Now showing items 1-20 of 106

    • Dynamics of the Earth's thermosphere across a range of spatial and temporal scales

      Itani, Rajan; Conde, Mark; Hampton, Donald; Bristow, William; Delamere, Peter (2023-12)
      This dissertation presents an investigation of aspects of the dynamics of Earth's thermosphere that do not harmonize with the current understanding. Three distinct thermospheric phenomena that correspond to different spatial and temporal scales were examined using Fabry-Perot interferometer measurements of wind and temperature to investigate aspects of thermospheric dynamics. Earth's thermosphere has very high kinematic viscosity and is highly convectively stable. Theory and physics-based models suggest that structures in the horizontal wind on small spatial scales (∼500 km or less) are unlikely to occur in the upper thermosphere. By contrast, a large-scale, persistent, strong wind flow that transports air parcels from the dayside of the polar cap to the nightside, known as cross-polar jet, was found to stall abruptly upon exiting the polar cap above Alaska. Stalling was observed most frequently around mid-winter during periods of low solar activity. The stalling of the crosspolar jet is considerably more abrupt than the first principle models would predict. This phenomenon was investigated as an example of dynamics occurring at intermediate spatial and temporal scales. Along with this meso-scale phenomena, oscillatory perturbations in observed winds and temperatures were examined to test whether these could be the signatures of gravity wave activity. These waves represent a local to synoptic scale behavior. Wave periods ranging from ∼30 min to ∼3 hr were observed. Our data show that gravity wave activity was a nearly ubiquitous feature of winds observed at ∼240 km altitude at auroral latitudes. The actual wave field appears to be complicated, presumably resembling ocean surface waves if they could be visualized. Oscillation amplitudes were typically found to increase with increasing geomagnetic activity and the wave response to geomagnetic activity was similar in both hemispheres. Periods and horizontal wavelengths of the observed oscillations fall within the previously reported range for thermospheric gravity waves. In addition, we examined the thermospheric neutral temperature data to see whether there is any temporal trend in measured temperatures on a time scale of decades. Studies suggest that the Earth's troposphere is warming globally because of anthropogenic emission of greenhouse gases. First principle models suggest that, unlike in the troposphere, greenhouse gases are expected to cool the thermosphere. Consistent with model expectations our data also show a thermospheric cooling trend and the rate of cooling is -27.4 ± 6.2 K/decade. The estimated rate of cooling is more than four times the corresponding uncertainty, indicating that the cooling is statistically significant. However, the observed cooling rate is up to an order of magnitude higher than suggested by simulation studies considering only the effect of CO₂. Nevertheless, the observed trend agrees well with observations of ionospheric temperature at these altitudes using incoherent scatter radar. This long-term temperature trend is an indicator of behavior at the largest spatial and temporal scales. Overall, these three studies together suggest significant shortcomings in our current paradigm for understanding the behavior of the thermosphere. Specifically, stalling of the cross-polar jet shows that spatial structures can arise on much more localized scales than currently appreciated. The ubiquitousness of gravity waves suggests that their role in thermospheric dynamics is probably much more significant than currently appreciated. The effect of anthropogenic changes in atmospheric chemistry appear to be much more substantial than expected. The lowest altitudes at which spacecraft can orbit for an operational useful length of time occur in the Earth's thermosphere. Conditions in the thermosphere impact spacecraft orbits. It is thus necessary to account for the thermospheric dynamics in order to predict the orbit with lowest possible uncertainty and for collision avoidance.
    • Exploring the robustness of a surrogate-based ice flow model calibration

      Blum, Kyle; Aschwanden, Andy; Newman, David; Truffer, Martin; Wackerbauer, Renate (2023-12)
      When simulating ice sheets using numerical models, model parameters have a great impact on the ice flow, velocity, and response to external forces, making them key factors in accurately predicting ice sheet behavior. In order to have confidence in modeled predictions of ice sheets, we must first have confidence in the physical models we use to simulate them. In order to have confidence in these physical models, we must first have confidence in our calibrations of these ice dynamics parameters. Researchers have been training neural networks to emulate expensive ice sheet models. These surrogate ice sheet models are designed to take ice flow parameters as input, and to output ice surface speed fields that closely resemble modeled fields that physically based ice sheet models would calculate. In this way, these surrogate models act as a computationally inexpensive alternative to map model parameter values to the resulting calculated ice speeds. These surrogates have been implemented and leveraged for Bayesian statistically based approaches to ice flow parameter calibrations that would otherwise be intractable using a high fidelity ice sheet model. By examining the methods we use to train these surrogates, and the extent to which the architecture of these surrogates affect their performance, we can have more confidence in our calibrations that make use of them. Here we present an analysis of the methods with which we train surrogate models as a means of calibrating important ice flow parameters. We focus on determining desirable characteristics for data over which the neural networks are trained, as well as the architecture of the surrogates themselves.
    • Applying information theory to GAMERA simulations of Jupiter-like magnetospheres

      Mino, Blake; Delamere, Peter; Ng, Chung-Sang; Zhang, Hui; Damiano, Peter (2023-08)
      Magnetospheres are poorly understood, largely due to lack of significant amounts of in-situ data and the complexity of the systems. Accurate numerical magnetohydrodynamic (MHD) magnetosphere simulation codes can bolster this lack of observational data. In this thesis, we explore applying mutual information theory techniques to a global-scale Jovian-like magnetosphere simulation provided by the Grid Agnostic MHD for Extended Research Applications (GAMERA) code. The mutual information between two variables is a measure of their dependency and is even capable of detecting nonlinear relationships, unlike traditional linear correlation calculations. Time shifted mutual information is a useful metric for identifying possible causal relationships and improving understanding of overall dynamics within the magnetosphere system. Data from the Juno spacecraft exhibits a 2-3 hour periodicity. Corotating density arm structures seen in the GAMERA simulation, if present at Jupiter, could contribute to this periodicity. We investigate how these density structures modulate the internal dynamics of the magnetosphere. Preliminary analysis indicates that the dynamics of the simulation are largely driven by the advection of the corotating density arm structures. A consistent 3-5 hour periodicity is noticeable and likely caused or contributed to by these density structures. Relationships at 10-20 hours are on the subcorotation timescale and are consistently more nonlinear than relationships found at shorter periodicities.
    • Design, calibration, and experiments to improve the feasibility of student-built magnetometers for heliophysics research

      Cohen, Austin David; Ozturk, Dogacan; Hampton, Don; Delamere, Peter; Hull, Michael (2023-08)
      Dynamic interactions between the solar wind and the Earth's magnetosphere can create strong geomagnetic field disturbances and trigger geomagnetically induced currents. Geomagnetically induced currents may cause damage to infrastructure such as damage to high-voltage power transformers and increased corrosion of pipelines. Ground observations of geomagnetic fields are widely used for geomagnetically induced current studies; however, there is insufficient information on the spatial extent of the localized geomagnetically induced current events due to a lack of spatial coverage. The Space Weather UnderGround is an education and outreach program, first initiated by Dr. Charles Smith at the University of New Hampshire and expanded to the University of Alaska Fairbanks. The Space Weather UnderGround program is aimed to educate high school students on space weather phenomena while equipping them with STEM skills. Students who participate learn to build a semi-professional magnetometer kit which is then used by researchers as a cost-effective and research-capable array of magnetometers across Alaska and New Hampshire. The Space Weather UnderGround magnetometer array provides high resolution geomagnetic field data with 1nT/s accuracy, and the data is made publicly available for improving our understanding and prediction of geomagnetically induced currents. Several University of Alaska Fairbanks Space Weather UnderGround magnetometer designs have been developed between 2021 and 2023, and various experiments and calibrations have been conducted to improve their applicability towards heliophysics research. This thesis will give a brief introduction to magnetometers and space weather, while focusing on the Simple Aurora Monitor and its use in the Space Weather UnderGround program. Next, sensor and deployment experiments will be described, including deployment vessels developed by the University of Alaska Fairbanks Space Weather UnderGround team. Finally, the data acquisition process, along with the educational outcomes of the project, will be discussed.
    • Generalized modeling of complex dynamical systems: an application to the stability of ecological networks

      Awender, Stefan; Wackerbauer, Renate; Breed, Greg; Newman, David; Doak, Pat (2023-05)
      Understanding the stability of food webs is crucial for resource sustainability and conservation of ecosystems, especially in the context of climate change. Specific models describe the biomass flow in food webs by a set of ordinary differential equations that require the explicit reconstruction of a mathematical expression for each of the interactions or processes, like predator-prey interactions, primary production, and mortality. Although specific models are rich with time-evolution information, limited access for empirical observation of these typically immense systems induce uncertainty in the data and approximations in the corresponding models that can threaten robustness or relevance of results. Generalized models can produce the stability of all the equilibria of specific models that have the same vague structure and bypasses the requirement to specify every function by evoking a normalizing transformation. The analysis is subsequently computationally efficient and can be used to study large food webs with a great number of replicates. Often generalized ecological network studies confine the scope to a small subset of the variable dynamical scenarios, but this limits the interpretations that can be inferred. With this in mind, we develop a deterministic food-web generator that can be used to compare large food webs that differ by only a single link and maintain an expansive dynamical scope. We found behavior that indicates the existence of critical links and a grander theory on topological equivalence. We explicitly show how we can create hypothetical paths the system may traverse upon enrichment of lower trophic levels using the expanded dynamical scope. Generalized modeling is unable to produce evolution solutions among other things, but it has an unlimiting access to the stability of equilibria while specific models provide only a subset of stability data. Generalized modeling is a relatively new method and its relation to specific model outcomes/results is not clearly understood. Specific models can inform generalized modeling studies on properties like coexistence of fixed points or actually occurring relative weighting of flows between ecosystem members. We combined the methods and demonstrated the validity of the abstract technique of generalized modeling in emphasis to its usefulness/power for the analysis of network stability. The specific model provided a unifying explanation to a conglomerate of related microcosm experiments that showed conflicting results on enrichment and implied stabilization upon the hampering of predatory efficiency. We identified the conditions by which enrichment is stabilizing to a steady state when basal species are in a resource-deprived environment but destabilizing if resources become more abundant. A prevalent issue in ecology involves discrepancies between simulation and empirical observation about food-web stability such as how intuition says enrichment or complexity in some way are favorable to stability but mathematical models find it predominately the opposite. A common rationalization for these discrepancies includes discourse on reductionistic versus holistic rhetoric. The idea being that as models become better representations of ecosystems that capture more intricacies and detail, they will help to resolve the issue. We constructed over a million food webs that reveal positive effects on fixed-point stability from the incorporation of more realistic ecosystem features that include species specialization, habitat modularity, and predator's prey preferences. Arctic warming is a portent to changes in species composition and ecological theory predicts the existence of key ecosystem members that have extraordinary influence on overall ecosystem function or the state of the system. Motivated by sea-ice loss and northward expansion of species distributions, ice-obligate species are removed from the food webs and southern competitors are introduced. Although it is common understanding that apex predators can enhance biodiversity, we find the presence of "super killers" significantly destabilizes food webs. Ecosystems have immense complexity with thousands of species, but ecosystem models condense and consider only a few species that are of the most interest or abundance, neglecting the many weak interactions comprising the larger ecosystem. Considering this, we suppose a food web is subsumed by a larger phantom ecological network that represents hypothetically rare species or predator-prey relationships. Each link from the phantom network contributes a variably weak perturbation, but collectively, induce a net positive effect on the average stability of the food webs, considerably so near the optimal perturbation strength.
    • A wiggle around a giant: exploring the hot electrons within the Io torus

      Coffin, Drew; Delamere, Peter; Demiano, Peter; Zhang, Hui; Newman, David (2022-12)
      Jupiter exhibits a fundamental rotational periodicity known as System IV that has no widely-accepted explanation, yet is easily observed in ultraviolet emission from the plasma torus generated by the innermost Galilean moon Io. This periodicity around Jupiter maps to a persistent, radially independent subcorotation within the Io plasma torus. In this thesis, we explore the origin and consequences of this periodicity. Using an equatorial chemistry and diffusive transport model, we demonstrate that a prescribed hot electron population produces a coherent wave of heightened energy flow that induces a consistent subcorotation. This additional hot electron population is consistent with the energization produced at high latitude by parallel electric fields induced by Alfvén waves propagating to the planet. The radial independence of this period means that while the generating mechanisms are likely in close proximity to the Io flux tube, the periodicity has dramatic consequences for plasma out at Europa, the next moon outwards and a subject of intense scientific curiosity.
    • Properties and formation mechanisms of foreshock transients

      Vu, Andrew; Zhang, Hui; Delamere, Peter; Otto, Antonius; Sibeck, David (2022-08)
      Interactions between bow shock-reflected (foreshock) ions with solar wind particles can lead to the formation of foreshock transients, frequently observed upstream of the bow shock. Foreshock transients, such as foreshock bubbles, hot flow anomalies, and spontaneous hot flow anomalies, display heated, tenuous cores with large flow deflections bounded by compressional boundaries or shocks. They aid in particle acceleration at the bow shock and their significant dynamic pressure depletions can disturb the magnetosphere-ionosphere system. Thus, studying foreshock transients will expand our understanding of shocks and of the solar wind-planetary magnetosphere coupling throughout the universe. The dissertation presents an observational statistical study and numerical simulations of foreshock transients to investigate their solar wind conditions, properties, and formation mechanisms. The statistical study shows that occurrences of foreshock transients are higher for lower magnetic field strengths and higher solar wind speeds and Alfvén Mach numbers. A geometrical approach with a model bow shock reveals that they typically span up to 3 Earth radii along and extend up to 6 Earth radii from the bow shock. The study also finds that foreshock transients with local density enhancements (substructures), in otherwise density-depleted cores, are often larger in size than those without substructures, and provide variances to the dynamic pressure profiles that could further deform the bow shock surface and lead to more complicated geoeffects. In addition, foreshock transients with and without substructures occur in the same solar wind conditions, thereby implying that substructures are an inherent property of foreshock transients during their evolution. In numerical simulations where injected foreshock ions perform partial gyrations around a discontinuity to generate Hall currents that change the magnetic field topology, foreshock bubbles form from thin discontinuities, and hot flow anomalies form from thick discontinuities. The simulations prove that the foreshock ion current configuration, controlled by the magnetic field change the foreshock ions experience within their gyromotions, determines the magnetic field profile of the structure. Furthermore, the parameter scan results show that the initial foreshock ion distribution types determine how easily foreshock ions can cross the discontinuity and thus how strong the structure forms; parameters that increase their densities or speeds perpendicular to the magnetic field across the discontinuity leads to stronger Hall currents and more significant magnetic field variations. We also find that the initial foreshock ion densities, thermal speeds, and beam speeds all positively and linearly correlate with the expansion speeds and the density compression ratios of the formed structures. This, thereby, provides a possibility to quantify the role of these parameters in the formation and expansion model of foreshock transients and to forecast their aforementioned particle acceleration and geoeffects.
    • Exospheric neutral density study using XMM-Newton soft x-ray observations and MHD-based magnetosheath model

      Jung, Jaewoong; Zhang, Hui; Connor, Hyunju K.; Carter, Jennifer A.; Sibeck, David G.; Newman, David (2022-08)
      Studying interactions between the solar wind and the Earth is important, and one future satellite mission, called Solar wind Magnetosphere Ionosphere Link Explorer (SMILE), will be launched in late 2024 or early 2025 to study these interactions. SMILE will image the magnetosheath using the charge exchange process and extract the location and motion of the bow shock and magnetopause. The subsolar magnetopause is typically around 10 Earth radii, and neutral density in this outer exosphere is poorly understood. For that reason, we study the neutral density around 10 Earth radii. We estimated the exospheric density at 10 Earth radii using XMM-Newton astrophysics observations, one taken during solar minimum and five taken during solar maximum. For the solar minimum case study, the lower limit of the exospheric density was estimated to be 36.8 ± 11.7 cm⁻³ at 10 Earth radii subsolar point. For solar maximum case studies, we estimated neutral density to be in the range of 42.5 - 65.1 cm⁻³ at 10 Earth radii subsolar point. This suggests weak dependence of neutral density on solar activity but more statistical analysis is needed. The neutral density behavior of the outer exosphere will help us understand the Earth's atmospheric loss due to the dynamic space environment and thus, infer the entire evolutionary history of the Earth's atmosphere as well as of other planetary atmospheres. Along with neutral density, plasma number density, velocity, and temperature in the magnetosheath are key parameters for predicting soft X-ray images. We developed a user-friendly model of magnetosheath parameters to help derivation of these parameters in future wide field-of-view soft X-ray missions. The model parameterizes number density, velocity, magnetic field, and temperature, by using OpenGGCM MHD simulation results as seed data. We made a suite of magnetosheath models, by compiling pre-existing magnetosheath models (analytic, gas-dynamic) with our MHD-based model. This parameterized model is expected to enable researchers to reconstruct expected soft X-ray images and also use these images for analysis of observed images from future satellite missions including SMILE.
    • Phase effects on turbulent transport in the magnetic confinement of plasmas for nuclear fusion

      Rogers, Dempsey; Newman, David; Delamere, Peter; Truffer, Martin (2021-12)
      With climate change effects on the rise, the global energy infrastructure requires revision. We first provide a brief review of common energy resources as well as their safety and climate effects. We then compare and contrast nuclear fission and fusion based energy schemes. Difficulties based on the requirements of the fusion triple product, as well as the fast neutrons from the deuterium and tritium reaction are also discussed. The lack of sufficient experimental controls in enhanced confinement modes like the I-mode and the H-mode, lead to difficulties satisfying the restrictions imposed by the Greenwald density limit. These combined with several operational needs like ash and impurity removal, enhanced density control, the ability to access other confinement modes at reduced energy thresholds, motivates the search for a barrier capable of variable energy and density confinement. Self consistent models suggest that unique phase relationships exist between different turbulent instabilities and plasma profiles like temperature and density, that determine the turbulent transport of the quantity. Two common instabilities, driven by the electron and ion temperature gradient, and their unique phase relations are used to arrive at a net phase relation for temperature and for density. Then, using electron and ion radio frequency heating, the difference in phase of the turbulent transport may be locally changed, altering transport dynamics. Methods to increase core temperature while simultaneously increasing density transport, thereby avoiding the Greenwald limit, are discussed. The proposed transport controls are based upon characteristics of the localized radio frequency heating including amplitude, location, and duration. These parameters determine the power deposited in the plasma, and therefore the local ratios of the electron and ion temperature driven instabilities. Aspects of each parameter's effect on radial transport are summarized, with the strongest phase barrier allowing for a ∼ 15% increase of core ion temperature and ∼ 30% decrease of core density.
    • The distribution of nitric oxide at 150 km

      Stern, Timothy E. (2008-12)
      "The objectives of this thesis are to determine the morphology of nitric oxide at the altitude of 150 km and to determine what drives the observed variability. Those objectives are accomplished by characterizing satellite observations of nitric oxide at that altitude and comparing them with those at 106 km, the altitude of peak density. The global distribution of nitric oxide and its response to geomagnetic activity vary between the two altitudes. At 150 km, nitric oxide is most abundant at high latitudes in the sunlit summer hemisphere, in contrast to nitric oxide at 106 km, which is most abundant at high latitudes in the winter hemisphere. The high-latitude component of nitric oxide at both altitudes is associated with geomagnetic activity, although the primary production mechanisms differ between the two altitudes. At 106 km, high-latitude nitric oxide density enhancements are driven by particle precipitation. At 150 km, nitric oxide at high latitudes is enhanced by increased temperatures arising from Joule heating. Enhancements at 150 km occur more rapidly than those at 106 km. At both altitudes, the response of nitric oxide to geomagnetic activity exhibits a seasonal variation that is attributed to seasonal variations in the production mechanisms"--Leaf iii
    • Determination of the diffusion coefficient for trimethylaluminum in the thermosphere at altitudes 120 to 180 km

      Bhattacharya, Tapas (2009-05)
      "The object of this work is to determine the diffusion coefficient (D) of trimethylaluminum (TMA) in the lower thermosphere as a function of altitude (h). This is done by measuring the dispersion of chemiluminescent TMA that is released in discrete quantities, or puffs, from sounding rockets at altitudes 120 to 180 km. Diffusing TMA, which glows in contact with atmospheric oxygen, is observed with stereoscopic ground-based imaging. Brightness profiles across a puff are found to be Gaussian in shape, with width parameter [sigma](t, h) that increases with age (t) of the puff leading to D = [sigma]² (t, h)/2t, independent of time, which is in good agreement with some past results. For example D = (2.5 ± 0.2) x 10³m²s⁻¹ at an altitude of 128 km for the state of the thermosphere at that time. A constant A links three altitude-dependent terms, the diffusion coefficient, temperature and density, at a particular location of the atmosphere, via D(h) = ATS (h)/n(h). It is determined from this study to be A=(4.42±0.05)x10¹⁸(m·s)⁻¹ for s = 0.75. Using these values for A and s, and temperatures and the densities determined from the MSIS-90 thermospheric model, diffusion coefficients for TMA can be determined at other locations and under different geomagnetic conditions"--Leaf iii
    • Plasma transport and magnetic flux circulation in Saturn's magnetosphere

      Neupane, Bishwa Raj; Delamere, Peter; Ng, Chung-Sang; Newman, David; Wackerbauer, Renate (2021-08)
      The magnetospheres of outer planets are very different than the terrestrial magnetosphere. The magnetosphere of Saturn is rapidly rotating, and has its own plasma source. Enceladus located around 4Rs is the main source of plasma. The strong magnetic field of Saturn's magnetosphere picks up the plasma which experiences a strong centrifugal force in the non-inertial reference frame. The plasma produced in the inner magnetosphere has to be transported radially outward and lost to the solar wind. The transport of plasma in Saturn's magnetosphere is not fully understood. It is believed that transport is centrifugally-driven, occurring via flux tube interchange motions in the inner magnetosphere and via plasmoid expulsion in the magnetotail due to reconnection. It has been found that these mechanisms are not sufficient to explain the transport. We tried to determine different possible transport mechanisms that could exist in the outer planetary magnetosphere. Ma et al. (2019a) showed the low-specific entropy plasma with a narrow distribution in Saturn's inner magnetosphere and suggests a significant nonadiabatic cooling process during the inward motion while high specific entropy suggests the nonadiabatic heating during the outward transport. We have estimated the outward plasma transport rate about 55 kg s⁻¹. The calculation of magnetic flux transport and analysis of magnetic field data indicates that plasma transport in the Saturn magnetosphere could be dominated by small scale magnetic reconnection.
    • Modeling supraglacial lake drainage and its effects on the seasonal evolution of the subglacial drainage system in a tributary glacier setting

      Franco, Nevil Arley; Truffer, Martin; Wackerbauer, Renate; Delamere, Peter (2021-08)
      This work aims to gain a better understanding of the relationship between glacier motion and water distributed through subglacial drainage systems. A numerical scheme (GlaDS) is used to model both inefficient and efficient drainage systems to see which dominates after the draining of a supraglacial lake on a synthetic glacier that is made up of an outline that features a main branch and a tributary. The geometry is based on the surgetype Black Rapids Glacier (Ahtna Athabascan name: Da lu'itsaa'den) in Alaska, where a lake develops in the higher ablation zone, and drains rapidly early in the melt season. It has also been observed that this lake drainage causes a twofold or threefold speed-up of the main branch, with some acceleration of the lower-lying Loket tributary. This speed-up can be considered a surrogate for a surge, which also initiates in the main branch, while, during times of quiescence, the ice flow on the tributary is dominant. We investigate the effects of varying timing and volume inputs of lake drainage with a focus on its effects beneath the tributary. We find that the response of the glacier depends on the seasonal timing, the amount of water from the draining lake, and its location on or near the margins of the glacier. Results show that an inefficient drainage system is the cause of the glacier speed-up, both when the lake drains rapidly or when there is an extended time in drainage, at any time of the season. The speed signals vary throughout the glacier depending on the location of the lake relative to that of an evolved efficient drainage system.
    • Control of internal transport barriers in magnetically confined tokamak fusion plasmas

      Panta, Soma Raj; Newman, David; Wackerbauer, Renate; Ng, Chung Sang; Sanchez, Raul (2020-07)
      In the Tokamak plasma, for fusion to be possible, we have to maintain a very high temperature and density at the core at the same time keeping them low at the edge to protect the machine. Nature does not favor gradients. Gradients are source of free energy that causes instability. But we require a large gradient to get energy from plasma fusion. We therefore, apply a huge magnetic field on the order of few Tesla (1 T-10 T) that confines the plasma in the core, maintaining gradients. Due to gradients in density of charged particles (ions and electrons), there is an electric field in the plasma. Heat and particle transport takes place from core to edge mainly through anomalous transport while the E x B velocity sheer acts to reduce the transport of heat and particles. The regime at which the E x B velocity shear exceeds the maximum linear instability growth rate, as a result, the transport of particles and heat gets locally reduced is termed as the formation of a transport barrier. This regime can be identified by calculating the transport coefficients in the local region. Sometimes it can be observed in the edge where it is called an edge barrier while if it is near the core it is an internal transport barrier. There is a positive feedback loop between gradients and transport barrier formation. External heating and current drives play an important role to control such barriers. Auxiliary heating like neutral beam injection (NBI) and radio frequency (RF) heating can be used at a proper location (near the core of the plasma) to trigger or (far outside from the core) to destroy those barriers. Barrier control mechanism in the burning plasmas in international thermonuclear test reactor (ITER) parameter scenarios employing fusion power along with auxiliary heating source and pellets are studied. Continuous bombardment with pellets in the interval of a fraction of a second near the core of the burning plasma results in a stronger barrier. Frozen pellets along with auxiliary heating are found to be helpful to control the barriers in the tokamak plasmas. Active control mechanism for transport barriers using pellets and auxiliary heating in one of tokamaks in United States (DIII-D) parameter scenarios are presented in which intrinsic hysteresis is used as a novel control tool. During this process, a small background NBI power near the core assists in maintaining the profile. Finally, a self-sustained control mechanism in the presence of core heating is also explored in Japanese tokamak (JT-60SA) parameter scenarios. Centrally peaked narrow NBI power is mainly absorbed by ions with a smaller fraction by the electrons. Heat exchange between the electron and ion channels and heat conduction in the electron channel are found to be the main processes that govern this self control effect. A strong barrier which is formed in the ion channel is found to play the main role during the profile steepening while the burst after the peaked core density is found to have key role in the profile relaxation.
    • Regional modeling of Greenland's outlet glaciers with the parallel ice sheet model

      Della-Giustina, Daniella N. (2011-12)
      The most recent report from the Intergovernmental Panel on Climate Change cites ice sheet dynamics as the greatest source of uncertainty for predicting current and future rates of sea level rise. This has prompted the development and use of ice sheet models that are capable of simulating the flow and evolution of ice sheets and their corresponding sea level contribution. In the Arctic, the Greenland ice sheet appears to be responding to a warming climate more quickly than expected. In order to determine sea level contribution from Greenland, it is necessary to capture the regional dynamics of the fast flowing outlet glaciers that drain the ice sheet. This work has developed a novel regional model capable of simulating an outlet glacier, and its associated drainage basin, as a mode of using the Parallel Ice Sheet Model. Specifically, it focuses on modeling the Jakobshavn Isbrae as a demonstration. The Jakobshavn Isbrae is one of the world's fastest flowing outlet glaciers, and accounts for nearly 5% of ice loss from the Greenland Ice Sheet. Additionally, the Jakobshavn Isbrae has been widely studied for several decades, and a wealth of remotely sensed and in situ data is available in this region. These data are used as model input and for model validation. We have completed a parameter study in this work to examine the behavior of the regional model. The purpose of this study was not to tune the model to match observations, but rather to look at the influence of parameter choices on the ice dynamics. Model results indicate that we have identified the subset of the model parameter space that is appropriate for modeling this outlet glacier. Additionally, we are able to produce some of this more interesting features that have been observed at Jakobshavn, such as the development and disintegration of a floating ice tongue and the distribution of observed surface velocities. We validate these model results by comparison with recent spatially rich measurements of ice surface speeds, as well as ice geometry.
    • Transient spatiotemporal chaos in a Morris-Lecar neuronal ring network collapses to either the rest state or a traveling pulse

      Keplinger, Keegan (2012-12)
      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.
    • Two-dimensional Bernstein-Greene-Kruskal modes in a magnetized plasma with kinetic effects from electrons and ions

      Tang, Han; Chung-Sang, Ng; Delamere, Peter; Newman, David (2020-05)
      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.
    • Solar magnetic fields: source, evolution, and interaction with planetary magnetospheres

      Burkholder, Brandon; Delamere, Peter; Otto, Antonius; Newman, David; Ng, Chung-Sang; Connor, Hyunju (2019-08)
      Magnetized plasmas with twisted and filamented magnetic fields are pervasive throughout the heliosphere. In the solar magnetic field, photospheric convection on scale sizes from granules to differential rotation is responsible for driven magnetic reconnection. These reconnection sites are closely related to the magnetic topology, which is highly complex as the magnetic field is structured by a network of many thousands of magnetic flux concentrations. The coronal plasma overlying this "magnetic carpet" is the source of the solar wind flow, which has been found to be turbulent as close to the sun as our observations can currently resolve. At 1 AU, observations have also revealed a highly structured solar wind which we posit in this thesis originates in the corona rather than forming in-transit. Further, the solar wind-magnetosphere interaction depends on variability in the solar wind. When the boundary between solar wind plasma and magnetospheric plasma is unstable to the growth of Kelvin-Helmholtz waves, driven magnetic reconnection can occur on the magnetopause boundary. Such reconnection allows magnetic field to thread the boundary and transport can take place. We quantify the solar wind interaction for a corotation dominated system in terms of the mass and momentum transport driven by Kelvin-Helmholtz instabilities. Model-data comparisons are performed in this thesis using both the magnetohydrodynamic and hybrid-kinetic approaches for fluid simulations.
    • Risk analysis of Cordova's microgrid from a complex systems viewpoint

      Bowker, Anna Lipetzky; Newman, David; Huang, Daisy; Wackerbauer, Renate (2019-08)
      Cordova is a town of approximately 2,000 people located on the southern coast of Alaska. A power grid for a town this size, with a large seasonal fishing economy, is considered a moderate to large sized microgrid in terms of power produced. Understanding the vulnerabilities and risks of failures in such a grid is important for planning and operations and investigating these characteristics in the context of complex system dynamics is novel. The analysis of Cordova's microgrid is a case study relevant to a large class of microgrid communities that could benefit from this work. Our analysis of this grid began by looking at the distribution of all outages from 2003 - 2017 by size, followed by splitting up outages based on certain characteristics and again looking at outage size distribution based on different characteristics. Following this we correlated the outages with different weather patterns and then with the hourly load demand on the system. After doing these analyses we developed a risk metric to give a single numerical value to the risk of an outage occurring during certain time periods and under certain conditions. We looked at risk in the summer versus the winter due to the summer having a much larger load demand, and we also looked at the risk before and after all cables in the grid were buried underground. This gives us an idea of when/under what circumstances the most outages are likely to occur and allows us to run our model of the system, make changes, and determine if those changes were beneficial to the system or not.
    • The effect of rate, frequency, and form of migration on host parasite population dynamics

      Mottet, Geneva; Drown, Devin M.; Newman, David; Wackerbauer, Renate (2019-08)
      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.