Now showing items 79-91 of 91

• #### The Characterization Of The Infrasonic Noise Field And Its Effects On Least Squares Estimation

Localization of the source of an acoustic wave propagating through the atmosphere is not a new problem. Location methods date back to World War I, when sound location was used to determine enemy artillery positions. Since the drafting of the Comprehensive Nuclear-Test-Ban Treaty in 1996 there has been increased interest in the accurate location of distant sources using infrasound. A standard method of acoustic source location is triangulation of the source from multi-array back azimuth estimates. For waves traveling long distances through the atmosphere, the most appropriate method of estimating the back azimuth is the least squares estimate (LSE). Under the assumption of an acoustic signal corrupted with additive Gaussian, white, uncorrelated noise the LSE is theoretically the minimum variance, unbiased estimate of the slowness vector. The infrasonic noise field present at most arrays is known to violate the assumption of white, uncorrelated noise. The following work characterizes the noise field at two infrasound arrays operated by the University of Alaska Fairbanks, The power distribution and coherence of the noise fields was determined from atmospheric pressure measurements collected from 2003-2006. The estimated power distribution and coherence of the noise field were not the white, uncorrelated noise field assumed in the analytic derivation of the LSE of the slowness vector. The performance of the LSE of azimuth and trace velocity with the empirically derived noise field was numerically compared to its performance under the standard noise assumptions. The effect of violating the correlation assumption was also investigated. The inclusion of clutter in the noise field introduced a dependence to the performance of the LSE on the relative signal amplitude. If the signal-to-clutter ratio was above 10 dB, the parameter estimates made with the correlated noise field were comparable to the estimates made with uncorrelated noise. From the results of these numerical studies, it was determined that the assumption of Gaussian, white, uncorrelated noise had little effect on the performance of the LSE at signal-to-noise ratios greater than 10 dB, but tended to over estimate the performance of the LSE at lower signal-to-noise ratios.
• #### The Dynamics And Morphology Of Sprites

In 1999 the University of Alaska Fairbanks fielded a 1000 fields-per-second intensified CCD camera to study sprites and associated upper atmospheric phenomena occurring above active thunderstorms as part of the NASA Sprites99 campaign. The exceptional clarity and definition obtained by this camera the night of August 18, 1999, provides the most detailed image record of these phenomena that has been obtained to date. The result of a frame-by-frame analysis of the data permits an orderly classification of upper atmospheric optical phenomena, and is the subject matter of this thesis. The images show that both elves and halos, which are diffuse emissions preceding sprites, are largely spatially unstructured. Observations of sprites initiating outside of main parts of halos, and without a halo, suggest sprites are initiated primarily from locations of atmospheric composition and density inhomogeneities. All sprites appear to start as tendrils descending from approximately 75 km altitude, and may form other dynamic or stationary features. Dynamic features include downward developing tendrils and upward developing branches. Stationary features include beads, columns, and diffuse "puffs," all of which have durations greater than 1 ms. Stationary sprite features are responsible for a significant fraction of the total optical emissions of sprites. Velocities of sprite tendrils were measured. After initial speeds of 106--107 m/s, sprite tendrils may slow to 105 m/s. Similarly, on some occasions the dim optical emission left behind by the descending tendrils may expand horizontally, with speeds on the order of 105 m/s. The volume excited by the sprite tendrils may rebrighten after 30--100 ms in the form of one of three different sprite after effects collectively termed "crawlers." A "smooth crawler" consists of several beads moving upward (~105 m/s) without a large vertical extent, with "smooth" dynamics at 1 ms timescale. "Embers" are bead-like forms which send a downward-propagating luminous structure towards the cloudtop at speeds of 106 m/s, and have irregular dynamics at 1 ms timescales. In TV-rate observations, the downward-propagating structure of an ember is averaged out and appears as a vertically-extended ribbon above the clouds. The third kind of crawler, so-called "palm tree," appears similar to an ember at TV-rates, but with a wider crown at top.
• #### The effect of rate, frequency, and form of migration on host parasite population dynamics

What is the effect of migration on host-parasite population dynamics? Animals live in a landscape where they move between patches. They are also locked in host-parasite conflicts. Host-parasite interactions are modeled with consumer resource functions. I constructed models using two different consumer resource functions (the Lotka Volterra system and the Saturating Type II system). The first model was a conservative system. The second was dissipative and more biologically realistic. I examined the effect of rate of migration, time between migration events, and form of migration. I found that the time between migration events had the largest effect on the synchronization in host-parasites population dynamics between the patches. Decreased time between migration events increased the fraction of simulation to completely synchronize and decreased the time it took to do so. In the first model, I observed simulations with a low rate of migration took a long time to synchronization and with a high rate of migration took a short time to synchronize. There was a phase transition between these two amounts of time it took to synchronize. In the second model, simulations done at low rates of migration did not synchronize while with increased migration rates the fraction of simulations to synchronize increased. I found in some simulations of parasite only migration that the patches synchronized faster. My results imply that parasite only migration to islands could have a greater impact on the extinction risk on islands further from the mainland than other forms of migration.
• #### The generalized Ohm's law in collisionless magnetic reconnection

Magnetic reconnection is an important process in space environments. As a result of magnetic reconnection, the magnetic field topology changes, which requires the breakdown of the frozen-in condition in ideal magnetofluids. In a collisional plasma, the resistivity associated with Coulomb collisions of charged particles is responsible for the breakdown of frozen-in condition. In a collisionless plasma, however, the cause of the breakdown of frozen-in condition remains unanswered. We address this problem by investigating the generalized Ohm's law and the force balance near magnetic neutral lines based on two-dimensional particle simulations. In a particle simulation with one active species, it is found that a weakly anisotropic and skewed velocity distribution is formed near the magnetic X line, leading to the presence of off-diagonal elements of plasma pressure tensor. The gradients of the off-diagonal pressure terms transport plasma momentum away from the X line to balance the reconnection electric field. The presence of the reconnection electric field results in the breakdown of frozen-in condition. The importance of both electron and ion off-diagonal pressure tensor terms in the generalized Ohm's law near neutral lines is further confirmed in full particle simulations. The generation of the off-diagonal pressure terms can be explained in terms of the thermal dispersion of particle motions and the response of particle distribution function in the electric and magnetic fields near the neutral lines. In the particle simulations, we also find the presence of a new dynamo process, in which a large amount of new magnetic flux near the magnetic O line is generated. This dynamo process is not allowed in resistive magnetofluids. However, in a collisionless plasma, the plasma inertia and momentum transport due to the off-diagonal plasma pressure terms can lead to E $\cdot$ J < 0 near the magnetic O line and make the dynamo process possible.
• #### The morphology and electrodynamics of the boreal polar winter cusp

The major result of this thesis is the magnetic signatures of the dayside cusp region. These signatures were determined by comparing the magnetic observations to optical observations of different energy particle precipitation regions observed in the cusp. In this thesis, the cusp is defined as the location of most direct entry of magnetosheath particles into the ionosphere. Optical observations show that the observing station rotates daily beneath regions of different incident energy particles. Typically, the station passes from a region in the morning of high energy particles into a region near magnetic noon of very low energy precipitation, and then returns to a region of high energy precipitation after magnetic noon. A tentative identification of the cusp is made on the basis of these observations. The optical observations also are used to determine the upward field aligned current density, which is found to be most intense in the region identified as the cusp. The magnetic field measurements are found to correlate with the optical measurements. When the characteristic energy is high, the spectrogram shows large amplitude broad band signals. The Pc5 component of these oscillations is right hand polarized in the morning, and left hand polarized in the afternoon. During the time the optics detect precipitation with a minimum characteristic energy, the magnetic spectrogram shows a unique narrow band tone at 3-5 mHz. The occurrence statistics of the magnetic oscillations are compared to DMSP satellite observations of the cusp and low latitude boundary layer. The pulses that make the narrow band tone are found to come in wave trains that are phase coherent. These trains of coherent pulses are found to be separated by phase jumps from adjacent wave trains. These jumps in phase occur when a new field aligned current appears on the equatorward edge of the cusp. This combination of phase coherent wave trains associated with poleward propagating auroral forms which are shown to contain intense field aligned currents may be the signature of newly reconnected flux tubes in the ionosphere.
• #### The quasiparallel collisionless shock wave: A simulation study

The structure of the quasi-parallel collisionless shock wave is studied via a numerical simulation model. The model is compared to observations and theoretical predictions and within its limitations appears to reproduce the true shock structure reasonably well. Three electron equations of state and their effects on the simulation are examined. It is found that only the isotropic-adiabatic electron equation of state yields acceptable results in the simulation at high Mach numbers. The scale lengths of the shock are measured, normalized by the natural scale lengths of the plasma, and plotted as a function of the Alfven Mach number. It is found that the wavelength of the upstream waves follows that predicted for a phase standing whistler quite well and the scalelength of the jump in the magnitude of the magnetic field is generally greater than, but approximately equal to this wavelength. For Alfven Mach numbers $M\sb{A} >$ 2.5, waves are generated in the downstream region. Their wavelength and the scale length of the plasma transition are larger than the natural scale lengths of the plasma. The ion heating is seen to occur in two stages. In the first stage which occurs upstream of the principal shock ramp, the heating can be characterized by a polytropic power law equation of state with an exponent much greater than the isentropic-adiabatic rate of $\gamma$ = 5/3. The second stage of heating which occurs from the principal shock ramp to the downstream region is characterized by an exponent on the order of the isentropic-adiabatic rate. The results show that the ion heating occurs mainly around the principle density jump near the center of the shock transition region, while the increase in entropy takes place mainly in the upstream side of the shock transition region. It is suggested that the ion heating is a consequence of the non-adiabatic scattering of the ions through the magnetic field of the shock and its upstream precursor wave.
• #### Transient spatiotemporal chaos in a diffusively and synaptically coupled Morris-Lecar neuronal network

Transient spatiotemporal chaos was reported in models for chemical reactions and in experiments for turbulence in shear flow. This study shows that transient spatiotemporal chaos also exists in a diffusively coupled Morris-Lecar (ML) neuronal network, with a collapse to either a global rest state or to a state of pulse propagation. Adding synaptic coupling to this network reduces the average lifetime of spatiotemporal chaos for small to intermediate coupling strengths and almost all numbers of synapses. For large coupling strengths, close to the threshold of excitation, the average lifetime increases beyond the value for only diffusive coupling, and the collapse to the rest state dominates over the collapse to a traveling pulse state. The regime of spatiotemporal chaos is characterized by a slightly increasing Lyapunov exponent and degree of phase coherence as the number of synaptic links increases. In contrast to the diffusive network, the pulse solution must not be asymptotic in the presence of synapses. The fact that chaos could be transient in higher dimensional systems, such as the one being explored in this study, point to its presence in every day life. Transient spatiotemporal chaos in a network of coupled neurons and the associated chaotic saddle provide a possibility for switching between metastable states observed in information processing and brain function. Such transient dynamics have been observed experimentally by Mazor, when stimulating projection neurons in the locust antennal lobe with different odors.
• #### Transient spatiotemporal chaos in a Morris-Lecar neuronal ring network collapses to either the rest state or a traveling pulse

Transient spatiotemporal dynamics exists in an electrically coupled Morris-Lecar neuronal ring network, a theoretical model of an axo-axonic gap junction network. The lifetime of spatiotemporal chaos was found to grow exponentially with network size. Transient dynamics regularly collapses from a chaotic state to either the resting potential or a traveling pulse, indicating the existence of a chaotic saddle. For special conditions, a chaotic attractor can arise in the Morris-Lecar network to which transient chaos can collapse. The short-term outcome of a Morris-Lecar ring network is determined as a function of perturbation configuration. Perturbing small clusters of nearby neurons in the network consistently induced chaos on a resting network. Perturbation on a chaotic network can induce collapse in the network, but transient chaos becomes more resistant to collapse by perturbation when greater external current is applied.
• #### Transient spatiotemporal chaos on complex networks

Some of today's most important questions regard complex dynamical systems with many interacting components. Network models provide a means to gain insight into such systems. This thesis focuses on a network model based upon the Gray-Scott cubic autocatalytic reaction-diffusion system that manifests transient spatiotemporal chaos. Motivated by recent studies on the small-world topology discovered by Watts and Strogatz, the network's original regular ring topology was modified by the addition of a few irregular connections. The effects of these added connections on the system's transience as well on the dynamics local to the added connections were examined. It was found that the addition of a single connection can significantly effect the transient time of spatiotemporal chaos and that the addition of two connections can transform the system's spatiotemporal chaos from transient to asymptotic. These findings suggest that small modifications to a network's topology can greatly affect its behavior.
• #### Two- and three-dimensional study of the Kelvin-Helmholtz instability, magnetic reconnection and their mutual interaction at the magnetospheric boundary

Magnetic reconnection and the Kelvin-Helmholtz (KH) instability regulate the transport of magnetic flux, plasma, momentum and energy from the solar wind into the magnetosphere. In this thesis, I use two-dimensional and three-dimensional MHD simulations to investigate the KH instability, magnetic reconnection, and their relationship. Two basic flow and magnetic field configurations are distinguished at the Earth's magnetopause: (1) configurations where the difference in plasma velocity between the two sides of the boundary $\Delta$v (velocity shear) is parallel to the difference of the magnetic field $\Delta$b (magnetic shear), and (2) configurations where the velocity shear is perpendicular to the magnetic shear. For configuration (1), either magnetic reconnection is modified by the shear flow, or the KH instability is modified by the magnetic shear and resistivity. The evolution of the basic configuration (2) requires three dimensions. In this case, both processes can operate simultaneously in different planes. If the KH instability grows faster initially, it can wrap up the current layer and thereby initiate a very fast and turbulent reconnection process. The resulting magnetic turbulence can provide the first explanation of often very turbulent structures of the magnetopause current layer. For the first time, it is quantitatively confirmed that the KH instability operates at the magnetospheric boundary at low latitudes.
• #### Two-dimensional Bernstein-Greene-Kruskal modes in a magnetized plasma with kinetic effects from electrons and ions

Electrostatic structures are observed in various of space environments including the auroral acceleration region, the solar wind region and the magnetosphere. The Bernstein-Greene-Kruskal (BGK) mode, one of the non-linear solutions to the Vlasov-Poisson system, is a potential explanation to these phenomena. Specifically, two dimensional (2D) BGK modes can be constructed through solving the Vlasov-Poisson-Ampère system with the assumption of a uniform ion background. This thesis discusses the existence and features of the 2D BGK modes with kinetic effects from both electrons and ions. Specifically, we construct electron or ion BGK modes with finite temperature ratio between ions and electrons. More general cases, the electron-ion 2D BGK mode with the participation of both non-Boltzmann electron and ion distributions are constructed and analyzed as well.
• #### Variation of electron and ion density distribution along earth's magnetic field line deduced from whistler mode (wm) sounding of image/rpi satellite below altitude 5000 km

This thesis provides a detailed survey and analysis of whistler mode (WM) echoes observed by IMAGE/RPI satellite during the years 2000-2005 below the altitude of 5000 km. Approximately 2500 WM echoes have been observed by IMAGE during this period. This includes mostly specularly reflected whistler mode (SRWM) echoes and ~400 magnetospherically reflected whistler mode (MRWM) echoes. Stanford 2D raytracing simulations and the diffusive equilibrium density model have been applied to 82 cases of MRWM echoes, observed during August-December of the year 2005 below 5000 km to determine electron and ion density measurements along Earth's magnetic field line. These are the first results of electron and ion density measurements from WM sounding covering L-shells ~1.6-4, a wide range of geomagnetic conditions (Kp 0+ to 7), and during solar minima (F10.2~70-120) in the altitude range 90 km to 4000 km. The electron and ion density profiles obtained from this analysis were compared with in situ measurements on IMAGE (passive recording; electron density (Ne)), DMSP (~850 km; Ne and ions), CHAMP (~350 km; Ne), Alouette (~500-2000 km; Ne and ions), ISIS-1, 2 (~600-3500 km; Ne, ions), AE (~130-2000 km; ions) satellites, bottom side sounding from nearby ionosonde stations (Ne), and those by GCPM (Global Core Plasma Model), IRI-2012 (International Reference Ionosphere). Based on this analysis it is found that: (1) Ne shows a decreasing trend from L-shell 1.6 to 4 on both the day and night sides of the plasmasphere up to altitude ~1000 km, which is also confirmed by the GCPM and IRI-2012 model. (2) Above ~2000 km altitude, GCPM underestimates Ne by ~30-90% relative to RPI passive measurements, WM sounding results. (3) Below 1500 km, the Ne is higher at day side than night side MLT (Magnetic Local Time). Above this altitude, significant MLT dependence of electron density is not seen. (4) Ion densities from WM sounding measurements are within 10-35% of those from the Alouette, AE, and DMSP satellites. (5) The effective ion mass in the day side is more than two times higher than night side below altitude ~500 km. (6) The O⁺/H⁺ and O⁺/(H⁺+H⁺+) transition heights at day side are ~300-500 km higher than night side; the transition heights from the IRI-2012 model lie within the uncertainty limit of WM sounding for night side, but for day side (L-shell>2.5) they are 200 km higher than WM uncertainty limits. (7) foF2 (F2 peak plasma densities) from ionosonde stations and the IRI-2012 model are ~1.5-3 MHz higher than those from WM sounding during daytime. These measurements are very important as the ion density profile along geomagnetic field lines is poorly known. They can lead to a better understanding of global cold plasma distribution inside the plasmasphere at low altitude and thereby bridge the gap between high topside ionosphere and plasmasphere measurements. These results will provide important guidance for the design of future space-borne sounders in terms of frequency and virtual range, in order to adequately cover ion density measurements at low altitudes and wide range of MLTs, solar and geophysical conditions.
• #### Variational anodic oxidation of aluminum for the formation of conically profiled nanoporous alumina templates

Anodic oxidation of metals, otherwise known as anodization, is a process by which the metal in question is intentionally oxidized via an electrochemical reaction. The sample to be oxidized is connected to the anode, or positive side of a DC power source, while a sample of similar characteristics is attached to the cathode or negative side of the same power source. Both leads are then immersed in an acidic solution called the electrolyte and a current is passed between them. Certain metals such as aluminum or titanium anodized in this way form a porous oxide barrier, the characteristics of which are dependent on the anodization parameters including the type of acid employed as the electrolyte, pH of the electrolyte, applied voltage, temperature and current density. Under specific conditions the oxide formed can exhibit highly ordered cylindrical nanopores uniformly distributed in a hexagonal pattern. In this way anodization is employed as method for nanofabrication of ordered structures. The goal of this work is to investigate the effects of a varied potential difference on the anodization process. Specifically to affect a self-assembled conical pore profile by changing the applied voltage in time. Although conical pore profiles have been realized via post-processing techniques such as directed wet etching and multi-step anodization, these processes result in pore dimensions generally increasing by an order of magnitude or more. To date there has been reporting on galvanostatic or current variations which directly effected the resulting pore profiles, but to our knowledge there has not been a reported investigation of potentiostatic or voltage variation on the anodization process. We strive to realize a conical pore profile in process with the traditional two-step anodization method while maintaining the smallest pore dimensions possible. Pores having diameters below 20nm with aspect ratios about 1.0 would be ideal as those dimensions would be much closer to some of the characteristic lengths governing the quantum confined spatial domain. Thus we set out to answer the question of what effect a time varied potential difference will have on the traditional two-step anodization method, a technique we refer to as variational iodization, and if in fact conically profiled nanopores can be realized via such a technique.