• Formation of solar prominences and eruption of solar magnetic arcade systems

      Choe, Gwang-Son; Lee, Lou-Chuang; Akasofu, Syun-Ichi; Roederer, Juan G.; Swift, Daniel W.; Watkins, Brenton J. (1995)
      Formation and eruption of solar prominences, coronal mass ejections (CMEs) and solar flares are the most magnificent phenomena among solar activities. Observations show that there is an interrelationship among these events and that their manifestation is conditioned by certain common photospheric signatures. One of them is the increase in magnetic shear. In this thesis, the evolution of the solar atmosphere is studied by numerical simulations with photospheric motions as boundary conditions. Firstly, mechanisms of prominence formation are investigated. It is found that prominences can be formed by the development of a thermal instability (1) in a rapidly expanding magnetic arcade, (2) in a magnetic island created by magnetic reconnection or (3) in the current sheet between two bipolar arcades. Secondly, the quasi-static evolution of a magnetic arcade subject to footpoint shearing is studied under the ideal MHD condition. Three distinct evolutionary phases are found, in the last of which a current layer develops and grows indefinitely with the increasing shear. Force-free field solutions are also constructed and compared with dynamic solutions. Finally, resistive evolutions of magnetic arcades are investigated imposing resistivity on the pre-sheared magnetic fields. It is found that there is a critical amount of shear, over which magnetic reconnection can take place to create a magnetic island. The effects of different values and spatial patterns of resistivity are studied. With a localized resistivity, most of principal features in solar eruptive processes are reproduced. A comparative study is made between the numerical results and observations.
    • High frequency backscatter from the polar and auroral e-region ionosphere

      Forsythe, Victoriya V.; Bristow, William; Conde, Mark; Sahr, John; Zhang, Hui (2017-05)
      The Earth's ionosphere contains collisional and partially-ionized plasma. The electric field, produced by the interaction between the Earth's magnetosphere and the solar wind, drives the plasma bulk motion, also known as convection, in the F-region of the ionosphere. It can also destabilize the plasma in the E-region, producing irregularities or waves. Intermediatescale waves with wavelengths of hundreds of meters can cause scintillation and fading of the Global Navigation Satellite System (GNSS) signals, whereas the small-scale waves (< 100 m) can scatter radar signals, making possible detection of these plasma structures and measurements of their characteristics such as their phase velocity and intensity. In this work, production of the decameter-scale (10 m) irregularities in the ionospheric E-region (100-120 km in altitude) at high latitudes is investigated both theoretically, using linear fluid theory of plasma instability processes that generate small-scale plasma waves, and experimentally, by analyzing data collected with the newly-deployed high-southern-latitude radars within the Super Dual Auroral Radar Network (SuperDARN). The theoretical part of this work focuses on symmetry properties of the general dispersion relation that describes wave propagation in the collisional plasma in the two-stream and gradient-drift instability regimes. The instability growth rate and phase velocity are examined under the presence of a background parallel electric field, whose influence is demonstrated to break the spatial symmetry of the wave propagation patterns. In the observational part of this thesis, a novel dual radar setup is used to examine E-region irregularities in the magnetic polar cap by probing the E-region along the same line from opposite directions. The phase velocity analysis together with raytracing simulations demonstrated that, in the polar cap, the radar backscatter is primarily controlled by the plasma density conditions. In particular, when the E-region layer is strong and stratified, the radar backscatter properties are controlled by the convection velocity, whereas for a tilted E-layer, the height and aspect angle conditions are more important. Finally, the fundamental dependence of the E-region irregularity phase velocity on the component of the plasma convection is investigated using two new SuperDARN radars at high southern latitudes where plasma convection estimates are accurately deduced from all SuperDARN radars in the southern hemisphere. Statistical analysis is presented showing that the predominance of the E-region echoes of a particular polarity is strongly dictated by the orientation of the convection plasma ow which itself has a significant asymmetry towards westward zonal flow.
    • Hot flow anomalies at earth's bow shock and their magnetospheric-ionospheric signatures

      Chu, Christina Seiman; Zhang, Hui; Otto, Antonius; Ng, Chung-Sang; Sibeck, David (2017-08)
      Hot flow anomalies (HFAs) are typically observed upstream of bow shocks. They are characterized by a significant increase in particle temperature and substantial flow deflection from the solar wind flow direction coinciding with a decrease in density. HFAs are important to study and understand because they may play an important role in solar wind-magnetosphere coupling. They may drive magnetopause motion, boundary waves, and flux transfer events. They can excite ultra low frequency waves in the magnetosphere, drive magnetic impulse events in the ionosphere, and trigger aurora brightening or dimming. Studying HFAs will aid in the understanding of fundamental processes that operate throughout the heliosphere such as particle energization and shocks. This dissertation presents statistical and case studies of hot flow anomalies identified in Time History of Events and Macroscale Interactions During Substorms (THEMIS) satellite data from 2007-2009. The characteristics and occurrence of HFAs, their dependence on solar wind/interplanetary magnetic field (IMF) conditions and location, and their magnetospheric-ionospheric signatures, have been investigated using in-situ spacecraft observations and ground based observations. THEMIS observations show that HFAs span a wide range of magnetic local times (MLTs) from approximately 7 to 16.5 MLT. HFAs were observed up to 6.3 Earth radii (RE) upstream from the bow shock. It has been found that the HFA occurrence rate depends on solar wind and interplanetary magnetic field (IMF) conditions as well as distance from the bow shock. HFA occurrence decreases with distance upstream from the bow shock. HFAs are more prevalent when there is an approximately radial interplanetary magnetic field. No HFAs were observed when the Mach number was less than 5, suggesting there is a minimum threshold Mach number for HFAs to form. HFAs occur most preferentially for solar wind speeds from 550-600 km/s. Multiple THEMIS spacecraft observations of the same HFA provide an excellent opportunity to perform a spatial and temporal analysis of an HFA. The leading edge, tangential discontinuity inside the HFA, and trailing shock boundaries for the event were identified. The boundaries' orientations and motion through space were characterized. The HFA expansion against the solar wind was 283 km/s. The spatial structure of the HFA was deduced from multiple spacecraft observations. The HFA is thicker closer to the bow shock. The magnetospheric-ionospheric signatures of an HFA have been investigated using in-situ spacecraft observations and ground based observations. Magnetic field perturbations were observed by three GOES spacecraft at geostationary orbit and high-latitude ground magnetometers in both hemispheres. Observations from magnetometers located at different MLTs showed that the perturbation propagates tailward at 0.32°/s or 9 km/s (1.27°/s or 21 km/s) for the northern (southern) hemisphere, which is consistent with an HFA propagating tailward along the dawn flank. SuperDARN radar observations showed a change in plasma velocity shortly after the HFA was observed by THEMIS.
    • Improved Modeling Of Turbulent Transport: From Noise In Transport Models To The Parareal Algorithm Applied To Full Turbulence Codes

      Samaddar, Debasmita; Newman, David (2010)
      Turbulence and turbulent transport are ubiquitous in nature and are of fundamental importance in everything from the spread of pollution to confinement in fusion plasmas. In order to study this, turbulence models need to be as realistic as possible and one must also be able to evolve the turbulence and the profiles of the quantities of interest on transport (long) time scales. Improving turbulence simulations by the introduction of new techniques forms the basis of this research. One part of this work involved improving the performance of a 1D transport model by the addition of noise. On a more fundamental level, studying long time dynamics for turbulence simulations is very difficult even with the fastest computers available now or in the near future. To help overcome this difficulty, a new way of simulating turbulence has been presented, namely parallelizing in time. Time parallelization of a fully developed turbulent system is a new application. Parallelizing the space domain to computationally solve partial differential equations has been extensively used and is one of the most common forms of parallelization. In contrast, the Parareal Algorithm parallelizes the time domain and has been found to significantly reduce the computational wall time in many simpler systems. Despite its success in other less complex problems, it has not yet been successfully applied to a turbulent system (to the best of our knowledge). If efficiently applied, this algorithm will allow study of the turbulent transport dynamics on transport time scales - something that has heretofore been very difficult. In this work, the results of applying the Parareal Algorithm to simulations of drift wave turbulence in slab geometry in which the relative dominance of the polarization and E x B nonlinearities are tuned artificially, are presented. These turbulent systems are in many ways similar to neutral fluid turbulence models, so success of the Parareal scheme in them expands the prospect of a broader range of application to many other turbulent problems. This thesis also presents the results of a modification to the algorithm. A model to study and predict the parameters governing the convergence of the scheme is also explored.
    • The interaction of Io and the Jovian magnetic field: Io's Alfven wings and particle acceleration

      Dols, Vincent (2001-08)
      Conditions for the formation of an electric field along the field lines of Jupiter crossing the satellite Io are investigated by examining the properties of Io's Alfven wave. A three-dimensional self-consistent MHD model, using a simplified magnetosphere description, illustrates the formation of this electric field and of Io's related auroral emission in the Jovian ionosphere. The Alfven wing properties between Io and Jupiter are studied with a one-dimensional MHD model and a realistic magnetosphere. Any change in the Io/Jupiter system affects the structure of the Alfven wing and likely affects the structure of Io's auroral emissions. This emission is likely structured in multi-spots and the angle between the first spot and the instantaneous projection of Io is less than 3°. In the limited context of the 1D approximation, the acceleration mechanism is expected close to Jupiter.
    • Interaction of two tributary glacier branches and implications for surge behavior

      Knowles, Christopher P.; Truffer, Martin; Larsen, Chris; Newman, David; Wackerbauer, Renate (2018-05)
      A glacier surge is a dynamic phenomenon where the glacier after a long period of quiescence, increases its velocities by up to two orders of magnitude. These surges tend to have complex interactions with tributaries, yet the role of these tributary interactions towards glacier surging has yet to be fully investigated. In this work we construct a synthetic glacier with an adjustable tributary intersection angle to study tributary interaction with the trunk glacier. The geometry we choose is loosely based on the main trunk and tributary interaction of Black Rapids Glacier, AK, USA, which last surged in 1936-1937. We investigate surface elevations, medial moraine locations, and erosive power at the bed of the glacier in response to our adjustable domain and relative flux. A nonlinear relationship between tributary flux and surface elevations is found that indicates flow restrictions can occur with geometries like Black Rapids Glacier. These flow restrictions cause increased ice thicknesses up-glacier which can lead to surges via increased stresses.
    • Investigation of strongly ducted infrasonic dispersion using a vertical eigenfunction expansion of the Helmholtz equation in a modal broad band acoustic propagation code

      Edon, Robert Alexander; Olson, John V.; Fee, David E.; Szuberla, Curt A. (2015-12)
      This study investigates an infrasound propagation model created by the National Center for Physical Acoustics (NCPA) which is applied to atmospheric data with a strong temperature inversion in the lower atmosphere. This temperature inversion is believed to be the primary cause of a dispersed infrasonic signal recorded by an infrasound sensor array located on the Southern California coast in August, 2012. The received signal is characterized by initial low frequency content followed by a high frequency content tail. It is shown the NCPA model is hindered by limited atmospheric data and no ground truth for the source function which generated the received signal. The results of the NCPA model are shown to not reproduce the recorded signal and provide inconclusive evidence for infrasonic dispersion.
    • Kelvin-Helmholtz Instability And Magnetic Reconnection At The Earth's Magnetospheric Boundary

      Ma, Xuanye; Otto, Antonius; Lummerzheim, Dirk; Newman, David; Ng, Chung-Sang; Zhang, Hui (2012)
      Magnetic reconnection and Kelvin-Helmholtz (KH) instability are the two most important mechanisms for plasma transport across the Earth's magnetospheric boundary layer. Magnetic reconnection is considered as the dominant process for southward interplanetary magnetic field (IMF), and the KH instability is suggested to play an important role for northward IMF. It is interesting to note that this plasma entry is associated with a dramatic entropy increase, which indicates the existence of strong nonadiabatic heating during the entry process. Observations indicate a plasma entropy increase by two orders of magnitude during the transport from solar wind into the Earth's magnetosphere. Therefore, it is important to examine whether magnetic reconnection can provide sufficient nonadiabatic heating to explain the observed plasma properties and to identify plasma conditions that allow strong nonadiabatic heating. This thesis demonstrates that the entropy can indeed strongly increase during magnetic reconnection provided that the plasma beta, i.e., the ratio of thermal to magnetic energy density is small. A realistic three-dimensional configuration of the Earth's magnetopause for southward IMF conditions includes large anti-parallels magnetic components with a fast perpendicular shear flow. Thus, it is expected that KH modes and magnetic reconnection operate simultaneously and interact with each other. This thesis provides a systematic study on this interaction between reconnection and KH modes by means of three-dimensional MHD and Hall MHD numerical simulations. It is demonstrated that both reconnection and nonlinear KH waves change the other modes onset condition by changing the width of the transition layer. It is shown that dynamics of the system can be strongly modified by a guide field or Hall physics. In the presence of plasma flow, magnetic reconnection is also associated with the generation of field-aligned currents (FACs), which play a critical role in the coupling between the magnetosphere and ionosphere. This thesis also examines systematically the generation of FACs. It is demonstrated that such currents are generated either by a guide magnetic field, by shear flow, or by the inclusion of Hall physics already in two-dimensional magnetic reconnection.
    • Linking climate history and ice crystalline fabric evolution in polar ice sheets

      Kennedy, Joseph Huston; Pettit, Erin; Truffer, Martin; Bueler, Ed; Newman, David; Szuberla, Curt (2015-08)
      An ice sheet consists of an unfathomable number of ice crystallites (grains) that typically have a preferred orientation of the crystalline lattices, termed fabric. At the surface of ice sheets, the microstructural processes that control the grain structure and fabric evolution are influenced by climate variables. Layers of firn, in different climate regimes, may have an observable variation in fabric which can persist deep into the ice sheet; fabric may have 'memory' of these past climate regimes. To model the evolution of a subtle variation in fabric below the firn-ice transition, we have developed and released an open-source Fabric Evolution with Recrystallization (FEvoR) model. FEvoR is an anisotropic stress model that distributes stresses through explicit nearest-neighbor interaction. The model includes parameterizations of grain growth, rotation recrystallization and migration recrystallization which account for the major recrystallization processes that affect the macroscopic grain structure and fabric evolution. Using this model, we explore the evolution of a subtle variation in near-surface fabric using both constant applied stress and a stress-temperature history based on data from Taylor Dome, East Antarctica. Our results show that a subtle fabric variation will be preserved for ~200ka in compressive stress regimes with temperatures typical of polar ice-sheets. The addition of shear to compressive stress regimes preserves fabric variations longer than in compression-only regimes because shear drives a positive feedback between crystal rotation and deformation. We find that temperature affects how long the fabric variation is preserved, but does not affect the strain-integrated fabric evolution profile except when crossing the thermal-activation-energy threshold (~-10°C). Even at high temperatures, migration recrystallization does not rid the fabric of its memory under most conditions. High levels of nearest-neighbor interactions between grains will rid the fabric of its memory more quickly than low levels of nearest-neighbor interactions. Because FEvoR does not compute flow, an integrated fabric-flow model is needed to investigate the flow-fabric feedbacks that arise in ice sheets. Using the open-source Parallel Ice Sheet Model (PISM) and FEvoR, we develop a combined flow-fabric model (PISM-FEvoR). We provide the first integrated flow-fabric model that explicitly computes the fabric evolution and includes all three major recrystallization processes. We show that PISM-FEvoR is able to capture the flow enhancement due to fabric by modeling a slab-on-slope glacier, initialized with a variety of fabric profiles. We also show that the entire integrated fabric-flow history affects the final simulated flow. This provides a further, independent validation of using an integrated fabric-flow model over a constant enhancement factor in ice-sheet models.
    • Local scale structures in earth's thermospheric winds and their consequences for wind driven transport

      Dhadly, Manbharat Singh; Conde, Mark; Collins, Richard; Olson, John; Hampton, Donald; Smith, Roger (2015-12)
      In the traditional picture of Earth's upper thermosphere (~190-300 km), it is widely presumed that its convective stability and enormous kinematic viscosity attenuate wind gradients, and hence smooth out any structure present in the wind over scale size of several hundreds of kilometers. However, several independent experimental studies have shown that observed upper thermospheric wind fields at high latitudes contain stronger than expected local-scale spatial structures. The motivation of this dissertation is to investigate how the resulting local-scale gradients would distort neutral air masses and complicate thermospheric wind transport. To achieve this goal, we examined the behavior of a simple parameter that we refer to as the "distortion gradient". It incorporates all of the wind field's departures from uniformity, and is thus capable of representing all resulting contributions to the distortion or mixing of air masses. Climatological analysis of the distortion gradient using 2010, 2011, and 2012 wind data from the All-sky Scanning Doppler Imager (SDI) located at Poker Flat (65.12N, 147.47W) revealed the diurnal and seasonal trends in distortion of thermospheric masses. Distortion was observed to be dependent on geomagnetic activity and orientation of the interplanetary magnetic field. To understand the time-cumulative influence of these local-scale non-uniformities on thermospheric wind driven transport, time-resolved two-dimensional maps of the thermospheric vector wind fields were used to infer forward and backward air parcel trajectories. Tracing air parcel trajectories through a given geographic location indicates where they came from previously, and where they will go in the future. Results show that wind driven transport is very sensitive to small-scale details of the wind field. Any local-scale spatial wind gradients can significantly complicate air parcel trajectories. Transport of thermospheric neutral species in the presence of the local-scale wind gradients that we observed was found to be far more complicated than what current models typically predict. To validate these findings, we cross-compared the upper thermospheric neutral winds inferred from a narrow field of view Fabry-Perot interferometer with winds measured by our all-sky SDI. A high degree of correlation was present between their measurements. This cross-validation study suggests the presence of small-scale short-lived, and previously unobserved wind features in the upper thermosphere, with typical length scales less than ~40 km. The spatially and temporally localized wind features implied by this study represent a new and unexplored regime of dynamics in the thermosphere.
    • Longwave Radiative Transfer In The Atmosphere: Model Development And Applications

      Delamere, Jennifer Simmons; Stamnes, Knut H. (2003)
      A FLexible Radiative Transfer Tool (FLRTT) has been developed to facilitate the construction of longwave, correlated k-distribution, radiative transfer models. The correlated k-distribution method is a technique which accelerates calculations of radiances, fluxes, and cooling rates in inhomogeneous atmospheres; therefore, correlated k-distribution models are appropriate for simulations of satellite radiances and inclusion into general circulation models. FLRTT was used to build two new rapid radiative transfer models, RRTM_HIRS and RRTM_v3.0, which maintain accuracy comparable to the line-by-line radiative transfer model LBLRTM. Iacono et al. [2003] evaluated upper tropospheric water vapor (UTWV) simulated by the National Center for Atmospheric Research Community Climate Model, CCM3, by comparing modeled, clear-sky brightness-temperatures to those observed from space by the High-resolution Radiation Sounder (HIRS). CCM3 was modified to utilize the rapid radiative transfer model RRTM and the separate satellite-radiance module, RRTM_HIRS, which calculates brightness temperatures in two HIRS channels. By incorporating these accurate radiative transfer models into CCM3, the longwave radiative transfer calculations have been removed as a significant source of error in the simulations. An important result of this study is that CCM3 exhibits moist and dry discrepancies in UTWV of 50% in particular climatic regions, which may be attributed to errors in the CCM3 dynamical schemes. RRTM_v3.0, an update of RRTM, is a rapid longwave radiative transfer appropriate for use in general circulation models. Fluxes calculated by RRTM_v3.0 agree with those computed by the LBLRTM to within 1.0 W/m2 at all levels, and the computed cooling rates agree to within 0.1 K/day and 0.3 K/day in the troposphere and stratosphere, respectively. This thesis also assessed and improved the modeling of clear-sky, longwave radiative fluxes at the Atmospheric Radiation Measurement Program North Slope of Alaska site by simultaneously addressing the specification of the atmosphere, radiometric measurements, and radiative transfer modeling. Consistent with findings from other field sites, the specification of the atmospheric water vapor is found to be a large source of uncertainty in modeled radiances and fluxes. Improvements in the specification of carbon dioxide optical depths within LBLRTM resulted, in part, from this analysis.
    • Magnetic Reconnection As A Chondrule Heating Mechanism

      Lazerson, Samuel A.; Wiechen, Heinz (2010)
      The origin of chondrules (sub-millimeter inclusions found in stony meteorites) remains today an open question despite over century of examination. The age of these proto-solar relics shows a well defined cutoff of around 4.5 billion years ago. This places them as the oldest solids in the solar system. Chemical examination indicates that they experienced heating events on the order of 5000 K/hr for periods of around 30 minutes, followed by extending periods of cooling. Additional examination indicates the presence of large magnetic fields during their formation. Most attempts to explain chondrule formation in the proto-solar nebula neglect the existence of a plasma environment, with even less mention of dust being a charge carrier (dusty plasma). Simulations of magnetic reconnection in a dusty plasma are forwarded as a mechanism for chondrule formation in the proto-solar nebula. Here large dust-neutral relative velocities are found in the reconnection region. These flows are associated with the dynamics of reconnection. The high Knudsen number of the dust particles allows for a direct calculation of frictional heating due to collisions with neutrals (allowing for the neglect of boundary layer formation around the particle). Test particle simulations produce heating equivalent to that recorded in the chondrule mineral record. It is shown that magnetic reconnection in a dusty plasma is of fundamental importance to the formation of the most primitive solids in the solar system.
    • Magnetic reconnection in the presence of sheared plasma flow

      La Belle-Hamer, Annette Louise; Lee, L. C. (1994)
      Classical models of magnetic reconnection consist of a small diffusion region bounded by two slow shocks, across which the plasma is accelerated. Most space plasma current sheets separate two different plasmas, violating symmetry conditions across the current sheet. One form of asymmetry is a sheared plasma flow. In this thesis, I investigate the magnetic reconnection process in the presence of a shear flow across the current sheet using two-dimensional magnetohydrodynamic (MHD) simulations. The results show that only for sheared flow below the average Alfven velocity of the inflow regions can steady state magnetic reconnection occur. A detailed examination of the Rankine-Hugoniot jump conditions reveals that the two slow shocks of earlier models are replaced by a strong intermediate shock and a weaker slow shock in the presence of shear flow. Both symmetric and asymmetric density profiles are examined. Depending upon the direction of the flow in the adjacent inflow region, the effects from the sheared flow and the effects from the density asymmetry will compete with or enhance each other. The results are applied to the dayside and flank regions of the magnetosphere. For tailward flow in the flanks, the two asymmetries compete making the magnetic field transition layer broad with the high speed flow contained within the transition region. For the dayside region, the magnetic field transition region is thin and the accelerated flow is earthward of the sharp current layer (magnetopause). These results are consistent with the data. A velocity shear in the invariant direction was examined under otherwise symmetric conditions. With the magnetic field initially only in the $x-y$ plane, $B\sb{z},$ and consequently field-aligned current, is generated by the initial $v\sb{z}.$ The field-aligned current depends on the velocity profiles in all directions. For a velocity sheared in both the z and the y direction, the results show a very localized region of large field-aligned currents.
    • Magnetohydrodynamic Simulations Of Plasma Dynamics In The Magnetospheric Cusp Region

      Adamson, Eric T.; Otto, Antonius (2012)
      The Earth's magnetospheric cusp regions are rich in interesting plasma physics. The geomagnetic cusps offer solar wind plasma a relatively easy entry point into the magnetosphere through magnetic reconnection with the interplanetary magnetic field. The cusp regions are characterized by various interesting and important observations such as low energy particle precipitation, significant outflow of ionospheric material, and the frequent presence of energetic particles in regions of depressed magnetic field strength. The physical mechanisms that lead to these observations is often unresolved, for instance the acceleration mechanism for energetic cusp populations is not understood, nor is it known what implications they may have on magnetospheric dynamics. It is however, well accepted that magnetic reconnection plays a critical role in the vicinity of the cusps and is likely responsible for much of the dynamics in the region. Modeling of the geomagnetic cusps is notoriously challenging. Global magnetospheric models have proven indispensable in the study of the interaction of the solar wind plasma with the Earth's magnetosphere, however, the exterior cusp region poses a significant challenge for these models due to their relatively small scale. I have developed a mesoscale cusp-like magnetic field model in order to provide a better resolution (up to 300 km) of the entire cusp region than is possible in these global models. Typical observational features of the high-altitude cusps are well reproduced by the simulation. Results for both strongly northward and strongly southward interplanetary magnetic field indicate extended regions of depressed magnetic field and strongly enhanced plasma beta (cusp diamagnetic cavities). The Alfvenic nature of the outer boundary between the cusp and magnetosheath, in addition to the flow characteristics in the region, indicate that magnetic reconnection plays an important role in structuring the high-altitude cusp region. The inner boundaries with magnetosphere are gradual transitions forming a clear funnel. These cavities further present a unique configuration in which reconnecting magnetic flux tubes may gain a significant amount of flux tube entropy (H = p1/gammaV) through topological changes due to magnetic reconnection.
    • Magnetospheric imaging of EUV emissions at 83.4 and 30.4 nm wavelengths

      Garrido, Dante Espino; Smith, R. W. (1994)
      Magnetospheric images are constructed from resonant scattering of emissions by He$\sp+$ 30.4-nm and O$\sp+$ 83.4-nm ions from different spatial locations to study the structure of the intensities and its relation to the distribution of He$\sp+$ and O$\sp+$ ions around the Earth. The image intensities at these EUV wavelengths were obtained from a knowledge of ion scattering rates and available data on ion densities. This particular approach is called forward modelling and consists of the calculation of simulated EUV images of the magnetosphere. Different regions in the magnetosphere have been considered in this study to determine the dependence of the image intensities on ion energies and ion drift speeds with respect to the Sun-Earth line. Hot O$\sp+$ ions in the energy range from 1 keV to 50 keV are present in the plasma sheet with typical densities of the order of 0.1 ions cm$\sp{-3}$ arising during disturbed times. Image intensities of the order of a few millirayleighs were obtained in our simulations for these densities. During quiet times the densities are of the order of 0.05 ions cm$\sp{-3}.$ The reduction of the image intensities as a result of Doppler shifts caused by ion motion relative to the Sun-Earth line is discussed in detail and the effects of ion dynamics (particle acceleration) in the polar cap on the image intensities have also been analyzed for both He$\sp+$ and O$\sp+$ ions. The possibility of detecting polar outflows may also depend on the location of the imager. Simulated images of the plasmasphere and trough regions in both 30.4-nm and 83.4-nm wavelengths have been obtained to reflect the relative abundance of the ions in these regions. Photometric intensities of He$\sp+$ at 30.4 nm were obtained from a spinning rocket at an altitude of 435 km. The different viewing angles covered a wide range of regions in the magnetosphere, and this particular rocket geometry offered the possibility of obtaining the He$\sp+$ ion distribution from the measured intensities. This method (forward inversion) can be applied to 2-D images and it is shown that it is possible to extract 3-D ion distributions from the images.
    • Measurement of field aligned electron density distribution, ducts, and Z mode cavities from ducted and nonducted fast Z mode echoes observed on the image satellite

      Mayank, Kumar; Newman, David; Simpson, William; Truffer, Martin; Braddock, Joan (2015-05)
      Z mode (ZM) sounding from the Radio Plasma Imager (RPI) onboard the Imager for Magnetopause to Aurora Global Exploration (IMAGE) satellite has provided a new method to measure the geomagnetic field aligned electron density (Ne) distribution, magnetospheric ducts, and Z mode cavities in the low- to mid-latitude region of the magnetosphere from the ducted and nonducted fast ZM echoes observed from radio sounding at 20-1000 kHz. Z mode is a trapped wave mode of the plasma confined in frequency between the ZM cutoff frequency, fz, and the upper hybrid resonance frequency, fuh. In the past, trapped ducted Z mode echoes in the ZM cavity were used to measure field aligned electron density thousands of kilometers above the satellite altitude. However, no attempt was made to study the properties of the ducts that can guide ZM waves. Magnetospheric ducts play an important role in the propagation of plasma waves, wave particle interactions, particle acceleration, and precipitation. In the past, ducts have been known since the discovery of the plasmasphere, but there is limited knowledge of the properties of ducts, their origins and occurrence patterns, and their distribution on a global scale in the magnetosphere especially for ducts extending to high altitudes (above 1000 km). Analysis of ducted echoes from the conjugate and local hemispheres has enabled nearly instantaneous magnetic field aligned electron density profiles. Field aligned density distribution plays an important role in magnetospheric dynamics. Despite its importance, there is a lack of accurate representation of the latitudinal dependence of density distribution along the field lines by the existing empirical models, due to limited measurement of off-equatorial electron density profiles and availability of relatively fewer measurements of Ne at higher altitudes. There is also a need to describe the transition of field aligned electron densities from the topside ionosphere into the plasmasphere properly. RPI on IMAGE, designed to sweep from 3 kHz to 3 MHz, has observed both ducted and nonducted ZM echoes in the frequency range of 40 - 800 kHz in an altitude range of ~600-10,000 km, invariant latitude of ~20°-90°, at all magnetic local times (MLTs) and for an fpe/fce range of 0.25-6, where fpe and fce are electron plasma frequency and electron gyro frequency respectively. About 72,000 plasmagrams have been surveyed, out of which ~1500 cases of ducted- and ~3500 cases of nonducted fast ZM echoes have been identified. Two cases of ducted, two cases of nonducted, and a set of seven successive cases of nonducted and ducted fast ZM echoes have been analyzed to study the propagation, reflection, and guidance of fast ZM waves in the magnetosphere and measure: (1) field aligned electron density distribution; (2) duct parameters (half-width ΔL and density variation ΔN/N); and (3) Z mode cavity parameters. In absence of ducts, four nonducted echoes are obtained, each reflecting from locations where fz ~f, where f is the wave frequency. Three of the echoes retrace their path after reflection and one forms a loop. When a Ne depletion duct is present inside a Z mode cavity, fast Z mode waves can be guided within the duct and propagate back and forth between their reflection points above and below the satellite altitude. Ray tracing calculations of ducted Z mode echoes show that: (1) in the presence of ducts, both nonducted and ducted echoes are obtained, and echoes are formed from both looping as well as retracing paths; (2) average time delays (tg) depend upon the electron density distribution along geomagnetic field lines and the shape of the Z mode cavity; (3) time delay spread (Δtg) and upper cutoff frequency (fuc) depend upon duct half-width (ΔL) and density perturbation (ΔN/N); (4) echoes are obtained from local as well as conjugate hemispheres when the duct extends to the conjugate hemisphere; and (5) the discovery of a new phenomenon of gap in frequency for echoes reflecting from below the satellite altitude. The gap frequency range depends upon the duct parameters and is a consequence of the peculiar shape of the refractive index surface for f~fpe. From ZM soundings of 11 case studies, it is found that the measured ducts have half-widths of ~160-500 km at the equator and density depletions of ~1%-6%, covering an altitude range of ~1000-10,000 km. The first quantitative measurements of ZM cavities have been presented in this thesis. The bandwidth and the length of ZM cavities lie in the range of ~10-50 kHz and ~3800-15,000 km, respectively. From the inversion of time delay versus frequency dispersion of the fast ZM echoes, field aligned electron density distributions both above and below the satellite from an altitude of ~1000-10,000 km (and ~90- 10,000 km if accompanied by Whistler mode echoes) are obtained. The electron density profiles obtained from ZM sounding are in good agreement with in-situ electron density measurements from the CHAMP (~350 km) and DMSP (~850 km) satellites, from RPI passive measurements, and give a better estimate of electron density as compared to empirical density models. Our results demonstrate that ZM sounding is a new and powerful method to measure field aligned electron density distribution, magnetospheric ducts, and ZM cavities in the altitude range of ~1000-10,000 km (and ~90-10,000 km if accompanied by Whistler mode echoes) at low- to mid-latitude region of the magnetosphere. These measurements can improve the existing empirical density models and provide a new understanding of the magnetospheric ducts as well as of waves propagating therein.
    • Modeling the generation and propagation of dispersive waves in the giant magnetospheres through mass loading and transport using hybrid simulation

      Stauffer, Blake; Delamere, Peter; Otto, Antonius; Zhang, Hui; Newman, David (2018-05)
      The magnetodiscs of Jupiter and Saturn are characterized by turbulence in the magnetic field. Broadband spectra of precipitating electrons at Jupiter suggest that a process is underway whereby large scale perturbations undergo a turbulent cascade in the magnetodisc. The cascade couples large perturbations to dispersive scales (kinetic and inertial Alfvén waves). Plasma transport in the rapidly rotating giant magnetospheres is thought to involve a centrifugally-driven flux tube interchange instability, similar to the Rayleigh-Taylor (RT) instability. Mass loading from satellites such as Io and Enceladus also cause dispersive wave formation in the magnetosphere, which is a source for broadband aurora. This dissertation presents a set of hybrid (kinetic ion/fluid electron) plasma simulations of the RT instability and the Io flux tube using conditions appropriate for the magnetospheres of Jupiter and Saturn. Both the Io torus and the planetary magnetodisc act as resonant cavities for counter propagating waves, which creates turbulence. The transmission ratio of wave power from the Io torus is 53%, an improvement from previous models (20% transmission), which is important to the generation of the Io auroral footprint. The onset of the RT instability begins at the ion kinetic scale and cascades to larger wavelengths. Strong guide field reconnection is a mechanism for radial transport of plasma in the magnetodisc. Counter propagating waves within the RT instability is the origin of turbulence within the magnetodisc.
    • NCPA propagation code users manual

      Winkelman, Andrew T.; Szuberla, Curt A.; Fee, David E.; Olson, John V. (2015-12)
      This manual was written for University of Alaska Fairbanks infrasound group to assist researchers in using the National Center for Physical Acoustics (NCPA) code suite to further investigate observed infrasonic phenomena. The NCPA code suite is designed to simulate various aspects of infrasound propagation through a model atmosphere. This suite was developed and tested by the University of Mississippi National Center for Physical Acoustics infrasound group. Included are raytrace routines to initially establish signal paths, both single frequency and broadband modal routines to calculate pressure fields and transmission losses, and a parabolic method to calculate pressure fields and transmission losses in model atmospheres.
    • Neural Network Approach To Classification Of Infrasound Signals

      Lee, Dong-Chang; Szuberla, Curt (2010)
      As part of the International Monitoring Systems of the Preparatory Commissions for the Comprehensive Nuclear Test-Ban Treaty Organization, the Infrasound Group at the University of Alaska Fairbanks maintains and operates two infrasound stations to monitor global nuclear activity. In addition, the group specializes in detecting and classifying the man-made and naturally produced signals recorded at both stations by computing various characterization parameters (e.g. mean of the cross correlation maxima, trace velocity, direction of arrival, and planarity values) using the in-house developed weighted least-squares algorithm. Classifying commonly observed low-frequency (0.015--0.1 Hz) signals at out stations, namely mountain associated waves and high trace-velocity signals, using traditional approach (e.g. analysis of power spectral density) presents a problem. Such signals can be separated statistically by setting a window to the trace-velocity estimate for each signal types, and the feasibility of such technique is demonstrated by displaying and comparing various summary plots (e.g. universal, seasonal and azimuthal variations) produced by analyzing infrasound data (2004--2007) from the Fairbanks and Antarctic arrays. Such plots with the availability of magnetic activity information (from the College International Geophysical Observatory located at Fairbanks, Alaska) leads to possible physical sources of the two signal types. Throughout this thesis a newly developed robust algorithm (sum of squares of variance ratios) with improved detection quality (under low signal to noise ratios) over two well-known detection algorithms (mean of the cross correlation maxima and Fisher Statistics) are investigated for its efficacy as a new detector. A neural network is examined for its ability to automatically classify the two signals described above against clutter (spurious signals with common characteristics). Four identical perceptron networks are trained and validated (with >92% classification rates) using eight independent datasets; each dataset consists of three-element (each element being a characterization parameter) feature vectors. The validated networks are tested against an expert, Prof. Charles R. Wilson, who has been studying those signals for decades. From the graphical comparisons, we conclude that such networks are excellent candidate for substituting the expert. Advantages to such networks include robustness and resistance to errors and the bias of a human operator.
    • A new model for the substorm growth phase

      Hsieh, Min-Shiu; 謝旻秀; Otto, Antonius; Bristow, William; Ng, Chung-Sang; Zhang, Hui (2014-08)
      The physics of geomagnetic substorms has been under debate for a long time. In particular, the formation of a thin current sheet (CS) is a central unresolved problem because it provides the magnetotail conditions for the expansion phase onset. This dissertation presents a new CS thinning mechanism based on midnight magnetic flux depletion (MFD), which is caused by sunward convection to balance dayside reconnection during periods of southward interplanetary magnetic field. The results demonstrate that MFD is a highly efficient mechanism to generate a very thin CS in the near-Earth tail. This study also examines CS formation under the influence of adiabatic lobe compression in combination with MFD and proposes a double-current sheet evolution at distinct locations in the near-Earth region and mid-tail region. The results suggest that substorm expansion onset is associated only with near-Earth onset of magnetic reconnection, while mid-tail reconnection causes bursty bulk flows. In addition, this dissertation investigates the changes of the auroral morphology associated with the magnetotail evolution. An ionospheric map is constructed based on Tsyganenko 96 magnetic field model corrected by magnetic flux conservation. By employing MFD, the mapping results such as the equatorward expansion of the open/closed field boundary, the convergent motion of strong field-aligned currents, and the location of electron and ion isotropy boundaries are consistent with typical ionospheric observations. These results demonstrate that MFD is the first model that can consistently explain and predict the typical magnetotail and ionospheric evolution during the substorm growth phase and shed light on the physics of the growth phase aurora.