• Radial and azimuthal dynamics of the io plasma torus

      Copper, Matthew; Delamere, Peter; Ng, Chung-Sang; Otto, Antonius (2015-05)
      The moon Io orbits Jupiter emitting neutral particles from its volcanic surface. This emission is ionized and forms the Io plasma torus around Jupiter. The variation of conditions at Io and Jupiter lead to variations in the content of the plasma in the torus. Volcanoes on Io's surface erupt and change the rate of neutral input. Hot electrons (30-100 eV), whose abundances vary in azimuth, create highly ionized species. Radial variation in subcorotation velocities, velocities less than than that of the motion of the dipole magnetic field, creates shears while maintaining coherent radial structure in the torus. Poorly understood changes in plasma density circulate through the torus creating the anomalous System IV behavior that has a period slightly longer than the rotation of Jupiter's magnetic field. This thesis summarizes the research that has produced a two-dimensional physical chemistry model, tested several existing theories about subcorotation velocities, System IV variation, and hot electrons, and adopted new methods of Io plasma torus analysis. In an attempt to understand important dynamics, the thesis modeled differing scenarios such as an initialized two-peak structure, a subcorotation profile dictated by mass loading and ionospheric conductivity, and a critical combination of two populations of hot electrons that accurately mimics the observed System IV phenomenon. This model was also used to solve the inverse problem of determining the best fit for the model parameters, neutral source input rate and radial transport rate, using observations of density, temperature, and composition. In addition the thesis shows the need for multi-dimensional modeling and the results from its groundbreaking two-dimensional model.
    • Radiation transport in cloudy and aerosol loaded atmospheres

      Kylling, Arve; Stamnes, Knut; Shaw, Glenn E.; Weeks, Wilford W.; Rees, Manfred H.; Smith, Roger W. (1992)
      The equation for radiation transport in vertical inhomogeneous absorbing, scattering, and emitting atmospheres is derived from first principles. It is cast in a form amenable to solution, and solved using the discrete ordinate method. Based on the discrete ordinate solution a new computationally efficient and stable two-stream algorithm which accounts for spherical geometry is developed. The absorption and scattering properties of atmospheric molecules and particulate matter is discussed. The absorption cross sections of the principal absorbers in the atmosphere, H$\sb2$O, CO$\sb2$ and O$\sb3,$ vary erratically and rapidly with wavelength. To account for this variation, the correlated-k distribution method is employed to simplify the integration over wavelength necessary for calculation of warming/cooling rates. The radiation model, utilizing appropriate absorption and scattering cross sections, is compared with ultraviolet radiation measurements. The comparison suggests that further experiments are required. Ultraviolet (UV) and photosynthetically active radiation (PAR) is computed for high and low latitudes for clear and cloudy skies under different ozone concentrations. An ozone depletion increases UV-B radiation detrimental to life. Water clouds diminish UV-B, UV-A and PAR for low surface albedos and increase them for high albedos. The relative amount of harmful UV-B increases on overcast days. The daily radiation doses exhibit small monthly variations at low latitudes but vary by a factor of 3 at high latitudes. Photodissociation and warming/cooling rates are calculated for clear skies, aerosol loaded atmospheres, and atmospheres with cirrus and water clouds. After major volcanic explosions aerosols change O$\sb3$ and NO$\sb2$ photodissociation rates by 20%. Both aged aerosols and cirrus clouds have little effect on photodissociation rates. Water clouds increase $(\sim$100%) photodissociation rates that are sensitive to visible radiation above the cloud. Solar warming rates vary by 50% in the stratosphere due to changing surface albedo. Water clouds have a similar effect. The net effect of cirrus clouds is to warm the troposphere and the stratosphere. Only extreme volcanic aerosol loadings affect the terrestrial warming rate, causing warming below the aerosol layer and cooling above it. Aerosols give increased solar warming above the aerosol layer and cooling below it.
    • Radiation transport in the atmosphere - sea ice - ocean system

      Jin, Zhonghai; Stamnes, Knut; Lynch, Amanda; Rees, Manfred H.; Shaw, Glenn E.; Tsay, Si-Chee; Weeks, Wilford F. (1995)
      A comprehensive radiative transfer model for the coupled atmosphere-sea ice-ocean system has been developed. The theoretical work required for constructing such a coupled model is described first. This work extends the discrete ordinate method, which has been proven to be effective in studies of radiative transfer in the atmosphere, to solve the radiative transfer problem pertaining to a system consisting of two strata with different indices of refraction, such as the atmosphere-ocean system and the atmosphere-sea ice-ocean system. The relevant changes (as compared to the standard problem with constant index of refraction throughout the medium) in formulation and solution of the radiative transfer equation, including the proper application of interface and boundary conditions, are presented. This solution is then applied to the atmosphere-sea ice-ocean system to study the solar energy balance in this coupled system. The input parameters required by the model are observable physical properties (e.g., the profiles of temperature and gas concentrations in the atmosphere, and the profiles of temperature, density, and salinity in the ice). The atmosphere, sea ice and ocean are each divided into a sufficient number of layers in the vertical to adequately resolve changes in their optical properties. This model rigorously accounts for the multiple scattering and absorption by atmospheric molecules, clouds, snow and sea water, as well as inclusions in the sea ice, such as brine pockets and air bubbles. The effects of various factors on the solar energy distribution in the entire system have been studied quantitatively. These factors include the ice salinity and density variations, cloud microphysics as well as variations in melt ponds and snow cover on the ice surface. Finally, the coupled radiative transfer model is used to study the impacts of clouds, snow and ice algae on the light transport in sea ice and in the ocean, as well as to simulate spectral irradiance and extinction measurements in sea ice.
    • Radiative transfer modeling in the coupled atmosphere-ocean system and its application to the remote sensing of ocean color imagery

      Yan, Banghua; Stamnes, Knut; Nielsen, Hans; Watkins, Brenton; Olson, John (2001-08)
      Ocean color is the radiance emanating from the ocean due to scattering by chlorophyll pigments and particles of organic and inorganic origin. Thus, it contains information about chlorophyll concentrations which can be used to estimate primary productivity. Observations of ocean color from space can be used to monitor the variability in marine primary productivity, thereby permitting a quantum leap in our understanding of oceanographic processes from regional to global scales. Satellite remote sensing of ocean color requires accurate removal of the contribution by atmospheric molecules and aerosols to the radiance measured at the top of the atmosphere (TOA). This removal process is called 'atmospheric correction.' Since about 90% of the radiance received by the satellitee sensor comes from the atmosphere, accurate removal of this portion is very important. A prerequisite for accurate atmospheric correction is accurate and reliable simulation of the transport of radiation in the atmosphere-ocean system. This thesis focuses on this radiative transfer process, and investigates the impact of particles in the atmosphere (aerosols) and ocean (oceanic chlorophylls and air bubbles) on our ability to remove the atmospheric contribution from the received signal. To explore these issues, a comprehensive radiative transfer model for the coupled atmosphere-ocean system is used to simulate the radiative transfer process and provide a physically sound link between surface-based measurements of oceanic and atmospheric parameters and radiances observed by satellite-deployed ocean color sensors. This model has been upgraded to provide accurate radiances in arbitrary directions as required to analyze satellite data. The model is then applied to quantify the uncertainties associated with several commonly made assumptions invoked in atmospheric correction algorithms. Since Atmospheric aerosols consist of a mixture of absorbing and non-absorbing components that may or may not be soluble, it becomes a challenging task to model the radiative effects of these particles. It is shown that the contribution of these particles to the TOA radiance depends on the assumptions made concerning how these particles mix and grow in a humid environment. This makes atmospheric correction a very difficult undertaking. Air bubbles in the ocean created by breaking waves give rise to scattered light. Unless this contribution to the radiance leaving the ocean is correctly accounted for, it would be mistakenly attributed to chlorophyll pigments. Thus, the findings in this thesis make an important contribution to the development of an adequate radiative transfer model for the coupled atmosphere-ocean system required for development and assessment of algorithms for atmospheric correction of ocean color imagery.
    • Recurrence analysis methods for the classification of nonlinear systems

      Graybill, Mark; Wackerbauer, Renate; Chowdhury, Ataur; Newman, David (2014-05)
      Recurrence is a common phenomenon in natural systems: A system enters and leaves a state, but after a given period of time, passes near that same state again. Many complex signals, such as weather cycles, heartbeats, or neuron firing patterns, all show recurrence. The recurrence plot (RP) displays all times j where a system returns near a state it has occupied at time i, giving rise to upward-sloping diagonal lines where a system follows a recurrent path, orthogonal lines when the system changes very slowly, or many disconnected points where a system's behavior is unpredictable. Investigation of the RP can then proceed through recurrence quantification analysis (RQA). Three new measures for RQA were developed: diagonality, quantifying diagonal lines, verticality, quantifying vertical lines, and periodicity quantifying the arrangement of recurrence points in periodic structures. These new measures were applied alongside classical recurrence measures to explore trends in random data, identify periodicity and chaotic behavior in the logistic map, estimate the dimensionality of the Lorenz attractor, and discriminate between persistent data signals. In collaboration with biologist Dr. Michael Harris, RQA methods were applied to the discrimination of two neuron types: serotonergic cells are believed to stimulate respiration, while nonserotonergic cells are implicated in respiratory inhibition. Typical discrimination methods compare mean and standard deviation of firing rates to a reference line, which correctly classifies serotonergic cells but incorrectly classifies many nonserotonergic cells. Voltage signals from such cells were converted into inter-spike intervals. Convergence required trials containing over 300 spikes for biological methods, and over 1000 for full investigation using RQA. Whether such cells can be discriminated from baseline firing patterns remains an open question.
    • 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.
    • Remote sensing of surface albedo and cloud properties in the Arctic from AVHRR measurements

      Han, Wei; Stamnes, Knut; Bowling, Sue Ann; Harrison, William; Li, Shusun; Lubin, Dan; Watkins, Brenton (1996)
      Based on a comprehensive radiative transfer model, algorithms suitable for arctic conditions are developed to retrieve broadband surface albedo and water cloud properties from National Oceanic and Atmospheric Administration (NOAA) Advanced Very High Resolution Radiometer (AVHRR) narrowband measurements. Reflectance anisotropy of snow surfaces is first simulated by an discrete ordinates radiative transfer formulation, and is then included in the comprehensive model for the retrieval. Ground-based irradiance measurements made by NOAA Climate Monitoring and Diagnostics Laboratory (CMDL) in Barrow, Alaska are compared with retrieved albedo and downwelling irradiances computed from retrieved cloud optical depth and effective radius. Good agreement is found between satellite estimates and ground-based measurements, which indicate that the retrieval algorithms proposed in this thesis are suitable for arctic conditions. It is found that the effects of snow bidirectional reflectance on the retrieval of the broadband albedo are significant, and that the Lambertian approximation could lead to a 30% underestimate of the surface albedo. It is also found that cloud effective radius in the Arctic is generally smaller as compared with mid- and low-latitudes.
    • Reversability Of Arctic Sea Ice Retreat - A Conceptual Multi-Scale Modeling Approach

      Mueller-Stoffels, Marc; Wackerbauer, Renate (2012)
      The ice-albedo feedback has been identified as an important factor in the decay of the Arctic sea ice cover in a warming climate. Mechanisms of transition from perennial ice cover to seasonal ice cover are discussed in the literature; the existence of a tipping point is disputed. A newly developed regular network model for energy exchange and phase transition of an ice covered ocean mixed layer is introduced. The existence of bistability, a key ingredient for irreversibility, on local and regional scales is explored. It is shown in a spatially confined model that the asymptotic behavior and the existence of a parameter region of bistability strongly depend on the albedo parametrization. The spatial dynamics of sea ice retreat are studied for a high resolution latitudinal model of the ocean mixed layer. This regional model suggests that sea ice retreat is reversible. It is shown that laterally driven melt of thick multi-year sea ice, and thus, ice-albedo feedback, is an important mechanism in the transition from perennial to seasonal ice cover at the pole. Results are used to interpret observed changes in the recent ice extent and ice volume record. It is shown that the effectiveness of ice-albedo feedback strongly depends on the existence of lateral heat transfer mechanisms in the ocean.
    • 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.
    • Role Of Conductivity Spatial Structure In Determining The Locations Of Sprite Initiation

      Tavares, Fernanda De Sao Sabbas; Jeffries, Martin O. (2003)
      Sprites are transient optical signatures of mesospheric electrical breakdown in response to lightning discharges. Multiple sprites are often observed to occur simultaneously, laterally displaced from the underlying causative cloud-to-ground (CG) lightning discharge. The causes of this lateral displacement are presently not understood. This dissertation investigates the role of neutral density perturbations in determining the locations of sprite initiation. The work was performed in three interrelated studies. (1) A detailed statistical study of the temporal-spatial relationships between sprites and the associated CG was performed for July 22, 1996. The distribution of sprite offsets relative to the underlying lightning had a mean of ~40 km. The distribution of sprite onset delays following the parent lightning had a mean of ~20--30 ms, consistent with theoretical estimates for the electron avalanche-to-streamer transition in the mesosphere. (2) A follow-up study for the same observations was performed to investigate the relationship of the sprites to convective activity in the underlying thunderstorm, using GOES-8 infrared imagery of cloud-top temperatures. The sprite generating thunderstorm was a Mesoscale Convective System (MCS). The maximum sprite and -CG production of the system were simultaneously reached at the time of maximum contiguous cloud cover of the coldest region, corresponding to the period of greatest convective activity of the system. Thunderstorm convective activity is a potential source of gravity waves and mesospheric turbulence. (3) Computer simulations of the temporal-spatial evolution of lightning-induced electric fields in a turbulent upper atmosphere were performed. The modeled turbulence in the simulations spanned the amplitude range 10% to 40% of the ambient background neutral density, with characteristic scale sizes of 2 km and 5 km, respectively. The results indicate that neutral density spatial structure, similar to observed turbulence in the mesosphere, facilitates electrical breakdown in isolated regions of density depletions at sprite initiation altitudes. These spatially distributed breakdown regions provide the seed electrons necessary for sprite generation, and may account for the observed sprite offsets.
    • Role of ionospheric conductance in magnetosphere-ionosphere coupling

      Bhattacharya, Tapas; Otto, Antonius; Bristow, William; Conde, Mark; Lummerzheim, Dirk; Ng, Chung-Sang (2014-08)
      Magnetosphere-ionosphere (MI) coupling has been studied for a long time. However, not much work has been done on a systematic understanding of the relation between ionospheric Pedersen conductance, its effect on the evolution and modification of field-aligned currents (FACs), and the influence of conductance and FACs on the formation of parallel electric fields which cause particle precipitation. Though the roles of ionospheric conductance gradients for FACs and parallel electric field evolution are directly related, they are poorly understood. This dissertation advances the understanding of these areas and all results of this study are based on numerical simulations that employ a three-dimensional - two-fluid (ions and neutrals) simulation code. The first part of this dissertation presents a systematic study of the magnetospheric and ionospheric influences on the evolution and modification of FACs with focus on the role of ionospheric Pedersen conductance and its gradients. FACs are typically generated in the magnetosphere and are carried into the ionosphere by Alfvén waves. During their reflection from the ionosphere these FACs are modified depending on the magnitude and distribution of ionospheric conductance. For conductance gradients along the polarization of the wave, strong Pedersen currents can be generated which in turn enhance the FAC as well. The second part of this dissertation addresses the properties and evolution of parallel electric fields in an attempt to better understand the formation of discrete auroral arcs in response to the evolution of FACs for predetermined ionospheric conductance patterns. Frequently, auroral acceleration is believed to occur through U or V shaped potentials. Therefore, this part examines the properties of localized parallel electric fields in a uniform magnetic field. It is demonstrated that localized parallel electric fields generate magnetic flux in the absence of source of free energy. It is also shown that parallel electric fields generated in a FAC in the presence of a (anomalous) resistivity represent a load and can provide physical explanation for the auroral acceleration geometry. The results demonstrate that such electric fields can be significantly enhanced by Alfvén wave reflection where both magnitude and gradients of the ionospheric conductance are important. The strongly enhanced parallel electric field is associated with magnetic reconnection and modifies the FAC system such that thin current layers (with curls and folds) are observed to be embedded in the large scale current system.
    • A simulation study of magnetic reconnection processes at the dayside magnetopause

      Shi, Yong; Lee, L. C.; Akasofu, S-I.; Gatterdam, R.; Gosink, J.; Swift, D. W. (1989-12)
      In this thesis, the day side reconnection processes are studied by using computer simulations. First, the global magnetic reconnection patterns at the dayside magnetopause are studied based on a two-dimensional incompressible magnetohydrodynamic (MHD) code. It is found that multiple X line reconnection may prevail at the dayside magnetopause when the magnetic Reynolds number is large (> 200). The formation and subsequent poleward convection of magnetic islands are observed in the simulation. The Alfvén Mach number of the solar wind, MAsw , cam also change the reconnection patterns. For a large reconnection tends to occur at the higher latitude region. Secondly, the structure of the dayside reconnection layer is studied by a two-dimensional compressible MHD simulation. In a highly asymmetric configuration typical of the dayside magnetopause, the pair of slow shocks bounding the reconnection layer in Petschek’s symmetric model is found to be replaced by an intermediate shock on the magnetosheath side and a weak slow shock on the magnetospheric side. In addition, a mechanism for the enhancement of By, which is observed in the magnetopause current layer and magnetic flux tubes, is proposed.
    • Solar Flare Soft X -Ray Irradiance And Its Impact On The Earth's Upper Atmosphere

      Rodgers, Erica M.; Bailey, Scott (2007)
      Solar flares dramatically enhance the soft X-ray region of the solar spectrum. The enhancement is more significant than previously thought, and the solar soft X-ray instruments aboard the Thermosphere Ionosphere Mesosphere Energetics Dynamics (TIMED) and Solar Radiation and Climate Experiment (SORCE) satellites have observed more flares than expected. This dissertation presents a state-of-the-art analysis used to determine flare spectra from TIMED and SORCE solar observations. A relationship is established between Geostationary Operational Environmental Satellite (GOES) flare 0.1-0.8 nm irradiances and XPS flare 0.1-2 and 0.1-7 nm irradiances. Solar flares primarily enhance the soft X-ray irradiance in the 0.1-2 nm range, and rapidly modify the energy input to the lower thermosphere. Most of the excess flare 0.1-2 nm irradiance comes from 1-2 nm. Thus, flares deposit a large amount of their energy between 100-110 km. One of the key effects of this energy deposition is to modify nitric oxide (NO), which plays an important role in the energy balance of the thermosphere as it is a source of radiative cooling through infrared emissions. The density of NO is highly variable as a function of time and latitude, and reaches a maximum in the same altitude region where the flare irradiance is absorbed. This dissertation also presents valid comparisons between Student Nitric Oxide Explorer (SNOE) satellite NO observations and those predicted by a photochemical thermospheric model to provide a better understanding of low latitude flare enhanced NO column density. Large flares can deposit the same amount of 0.1-2 and 0.1-7 nm energy to the thermosphere during a relatively short time as the Sun normally deposits in one day. The NO column density doubles as the daily integrated energy to the thermosphere doubles.
    • 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.
    • Spectroscopic study of sprites

      Kanmae, Takeshi; 神前 丈; Stanbaek-Nielsen, Hans; Olson, John; Lummerzheim, Dirk; Simpson, William; Hampton, Donald (2014-12)
      Optical emissions from sprites--large electric discharges in the mesosphere caused by intense lightning strokes--have been studied for decades. Studies have identified that sprite emissions are primarily composed of molecular band emissions of nitrogen and notably identified the near ultraviolet and blue emission from the N₂⁺ First Negative system, which provided direct evidence of ionization in sprites. This implies that further evidence of the ionization may be provided by the visible and near infrared emission from the N₂⁺ Meinel system, which is more accessible from ground-based platforms, though anticipated strong quenching in the mesosphere and below have made the presence of the emission somewhat controversial. To investigate the presence of the Meinel emission along the vertical extent of sprites, we made ground-based spectral observations in 2005. The observed spectra were mainly composed of the N₂ First Positive system, and no or little indication of the Meinel bands were found. This study suggests that the quenching is indeed severe at sprite altitude, and it is difficult to study the ionization process in sprites via the Meinel emission. In addition, the data allowed us to investigate details of the First Positive emission from sprites. The observed First Positive spectra showed that the vibrational distribution of the upper state varies along the vertical extent of sprites, which is in agreement with previous reports, and furthermore this study indicates that the variation is associated with altitude, implying that collisional energy transfer processes play roles in exciting the First Positive emission, particularly at lower altitudes. Recent high-speed imaging observations have revealed the very dynamic nature of sprites: they develop within a few to 10 ms in forms of streamers and columnar glows. The underlying electron energies in these features have been inferred from their emissions in previous measurements, but they lacked either sufficient temporal or spatial resolution. To investigate the underlying electron energies, we made airborne spectral observations in 2009 with a slitless spectrograph, which provided temporal and spatial resolution improved over the previous measurements. The observed spectra clearly showed that the streamers consistently had a higher fraction of blue emission compared to the glows, indicating that the more energetic nature of the streamers. From the fractional blue emissions, the local electric fields were inferred to be 0.7 to 1.5Ek in the streamers and 0.3 to 0.6Ek in the glows, where Ek is the conventional breakdown electric field. The results support the interpretation of sprites as scaled analogs of streamer discharges observed in laboratory experiments.
    • Spectroscopy of the N₂ Vegard-Kaplan bands in the dayglow

      Yonker, Justin (2005-05)
      A synthetic spectrum of the N₂ Vegard-Kaplan (VK) bands is developed for dayglow conditions at thermospheric altitudes. Due to the change in electron spin, the A³Eu state is metastable (lifetime 2-3 s.) and excited from the X¹Eg⁺ ground state primarily by photo-electron impact. Cascade from higher-energy triplets contributes to the A³Eu⁺ population and, due to its long lifetime, losses due to quenching are significant. Because of quenching, the VK bands were the last of the major N₂ emissions to be observed, as is explained in an historical review of the work of Vegard and Kaplan. Taking as inputs the solar soft x-ray measurements of the Student Nitric Oxide Explorer (SNOE), the model computes steady-state, ro-vibrational A³Eu⁺ populations and generates the synthetic VK spectrum in Rayleighs. Comparison with observations is hampered by a lack of VK data for days within the SNOE mission (1998-2003). As such, model results for a day in 1999 are compared with VK observations from 1992. The error is within 50% for altitudes below 150 km, above which it steadily decreases to 8% at 280 km. These errors are reasonable considering the unknown solar conditions of the 1992 observations.
    • Structure of reconnection layers in the magnetosphere

      Lin, Yu; Lee, Lou-Chuang; Hawkins, J. G.; Sentman, D. D.; Smith, R. W.; Swift, D. W. (1993)
      Magnetic reconnection can lead to the formation of observed boundary layers at the dayside magnetopause and in the nightside plasma sheet of the magnetosphere. In this thesis, the structure of these reconnection layers is studied by solving the one-dimensional Riemann problem for the evolution of a current sheet. Analytical method, resistive MHD simulations, and hybrid simulations are used. Based on the ideal MHD formulation, rotational discontinuities, slow shocks, slow expansion waves, and contact discontinuity are present in the dayside reconnection layer. Fast expansion waves are also present in the solution of the Riemann problem, but they quickly propagate out of the reconnection layer. Our study provides a coherent picture for the transition from the reconnection layer with two slow shocks in Petschek's model to the reconnection layer with a rotational discontinuity and a slow expansion wave in Levy et al.'s model. In the resistive MHD simulations, the rotational discontinuities are replaced by intermediate shocks or time-dependent intermediate shocks. In the hybrid simulations, the time-dependent intermediate shock quickly evolves to a steady rotational discontinuity, and the contact discontinuity does not exist. The magnetotail reconnection layer consists of two slow shocks. Hybrid simulations of slow shocks indicate that there exists a critical number, $M\sb{c}$, such that for slow shocks with an intermediate Mach number $M\sb{I} \ge M\sb{c}$, a large-amplitude rotational wavetrain is present in the downstream region. For slow shocks with $M\sb{I} < M\sb{c}$, the downstream wavetrain does not exist. Chaotic ion orbits in the downstream wave provide an efficient mechanism for ion heating and wave damping and explain the existence of the critical number $M\sb{c}$ in slow shocks.
    • Studying auroral microphysics using multiple optically tracked rocket sub-payloads

      Vann, Joshua M.; Conde, Mark; Delamere, Peter; Hampton, Donald (2018-12)
      There is insufficient knowledge of scale length parameters associated with ionospheric plasma structures. Using a novel technique combining rocket-based instrument data with ground-based optical and instrumental data measurements, ISINGLASS attempts to determine the spatial scale lengths over which parameter differences in auroral arcs present in the upper ionosphere. Determination of such scale lengths has the propensity to strengthen preexisting models of magnetosphere-ionosphere interactions. While analysis is not complete and the extent of such scale lengths is still unknown, after completion of the experiment phase of the mission, differences in measurements have been found that cannot be accounted for through experimental error. This shows the existence of a critical scale length within the distances measured, and the techniques used present a reliable method with which to launch a future campaign.
    • Suprathermal electron tails in a beam-plasma instability

      Hollerbach, Uwe; Swift, W.; Stenbaek-Nielsen, H. C.; Rees, M. H.; Kan, J. R. (1987-09)
      This study investigated the suprathermal electron tails produced in a beam-plasma instability, and their scaling with beam and background densities. A periodic one-dimensional electrostatic simulation was used to study the suprathermal tails. Electrons were treated as particles, and ions were treated as a fluid. The simulation showed that ion dynamics are required for the formation of the suprathermal tails, as expected from the theory of the oscillating two-stream instability. The energy of the suprathermal tails is directly proportional to the beam density, and does not depend strongly on the background density. There is a slight decrease in the energy of the suprathermal tails as the background density increases. A novel numerical effect was also found: a three-plasmon interaction caused by the modification of the Langmuir wave dispersion relation when high-order splines are used as particle shape factors.
    • Synchronization in biological systems

      Klaas, Jonathan P. (2004-12)
      Synchronization, the adjustment of rhythms via coupling, is an essentially nonlinear effect in coupled dynamical systems. Synchronization is observed in many systems, for example the moon's periods of rotation and revolution, in pendulums suspended from a common support, in swarms of fireflies that flash in unison, and in biological circadian rhythms. Circadian rhythms are periodic fluctuations in multiple physiological systems that have evolved as a consequence of the daily rotation of the earth. These rhythms have been observed in organisms ranging from cyanobacteria to man. In this thesis we will present a conceptually simple model of circadian rhythms to yield insight into the activity patterns of mice in light and food restriction experiments. The model consists of two coupled van der Pol oscillators that are driven by an external periodic influence representing food availability. The results of the model are compared to circadian data of mice collected by Dr. Abel Bult-Ito (Institute of Artic Biology).