Department of Physics (Fredericton)

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A cluster multi-spacecraft study of Earth's bow shock
A cluster multi-spacecraft study of Earth's bow shock
The location, shape and motion of Earth’s bow shock are investigated using observations based on measurements made by the Cluster spacecraft quartet. Several bow shock crossings have been identified and carefully characterized according to relevant plasma parameters; a collection of 133 shocks has been selected and analysed using a timing method. The shock crossings cover orbits in which the spacecraft separation is of the order of ∼ 600 km or less. When present, the magnetic field fluctuations are suppressed using the conventional low-pass filtering technique prior to implementing timing method. The results of this investigation are compared with both Gas Dynamics and Magnetohydrodynamics (MHD) bow shock models.We have found, on a statistical basis, that the shock standoff position derived from the timing method agrees well with the Gas Dynamics predictions for high Mach-number cases only. We have also found that for half the crossings, the timing and the conic-based shock normals agree within an 11 degree-angle. Our results strongly indicate that the motion of the shock is predominantly along the Sun-Earth direction; a departure from this direction is not related to the shock-crossing location. Shock velocities below ∼ 80 km/s satisfactorily follow a nearly Gaussian distribution with zero mean and a standard deviation of ∼ 42 km/s. We show that high speed motions are correlated with sharp increases in the solar wind upstream ram pressure, and are consistent with gas dynamics model predictions., Electronic Only (UNB thesis number) Thesis 9504 (OCoLC)958462579, by Thamer Yousef Saeed Alrefay, Ph.D. University of New Brunswick, Graduate Academic Unit of Physics, 2014.
An empirical model of ionospheric scintillation at high latitudes
An empirical model of ionospheric scintillation at high latitudes
by Hichem Mezaoui, Trans-ionospheric radio signals experience both amplitude and phase variations as they propagate through a turbulent ionosphere, this phenomenon is known as scintillation. As a result of these fluctuations, GPS receivers lose track of signals and consequently induce positioning and navigational errors. Therefore, there is a need to study scintillation and their causes in order to not only resolve the navigational problem but in addition develop analytical and numerical radio propagation models. This thesis presents the work that has been done to develop an empirical model of ionospheric scintillation at high latitudes. In this study, GPS L1 signals were recorded and characterized using the Canadian High Arctic Ionospheric Network (CHAIN).We developed new indices to quantify scintillation and the chaoticity of the turbulent ionosphere. More particularly, we used the multi-fractal aspect of the scintillating GPS signal to compute the corresponding wavelet-based entropy and fractal dimension. These indices were used to construct scintillation maps in the geomagnetic domain. It has been found that the chaoticity of the scintillating signal exhibits a dependence on geomagnetic conditions and a seasonal cycle, suggesting the possibility to quantify the ionospheric turbulence using the proposed indices. In the second part of the thesis, a simulator of the trans-ionospheric channel was developed. The model takes into account the case of strong scintillation, where the amplitude fluctuations start to build up inside the ionospheric slab. The features of the power spectra of the observed scintillation events were reproduced: it has been found that the amplitude fluctuations are characterized by a power spectral density that obeys a power law with a break down at the Fresnel scale. The phase, on the other hand, does not exhibit a breakdown of the power law, which is in agreement with the observations.
Analysing neural firing with the Fitz-Hugh Nagumo model
Analysing neural firing with the Fitz-Hugh Nagumo model
by Ahmed Yahia Khiari, Alan Lloyd Hodgkin and Andrew Huxlley's pioneering work in the early 1950s revolutionized the fields of biophysics and neuroscience. Earning them a Nobel Prize in Medicine Physiology in 1963, their Hodgkin-Huxley (H-H) model was the first model offering an apt mathematical and quantitative description of neural action potential propagation. Its physiological relevance notwithstanding, what stands as the arguably most profound result of their work is the unveiling of the innate dynamical nature of neurons. Since then, numerous efforts have gone into simplifying the H-H model and creating novel dynamical models of neural firing. One of the most prominent of such models is the 2D FitzHugh-Nagumo (FHN) model. Given its simplicity, the FHN model fails to simulate a rich set of neural firing behaviour. This thesis aims to extend on the FHN model by using it as a basis for a 3D model capable of simulating a richer set of neural behaviours. To that end, tools of dynamical systems analysis, —such as linear stability and bifurcation analysis— are used to study the dynamical workings of the 3D model, and how they pertain to the different neural behaviours exhibited.
Cosmological perturbation theory in a matter-time gauge
Cosmological perturbation theory in a matter-time gauge
by Mustafa Saeed, This work examines cosmological perturbations in a Hamiltonian framework with a matter-time gauge. Einstein's field equations are written in a matter-time gauge. The perturbed three-metric of cosmology, its conjugate momentum and the shift are substituted in these equations. The equations of motion of the perturbations to linear order are derived. These equations are expanded in terms of spatial Fourier modes and are then decomposed into scalar, vector and tensor components. After fixing gauges and solving constraints we find that the scalar mode is ultralocal and that the vector modes vanish. We also see that the traceless transverse tensor modes give the known propagation equation for gravitational waves in an expanding, spatially at, homogeneous and isotropic background.
Data sampling and reconstruction strategies for rock core magnetic resonance imaging
Data sampling and reconstruction strategies for rock core magnetic resonance imaging
by Dan Xiao, Magnetic resonance imaging (MRI) is uniquely well suited for studies of sedimentary rocks as it allows direct non-invasive detection of fluid content and fluid interactions in the pore space. Pure phase encoding MRI methods have proven to be robust in their ability to generate quantitative images in porous media. However, the sensitivity is low for pure phase encoding, especially with low magnetic field MRI systems that are common for porous media studies. Novel sampling strategies and data reconstruction methods, described in this thesis, improve measurement sensitivity with no hardware modifications required. Pure phase encode MRI methods acquire a single k-space data point with each radio frequency (RF) excitation. Reducing the number of acquired data points will significantly increase the measurement sensitivity. The goal is to look for data sampling and image reconstruction methods that ensure good image quality with reduced data. These methods are based on the inherent sparsity of MRI data, either in k-space or in transformed image spaces. Sample geometry based restricted sampling exploits k-space redundancy, with simple and reliable linear image reconstruction. The sampling patterns that collect regions of high intensity signal while neglecting low intensity regions can be naturally applied to a wide variety of pure phase encoding measurements. An important application is T 2 mapping spin-echo single point imaging (SES PI) that reveals different bedding plane structures within the rock core plug sample. In compressed sensing, spatial or spatiotemporal correlations of the static and dynamic MR images are exploited by transforming the images to sparse representations. Incoherent sampling and non-linear reconstruction are required. Imaging speed can also be improved by more efficient data collection. This can be achieved by combining phase and frequency encodings. A novel k-space trajectory, with rapid and accurate linear image reconstruction, is employed for high quality quantitative density images. In this thesis, new MRI data sampling and image reconstruction methods, for application to porous media, have been developed. These methods significantly improve the measurement sensitivity of quantitative MR imaging.
Determination and analysis of the sub-second fluctuations in the ionospheric electron density using the GPS
Determination and analysis of the sub-second fluctuations in the ionospheric electron density using the GPS
by Anthony Mark McCaffrey, Structures in the high latitude ionospheric density are responsible for variations in the phase and amplitude of trans-ionospheric signals. These include space-based navigation systems like the Global Positioning System (GPS). The ionosphere is dispersive, and the refractive index is dependent on the electron density. Thus, variations in the ionosphere will cause changes in the refractive index of the medium. This will lead directly to fluctuations in the phase of the signal, as observed at ground-based receivers. If the spatial scale of these changes in the electron density is sufficiently small, diffraction can occur. This causes rapid variations in the phase and amplitude of the signal. The study of these refractive and diffractive variations (scintillation) has proven vital in monitoring and studying the ionospheric medium. It is known that the diffractive variations in the signal’s amplitude and phase are observed in the high-frequency regime; in the typical modern analysis, the refractive variations are assumed to be of low frequency and thus removed by high pass filtering the received signal. This assumption is based on longstanding low latitude measurements. In the high latitude, these assumptions must be reevaluated because of the increase in ionospheric drift speeds observed in the region. Without careful evaluation, the high-frequency refractive variations can be wrongly classified as diffraction. On the other hand, in studying high-frequency variations in the electron density, diffractive artifacts may appear as refractive. This study verifies the existence of diffractive-like high-frequency refractive variations in the high latitude and outlines methods to determine whether the high-frequency variations observed in the GPS carrier phase observable are purely refractive. These methods rely on recent advances in the GPS satellite and receivers; the advances and their impact on ionospheric monitoring are discussed. The outlined methods of distinguishing these refractive variations are used to study the high-frequency changes in the ionosphere electron density. Electron precipitation is shown to be a likely cause of the small-scale structures which induce the high-frequency refractive variations in the GPS carrier phase.
Development of an automated technique to pursue a climatology of atmospheric gravity waves above Eureka, Nunavut
Development of an automated technique to pursue a climatology of atmospheric gravity waves above Eureka, Nunavut
by Christopher Vail, A new automated technique developed to detect atmospheric gravity waves in images of an All Sky Imager (ASI) is described. The instrument used for this analysis is the Polar Environment Atmospheric Research Laboratory (PEARL) ASI (PASI) located at Eureka, Nunavut (80°N, 86°W) which has been operating since 2007. This imager data set is analyzed using this technique and the following wave parameters are determined: background wind corrected wave speed, intrinsic period, propagation direction, and horizontal and vertical wavelengths. The results of the analysis are collected over every season of operation and combined to determine the typical wave characteristics for this location and the seasonal variations. The preferred direction of gravity waves for each month is described. The first high Arctic observation of the daily variation in brightness and direction of gravity waves during a Sudden Stratospheric Warming (SSW) are described in detail.
Empirical model in the characterization of high frequency propagation in the Arctic region
Empirical model in the characterization of high frequency propagation in the Arctic region
by Rachel Athieno, The use of high frequency (HF) communication at high latitudes still forms the backbone of many systems, owing to the fact that there are few alternatives to HF radio for the merchant and fishing fleets, military forces (land, sea and air) and for the civil aviation industry for these latitudes. HF communication uses the ionosphere as a medium of propagation, but its variable nature can be a disadvantage to both the radio operators and users. The choice of a suitable usable frequency is thus dependent on the ability to predict the conditions of the ionosphere. Thus, HF propagation predictions can be inferred from the predicted conditions of the ionosphere. In fact, in most cases, the predictions of ionospheric conditions and HF propagation are often assumed to be identical. Hence, the need to develop ionospheric model(s) suitable for high latitudes to enable a relatively smooth operation of ionospheric-dependent radio communication. Most of the available models, particularly the empirical ones were developed with little data at high latitudes. Data availability has improved over the years due to technological advancement and continued research. For instance, the Canadian High Arctic Ionospheric Network (CHAIN) provides a wealth of data for the polar and auroral regions. Using CHAIN data, the performance of the Voice of America Coverage Analysis Program (VOACAP), Ionospheric Communication Enhanced Profile Analysis and Circuit (ICEPAC) and the recommendation 533 (REC533) propagation models was evaluated and some inconsistences were identified for each of the models. An additional study of ionospheric variability in the arctic region revealed that the currently available International Telecommunication Union Recommendation (ITU-R) variability factors demonstrate notable differences from observations during the winter and equinoxes. A model for the critical frequency of the F2 layer was developed using both CHAIN and Space Physics Interactive Data Resource (SPIDR) data. The model results show an improvement compared to the available baseline International Reference Ionosphere (IRI) model.
Far-infrared synchrotron-based spectroscopy of proton tunnelling in malonaldehyde
Far-infrared synchrotron-based spectroscopy of proton tunnelling in malonaldehyde
Although the internally hydrogen-bonded species malonaldehyde (C3O2H4) is considered an important prototype molecule for intramolecular proton transfer, its far-IR spectrum is not well understood. Using high-resolution spectra obtained from the Canadian Light Source synchrotron in Saskatoon, Saskatchewan, I have made significant progress in understanding its low-energy vibrational structure. A new rotational characterization of the vibrational ground state tunnelling-split pair is presented here, which benefits from these new IR measurements covering a more complete range of rotational parameter space than was reported previously. Full rotational analyses have been performed for three low-energy vibrational states at 241, 390, and 405 cm-1 and these states (as well as states at 273 and 282 cm-1) have been conclusively matched to early microwave measurements [W. F. Rowe, Ph.D. Thesis, Harvard University, 1975]. Progress has been made toward developing a theoretical treatment of malonaldehyde using the Generalized Semi-Rigid Bender Hamiltonian to describe the large-amplitude tunnelling motion.
GPS total electron content techniques for observing the structure and dynamics of the high latitude ionosphere
GPS total electron content techniques for observing the structure and dynamics of the high latitude ionosphere
by Chris Watson, The solar wind, magnetosphere and ionosphere (SW-M-I) constitute a highly coupled system, where solar wind energy drives dynamic processes of the magnetospheric cavity and embedded ionosphere. The high latitude ionosphere has an inherently complex structure and dynamic behavior due to direct coupling to the solar wind and outer magnetosphere. Energy and charged particles deposited in the high latitude ionosphere via SW-M-I coupling processes generate ionization structures with variable spatial scales and dynamic behavior. Characteristics of ionization structures, such as their source, generation mechanisms, and evolution, are not well understood, largely due to past inadequacies in observational capabilities at high latitudes. From a physics perspective, the observation of high latitude ionospheric processes and structures is a proven technique for studying physical processes in the magnetosphere and solar wind. From an industrial and societal perspective, the dynamic high latitude ionosphere is potentially problematic due to significant impacts on electromagnetic signals used for navigation and communication. Recent installation of high data rate Global Positioning System (GPS) receivers of the Canadian High Arctic Ionospheric Network (CHAIN) has provided new, high resolution observations of the high latitude ionosphere. This thesis uses a statistical study and case studies to investigate the characteristics and source regions of ionospheric total electron content (TEC) variations observed by CHAIN GPS receivers. In the statistical study, occurrence rate, amplitude, and frequency of TEC variations were examined, in order to investigate the dependence of these characteristics on magnetic local time, latitude, and solar wind conditions. In three case studies, variations in TEC resulting from ultra-low frequency (ULF) waves in Earth’s geomagnetic field were observed. It is shown that GPS TEC has a significant response to various ULF wave classifications and intensities, and is a potentially valuable tool for high resolution study of ULF activity and the associated ionospheric response.
High data rate global positioning system receiver performance analysis for ionospheric monitoring within the Canadian high arctic region
High data rate global positioning system receiver performance analysis for ionospheric monitoring within the Canadian high arctic region
by Anthony Craig Mark McCaffrey, The Canadian High Arctic Ionospheric Network (CHAIN) has recently begun an expansion of Global Positioning System (GPS) receiver stations within the Canadian auroral and high latitude regions which will be used to monitor the ionosphere. The stations included in this expansion will utilize the Septentrio PolaRxS Pro Global Navigation Satellite System (GNSS) receiver. This receiver is new to the CHAIN network and work regarding its performance as well as newly obtainable data thanks to its improved specifications, specifically the increased sampling rate of 100 Hz, must be analyzed. One method of high arctic ionospheric monitoring in which the receiver will take part involves the calculation of G PS-derived total electron content (TEC). The accuracy of GPS-derived TEC relies heavily on the accurate estimation of the instrumental biases, known as the differential code bias (DCB). The most appropriate methods of estimating the DCBs for the Septentrio PolaRxS Pro receivers are determined and tested. These include two variations of the minimization of standard deviations method as well as two variations of bias estimation through the comparison of stationderived TEC and TEC maps provided by the International GNSS Service (IGS). Biases obtained using the minimization of standard deviations methods range from -9.81 TECU to 9.36 TECU. Methods involving the comparison of station-derived TEC and TEC maps return bias values ranging from -4.01 TECU to 18.05 TECU and -8.84 TECU to 11.57 TECU for a least squares comparison and a direct, per epoch, comparison method, respectively. Another method of ionospheric monitoring in which the receiver will be used involves the logging and analysis of signal intensity and phase scintillation. The PolaRxS Pro is capable of sampling GPS carrier signal intensity and phase at a maximum rate of 100 Hz, double that of previous receivers. The characteristics of the amplitude and phase scintillation spectra at 100 Hz sampling rates are described. Results are obtained specifically within the auroral region during May 24th through 31st 2013. Wavelet and Fourier transform methods of analysis are described for a qualitative and quantitative comparison of the higher frequency spectral range with the current theoretical predictions. Higher frequency amplitude spectra shows an abrupt deviation from theoretical predictions. Temporal analysis shows no dominant characteristics during the scintillation event in the higher frequency region where static analysis shows a near zero spectral slope before, during and after the event. This constant spectrum in the high frequency amplitude alludes to noise in the region. Phase spectral analysis shows a more subtle deviation from theoretical predictions in the higher frequency regime. In the lower frequency portion, up to about 20 Hz, the expected behavior based on previous work is observed, a power law behavior with a mean spectral slope around -2. In the higher frequencies the mean spectral slope becomes increasingly more positive up to a value of -0.4394 seen in the 40 Hz to 50 Hz range.
Magnetic resonance study of two-phase gas-liquid systems: acoustic cavitation and vertical bubbly flow
Magnetic resonance study of two-phase gas-liquid systems: acoustic cavitation and vertical bubbly flow
by Aidin Arbabi, We performed magnetic resonance studies of gas-liquid systems: acoustic cavitation and vertical bubbly flow to obtain information about the water molecules mobility, bubble size, and void fraction, critical for a better understanding of these complex dynamic systems. A varying magnetic susceptibility between the phases in gas-liquid systems fundamentally limits the employment of conventional magnetic resonance imaging methods for quantitative measurements. Susceptibility effects can be removed with the addition of paramagnetic salts to the solution. We, however, decided to use the susceptibility difference to extract useful information about bubble size, provided its image distortion effects are overcome. Pure phase encode imaging methods are suitable means for the quantitative measurement of these heterogeneous systems, as they incorporate fixed encoding times. A significant modulation of the magnetic resonance parameters, as driven by susceptibility, necessitates short signal acquisition times. The fast dispersion of water molecules in the acoustically cavitating liquid permitted the employment of pure phase encode imaging methods with short phase encoding times to obtain 3D voidage and velocity maps. Compressibility, divergence, and vorticity maps of the cavitating medium were also created. The average bubble size, both bulk and spatially resolved, was extracted for the vertical bubbly flow from the susceptibility-modulated Ti. For that, an analytical approach wit}:i two significantly different measurement regimes was employed. The fast mobility of water spins with respect to the short encoding times in the employed pure phase encode methods satisfied the fast diffusion regime requirements. A good agreement was observed between magnetic resonance and optics-based estimates of bubble sizes for the slower airflow rates. Bulk measurements of vertical bubbly flow were also performed at low magnetic field. We analyzed the susceptibility-induced effects in the Carr-Purcell-Meiboom-Gill signal using new analytical approaches, and derived two simplified equations for voidage and inclusion size estimation with basic relaxometry. The approach introduced was validated through control measurements on silica bead solutions with known bead sizes, and through comparison with measurements performed using an optics-based technique. The developed approach worked well for both control samples and bubbly flows. Our approach can also be used to study other gas-liquid systems with spherical non-interacting inclusions, like gas hydrates and sprays, provided all the theoretical requirements are met.

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