of Co nanoparticles embedded in a Fe matrix. The average magnetic moment per atom in these novel nanostructured materials was obtained from magnetisation curves as measured by a VSM. The results are compared to the Slater-Pauling curve for conventional Fe-Co alloys, and show that the magnetic moment per atom in samples containing up to about 20 at.% Fe nanoclusters embedded in a Co matrix (80 at.%) is larger than in the corresponding conventional Fe-Co alloy. There is a sharp drop in the moment above this concentration. Similarly, for samples with up to 20 at.% Co clusters embedded in a Fe matrix the moment per atom is above the Slater-Pauling curve. However, in the case of the latter, the enhancement in the magnetisation is significantly above the peak of the Slater-Pauling curve.;In order to determine the atomic structure of the embedded clusters, EXAFS measurements were carried out on both types of sample described in the previous paragraph. These experiments were performed on beamline 7.1 at the SRS in Daresbury. Fe nanoclusters embedded in Co were found to have the bcc structure, as in bulk Fe. However, simulations of the EXAFS data for Co clusters embedded in Fe show that the Co clusters adopt a bcc structure, as compared to hcp in bulk Co. 2014-12-15T10:41:21Z An XMM-Newton view of the x-ray properties of star-forming galaxies http://hdl.handle.net/2381/30686 Title: An XMM-Newton view of the x-ray properties of star-forming galaxies Authors: Jenkins, Leigh Abstract: This thesis presents X-ray observations from the XMM-Newton X-ray observatory of a sample of nearby star-forming galaxies: M101, NGC 3256, NGC 3310, NGC 7771/1 and NGC 2342/1. The spectral analysis examines the contributions from the various stellar and non-stellar processes occurring in these systems, utilizing complementary high spatial resolution data from the Chandra observatory to assess contributions from compact sources not resolved with XMM-Newton. The work begins with an investigation into the X-ray properties of the discrete source population of the supergiant spiral galaxy M101.;Spectroscopic and timing studies of the brightest source population show that they have properties consistent with accreting stellar-mass X-ray binary systems. Based on X-ray colour classification schemes, timing properties and multi-wavelength counterparts, the complete source population is shown to be dominated by accreting binary systems, with a smaller contribution from thermal supernova remnants. The analyses of the integrated emission from the powerful starburst merger galaxies NGC 3256 and NGC 3310 show contributions from non-thermal emission from populations of X-ray binaries, plus substantial emission from thermal plasmas with temperatures ranging between 0.2-0.9keV, representing supernova-heated gas and starburst driven superwinds. The X-ray/far-infrared (FIR) luminosities and plasma temperatures show correlations with the type of merger event and the age of the starburst component. Finally, two pairs of interacting galaxies are studied. In NGC 7771, discrete sources are resolved at the end of the galactic bar, and the first evidence of a low-luminosity active galactic nucleus (AGN) is detected spectroscopically. The remaining galaxies show similar spectral forms to the more powerful merger systems, and comparisons with other isolated, interacting and merging systems show that all galaxies follow established correlations for starburst galaxies between X-ray, FIR and radio luminosities. 2014-12-15T10:41:20Z A novel approach to ultraviolet auroral imaging using microchannel plate technology http://hdl.handle.net/2381/30685 Title: A novel approach to ultraviolet auroral imaging using microchannel plate technology Authors: Hamilton, Fraser Jackson Abstract: This thesis is an account of the work undertaken in proving that the imaging of the Earth's aurora in far ultraviolet wavelengths using microchannel plate (MCP) optics from a platform in low-Earth-orbit (LEO) is feasible. Using POLAR UVI imager data and particle flux data from the FAST spacecraft, the relationship between electron and proton energy flux and the UV auroral intensity during the substorm of August 26, 1998 is investigated. The findings show that protons are a significant contributor to the electron aurora. The analysis of this data coupled with a review of previous imagers demonstrates the ongoing need for auroral imaging allowing the science case for a LEO imager to be presented alongside baseline goals for the required performance of the MCP optics, to be established. Illumination of an early, prototype optic with a quasi-parallel beam of ultraviolet light determines the suitability of MCP optic technology for auroral imaging from LEO. Successful focussing of UV light using the optic is demonstrated and verified by Monte Carlo simulation. The results of this work demonstrate that MCPs are a suitable technology with which to produce a LEO auroral imager. A conclusion supported by the preliminary analysis of additional MCPs specifically deigned for such an imager. Tests of a slumped microchannel plate detector of small curvature radius, deigned for use with a slumped MCP optic, are described. The use of a slumped detector provides the required curved focal surface without the expense of the more common grinding technique, and the results prove that the technique is viable. A preliminary design study of an MCP-based imager for use in an 800 km LEO is conducted, and its performance compared to baseline goals derived in part from the analysis of POLAR/FAST data with encouraging results. 2014-12-15T10:41:19Z An investigation of fine structure effects in CCDs developed for JET-X http://hdl.handle.net/2381/30684 Title: An investigation of fine structure effects in CCDs developed for JET-X Authors: Keay, Adam. Abstract: Charge Coupled Devices (CCDs) are low power, robust detectors promising medium spectral resolution, high spatial resolution and a high signal to noise ratio. It is for this reason that CCDs have become the preferred focal plane detector in all x-ray astronomy missions due for launch in the 1990s and early in the next millennium. The absorption of x-rays by a CCD is modulated by the transmission of the complex electrode structures and passivation layers on its front surface. Since the path length of soft x-rays (0.l1 - 2 keV) is of the same order as the thickness of these layers, fine structure in the quantum efficiency (Q(E)) around absorption edges can have a profound influence on the response of the CCD. This thesis presents the work done in investigating fine structure effects in the Q(E) of x-ray CCDs and the impact that these effects will have on astronomy missions. The state of x-ray astronomy as we move into the CCD-mission era is reviewed in Chapter 2. the operation of a CCD as an x-ray detector is then described.;The extensive set of experiments carried out at the Daresbury Synchrontron Radiation Source (SRS) to map the Q(E) of JET-X CCDs are detailed, including a description of the first use of the new, low beam current mode of SRS operation. Chapter 5 describes the origins and expression of X-ray Absorption Fine Structure (XAFS) in the CCD response. The instruments that make up the JET-X Focal Plane Cameras are then reviewed. In Chapter 7 the importance of calibrating JET-X accurately is shown by analysing the impact of XAFS in the detector sub-systems. It is shown that XAFS threaten the scientific return from current and future x-ray astronomy missions. In Chapter 8 the information contained by the fine structure in the CCD Q(E) is extracted and analysed to obtain the thickness and structure of the inert layers on the surface of the CCD, and also to improve models of the CCD response. Finally the work is summarised and additional further work is suggested. 2014-12-15T10:41:18Z Multi-instrument magentospheric substorm studies http://hdl.handle.net/2381/30683 Title: Multi-instrument magentospheric substorm studies Authors: Draper, Natalie Caroline Abstract: The work presented in this thesis is intended to improve our understanding of the magnetospheric substorm process and in particular the substorm onset mechanism. Studying the timing of known substorm expansion phase onset signatures from a variety of instruments collecting data in key regions of the magnetosphere, ionosphere and on the ground allowed comparison of the events chosen to substorm onset models. Two substorm case studies are presented for intervals during which complementary data sets were available. The principal data sets came from the Fluxgate magnetometer (FGM), Plasma Electron and Current Experiment (PEACE), Cluster Ion Experiment (CIS) and Research with Adaptive Particle Imaging Detectors (RAPID) instruments on the Cluster spacecraft. It was only possible to determine the onset mechanism for one of the three substorms studied (the near-Earth neutral line model). Rapid thinning of the current sheet was detected prior to the third substorm studied. A tailward-moving travelling compression region was then detected in the magnetotail at substorm onset, and the near-Earth neutral line was located at 15 RE downtail. At onset the Region 1 current system was skewed to the pre-midnight sector, later returning to a more central location about the midnight sector. A new feature has been identified in the magnetotail and termed a magnetic cavity. This was not a singular feature; ten magnetic cavities were identified and four studied in detail. They are features located in the plasma sheet boundary layer and plasma sheet which have close to zero magnetic field and plasma characteristics similar to that of the central plasma sheet. There is evidence that magnetic cavities may be associated with the recovery phase of magnetospheric substorms. 2014-12-15T10:41:18Z Studies of AGN populations with XMM-Newton http://hdl.handle.net/2381/30682 Title: Studies of AGN populations with XMM-Newton Authors: Derry, Pamela Abstract: This thesis presents an X-ray and optical analysis of the Subaru XMM-Newton Deep Survey (SXDS), focusing on the analysis of the combined XMM-Newton and INT-WFC data. The SXDS has an unprecedented combination of depth and area coverage, covering 1.3 square degrees, making this survey the largest ultra-deep survey to date. The vast majority of the ≈ 1000 X-ray sources within the survey are found to be active galactic nuclei (AGN), making this an excellent resources for investigating the distribution of unobscured and obscured sources.;Concentrating on the AGN population, cross-correlation of X-ray, optical and spectroscopic data have enabled investigation into optical and X-ray colours, optical morphologies, redshift distribution and X-ray and optical obscuration. This thesis, utilizing underlying X-ray and optical properties of AGN such as central nucleus luminosity and obscuration also introduces a simple toy model with the aim of understanding the SXDS AGN X-ray-to-optical flux ratio distribution.;XMM-Newton's large field of view and SXDS field depth has also enabled investigations into the X-ray spectral properties of the brightest 130 sources (> 300 total EPIC counts) within the SXDS. These sources are shown to mainly comprise of unobscured and obscured AGN with a range of intrinsic absorptions and best filling photon indices of ? ≈ 1.8 -- 2.0, consistent with other X-ray surveys.;To illustrate the capabilities of XMM-Newton the X-ray source properties of two powerful, high redshift (z = 2.48) narrow line radio-loud type-2 quasars B3 0731+438 and 3C 257, observed separately from the SXDS are also presented.;The combined optical and X-ray analysis presented in this thesis reveals the SXDS AGN population comprise of a large population of unobscured and obscured AGN. These sources have a range of absorption columns, redshifts and luminosities, encompassing low luminosity AGN through to luminous quasars and demonstrate a strong relationship between optical and X-ray obscuration. 2014-12-15T10:41:17Z The infrared remote sensing of peroxyacetyl nitrate in the upper troposphere http://hdl.handle.net/2381/30681 Title: The infrared remote sensing of peroxyacetyl nitrate in the upper troposphere Authors: Allen, Grant Abstract: In this thesis, atmospheric limb-emission spectra measured by two Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) instruments, the MIPAS balloon-borne version 2 (MIPAS-B2) and MIPAS on Envisat (MIPAS-E), have been used to successfully detect PAN in the UT by virtue of its characteristic infrared spectral signature for the first time.;This is achieved using new infrared laboratory reference cross-sections for PAN also obtained in this thesis. This spectral reference dataset represents improved room temperature cross-sections (<4% uncertainty at the 95% confidence level) for PAN in the spectral range 550-2200 cm -1 and the measurement of low temperature (down to 250 K) and nitrogen broadened (up to 300 mb) cross-sections appropriate to the UT for the first time.;Analysis of spectra measured by the MIPAS-B2 instrument over the Mediterranean Sea region in April 1998 and May 1999 has yielded PAN concentrations as high as 350 parts per trillion by volume (pptv) in the UT using an iterative spectral fitting and residual analysis method developed in this thesis.;The detection of PAN was also achieved using this method for spectra measured by the MIPAS-E instrument during April 2003 for the Mediterranean Sea and East China Sea regions. Back trajectories of airmasses observed by MIPAS-E indicate that very high PAN concentrations (up to 668 pptv) detected over the East China Seas may be indicative of pollution outflow from Asia. Limits of detection for PAN in the UT are estimated to be as low as 40 pptv. 2014-12-15T10:41:15Z Characteristics of tropospheric carbon monoxide profiles retrieved from MOPITT measurements http://hdl.handle.net/2381/30680 Title: Characteristics of tropospheric carbon monoxide profiles retrieved from MOPITT measurements Authors: Richards, Nigel Andrew David Abstract: In this thesis, measurements of tropospheric carbon monoxide (CO) obtained by the Measurements of Pollution in the Troposphere (MOPITT) instrument are investigated. The MOPITT instrument and measurement techniques are discussed, and recent results from the MOPITT validation campaign are presented.;MOPITT phase I retrieved CO profile measurements are compared to in situ data obtained during the ACTO campaign. These comparison suggest that MOPITT CO profiles may possess a positive bias throughout the troposphere. The results also demonstrate the need to take into account the effects of vertical resolution and the influence of the a priori on MOPITT retrieved profiles through the use of the retrieval averaging kernels.;Comparisons of MOPITT and TOMCAT model CO profiles are also presented. The results suggest that MOPITT profiles are biased high compared to the model, and that these biases vary greatly from location to location. Simulations conducted using MOPITT operational averaging kernels to investigate the ability of the MOPITT instrument to measure layer enhancements of CO are then discussed. It is shown that MOPITT is sensitive to layer enhancements, although it is not always possible to determine the altitude of such enhancements. The seasonal cycle in MOPITT CO profile data is derived and compared to that of six CMDL surface measuring sites. A method for detecting long range transport events with anomalous CO using a regional analysis, which achieves limited success, is presented. Finally, an investigation of the combined use of the differing sensitivity of MOPITT between day and night in order to gain extra vertical information from MOPITT data is discussed. This is the first such study of its kind and the results are encouraging. Analysis shows that by examining the difference between day and night MOPITT retrieved 'surface' data it is possible, in a number of regions and seasons, to obtain further information on the vertical structure of CO. The results are validated using the TOMCAT model to represent the 'real' atmosphere. Examinations of day-night differences in phase I MOPITT data suggest that it is possible to use these differences to identify CO source regions such as biomass burning and industrialised regions. 2014-12-15T10:41:15Z Magnetosphere-ionosphere coupling currents in Jupiter's middle magnetosphere http://hdl.handle.net/2381/30679 Title: Magnetosphere-ionosphere coupling currents in Jupiter's middle magnetosphere Authors: Nichols, Jonathan D. Abstract: This thesis is a theoretical study of the magnetosphere-ionosphere coupling current system in Jupiter's middle magnetosphere associated with the breakdown of corotation of iogenic plasma and the jovian main auroral oval. The study initially investigates the effects of the ionospheric Pedersen conductivity and the iogenic plasma mass outflow rate. Wide ranges of values of these parameters are inputted to the model and the results are compared for dipole and current sheet field models. It is shown that previous results, obtained using 'reasonable' spot values, are generally valid over wide ranges of the parameters. The study then investigates the effect of precipitation-induced enhancements of the Pedersen conductivity. Previous models have assumed constant conductivity, whereas it is expected to be significantly elevated by strong field-aligned currents. A model of the dependence of the Pedersen conductivity on the field-aligned current is developed and incorporated into the model. The findings help reconcile the theoretical results with observation. Specifically, the plasma is maintained closer to rigid corotation out to much further distances than theory previously predicted, the equatorial radial current exhibits a sharp rise in the inner region of the middle magnetosphere and plateaus off thereafter, in line with Galileo magnetic field data, and the field-aligned current is concentrated in a peak in the inner region, which is to where the main oval is usually mapped. Finally, the study investigates the effect of self-consistently including magnetosphere-ionosphere decoupling due to field-aligned voltages. The results show that for typical jovian conditions the effect is very small, such that the modification to the system parameters is generally two orders of magnitude below the values of the parameters themselves. The conclusion is that the assumption made in previous work, that the effect of the field-aligned voltages is small, is generally valid. 2014-12-15T10:41:14Z Accretion discs around magnetic stars http://hdl.handle.net/2381/30678 Title: Accretion discs around magnetic stars Authors: Matthews, Owen Martin. Abstract: Accretion discs are commonplace around magnetic stars. They can be found in newly formed stars and in interacting binaries. The effects of the stellar magnetic field on these discs are manifold. For example it may act as a 'seed field' for the initiation of the magneto-rotational instability and thus drive viscosity. However this thesis explores the more direct effect of the stellar magnetic field on the global structure of circumstellar accretion discs, and how this structure may in turn influence the time dependent behaviour of the disc. This work is done through a combination of analytic and numerical techniques, by modelling a torque which represents the effect of the magnetic field.;Some young stellar objects exhibit disc outbursts, known as FU Orionis events, which are separated by recurrence times much longer than would be expected from the standard disc instability model. However, it is shown here that a moderate stellar magnetic field may truncate the accretion disc, and so stabilise the inner accretion disc against outbursts. This forces outbursts to begin in the outer disc where higher trigger densities must be reached, and so extends the recurrence time. This result is of great interest since planet formation may be strongly influenced by the structure of the protoplanetary disc.;The dwarf nova WZ Sagittae is unusual in that it also has very long recurrence times. In addition, a series of short echos have been observed to follow the main outburst. It has therefore been suggested that this object is surrounded by an accretion disc which is magnetically truncated in a similar manner to those around the FU Orionis stars. It is shown that such a truncated disc may, when placed within the binary Roche potential, exhibit echo outbursts. 2014-12-15T10:41:14Z X-ray scattering studies of self-assembled nanostructures http://hdl.handle.net/2381/30677 Title: X-ray scattering studies of self-assembled nanostructures Authors: Everard, Mark John Abstract: The structure and growth of self-assembled nanoparticle networks and epitaxial rare-earth thin-films have been studied using X-ray scattering.;A diffusive growth model was developed to model the specular reflected X-ray signal monitored at the anti-phase position, during the growth of two rare-earth (RE) metals, gadolinium and samarium onto molybdenum(110) single crystals. The model identifies atomic layer spacings and the degree of interlayer mass transport. Both RE elements are shown to grow in a layerwise manner but with significant roughness after the initial layer is occupied. The RE growth mode was modified by raised substrate temperatures. The presence of pre-deposition oxygen at the surface was found to encourage layer-by-layer growth for both Gd and Sm.;The structure of noble-metal nanoparticles passivated with thiolate organic ligands was studied using small and wide angle X-ray scattering (SAXS/WAXS). Ag nanoparticles were found to consist of a spherical metal core with fcc atomic packing. The passivating shell was modified to induce direct cross-linking between nanoparticles. The structure and development of the nanoparticle aggregates formed due to the interactions between functionalised thiol derivatives of porphyrin, benzene, C5 dithiol and MUA was monitored with SAXS. In all cases the structures were found to be open and fractal, with the size of the cross-link determined by the size of the functionalised ligand.;The structural quality of self-assembled noble-metal nanoparticle superlattices was investigated using GISAXS. Au nanoparticle networks were formed at the air/water interface. Fits to the GISAXS data, revealed that strain build-up in the layer can directly control the interparticle spacing. Larger Au nanoparticles were allowed to self-assembly onto a Si(111) substrate. The influence of the substrate temperature prior to assembly was investigated and revealed a striking phase transition below 16.7°C from disordered nanoparticle networks to highly ordered layered nanoparticle structures. It is thought that solvent volatility plays a crucial role in the ordering process. 2014-12-15T10:41:13Z Investigations of the Ionospheric Alfvén Resonator at auroral latitudes http://hdl.handle.net/2381/30676 Title: Investigations of the Ionospheric Alfvén Resonator at auroral latitudes Authors: Cash, Sophie Rose. Abstract: In this thesis both ground-based and space-based data are utilized to study the effects of the Ionospheric. Alfven Resonator (IAR) at auroral latitudes. Central to the investigation is a modulated heating experiment at Tromso, Norway on 8th October 1998 which constituted the first satellite detection of artificially excited ULF waves. Alfven waves were detected in electric field data collected by the FAST satellite. The downward electron flux data contains the first observations of electrons undergoing acceleration within the IAR due to parallel electric fields associated with the artificially stimulated Alfven waves. The time history and spectral content of the downward electron fluxes is investigated, and found to be consistent with the existence of a localized region of oscillating parallel electric fields at an altitude of ∼3400 km. This caused the periodic acceleration of suprathermal electrons towards the ionosphere. EISCAT UFT electron and ion temperature and density profiles were used in conjunction with neutral density profiles from the MSIS E-90 model and tristatic velocity measurements to determine the magnitude of the current source that launched the Alfven waves. This was used as an input into a simple 1-D model of the IAR first developed by Traktengerts et al. [2000] to investigate the range of conditions required for successful satellite-wave detection.;To support the IAR interpretation, natural resonance features of the IAR observed in pulsation magnetometer data from the high latitude station of Sodankyla are investigated during the month of October 1998. A new technique is developed to determine the diurnal evolution in frequency of IAR resonance features for 10 time intervals in October 1998. EISCAT UHF electron density profiles are used to determine the reflection coefficient at the lower IAR boundary, Rlower, which is compared to the magnetometer analysis results. The investigations in this thesis shed new light on the nature of the IAR and future field-line tagging experiments. 2014-12-15T10:41:13Z Diffuse and discrete sources of galactic x-ray emission http://hdl.handle.net/2381/30675 Title: Diffuse and discrete sources of galactic x-ray emission Authors: Hands, Alex. Abstract: Our Galaxy is a luminous X-ray source by virtue of its discrete X-ray source populations and various large-scale diffuse processes which produce the ultra-hot interstellar medium. This thesis presents an analysis of the properties of a variety of Galactic X-ray emitting components, based on data from the EPIC CCD cameras onboard ESA's XMM-NEWTON satellite observatory. In total 51 individual observations are analysed for which the combined exposure time is nearly 900 ks. X-ray spectra measured over a wide range in Galactic longitude and latitude are used to disentangle the different components of the diffuse soft X-ray background (SXRB). The SXRB is comprised primarily of a two-temperature Galactic halo, an unabsorbed local hot bubble (LHB) and, in certain directions, an extended plasma 'superbubble', all with temperatures in the range of 1-3 million K. Observations towards the Galactic Plane provide evidence that the LHB may be larger in extent than has previously been assumed, with some of the emission lying behind an absorption wall with a column density of NH ~ 3 x 1020 cm-2. The diffuse hard X-ray phenomenon known as the Galactic ridge is investigated at various locations both on and near to the Galactic Plane. The spectrum of the ridge at high energies is well described by a thermal plasma model with a temperature of 108.0 K. The exact nature of this emission is uncertain but several possibilities are discussed in this thesis. The surface brightness of the ridge is measured as ~ 10-10 erg s -1 cm-2 deg-2 (2-10 keV), of which ~ 15% is due to point sources and a further ~2% has been resolved and identified as extended supernova remnants and HII regions. In the region of XMM-NEWTON's X-ray Galactic Plane Survey (XGPS), a total of 424 discrete X-ray sources have been detected. Cumulative log N - log S distributions, including data from other satellites, illustrate how the dominating source population changes from Galactic X-ray binaries at high fluxes to extragalactic objects at fainter fluxes. However, an additional lower luminosity population, possibly associated with cataclysmic variables and RS CVns, appears to contribute significantly at intermediate X-ray fluxes. 2014-12-15T10:41:12Z Radiation damage in charge coupled devices http://hdl.handle.net/2381/30674 Title: Radiation damage in charge coupled devices Authors: Smith, David Ryan. Abstract: This thesis is concerned with the effects of radiation damage in CCDs used for space applications. The manufacturing process and operational principles of CCDs are presented in Chapter 2. The components of the space radiation environment, the two radiation damage mechanisms relevant to CCDs, and the effects of radiation on the operational characteristics of CCDs are described in Chapter 3. Chapter 4 presents a study to assess the suitability of novel L3Vision technology to applications in space. Two L3Vision CCDs were subjected to proton irradiations representative of doses expected to be received by spacecraft in low Earth orbit. Post-irradiation the devices were found to operate as expected, the effects of radiation on the operational characteristics of the devices being comparable to previous studies. The effect of low energy protons on CCDs is the subject of Chapter 5. The study was initiated in response to the finding that soft protons could be focused by the mirror modules of the XMM-Newton spacecraft onto the EPIC CCD detectors. Two EPIC devices were irradiated with protons of a few keV to find that soft protons cause more damage than that expected by the NIEL damage relationship, as they deposit most of their energy within the CCD. The observed change in CTI of the EPIC devices on XMM-Newton is however comparable to the pre-launch prediction, and the component attributable to low energy protons is small, <20%. Chapter 6 presents a study of a specific radiation induced phenomenon, `Random Telegraph Signals'. Development of analysis software and the irradiation of two CCDs are discussed before a detailed characterisation of the generated RTS pixels is presented. The study shows that the mechanism behind RTS involves a bi-stable defect linked with the E-centre, in combination with the high field regions of a CCD pixel. 2014-12-15T10:41:12Z On the nature and role of absorption and continuum reprocessing in the X-ray spectra of obscured Seyfert galaxies http://hdl.handle.net/2381/30672 Title: On the nature and role of absorption and continuum reprocessing in the X-ray spectra of obscured Seyfert galaxies Authors: Schurch, Nicholas Abstract: This thesis presents an investigation of the X-ray properties of four nearby galaxies, NGC 4151, NGC 7582, Mrk 3 and NGC 4945, all of which harbour a bright, heavily obscured Seyfert nucleus. It explores the implications of the results from these sources for heavily obscured active galactic nuclei (AGN) in general. The work focuses largely on data from the XMM-Newton X-ray satellite, concentrating initially on hard X-ray spectra from the EPIC CCD cameras. The analysis emphasises the complex nature of the X- ray absorption in NGC 4151 and highlights the massive impact that absorption has on the observed X-ray spectrum and variability of all four sources. The range of column densities exhibited (1023 cm-2 to >1024 cm-2) encompasses both Compton-thin, continuum-dominated sources (such as NGC 4151) and Compton-thick, reflection-dominated sources (e.g. Mrk 3 and NGC 4945). The often-subtle role that Compton reflection plays in the X-ray spectrum and variability of these sources is discussed in de tail. The source spectra, in each case, contain an intrinsically narrow, neutral, iron Ka line emission and a sharp iron K edge, adding to the growing body of evidence which suggests that iron reprocessing features are ubiquitous constituents of the hard X-ray spectra of AGN. The analysis of NGC 4945 demonstrates the enormous impact that a nuclear starburst can have on the X-ray properties of AGN. A preliminary analysis of the soft X-ray spectrum of NGC 4151, revealed by the high spectral resolution XMM-Newton RGS instruments, shows the spectrum to be completely dominated by line emission and radiative recombination continua from hydrogen-like and helium-like ions. It is argued that the soft emission originates in photoionized and photoexcited gas located in X-ray/optical ionization cones. The observational findings presented in this thesis are all generally consistent with the unified AGN picture, in which the major differences between the sources are produced by different orientations with respect to our line-of-sight. 2014-12-15T10:41:11Z A new multiple scattering technique for x-ray photoelectron diffraction http://hdl.handle.net/2381/30673 Title: A new multiple scattering technique for x-ray photoelectron diffraction Authors: Hart, John Francis. Abstract: This thesis is concerned with the development and application of a new method of performing multiple scattering calculations of X-ray Photoelectron Diffraction (XPD) from periodic surfaces. The technique is based on the dynamical theory of Reflection High Energy Electron Diffraction (RHEED), allowing more efficient access to high photoelectron energies (1000eV and above) than is available using present calculational schemes. Convergence tests have been performed to identify the limitations of the present method.;The method is applied to two overlayer systems for which XPD experimental data is available, c(2 x 2)S and c(2 x 2)O on Ni(001). For the c(2 x 2)S system, which is relatively well understood and therefore a good test system, data at four emission (take-off) angles is fitted, giving optimum agreement with the S atoms occupying hollow sites at a vertical adsorption height of between 1.30A and 1.35A. The agreement is improved slightly for the lowest two take-off angles by an outward relaxation in the top Ni-Ni layer spacing. Comparison is made with single scattering calculations for the system and the present method shown to be far superior at the lowest take-off angle. This clearly demonstrates the importance of multiple scattering at grazing emission angles.;For the c(2 x 2)O system, data at two take-off angles is fitted giving optimum agreement with the O atoms in hollow sites at an adsorption height of 0.75A. By allowing a small outward relaxation in the top Ni-Ni layer spacing, an almost equally good fit to the data is found at an adsorption height of 0.80A. An offcentre bonding site which has been proposed, in which the O atoms are displaced from their hollow sites by 0.3A in the [110] direction has been considered and ruled out. 2014-12-15T10:41:11Z XMM-Newton observations of Type-1 active galactic nuclei http://hdl.handle.net/2381/30671 Title: XMM-Newton observations of Type-1 active galactic nuclei Authors: Page, Kim Abstract: The X-ray spectra of active galaxies are, to a first approximation, power-law-like. Upon more detailed investigation, however, features become apparent: in particular, an emission line corresponding to iron, around 6.4 keV, and the so-called 'soft-excess' below ~ 2 keV, whereby an extrapolation of a power-law fit to the 2-10 keV energy band under-predicts the observed counts. In this thesis, the soft X-ray excesses in samples of both low- and high-luminosity Active Galactic Nuclei (Seyfert galaxies and QSOs respectively) are analysed. This allows the parameters of the soft excess, such as strength and temperature, to be compared with X-ray luminosity. It is found, however, that there are very few differences between Seyfert galaxies and QSOs, leading to the idea that the soft excess is not strongly linked to the luminosity of the object. Time variability of the soft excess is studied, by investigating three objects (3C 273, PKS 0558-504 and 1H 0419-577) which have each been observed a number of times in the X-ray band. The soft excess is found to vary in each, with the changes possibly due to the Compton cooling of the corona above the accretion disc, brought on by an increase in accretion rate. Another sample, of high-redshift objects, is also discussed, finding that there is no evidence for spectral evolution or intrinsic absorption in these radio-quiet QSOs. When considering this sample as a whole, there is an indication of spectral flattening at higher energies, suggesting the presence of a Compton reflection hump. Finally, the existence and strength of the neutral, narrow iron line is investigated, finding that, in the more luminous objects, the equivalent width is much lower, this is the X-ray equivalent of the Baldwin effect found in the optical and UV emission lines. 2014-12-15T10:41:10Z UV and optical studies of the physical properties of DAO white dwarfs http://hdl.handle.net/2381/30670 Title: UV and optical studies of the physical properties of DAO white dwarfs Authors: Good, Simon A. Abstract: An investigation into the physical properties and evolutionary status of DAO white dwarfs has been conducted, using optical and far-UV observations. Out of the 22 objects for which optical data were obtained, the spectrum of only one is best fitted by a stratified H+He composition model; this may be an object transferring between the He- and H-rich cooling sequences.;Homogeneous models are preferred for the remainder, but with 90% confidence in less than half the objects. The DAOs fall into two groups: The low Teff, high log g objects have temperatures and gravities that are similar to the majority of the DAs and mostly consist of DAO+dM binaries. The high T eff, low log g objects, which make up approximately two thirds of the sample, have relatively high luminosities for white dwarfs, and the most likely mechanism that is countering the gravitational settling for these objects is mass loss. When this ceases these objects will become low-mass DAs.;A search for evidence of binarity in the far-UV FUSE observations of 22 objects yielded positive results for only 5, suggesting that the majority of DAOs are descended from isolated stars. As seen in the DAs, the Teff derived from Balmer and Lyman lines for objects hotter than ∼50,000 K were found to be different, with the latter yielding the higher temperature. For 7 objects, the Lyman lines are so shallow that a model with Teff > 120,000 K is required to reproduce them. An investigation of the strength of photospheric absorption lines showed that the low log g objects have high heavy element abundances compared to DAs, whereas the DAO+dM binaries have slightly elevated lighter element abundances, with the heavier elements abundances similar to those seen in the DAs. This is qualitatively consistent with mass loss and accretion from a companions wind respectively, as the explanation for the helium that is observed in the spectra of the stars. 2014-12-15T10:41:10Z X-ray properties of cataclysmic variable stars http://hdl.handle.net/2381/30669 Title: X-ray properties of cataclysmic variable stars Authors: Baskill, Darren Stuart Abstract: In this thesis, I study the entire sample of non-magnetic cataclysmic variables observed with the Japanese satellite ASCA, presenting a detailed analysis of the spectral and temporal behaviour of these twenty-nine targets. The spectral analysis indicates that all the targets in the ASCA sample appear to be X-ray under-luminous, with only three possible exceptions. This indicates that energy is being lost from the accretion disk in a non-radiative way. Since a third of the observations require additional absorption above that expected from interstellar alone, both the X-ray under-luminosity and the excess absorption observed spectrally may be attributed to the existence of accretion disk-winds. The spectral analysis also indicates that the under-lying spectra of all non-magnetic cataclysmic variables may be more complicated than at first thought, since those spectra containing the highest number of counts require more sophisticated multi-temperature modelling of the X-ray source. Further spectra results suggests that there may be some previously unidentified magnetic systems in the ASCA sample. Such systems give away their identity through having a much harder spectrum than the non-magnetic systems, and both LS Peg and V426 Oph should be considered as weakly magnetic candidates. The temporal analysis reveals other unusual members. SS Cyg appears to be unusually faint during an observation made during quiescence, and a four-fold difference is observed in VW Hyi during two optically quiescent states, suggesting that the inner disk behaves almost independently of the outer disk. ASCA observations caught Z Cam during both an optical outburst, and during the transition to another outburst. The unique transition observation shows the X-ray count rate falling by a factor of three as the source becomes optically thick. High levels of absorption are detected in the X-ray spectra throughout both the outburst and transitional observations, greater than that expected from interstellar absorption alone. 2014-12-15T10:41:09Z Wave-particle interactions in the terrestrial magnetosphere http://hdl.handle.net/2381/30668 Title: Wave-particle interactions in the terrestrial magnetosphere Authors: Baddeley, Lisa Abstract: This thesis examines small scale poloidal mode magnetohydrodynamic waves which have an energy generation mechanism internal to the magnetosphere in the form of unstable particle populations. The energy from these particles is fed into a resonant wave mode and, ultimately, dissipated in the terrestrial ionosphere. By analysing Ion Distribution Functions (IDFs) the statistical occurrence of the driving magnetospheric particle populations is presented. Results indicate that the dominant driving particle populations are those of ~10 - 40 keV protons. The free energy of the particles has been quantified, revealing the dominance of the lower energy particle populations. The majority of unstable low energy protons contain > 1010 J of free energy in comparison to the higher energy protons which contain < 109 J. Using this method, one event, using conjugate ionospheric - magnetospheric data is examined and compared to a similar conjugate event of a rare subset of small scale waves called Giant Pulsations (Pgs). The available free energy is compared to the energy dissipated into the conjugate ionospheres. Estimates of the energy at the source and sink reveal that ~ 1010 J is transferred for the first event. Pgs are shown to transfer ten times this energy. The statistical study reveals that 1010 J is frequently available from unstable IDFs but 1011 J is not, thus providing an exploration for both the rarity of Pgs and the ubiquity of other small scale waves. Observations also suggest that drawn sector waves are driven by the drift-bounce resonance mechanism, while in the dark sector waves are driven by both drift and drift-bounce resonance mechanisms. Additionally, ionospheric observations indicate that the occurrence of small scale pulsations could be more abundant than previously thought. This implies that the quantity of energy being transported round the magnetospheric cavity and into the ionosphere via wave-particle interactions has previously been underestimated. 2014-12-15T10:41:09Z The atomic structure of fluoride superlattices and antimonide surfaces http://hdl.handle.net/2381/30667 Title: The atomic structure of fluoride superlattices and antimonide surfaces Authors: Alcock, Simon Graham Abstract: The atomic structure of the CaF2-Si(111) interface and the repeated unit cell of MnF2-CaF2-Si(111) superlattices have been determined using surface x-ray diffraction. Specular reflectivity and non-specular rod scans were measured for a number of superlattice samples. All samples had a type-B interface. Ultra-thin layers of MnF2, below a critical thickness of 5 monolayers, adopted the fluorite structure of CaF2. These layers are likely to have different magnetic properties to bulk anti-ferromagnetic MnF2, possibly leading to novel magnetic applications. We have developed software to interface with the structure factor calculation software ROD to calculate the semi-kinematical reflectivity and non-specular rods of a superlattice.;Ion bombardment and annealing were used to produce atomically clean, well ordered InSb(001)-c(4x4) and GaSb(001)-c(2x6) surface reconstructions from antimony capped and uncapped samples. Auger electron spectroscopy showed that a 1000A antimony capping layer was sufficient to prevent atmospheric oxidation of the substrate. A substantial re-ordering of the cap was observed at 180°C. Desorption of the cap was achieved by annealing the sample at 300°C under an Sb overpressure.;X-ray reflectivity measurements were used to determine the out-of-plane structure of the GaSbc(2x6) reconstruction. Theoretical fits indicate that both the top and the second layer of antimony dimers lie close to their bulk positions. In-plane x-ray measurements were used to verify the symmetry of the reconstruction.;The InSb anti-phase x-ray position was monitored during the growth of Tl on the InSb-c(4x4) reconstruction. Fits to the data showed that the growth mode is Stranski-Krastanow at room temperature, and revealed details about the formation of the initial wetting layer. A specular reflectivity ridge scan after deposition showed reflections produced by Tl and Tl 2O. In-plane x-ray diffraction peaks can be attributed to either Tl, TlSb, or Tl7Sb2 and indicate a preferential alignment of Tl. 2014-12-15T10:41:08Z Things that go bump in the night : the dynamics & hydrodynamics of stellar clusters http://hdl.handle.net/2381/30666 Title: Things that go bump in the night : the dynamics & hydrodynamics of stellar clusters Authors: Adams, Timothy James Abstract: I examine some of the dynamical interactions which occur in stellar clusters. I examine how a population of brown dwarfs will evolve within a cluster, in an attempt to explain the discrepancy between observational results and theoretical models regarding the number of brown dwarfs in a stellar cluster. I find that it is unlikely that the entire brown dwarf population could be expelled from the cluster in less than many dynamical relaxation times. Instead I propose that it is likely that the majority of the brown dwarfs are instead in binary systems with brighter companions which hide them from view. I have also modeled the formation of blue stragglers through the collision of two main-sequence stars. I propose that the angular momentum problem that other authors have found with blue stragglers formed via off-axis collisions can be resolved by considering the angular momentum loss associated with a stellar wind or locking to a disc. I show that both can quickly reduce the angular momentum of the merged object to the point where it can contract down to the main-sequence. I have examined the possibility of explaining the apparent paucity of red giants in the cores of post-core-collapse globular clusters through collisions with binary systems and with single stars. I show that such collisions often result in a bound system, where the red giant envelope acts as a common envelope and is eventually dispersed. I also show that such collisions can remove up to 17% of the red giant population. 2014-12-15T10:41:06Z XMM and ROSAT observations of clusters of galaxies http://hdl.handle.net/2381/30665 Title: XMM and ROSAT observations of clusters of galaxies Authors: Spurgeon, Louise Elizabeth Abstract: I present an investigation into the X-ray properties of galaxy clusters, consisting of a principal component analysis of ROSAT data and studies of three clusters observed with the XMM-Newton satellite. The principal component analysis provides an investigation into similarities between cluster surface brightness profiles. Initial results for 42 clusters are presented and compared to existing models. The cluster profiles are reproduced to good accuracy using three principal components. Correlation of the principal components to physical properties is investigated but the results are inconclusive. Observations of Abell 1413, Abell 665 and Abell 2163 made with XMM-Newton were investigated spectrally and spatially to determine cluster properties. Global temperatures were found to be 7.08 +/- 0.140.13 keV for Abell 665 and 11.98 +/- 1.2 keV for Abell 2163. The temperature and abundance profiles of the clusters are very different, with some differences attributed to the recent merging of subclusters in Abell 665 and Abell 2163. Spatial analysis of the cluster surface brightness profiles is undertaken with beta and NFW models. The profiles produced for the three clusters are used to estimate the variation of gas, gravitational and iron mass with radius. These are used to draw general conclusions, such supernovae numbers (NSNII 109--11). The cosmological density and matter density parameters are found; &OHgr;0 = 0.22 -- 0.33 +/- 0.1 and &OHgr; m ? 0.12+/-0.060.04. This suggests a low density universe, but is subject to uncertainty due to extrapolation to larger radii. 2014-12-15T10:41:06Z Studies of nightside spectral with behaviour from coherent high frequency radars http://hdl.handle.net/2381/30664 Title: Studies of nightside spectral with behaviour from coherent high frequency radars Authors: Woodfield, Emma Elizabeth. Abstract: The work presented in this thesis is aimed to improving our understanding of the HF Doppler spectral width parameter and how it can be used to identify ionospheric and magnetospheric boundaries, specifically the use of the frequently observed gradient between high (>200 m s-1) and low (<200 m s-1) spectral width. Locating the boundaries between regions is important to the study of how the magnetosphere responds to the solar wind and interplanetary magnetic field conditions. Three case studies and a statistical investigation are presented herein which investigate the nature of these regions. Data from the Co-operative U.K. Twin Located Auroral Sounding System (CUTLASS) HF radars and the European Incoherent SCATter (EISCAT) radars from the primary observations. The cases presented show that high spectral width can be observed both on open and closed magnetic field lines, and that there is a relationship between elevated electron temperature and high spectral width (although the reverse is not true) which appears to be restricted to the post 0300 MLT nightside region. The data also show that high spectral widths can be associated with both single- and multiple-peak HF spectra. The statistical study compares three years of data from the CUTLASS radars and the Syowa East radar (part of the Super Dual Auroral Radar Network, SuperDARN), magnetically conjugate to Iceland East. It is found that the mean spectral width is dependent on latitude, magnetic local time (MLT) and season. The data from the two hemispheres show similar dependence on these factors, although the Syowa East spectral widths are larger in general (instrumental variations are discounted). These results suggest that the physical mechanism(s) creating the high spectral widths must work both on open and closed field lines, be dependent upon latitude and MLT, and be less prevalent (or attenuated) in summer months. 2014-12-15T10:41:05Z Composition and morphology of substorm-associated ion injections http://hdl.handle.net/2381/30662 Title: Composition and morphology of substorm-associated ion injections Authors: Hall, Adrian Mark. Abstract: This thesis describes an investigation into the composition and morphology of substorm ion injections at magnetic latitudes and L-shells away from geostationary orbit. Furthermore, the work investigates the Dst-dependent conditions that affect these compositional and spatial characteristics. Count rate and composition measurements in the energy range 80 - 400 keV/q form the basis of the analysis. Near-equatorial observations by the CRRES spacecraft during 1991 have been used to study substorm and near-Earth magnetosphere composition, injection event distribution, and the injection region morphology. Measurements made by the Polar satellite during 1996 extend the study to high magnetic latitudes (50 - 60 °) and high L-shells (7 ≤ L ≤ 22), and are used to investigate the radial morphology of the injection region.;For the first time, substorm injection rate as a function of composition, Dst, and injection type has been quantified. In storm time the rate more than doubles. This appears to result from the observed increase in the azimuthal extent of the average injection region. Analysis has shown however, that substorm onset and type (single or multiple injection events) are (for the most part) not significantly composition dependent. The threefold increase in ionospheric-rich event frequency during storm time is principally a result of the increase seen in the equatorial, dusk flank event occurrence.;Ion injections have been observed over a large radial extent. Furthermore, ions drift coherently over a large range of L-shells, spanning, at times, from geostationary orbit to L ≈ 18. The inward transport of these ions is not composition dependent. The injected plasma depends on the magnetospheric composition tailward of the spacecraft. This is highly spatially variable and dependent on the past, and prevailing Dst conditions with large (fourfold) enhancements in ionospheric origin material observed inside of L ≈ 7 ( i.e. below the altitude of most substorm onsets) during storm time. The storm time nightside ring current is observed to expand both earthward and tail ward. Out of the equatorial plane, the injection rate is hypothesised to artificially decrease as a result of the tapered, crescent shaped form of the injection region. 2014-12-15T10:41:04Z Outburst behaviour of accretion discs in close binary systems http://hdl.handle.net/2381/30663 Title: Outburst behaviour of accretion discs in close binary systems Authors: Truss, Michael Robert. Abstract: I present the first two- and three-dimensional simulations of accretion disc-driven outbursts in cataclysmic variables and X-ray transients. A Smoothed Particle Hydrodynamics scheme is used to model the accretion disc, which is subject to the full tidal potential of a binary system. The scheme, which includes a time-varying viscosity, is applied to cataclysmic variables and X-ray transients with differing mass ratios and orbital periods.;Simulations of the dwarf nova SS Cygni, with mass ratio q = 0.6, produce coherent outbursts and a wide range of other observationally testable results. When applied to the tidally unstable system Z Chamaeleontis (q = 0.15), the scheme reproduces superoutbursts and superhumps. The tidal and thermal instabilities are shown to be entirely decoupled: a superoutburst is simply a normal outburst in the presence of a 3:1 eccentric inner Lindblad resonance.;The model is refined for X-ray transients to include a simple treatment of irradiation of the disc by the X-rays generated at the primary. In short periods systems with correspondingly small discs, if q is small tidal effects can influence the outburst behaviour. If a small region at the outer edge of the disc remains shielded from the X-rays, tidal forces can drive this gas inwards through the disc, producing the rebrightening that has been observed in several X-ray light curves of soft X-ray transient outbursts.;At longer periods, the disc cannot ever entirely be heated by the X-rays and more complex outburst behaviour is possible. Simulations show that this behaviour is determined by competition between the variable size of the irradiated region and the thermal instability operating in the cool region. 2014-12-15T10:41:04Z Radar observations of convection in the nightside high-latitude ionosphere http://hdl.handle.net/2381/30659 Title: Radar observations of convection in the nightside high-latitude ionosphere Authors: Grocott, Adrian. Abstract: The work discussed in this thesis concerns the ionospheric flow response to different magnetospheric phenomena which occur under various interplanetary and geomagnetic conditions. This was undertaken primarily using the SuperDARN HF radar network located in the auroral regions of both hemispheres, but also utilised magnetometer and spacecraft data from a variety of sources. A detailed case study is first presented concerning the nature of convection in the nightside ionospheric during an isolated substorm. The results of this study unequivocally demonstrate that twin-vortex flow can be excited in association with substorm expansion, a phenomenon which has previously been the subject of some controversy. Two further case studies are presented of the ionospheric flows which exist during intervals of northward, but (negative) By-dominated IMF. Under these conditions it is found that the substorm cycle dies away, but that Dungey-cycle flow persists for many hours. During these intervals, strong 'bursts' of flow were observed in the nightside ionosphere, which are found to be associated with episodic reconnection in the distant magnetic tail. This study has shed new light on the magnetospheric response to an IMF of intermediate clock angles, revealing that the Dungey-cycle and the substorm cycle are not, in fact, synonymous. Finally, a discussion of ongoing and possible future work is given, which is suggested by the results of the studies described above, with initial results from a number of pilot studies being presented. The flows during a number of substorm events have been studied, which reveal that the excitation of flow appears to accompany substorm expansions in general. Also, a study of the flows during another interval of northward IMF now dominated by positive By provides an appropriate counterpart to the negative By case. This preliminary investigation reveals flows of a similar nature but with the expected opposite sense of By-asymmetry. 2014-12-15T10:41:03Z A new determination of the charged pion mass an dmuon neutrino mass upper limit from exotic atom x-ray spectroscopy http://hdl.handle.net/2381/30661 Title: A new determination of the charged pion mass an dmuon neutrino mass upper limit from exotic atom x-ray spectroscopy Authors: Nelms, Nicholas Abstract: This thesis describes an experiment that has made a new determination of the charged pion mass and from which a new upper limit of the muon neutrino mass has been calculated. The experimental approach uses a high-intensity negative pion beam injected into a cyclotron trap and stopped inside a nitrogen/oxygen mix gas-filled target cell where highly excited pionic and muonic atoms are formed. The energy of photons emitted during de-excitation is directly proportional to the reduced mass of the system, from which the mass of the orbiting particle can be determined. X-ray spectra from pionic nitrogen and muonic oxygen are measured using a high-resolution Bragg crystal spectrometer arranged in Johann geometry. A new large area detector comprising six, high quantum efficiency charge coupled devices is positioned at the focus to measure the reflections from the spectrometer. By using muonic oxygen X-rays as calibration for the pionic nitrogen line the negative pion mass has been determined as 139.57176 0.000259 MeV/c2, a precision of 1.85 ppm. Although more precise, this value is approximately 11 ppm higher than the current world average. Consequently, the muon neutrino mass upper limit has been calculated as 0.33 MeV/c2 (90% confidence level) which offers no new information over the current value of 0.17 MeV/c2. Investigations are underway to determine the validity of the 11 ppm discrepancy. 2014-12-15T10:41:03Z Magnetism and magnetotransport in Fe and Co nanocluster assemblies http://hdl.handle.net/2381/30660 Title: Magnetism and magnetotransport in Fe and Co nanocluster assemblies Authors: Maher, Matthew John. Abstract: Vibrating sample magnetometry showed that dilute assemblies of unfiltered Fe and Co nanoclusters well above their blocking temperature are superparamagnetic and the magnetisation curves follow the Langevin function with a tight size distribution around a mean of 2.2 nm. Magnetisation curves of blocked non-interacting particles showed a small coercivity and a remanence value consistent with a uniaxial anisotropy in the particles. At low temperature, the films assume an asperromagnetic state and fits to this model indicated an enhanced anisotropy constant by a factor of ten over the bulk.;Cluster assembled films with packing densities up to 30 at % concentration are described by the Langevin function with an increased mean particle size. As the packing density is increased further, the behaviour deviates from the super-paramagnetic model towards a correlated spin glass. At low temperatures the anisotropy energy of the particles is greater than the exchange energy and the sample is asperromagnetic.;In pure Fe cluster assembled films (~500 A) thick capped by Ag a transition, as the temperature rises from 2 to 300K, from an asperromagnetic state to a correlated spin glass occurs. Uncapped Fe cluster films displayed only correlated spin glass behaviour even at 2K.;Giant magnetoresistance has been observed in the cluster assembled systems Fex-Ag1-x, FexCu1-x and CoxAg1-x and investigated for varying cluster packing densities. The results were compared to models formulated for granular alloys and were found to be in good agreement for < 16% cluster films. As the cluster interactions are increased, the peak of the GMR, at H~0, was found to be quenched due to the magnetic correlations among the particles.;Co cluster films created with Ag or SiO capping layers displayed a ferromagnetic - antiferromagnetic interaction caused by partial oxidation of the clusters. Magnetometry of an uncapped Co cluster film held in UHV has shown spin glass behaviour at 2K as found for Fe cluster films. 2014-12-15T10:41:03Z Coronal emission from late-type stars : a ROSAT and XMM view http://hdl.handle.net/2381/30656 Title: Coronal emission from late-type stars : a ROSAT and XMM view Authors: Briggs, Kevin. Abstract: In Chapter I, I introduce the framework of observational and theoretical evidence that has been constructed to understand the vast range of X-ray characteristics exhibited by late-type stars on the basis of the magnetic dynamo that is thought to power the Sun's own modest X-ray activity, and describe the spectral analysis techniques and X-ray instruments used in this work. In Chapter 2 I present a catalogue-based classification of X-ray sources detected in the ROSAT PSPC Galactic Plane Survey in the Third Quadrant (3QGPS) and use the results to infer a denser spatial distribution of young, X-ray active stars in the local Galaxy than has been traditionally assumed. In Chapter 3 I exploit the high coronal activity of young stars to search for unrecognised members of the Hyades-age open clusters NGC 6633 and IC 4756 in deep ROSAT HRI images, and make a novel comparison of the X-ray activity levels of these clusters with that of the Hyades. In Chapter 4 I explore the X-ray emission from very low mass stars in an XMM-Newton EPIC pointing in the Pleides, finding marginal support for a decline in X-ray activity for stars with spectral types later than " M4 that may be a signature of the changeover to a non-solar-like dynamo in fully-convective stars. I also undertake spectral and variability studies of the X-ray-bright members in the field, and present evidence that the detection of early-type stars in the cluster are fully consistent with emission from late-type companions. In Chapter 5 I perform a time-resolved spectroscopic study of the nearby active K0 star AB Dor using the XMM-Newton EPIC pn, with comparison to a study using the higher-resolution XMM-Newton RGS, investigating the temperature structure and elemental adundance changes during the course of a moderate flare. 2014-12-15T10:41:02Z Radar studies of natural and artificial waves and instabilities in the auroral ionosphere http://hdl.handle.net/2381/30657 Title: Radar studies of natural and artificial waves and instabilities in the auroral ionosphere Authors: Dhillon, Ranvir Singh. Abstract: The electron Pedersen conductivity instability (EPCI) was proposed by Dimant and Sudan (1995a, 1995b, 1995c) as an extension to accepted Farley-Buneman instability theory and was predicted to give rise to D-region radar echoes. Theoretical modelling of its growth rate and threshold velocity for varying flow and aspect angles is presented, together with evidence for a two-layer structure where the growth rate is maximised. Backscatter parameters obtained by two coherent radar systems, CUTLASS and STARE, are presented for two case studies where the data are consistent with an electrojet flow whose velocity exceeds the threshold value for EPCI excitation.;Backscatter parameters obtained from artificially generated FAIs for spatial sweeping and preconditioning experiments are also presented. Varying the illumination time of part of the heated patch affects the CUTLASS backscatter power corresponding to that patch. The variation in backscatter over CUTLASS range gates, for a heater beam with varying pointing direction, is shown to agree closely with the expected results obtained by modelling the heater beam intensity. It is shown that the CUTLASS backscatter power, for a given heater power, is dependent upon whether the ionosphere has previously been excited at a higher heater power.;A new longer-lag mode was run on CUTLASS for the October 1999 heating campaign and the ACF decorrelation and backscatter power decay time constants obtained from data collected when this mode was running were different by an order of magnitude. Turbulence characteristics were obtained from the artificial irregularity distribution. These are compared to a study preferred by Villain et al. (1996) and the results are consistent with an artificial irregularity distribution that remained correlated for longer times than naturally occurring irregularity distributions. 2014-12-15T10:41:02Z Nonlinear prediction and analysis of solar-geophysical disturbances within the ionosphere http://hdl.handle.net/2381/30658 Title: Nonlinear prediction and analysis of solar-geophysical disturbances within the ionosphere Authors: Francis, Nicholas. Abstract: This thesis presents novel and robust nonlinear techniques that can be used to analyse and predict geophysical time series. The techniques presented can cope with the problems of noise and non-continuity that are typical of solar-terrestrial data sets. Existing nonlinear analysis techniques are prone to given spurious indications of nonlinear behaviour in such circumstances. These new techniques can be applied as either an alternative to established techniques, to determine less fragile and more qualitative nonlinear properties that may be presented, or as a precursor - to determine whether or not it is viable to proceed with the application of standard nonlinear analysis methods. The methods in this thesis have a general applicability that extends to any time series prediction problem, and are not necessarily limited to geophysical applications. Furthermore, these techniques have been adapted to form the basis of a real-time ionospheric forecasting demonstrator. 2014-12-15T10:41:02Z X-ray diffraction of some technologically important surfaces http://hdl.handle.net/2381/30655 Title: X-ray diffraction of some technologically important surfaces Authors: Walker, Christopher John. Abstract: Surface X-ray diffraction (SXRD) has been used to study the effect of pressure and temperature on the Ni(110)-(2x1)-2CO reconstruction and the effect of temperature on the InSb(001)-c(8x2) reconstruction. The Ni(110)-(2x1)-2CO reconstruction was observed at a carbon monoxide pressure of 2.3 bar and has a structure unchanged from that at 1 x 10 -5 mbar. This structure was found to consist of CO molecules near short bridge sites in agreement with other recent studies in high vacuum conditions. Heating at 1 x 10-5 mbar causes a lifting of the (2x1) reconstruction due to desorption of the CO. In contrast, heating in 2.3 bar of CO leads to a massive rearrangement of the surface and the formation of {lcub}111{rcub} microfacets. The base of the microfacets is ≈100 A across and their surfaces are disrupted from the bulk terminated (111) surface. This is consistent with an enlargement of the lattice due to the dissolution of carbon in the facets.;SXRD and LEED experiments on the c(8x2) reconstruction of InSb (001) show no structural changes on cooling from room temperature to 74K. Key features of the reconstruction are determined which are at variance with the recent model of Jones et al.. 2014-12-15T10:41:01Z Transition-edge sensors for X-ray astronomy http://hdl.handle.net/2381/30653 Title: Transition-edge sensors for X-ray astronomy Authors: Trowell, Stephen P. Abstract: This thesis considers the development of a cryogenic Transition Edge Sensor (TES) microcalorimeter for X-ray astronomy. An optimum configuration for this application has yet to be established and some of the design criteria and fabrication methods are discussed on Chapter 5. Results from investigations into different TES material combinations are presented in Chapter 6. Degradation due to interdiffusion between materials has an impact on the reliability of TES detectors and analysis of Al/Ag, Ti/Au, Cu/Nb and Ir/Nb systems has formed a large part of our recent work. Such comparisons show the relative stability of iridium-based detectors. The superconducting properties of thin film iridium samples were investigated in the context of detector development. The measured superconducting-to-normal transitions are presented in Chapter 7. Changes in transition temperature from 157 mK to 52 mK were observed. A model based on the formation of thin iridium silicide layers during film deposition is proposed to describe this effect. Chapter 8 contains results from testing of a Ti/Au single pixel TES device. The capability of our digital signal processing system was demonstrated with the acquisition of X-ray spectra with photopeak energy resolutions of 25 eV FWHM at 6 keV, close to the detector limit. Imaging spectrometers are required for astronomy applications and our extended bismuth absorber with distributed read-out concept is described in Chapter 9. Photon propagation through bismuth at low temperatures (~100 mK) has been modelled using a Monte Carlo approach, and the results of the simulations indicate that large square bismuth absorbers (of side length ~5 mm) with corner TES read-out can achieve spatial resolutions better than 250 m. This is adequate to satisfy the current specification for the narrow-field imaging spectrometer instrument of ESA's proposed XEUS mission. 2014-12-15T10:41:00Z Properties of soft X-ray bright Active Galactic Nuclei http://hdl.handle.net/2381/30654 Title: Properties of soft X-ray bright Active Galactic Nuclei Authors: Vaughan, Simon. Abstract: Studies of the extremes of behaviour observed in Active Galactic Nuclei (AGN) constrain the physics of their emission processes by pushing existing models to their limits. Narrow-Line Seyfert 1 (NLS1s) represent one extreme of the AGN phenomenon, and soft X-ray selection is an efficient way to find NLS1s. This thesis examines the X-ray and optical properties of soft X-ray bright AGN and NLS1s in particular. The X-ray spectral properties, as observed by ASCA , of a sample of 22 NLS1s are discussed. The X-ray continua show a wide range in slope, with a mean only slightly steeper than that of 'normal' Seyfert Is and most show 'soft excess' emission, often containing a significant fraction of the X-ray luminosity. In addition, some NLS1s show spectral features in the range 0.7-0.9 keV and 1.1-1.4 keV, which are described in terms of absorption in photoionised material along the line-of-sight. The X-ray properties of the bright NLS1 Ark 564 are examined in detail and the X-ray spectrum is found to be consistent with a model comprising a power-law plus 'reflection' from ionised matter. A re-analysis of the ROSAT Wide Field Camera all-sky survey data is presented. A total of 19 AGN are detected in the extreme-ultraviolet, including eight NLS1s, making this the first complete sample to contain a high fraction of NLS1s. This work is extended using a larger, complete sample of AGN selected on the basis of 0.25 keV flux. Correlations between the measured X-ray and optical properties are discussed and the sample is used to isolate objects at the extreme ends of the parameter space explored. The luminosity function of the sample is presented, along with the luminosity functions of Seyferts classified on the basis of H/3 width and X-ray spectral slope. 2014-12-15T10:41:00Z Hyades, Pleides and Taurus Brown Dwarfs http://hdl.handle.net/2381/30650 Title: Hyades, Pleides and Taurus Brown Dwarfs Authors: Kenyon, Fraser Abstract: This thesis describes the search for brown dwarfs in Hyades and Pleiades open clusters and in the Taurus star forming region. Initial surveys were carried out with deep CCD observations. Infrared follow up work was then used confirm membership status. A first chapter provides a brief introduction to the fundamental properties of brown dwarfs and highlights the reasons why it is important to study them. The second chapter gives a current review of the theory of brown dwarfs and describes recent searches for brown dwarfs and their results. Chapter 3 then describes the basic principles of photometry, reviews the instrumentation in use and describes the data reduction techniques necessary for analysis. The results of the three projects carried out throughout the course of this Ph.D. are presented in chapters 4, 5 and 6. Chapter 4 presents a deep, large area, optical IZ survey of the Hyades in which no candidate brown dwarf members remain after follow up infrared JHK photometry, optical spectra and proper motion measurements. Chapter 5 presents the results of a deep CCD IZ survey of the Pleiades with follow up infrared K photometry implemented to confirm cluster membership. Luminosity and mass functions for the cluster are then derived with the mass function found to be slightly rising across the stellar/sub-stellar boundary as has been evident in recent work (Bouvier et al. 1998, Martin et al. 1998, Hodgkin & Jameson 2000). Chapter 6 presents the very preliminary results of a partial area reduction of a deep CCD RIZ survey of the Taurus star forming region with candidate member finder charts provided in Appendix A. Some 88 isolated brown dwarf candidates are found with masses down to 7Mjupiter. Further work will be needed to verify the status of these objects. Chapter 7 summarises the important conclusions from chapters 4, 5 and 6 and also give suggestions for future work. 2014-12-15T10:40:59Z Ionospheric signatures of dayside reconnection processes http://hdl.handle.net/2381/30652 Title: Ionospheric signatures of dayside reconnection processes Authors: McWilliams, Kathryn Anne. Abstract: This thesis presents a study of the ionospheric response to dayside magnetopause reconnection. The principal data set employed is the ionospheric convection velocities from the CUTLASS HF radars. In addition to this, images of the ultraviolet aurora from the VIS Earth Camera and the Far Ultraviolet Imager aboard the Polar spacecraft were examined. In situ measurements have shown that there is a time-dependent, periodic nature to magnetic reconnection, with time scales of the order of minutes; the transient nature of magnetopause reconnection is reflected in the ionospheres. In this thesis the plasma velocity fluctuations in the dayside ionosphere were found to be in full agreement with the repetition rates of bursts of reconnection at the magnetopause, as well as with the repetition rates of polarised moving visible auroral forms seen from Earth. The first-ever full vector measurements of ionospheric convection velocity within the footprint of the reconnecting flux tubes revealed that, despite being transient, magnetic reconnection was a large scale process during which layers of magnetic flux were successively peeled from the magnetopause. Directly measured particle precipitation revealed that particles originating the reconnection region was found to be present on the same magnetic field lines as the HF radar signature of the reconnected field lines. The multi-instrument study of the ionospheric responses to reconnection demonstrated that the measured ionospheric convection velocities are inextricably linked to measured ultraviolet aurora. Energetic particle precipitation from the magnetosphere into the atmosphere via field-aligned currents excites auroral emissions, and field-aligned current estimates from measured ionospheric convection velocities were found to be in excellent agreement with the aurora. 2014-12-15T10:40:59Z Auroral ion upflows in the F-region and topside ionosphere http://hdl.handle.net/2381/30651 Title: Auroral ion upflows in the F-region and topside ionosphere Authors: Foster, Craig. Abstract: Ion flow along geomagnetic field lines is an important mechanism in the coupling of the magnetosphere-ionosphere system. Over the last 25 years or so, a wealth of satellite and radar observations has revealed the ionosphere as a steady and significant source of magnetospheric plasma. In the auroral ionosphere, field-aligned O+ upflows are often detected with fluxes exceeding those observed at higher altitudes, indicating that the ionosphere actively supplies ions to the magnetosphere. A number of mechanisms, principally associated with disturbed ionospheric conditions, have been proposed to account for ion upflows. This thesis documents a study of ion upflows observed in the auroral F-region and topside ionosphere. The work is based almost exclusively on incoherent scatter measurements from the EISCAT facility.;More than 12 years' UHF Common Programme observations were employed in a study of the variation of upflow characteristics as a function of time, altitude, season and solar and geomagnetic activity. The study was extended in altitude with the inclusion of 5 years' vertical measurement from the EISCAT VHF radar. A quantitative comparison between upflow occurrence and plasma heating events was also performed. In addition, a case study is presented, examining three intervals of transient ion upflow detected during a run of the UK EISCAT Special Programme UFIS. The three events highlight some of the energization processes that can occur at different altitudes to generate field-aligned flow. Finally, a more detailed numerical investigation examined a day-side upflow observed by EISCAT in March, 1992. The event was modelled employing a simple 1-D approximation to the ion momentum-balance equation. The model explored the relative effects of plasma pressure, gravity and neutral wind on the field-parallel ion velocity; it was found that knowledge of the ion composition was necessary for accurate modelling of the observations. 2014-12-15T10:40:59Z Observations and multiwavelength modelling of active galactic nuclei and starburst galaxies http://hdl.handle.net/2381/30648 Title: Observations and multiwavelength modelling of active galactic nuclei and starburst galaxies Authors: Burston, Amanda. Abstract: This thesis contains observations and multiwavelength modelling of Active Galactic Nuclei (AGN) and Starburst (SB) galaxies. The drive behind this work is the identification of the powering mechanism of high luminosity galaxies, where obscuration due to gas and dust hide the central regions from direct view. I present near infrared (NIR) observations of Ultraluminous Infrared galaxies (ULIRGs) and X-ray luminous starburst galaxies (XLSBs). Observations in the NIR are particularly useful in the case of dust obscuration since much less attenuation is found at these wavelengths than in the optical (by a factor of around 10). ULIRGs are defined as galaxies with extremely high infrared luminosities, attributed variously to a hidden AGN, SB region, or combination of both. We examine the dominant power source in the sample observed, finding evidence for both star formation and active nuclei. XLSBs are a group of objects, with X-ray luminosities much higher than those of normal galaxies, thought to originate only from star formation. We investigate the properties of the SB regions and rule out hidden AGN as potential power sources, providing more evidence for the existence of this group. In addition to NIR based work, I detail the building and application of a computer model which aims to deconvolve multiwavelength spectral energy distributions (SEDs), encompassing radio to X-ray frequencies, in order to quantify the relative strength of the principal components, and hence better understand the physical processes involved. Emission components used include synchrotron emission, thermal emission at various temperatures from dust, stellar emission, recombination emission and accretion disk emission (when relevant). Preliminary results indicating that reasonable fits can be achieved, highlighting distinctions between stellar and non-stellar activity. 2014-12-15T10:40:58Z Large-scale current systems in the Jovian magnetosphere http://hdl.handle.net/2381/30647 Title: Large-scale current systems in the Jovian magnetosphere Authors: Bunce, Emma J. Abstract: The studies contained within this thesis focus on the large-scale azimuthal and radial current systems of Jupiter's middle magnetosphere, i.e. currents with radial ranges of 20-50 RJ. In the first study using magnetometer data from Pioneer-10 and -11, Voyager-1 and -2, and Ulysses, it is discovered that the azimuthal current in the middle magnetosphere is not axi-symmetric as had been assumed for the last twenty-five years, but that it is stronger on the nightside than on the dayside at a given radial distance. A simple empirical model is formulated, which reasonably describes the data in the domain of interest both in radial distance and local time, and allows direct calculation of the current divergence associated with the asymmetry. In a similar way, in the following chapter the radial currents have been computed for the dawn sector of the jovian magnetosphere along various fly-by trajectories. Combination of these radial current estimations with the azimuthal current model allows the total divergence of the equatorial current to be calculated. These current densities mapped to the ionosphere are surprisingly large at ~1A m-2. In order to carry the current, the magnetosphere electrons must be strongly accelerated along the field lines into the ionosphere by voltages of the order of 100 kV. The resulting energy flux is enough to produce deep, bright (Mega Rayleigh) aurora and thus provides the first natural explanation of the main jovian auroral oval. In the final study, newly-available data from the Galileo orbiter mission are combined with the fly-by data in order to compare them to the model derived in the first study. The model is then re-derived for the entire data set, which significantly improves the associated fractional errors. 2014-12-15T10:40:58Z