Browsing by Author "Simonetti, John H."
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- An Arcminute-Resolution Imaging Study of the H-alpha & [S II] Emission of the ISM from the Local Perseus Arm Using the Virginia Tech Spectral-Line SurveyNelson, Keith Phillip (Virginia Tech, 2011-08-08)The Virginia Tech Spectral-Line Survey (VTSS) is a series of 10-degree wide, arcminute-resolution images of the warm ionized interstellar medium (WIM) within the Milky Way for declinations greater than -15 degrees. The Wisconsin Hydrogen-Alpha Mapper (WHAM) studies the kinematics and distribution of this same material on an angular scale of one degree. Comparing the spatially-resolved images of the VTSS with WHAM's high spectral resolution images shows that the ISM's characteristics appear to be very similar at both degree and arcminute scales --- we see similar structures, and notice the same [S II]/H-alpha trend between those structures at both degree and arcminute scales. VTSS fields show three basic types of structures --- compact clouds with diameters greater than several degrees, those that are 1-degree or less in diameter, and extended filaments that differ from the clouds by spanning several degrees in length but having thicknesses of only a few tens of arcminutes. The latter two morphological types cannot be observed by WHAM. Additionally, VTSS data confirms that the [S II] intensity values are directly proportional to H-alpha intensities, a result that is also observed at degree resolutions. Finally, VTSS data show that [S II]/H-alpha ratios are, on average, nearly six times higher in the filaments. This would indicate that collisional excitation of singly-ionized sulfur ions is the dominant emission source within filaments. In clouds, the lower [S II]/H-alpha values observed are evidence that the H-alpha recombination line of photo-excited neutral hydrogen dominates. Because automating the VTSS was a key part of obtaining many of the images used in this project, I begin with a discussion of general observatory automation. I then address the specific processes and techniques used in automating the VTSS before discussing data collection and reduction techniques.
- Automation of the Spectral-Line Imaging Camera for the Virginia Tech Spectral-Line SurveyPortock, Kenneth Patrick (Virginia Tech, 2002-12-09)The Virginia Tech Spectral-Line Survey (VTSS) is a high sensitivity, high resolution survey of Hydrogen-$\alpha$ and Sulfur-II emission in the full northern hemisphere. The instrument used for the survey is the Spectral Line Imaging Camera (SLIC). SLIC uses a fast(f/1.2) lens attached to a cryogenically cooled, TK 512x512 CCD with 27 micron pixels. The focal length of the lens is 58mm which gives a pixel size of 1.6 arcminutes. The diameter of each field is 10degrees. A filter wheel ahead of the lens allows for imaging at different wavelengths. Automating the imaging system is desirable and advantageous due to the large scope of the survey. A variety of devices have been developed in order to implement automation of the SLIC observatory. They include an automated focusing mechanism, filter wheel, liquid nitrogen auto fill system, motorized roll-off roof, cloud monitor, and an equatorial mount. A PERL script, called SLICAR (Spectral Line Imaging Camera Automation Routine), was written to control and communicate with the various hardware and software components. The program also implements a user prepared Observing File, and makes decisions based on observing conditions.
- The Contribution Of Galactic Free-Free Emission to Anisotropies in the Cosmic Microwave Background Found by the Saskatoon ExperimentSimonetti, John H.; Dennison, B.; Topasna, G. A. (IOP PUBLISHING LTD, 1996-02)We made a sensitive, wide-field Ha image of the north celestial polar region. Using this image, we constrain the contribution of irregularities in interstellar free-free emission to the degree-scale anisotropies in the cosmic microwave background detected in recent observations at Saskatoon by the Princeton group. The analysis of the Her image mimics the Saskatoon data analysis: the resulting signal is the strength of irregularities sampled with the Saskatoon beam (i.e., degree-scale) along the 85 degrees declination circle. We found no such irregularities that could be attributed to Her emission. The implied upper bound on the rms variation in free-free brightness temperature is less than 4.6 mu K at 27.5 GHz. The observed cosmic microwave background anisotropies are much larger. Therefore, the contribution of irregularities in interstellar free-free emission to the observed anisotropies is negligible.
- Contribution of the First Electronically Excited State of Molecular Nitrogen to Thermospheric Nitric OxideYonker, Justin David (Virginia Tech, 2013-05-13)The chemical reaction of the first excited electronic state of molecular nitrogen, N₂(A), with ground state atomic oxygen is an important contributor to thermospheric nitric oxide (NO). The importance is assessed by including this reaction in a one-dimensional photochemical model. The method is to scale the photoelectron impact ionization rate of molecular nitrogen by a Gaussian centered near 100 km. Large uncertainties remain in the temperature dependence and branching ratios of many reactions important to NO production and loss. Similarly large uncertainties are present in the solar soft x-ray irradiance, known to be the fundamental driver of the low-latitude NO. To illustrate, it is shown that the equatorial, midday NO density measured by the Student Nitric Oxide Explorer (SNOE) satellite near the Solar Cycle 23 maximum can be recovered by the model to within the 20% measurement uncertainties using two rather different but equally reasonable chemical schemes, each with their own solar soft-xray irradiance parameterizations. Including the N₂(A) changes the NO production rate by an average of 11%, but the NO density changes by a much larger 44%. This is explained by tracing the direct, indirect, and catalytic contributions of N₂(A) to NO, finding them to contribute 40%, 33%, and 27% respectively. The contribution of N₂(A) relative to the total NO production and loss is assessed by tracing both back to their origins in the primary photoabsorption and photoelectron impact processes. The photoelectron impact ionization of N₂ is shown to be the main driver of the midday NO production while the photoelectron impact dissociation of N₂ is the main NO destroyer. The net photoelectron impact excitation rate of N₂, which is responsible for the N₂(A) production, is larger than the ionization and dissociation rates and thus potentially very important. Although the conservative assumptions regarding the level-specific NO yield from the N₂(A)+O reaction results in N₂(A) being a somewhat minor contributor, N₂(A) production is found to be the most efficient producer of NO among the thermospheric energy deposition processes.
- A Dedicated Search for Low Frequency Radio Transient Astrophysical Events using ETADeshpande, Kshitija Bharat (Virginia Tech, 2009-11-09)Astrophysical phenomena such as self-annihilation of primordial black holes (PBHs), gamma ray bursts (GRBs), and supernovae are expected to produce single dispersed pulses detectable in the low end of the radio spectrum. Analysis of these pulses could provide valuable information about the sources, and the surrounding and intervening medium. The Eight-meter-wavelength Transient Array (ETA) is a radio telescope dedicated to the search for these pulses in an 18 MHz bandwidth centered at 38 MHz. ETA consists of 10 dual-polarized active dipoles providing an all-sky field of view. This thesis describes the results of a search campaign using ETA, namely, a Crab giant pulse (CGP) search. CGPs are already known to exist, and thus provide an excellent diagnostic for system performance. We found 11 CGP candidates in 14 hours of data. Although there has not been a single compelling detection (signal-to-noise ratio > 6), our analysis shows that at least a few of these candidates may be CGPs. We also explain the analysis preparation for PBH and GRB searches. Additionally, we describe the instrument and a software "toolchain" developed for the analysis of data that includes calibration, radio frequency interference (RFI) mitigation, and incoherent dedispersion. A dispersed pulse simulation code was developed and used to test the toolchain. Finally, improvements are suggested.
- Detection in Hα of a Supershell Associated with W4Dennison, B.; Topasna, G. A.; Simonetti, John H. (IOP PUBLISHING LTD, 1997-01)From H I observations, Normandeau, Taylor, & Dewdney have identified a possible Galactic chimney emanating from W4. We observed a 10 degrees diameter field centered on this region in the Her line using a CCD camera sensitive to faint extended emission. Our image shows an apparent shell of H II, which we interpret as the ionized inner wall of a superbubble produced by stellar winds from the very young star cluster OCl 352. An analysis of the ionization balance indicates that much of the Lyman continuum radiation from the star cluster is absorbed and does not escape from the disk The shell appears to close 6 degrees (or about 230 pc) above the star cluster, and at a Galactic latitude of 7 degrees. The shell is quite elongated, with its major axis approximately perpendicular to the Galactic plane, as predicted for a superbubble formed in a stratified Galactic disk. The large size of the shell leads to an estimated age between 6.4 and 9.6 Myr, which exceeds that of OCl 352 (less than or similar to 2.5 Myr). The reason for this discrepancy is unclear, although it is possible that an earlier epoch of stellar outflow has contributed to the growth of the W4 superbubble.
- The Extreme Scattering Event toward PKS 1741-038: VLBI ImagesLazio, T. J. W.; Fey, A. L.; Dennison, B.; Mantovani, F.; Simonetti, John H.; Alberdi, A.; Foley, A. R.; Fiedler, R.; Garrett, M. A.; Hirabayashi, H.; Jauncey, D. L.; Johnston, K. J.; Marcaide, J.; Migenes, V.; Nicolson, G. D.; Venturi, T. (IOP PUBLISHING LTD, 2000-05)We report multiepoch VLBI observations of the source PKS 1741-038 (OT 068) as it underwent an extreme scattering event (ESE). Observations at four epochs were obtained, and images were produced at three of these. One of these three epochs was when the source was near the minimum flux density of the ESE, the other two were as the flux density of the source was returning to its nominal value. The fourth epoch was at the maximum flux density during the egress from the ESE, but the VLBI observations had too few stations to produce an image. During the event the source consisted of a dominant, compact component, essentially identical to the structure seen outside the event. However, the source's diameter increased slightly at 13 cm, from near 0.6 mas outside the ESE to near 1 mas during the ESE. An increase in the source's diameter is inconsistent with a simple refractive model in which a smooth refractive lens drifted across the line of sight to PKS 1741-038. We also see no evidence for ESE-induced substructure within the source or the formation of multiple images, as would occur in a strongly refractive lens. However, a model in which the decrease in flux density during the ESE occurs solely because of stochastic broadening within the lens requires a larger broadening diameter during the event than is observed. Thus, the ESE toward PKS 1741-038 involved both stochastic broadening and refractive defocusing within the lens. If the structure responsible for the ESE has a size of order I AU, the level of scattering within an ESE lens may be a factor of 10(7) larger than that in the ambient medium. A filamentary structure could reduce the difference between the strength of scattering in the lens and ambient medium, but there is no evidence for a refractively induced elongation of the source. We conclude that, if ESEs arise from filamentary structures, they occur when the filamentary structures are seen lengthwise. We are able to predict the amount of pulse broadening that would result from a comparable lens passing in front of a pulsar. The pulse broadening would be no more than 1.1 mu s, consistent with the lack of pulse broadening detected during ESEs toward the pulsars PSR B1937+21 and PSR J1643-1224. The line of sight toward PKS 1741-038 is consistent with a turbulent origin for the structures responsible for ESEs. The source PKS 1741-038 lies near the radio Loop I and is seen through a local minimum in 100 mu m emission.
- The field high-amplitude SX Phe variable BL Cam: results from a multisite photometric campaignFauvaud, S.; Sareyan, J. P.; Ribas, I.; Rodriguez, E.; Lampens, P.; Klingenberg, G.; Farrell, J. A.; Fumagalli, F.; Simonetti, John H.; Wolf, M.; Santacana, G.; Zhou, A. Y.; Michel, R.; Fox-Machado, L.; Alvarez, M.; Nava-Vega, A.; Lopez-Gonzalez, M. J.; Casanova, V. M.; Aceituno, F. J.; Scheggia, I.; Rives, J. J.; Hintz, E. G.; Van Cauteren, P.; Helvaci, M.; Yesilyaprak, C.; Graham, K. A.; Kral, L.; Kocian, R.; Kucakova, H.; Fauvaud, M.; Granslo, B. H.; Michelet, J.; Nicholson, M. P.; Vugnon, J. M.; Kotkova, L.; Truparova, K.; Ulusoy, C.; Yasarsoy, B.; Avdibegtovic, A.; Blazek, M.; Kliner, J.; Zasche, P.; Bartosikova, S.; Vilasek, M.; Trondal, O.; Van Den Abbeel, F.; Behrend, R.; Wucher, H. (EDP SCIENCES, 2010-06)Context. Short-period high-amplitude pulsating stars of Population I (delta Sct stars) and II (SX Phe variables) exist in the lower part of the classical (Cepheid) instability strip. Most of them have very simple pulsational behaviours, only one or two radial modes being excited. Nevertheless, BL Cam is a unique object among them, being an extreme metal-deficient field high-amplitude SX Phe variable with a large number of frequencies. Based on a frequency analysis, a pulsational interpretation was previously given. Aims. We attempt to interpret the long-term behaviour of the residuals that were not taken into account in the previous Observed-Calculated (O-C) short-term analyses. Methods. An investigation of the O-C times has been carried out, using a data set based on the previous published times of light maxima, largely enriched by those obtained during an intensive multisite photometric campaign of BL Cam lasting several months. Results. In addition to a positive (161 +/- 3) x 10(-9) yr(-1) secular relative increase in the main pulsation period of BL Cam, we detected in the O-C data short- (144.2 d) and long-term (similar to 3400 d) variations, both incompatible with a scenario of stellar evolution. Conclusions. Interpreted as a light travel-time effect, the short- term O-C variation is indicative of a massive stellar component (0.46 to 1 M(circle dot)) with a short period orbit (144.2 d), within a distance of 0.7 AU from the primary. More observations are needed to confirm the long-term O-C variations: if they were also to be caused by a light travel-time effect, they could be interpreted in terms of a third component, in this case probably a brown dwarf star (>= 0.03 M(circle dot)), orbiting in similar to 3400 d at a distance of 4.5 AU from the primary.
- The field high-amplitude SX Phoenicis variable BL Camelopardalis: results from a multisite photometric campaignRodriguez, E.; Fauvaud, S.; Farrell, J. A.; Zhou, A. Y.; Sareyan, J. P.; Lopez-Gonzalez, M. J.; Dupret, M. A.; Grigahcene, A.; de Ridder, J.; Klingenberg, G.; Wolf, M.; Van Cauteren, P.; Lampens, P.; Martinez, D.; Casanova, V. M.; Aceituno, F. J.; Helvaci, M.; Hintz, E. G.; Kral, L.; Fumagalli, F.; Simonetti, John H.; Granslo, B. H.; Kotkova, L.; Santacana, G.; Michelet, J.; Fauvaud, M.; Vugnon, J. M.; Sadsaoud, H.; Aliouane, A.; Grigahcene, Z. C.; Martin-Ruiz, S.; Amado, P. J.; Suarez, J. C.; Moya, A.; Kucakova, H.; Kocian, R.; Truparova, B.; Avdibegtovic, A.; Blazek, M.; Kliner, J.; Zasche, P.; Vilasek, M.; Bartosikova, S.; Yasarsoy, B.; Ulusoy, C.; Graham, K. A.; Nicholson, M. P.; Trondal, O.; Yesilyaprak, C.; Coker, D. (EDP SCIENCES, 2007-08)Context. BL Cam is an extreme metal-deficient field high-amplitude SX Phe-type variable where a very complex frequency spectrum is detected, with a number of independent nonradial modes excited, unusual among the high-amplitude pulsators in the Lower Classical Instability Strip. Aims. An extensive and detailed study has been carried out to investigate the pulsational content and properties of this object. Methods. The analysis is based on 283 h of CCD observations obtained in the Johnson V filter, during a long multisite photometric campaign carried out along the Northern autumn-winter of 2005-2006. Additionally, multicolour BI photometry was also collected to study the phase shifts and amplitude ratios, between light curves obtained in different filters, for modal discrimination of the main excited modes. Results. The detailed frequency analysis revealed a very rich and dense pulsational content consisting of 25 significant peaks, 22 of them corresponding to independent modes: one is the already known main periodicity f(0) = 25.5765 cd(-1) (Delta V = 153 mmag) and the other 21 are excited modes showing very small amplitudes. Some additional periodicities are probably still remaining in the residuals. This represents the most complex spectrum ever detected in a high-amplitude pulsator of this type. The majority of the secondary modes suspected from earlier works are confirmed here and, additionally, a large number of new peaks are detected. The amplitude of the main periodicity f(0) seems to be stable during decades, but the majority of the secondary modes show strong amplitude changes from one epoch to another. The suspected fundamental radial nature of the main periodicity of BL Cam is confirmed, while the secondary peak f(1) = 25.2523 cd(-1) is identified as a nonradial mixed mode g(4) with l = 1. The radial double-mode nature, claimed by some authors for the main two frequencies of BL Cam, is not confirmed. Nevertheless, the frequency f(6) = 32.6464 cd(-1) could correspond to the first radial overtone.
- Foundational Investigations & Astronomical Implications of Quantum GravityKavic, Michael James (Virginia Tech, 2009-10-12)In this thesis we consider foundational elements of quantum gravity as well as it possible observable astrophysical effects. In particular investigate a background independent formulation of Matrix Theory. We discuss a background independent formulation of a holographic theory of quantum gravity. The present thesis incorporates the necessary background material on geometry of canonical quantum theory, holography and spacetime thermodynamics, Matrix theory, as well as our specific proposal for a dynamical theory of geometric quantum mechanics, as applied to Matrix theory. At the heart of this thesis is a new analysis of the conceptual problem of time and the closely related and phenomenologically relevant problem of vacuum energy in quantum gravity. We also present a discussion of some observational implications of this new viewpoint on the problem of vacuum energy. as well as a novel solution to the low entropy and arrow of time puzzles of the initial state of the Universe. Our approach derives from the physics of the specific generalization of Matrix theory as the basis for a quantum theory of gravity considered here. The particular dynamical state space of this theory, the infinite dimensional analogue of the Fubini-Study metric over a complex non-linear Grassmannian, has recently been studied by Michor and Mumford. The geodesic distance between any two points on this space is zero. Here we show that this mathematical result translates to a description of a hot, zero entropy state and an arrow of time after the Big Bang. This is modeled as a far from equilibrium, large fluctuation driven, "freezing by heating" metastable ordered phase transition of a non-linear dissipative dynamical system. We also consider an evaporating black hole in the presence of an extra spatial dimension would undergo an explosive phase of evaporation. We show that such an event, involving a primordial black hole, can produce a detectable, distinguishable electromagnetic pulse, signaling the existence of an extra dimension of size L â ¼ 10â 18 â 10â 20 m. We derive a generic relationship between the Lorentz factor of a pulse-producing "fireball" and the TeV energy scale. For an ordinary toroidally compactified extra dimension, transient radio-pulse searches probe the electroweak energy scale (â ¼0.1 TeV), enabling comparison with the Large Hadron Collider.
- Galactic Foregrounds in Owens Valley Radio Observatory and UCSB South Pole 1994 Cosmic Microwave Background Anisotropy DataMukherjee, P.; Dennison, B.; Ratra, B.; Simonetti, John H.; Ganga, K.; Hamilton, J. C. (IOP PUBLISHING LTD, 2002-11)We study Galactic emission foreground contamination of the Owens Valley Radio Observatory and UCSB South Pole 1994 cosmic microwave background anisotropy data by cross-correlating with templates of infrared dust emission and new high-resolution Virginia Tech Spectral Line Survey (VTSS) and Southern Halpha Sky Survey Atlas (SHASSA) Halpha data. Halpha data provide rough upper limits on the level of free-free emission in the data sets. The cross-correlation analysis does not contradict a two-component foreground emission hypothesis, with the two dust-correlated components being free-free emission and spinning-dust emission.
- Gamma-ray emission from Galactic millisecond pulsars: Implications for dark matter indirect detectionSong, Deheng (Virginia Tech, 2022-01-18)The Fermi Large Area Telescope has observed a gamma-ray excess toward the center of the Galaxy at ~ GeV energies. The spectrum and intensity of the excess are consistent with the annihilation of dark matter with a mass of ~100 GeV and a velocity-averaged cross section of ~ 1e-26 cubic centimeter per second. In the meantime, a population of unresolved millisecond pulsars (MSPs) in the Galactic center remains a possible source of the excess. Furthermore, recent analyses have shown that the excess prefers the spatial morphology of the stellar bulge distribution in the Galactic center, supporting a MSP origin. The new discovery makes it imperative to further study the signals from MSPs. This dissertation studies the gamma-ray emission from Galactic millisecond pulsars to provide new insights into the origin of the Galactic center excess. Using the GALPROP code, we simulate the propagation of e± injected by the putative MSPs in the Galactic bulge and calculate the inverse Compton (IC) emission caused by the e± losing energy in the interstellar radiation field. We find recognizable features in the spatial maps of the IC. Above TeV energies, the IC morphology tends to follow the distribution of the injected e±. Then, we study the Cherenkov Telescope Array (CTA) sensitivity to the IC signal from MSPs. We find that the CTA has the potential to robustly discover the IC signature when the MSP e± injection efficiencies are in the range ≈ 2.9-74.1%. The CTA can also discriminate between an MSP and a dark matter origin for the radiating e± based on their different spatial maps. Next, we analyze the Fermi data from directions of Galactic globular clusters. The globular clusters are thought to be shining in gamma rays because of the MSP population they host. By analyzing their gamma-ray spectra, we reveal evidence for an IC component in the high-energy tail of Fermi data. Based on the IC component in the globular cluster spectra, the e± injection efficiency of millisecond pulsars is estimated to be slightly smaller than 10%. Finally, we study the spatial morphology of the 511 keV signal toward the Galactic center using data from INTEGRAL/SPI. We confirm that the 511 keV signal also traces the old stellar population in the Galactic bulge, which is similar to the Fermi GeV excess. Using a 3D smoothing kernel, we find that the signal is smeared out over a characteristic length scale of 150 ± 50 pc. We show that positron propagation prior to annihilation can explain the overall phenomenology of the 511 keV signal.
- How Do Quasars Impact Their Host Galaxies? From the Studies of Quasar Outflows in Absorption and EmissionXu, Xinfeng (Virginia Tech, 2020-05-27)"Quasar-mode feedback" occurs when momentum and energy from the environment of accreting supermassive black hole couple to the host galaxy. One mechanism for such a coupling is by high-velocity (up to ~0.2c) quasar-driven ionized outflows, appearing as blue-shifted absorption and emission lines in quasar spectra. Given enough energy and momentum, these outflows are capable of affecting the evolution of their host galaxies. This dissertation presents the studies of emission and absorption quasar outflows from different perspectives. (1). By conducting large broad absorption line (BAL) quasar surveys in both Sloan Digital Sky Survey and Very Large Telescopes (VLT), we determined various physics properties of quasar absorption outflows, e.g., the electron number density ((ne), the distance of outflows to the central quasar (𝑅), and the kinetic energy carried by the outflow (𝐸̇k). We demonstrated that half of the typical BAL outflows are situated at 𝑅 > 100 pc, i.e., having the potential to affect the host galaxies. (2). Our group carried out a Hubble Space Telescope program (PI: Arav) for studying the outflows in the Extreme-UV, collaborating with Dr. Gerard Kriss from Space Telescope Science Institute (STScI). We developed a novel method to fit the multitude of quasar absorption troughs efficiently and accurately. We have identified the most energetic quasar-driven outflows on record and discovered the largest acceleration and velocity-shift for a quasar absorption outflow. (3). By using the VLT data, Xu led the project to study the relationships between BAL outflows and emission line outflows. We found possible connections between these two types of quasar outflows, e.g., the luminosity of the [𝑂III λ5007 emission profile decreases with increasing ne derived from the BAL outflow in the same quasar. These findings are consistent with BAL and emission outflows being different manifestations of the same wind, and the observed relationships are likely a reflection of the outflow density distribution.
- Investigation of High Latitude Ionospheric Irregularities utilizing Modeling and GPS ObservationsDeshpande, Kshitija Bharat (Virginia Tech, 2014-07-10)Complex magnetosphere-ionosphere coupling mechanisms result in high latitude irregularities that are difficult to characterize. Until recently, the polar and auroral irregularities remained largely unexplored. Inadequate infrastructures to deploy and maintain advanced dual frequency Global Navigation Satellite System (GNSS) receivers at high latitudes, especially in the Southern hemisphere, makes such an investigation a formidable task. Additionally, the complicated geometry of the magnetic field lines in these regions pose challenges in designing global scintillation models. This dissertation takes some steps towards bridging these gaps while advancing the state-of-the-art high latitude irregularity studies. In the first part of this dissertation, we briefly describe the Autonomous Adaptive Low-Power Instrument Platforms (AAL-PIP) experimental setup. These space science instrument platforms are being deployed in remote locations in Antarctica, improving the coverage of GNSS data availability. We explain in detail the method developed for analyzing high rate (typically 50 Hz) data from a novel dual-frequency Global Positioning System (GPS) receiver called Connected Autonomous Space Environment Sensor (CASES). We also report first observations from CASES at high latitudes. From this study, we established that CASES can be reliably used as a science grade GPS scintillation monitor. Following this, a novel three dimensional (3D) electromagnetic (EM) wave propagation model called "Satellite-beacon Ionospheric-scintillation Global Model of the upper Atmosphere" (SIGMA) was developed to simulate GNSS scintillations on ground. GPS scintillation simulations of significantly high fidelity are now possible with this model. While the model is global, it is the first such model which accounts for the complicated geometry of magnetic field lines at high latitudes. Using SIGMA, a sensitivity study is presented to understand the effect of geographical, propagation and irregularity parameters on the phase scintillations. This allows us to reduce the dimensionality of the design space while solving the inverse problem described next. In the final part, we utilize the tools developed for GPS measurement analysis and SIGMA to characterize the high latitude irregularities. We propose an inverse modeling technique to derive irregularity parameters by comparing the high rate (50 Hz) GNSS observations to the modeled outputs. We consider interhemispheric high latitude datasets for this investigation. We also implement SIGMA for analyzing a substorm event observed by AAL-PIP stations. One of the unique contributions of this research is to demonstrate that such an inverse modeling technique can form a basis in the investigation of the ionospheric irregularities. Moreover, availability of ample auxiliary data from multi-instrument observations can assist in this quest of understanding the physics of high latitude irregularities and their generation mechanisms.
- Measurement of the Michel Parameter Rho Using the MEGA Positron SpectrometerLee, Fei-sheng (Virginia Tech, 2001-04-03)This experiment attempts to measure the Michel parameter Rho in normal muon decay Mu to e nu_{e} nu_{mu} with a precision of 0.001. The Standard Model with pure V-A interaction gives 0.75 for this parameter. The world average value is 0.7518 +/- 0.0026. A deviation of the value of Rho from 0.75 will indicate new physics. The experiment, RHO, was carried out at the Los Alamos Meson Physics Facility (LAMPF) of Los Alamos National Laboratory (LANL) in New Mexico, USA, in 1993. The existing experimental set-up that was built for the MEGA experiment was used for the RHO measurement. The experiment was a collaboration of about 50 physicists from Los Alamos National Laboratory, Stanford University, Fermi National Accelerator Laboratory, Virginia Tech, Texas A University, Indiana University, Valparaiso University, University of Houston, University of Chicago and University of Virginia. The result of the experiment is in agreement with Standard Model. The precision of this measurement was dominated by systematic uncertainties due to the fact that the positron spectrometer used in the measurement was designed for another experiment. Thus, the data analysis here focused mainly on the determination of systematic errors.
- Modeling mechanical dynamics in chain-mediated bacterial slidingMcMahon, Sean Gregory (Virginia Tech, 2023-01-11)Investigating the mechanical dynamics of bacterial motility has led to a deeper understanding of the behaviors and lifecycle of many bacterial species. We discuss chain driven sliding motility where the bacteria maintain connections between daughter cells following division, resulting in long chains that expand across the viscous substrate. These chains grow exponentially, suggesting the chain tips may accelerate to very fast speeds. We devise multiple mathematical frameworks encapsulating the key physical dynamics and interactions to investigate the dynamics of bacterial chains and the biological implications of this motility. Our first framework, the rigid rod model, provides a set of equations describing the chain growth dynamics. Analysis of these equations reveals the stress maintaining cell-cell linkages increases unsustainably at an exponential rate. We devise a perturbation analysis of the rigid rod model in order to predict the critical stress associated with mechanical failure of these linkages. A phenomenological population model reveals that repeated chain breakages limit the expansion of the entire population to linear growth. Through experimental observation and computer simulations, we identify two key mechanical instabilities that emerge in growing bacterial chains. The first is sharp localized kinking that leads to the chain breakage mentioned above. In the second dynamic, the chain buckles due to compressive drag forces resulting in the emergence of large curvatures throughout the chain. We devise a continuum mechanics framework to examine the curvature dynamics in the growing chain. Through linear stability analysis of the rigid rod model and the continuum mechanics framework, we predict the dominant instability dynamic based on the physical properties of the chain and its environment. We use rigid rod model simulations to investigate the biological implications of these dynamics. Lastly, we introduce a number of methods that extend the rigid rod model to allow for the investigation of interacting chains. We consider methods that implement forces due to the entanglement of cell body appendages as well as collision dynamics. In total these models provide generic frameworks for investigating mechanical dynamics of growing bacterial chains. Our models provide testable predictions and suggest biological motivations for the typical behaviors that are observed in these cell chains.
- Modification of Large Reflector Antennas for Low Frequency OperationHarun, Mahmud (Virginia Tech, 2011-11-28)Modifications of large reflector antennas, such that their observing capabilities are enhanced in the range of about 10-500~MHz without affecting operation of the pre-existing higher-frequency systems, are addressed in this dissertation. The major contributions of this dissertation can be divided into two parts: 1) designing new low frequency feeds, and 2) developing new analysis methodologies which, as opposed to traditional techniques, are suitable for analyzing low frequency systems. First, we consider the performance of existing schemes that provide low frequency capability. Then, a new class of dipole-based low frequency feeds - namely, the ``distributed feed array'' - is designed to cover the frequency range of interest without affecting operation at higher frequencies. As an example, distributed feed arrays are designed for the Expanded Very Large Array (EVLA) to cover the range of 50-250~MHz. A method of moments (MoM) model of an EVLA antenna is developed for this purpose. The new design shows performance comparable to the existing 4 m system on the EVLA in the range of 50-88~MHz, and introduces observing capabilities in the range of 110-250~MHz (currently not covered by the EVLA). Moreover, the blockage presented to the existing EVLA L-band system is reduced significantly when the existing 4 m system is replaced by the proposed system. At low frequencies, external noise can be a significant or dominant contribution to the total noise of the system. This, combined with mutual coupling between the array elements of the distributed feed array, makes it difficult to predict the sensitivity of these systems. This dissertation describes a system model and procedure for estimating the system equivalent flux density (SEFD) - a useful and meaningful metric of the sensitivity of a radio telescope - that accounts for these issues. We consider the efficiency of methods other than MoM - in particular, Physical Optics (PO), Uniform Geometrical Theory of Diffraction (UTD), and hybrid methods - for accelerated computation at low frequencies. A method for estimating the blockage presented by low frequency systems to the pre-existing higher-frequency systems is also described.
- Multi-messenger Observations of a Binary Neutron Star MergerAbbott, B. P.; Simonetti, John H.; Tsai, J.; Nathaniel, K. (2017-10-20)On 2017 August 17 a binary neutron star coalescence candidate (later designated GW170817) with merger time 12:41:04 UTC was observed through gravitational waves by the Advanced LIGO and Advanced Virgo detectors. The Fermi Gamma-ray Burst Monitor independently detected a gamma-ray burst (GRB 170817A) with a time delay of similar to 1.7 s with respect to the merger time. From the gravitational-wave signal, the source was initially localized to a sky region of 31 deg(2) at a luminosity distance of 40(-8)(+8) Mpc and with component masses consistent with neutron stars. The component masses were later measured to be in the range 0.86 to 2.26 M-circle dot. An extensive observing campaign was launched across the electromagnetic spectrum leading to the discovery of a bright optical transient (SSS17a, now with the IAU identification of AT 2017gfo) in NGC 4993 (at similar to 40 Mpc) less than 11 hours after the merger by the One-Meter, Two Hemisphere (1M2H) team using the 1 m Swope Telescope. The optical transient was independently detected by multiple teams within an hour. Subsequent observations targeted the object and its environment. Early ultraviolet observations revealed a blue transient that faded within 48 hours. Optical and infrared observations showed a redward evolution over similar to 10 days. Following early non-detections, X-ray and radio emission were discovered at the transient's position similar to 9 and similar to 16 days, respectively, after the merger. Both the X-ray and radio emission likely arise from a physical process that is distinct from the one that generates the UV/optical/near-infrared emission. No ultra-high-energy gamma-rays and no neutrino candidates consistent with the source were found in follow-up searches. These observations support the hypothesis that GW170817 was produced by the merger of two neutron stars in NGC4993 followed by a short gamma-ray burst (GRB 170817A) and a kilonova/macronova powered by the radioactive decay of r-process nuclei synthesized in the ejecta.
- New Concepts in Front End Design for Receivers with Large, Multiband Tuning RangesHasan, S. M. Shajedul (Virginia Tech, 2009-04-03)This dissertation presents new concepts in front end design for receivers with large, multiband tuning ranges. Such receivers are required to support large bandwidths (up to 10's of MHz) over very large tuning ranges (30:1 and beyond) with antennas that are usually narrowband, or which at best support multiple narrow bandwidths. Traditional techniques to integrate a single antenna with such receivers are limited in their ability to handle simultaneous channels distributed over very large tuning ranges, which is important for frequency-agile cognitive radio, surveillance, and other applications requiring wideband or multiband monitoring. Direct conversion architecture is gaining popularity due to the recent advancements in CMOS--based RFIC technology. The possibility of multiple parallel transceivers in RF CMOS suggests an approach to antenna--receiver integration using multiplexers. This dissertation describes an improved use of multiplexers to integrate antennas to receivers. First, the notion of sensitivity--constrained design is considered. In this approach, the goal is first to achieve sensitivity which is nominally dominated by external (environmental) noise, and then secondly to improve bandwidth to the maximum possible consistent with this goal. Next, a procedure is developed for designing antenna-multiplexer-preamplifier assemblies using this philosophy. It is shown that the approach can significantly increase the usable bandwidth and number of bands that can be supported by a single, traditional antenna. This performance is verified through field experiments. A prototype multiband multimode radio for public safety applications using these concepts is designed and demonstrated.
- The Non-Linear Electrodynamic Coupling Between the Solar Wind, Magnetosphere and IonosphereWilder, Frederick Durand (Virginia Tech, 2011-03-29)The polar electric potential imposed on the ionosphere by coupling between the earth's magnetosphere and the solar wind has been shown to have a non-linear response to the interplanetary electric field (IEF). This dissertation presents an empirical study of this polar cap potential saturation phenomenon. First, the saturation of the reverse convection potential under northward is demonstrated using bin-averaged SuperDARN data. Then, the saturation reverse convection potential is shown to saturate at a higher value at higher solar wind plasma beta. The reverse convection flow velocity is then compared with cross-polar cap flows under southward IMF under summer, winter and equinox conditions. It is demonstrated that the reverse convection flow exhibits the opposite seasonal behavior to cross polar cap flow under southward IMF. Then, an interhemispheric case study is performed to provide an explanation for the seasonal behavior of the reverse convection potential. It is found using DMSP particle precipitation data that the reverse convection cells in the winter circulate at least partially on closed field lines. Finally, SuperDARN and DMSP data are merged to provide polar cap potential measurements for a statistical study of polar cap potential saturation under southward IMF. It is found that the extent of polar cap potential saturation increases with increasing Alfvenic Mach number, and has no significant relation to Alfven wing transmission coefficient or solar wind dynamic pressure.