Now showing items 46-65 of 91

• #### NCPA propagation code users manual

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

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

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.
• #### Nitrogen oxides in the arctic stratosphere: Implications for ozone abundances

In the high latitude winter stratosphere, NO$\sb2$ sequesters chlorine compounds which are extremely efficient at destroying ozone. During the nighttime, NO$\sb2$ reacts with ozone to form $\rm N\sb2 O\sb5$ which acts as a reservoir of NO$\sb2$. Under heavy aerosol loading, $\rm N\sb2O\sb5$ may react with water on aerosol surfaces to form HNO$\sb3$, a reservoir more resistant to photolysis. This heterogeneous reaction results in reduced NO$\sb2$ concentration when the sun returns at the end of the winter. A spectrograph system has been developed to measure scattered zenith skylight and thereby determine stratospheric NO$\sb2$ slant column abundance. Conversion of the measured slant column abundance to vertical column abundance requires dividing by the air mass. The air mass is the enhancement in the optical path for the scattered twilight as compared to a vertical path. Air mass values determined using a multiple scattering radiative transfer code have been compared to those derived using a Monte Carlo code and were found to agree to within 6% at a 90$\sp\circ$ solar zenith angle for a stratospheric absorber. Six months of NO$\sb2$ vertical column abundance measured over Fairbanks during the winter 1992-93 exhibited the daylight diminished and increased as the sunlight hours lengthened. The overall seasonal behavior was similar to high-latitude measurements made in the Southern Hemisphere. The ratios of morning to evening column abundance were consistent with predictions based on gas-phase chemistry. The possible heterogeneous reaction of $\rm N\sb2O\sb5$ on sulfate aerosols was investigated using Fourier Transform Infrared Spectrometer measurements of $\rm HNO\sb3$ column abundance and lidar determinations of the aerosol profile. Using an estimated $\rm N\sb2O\sb5$ column abundance and aerosol profile as input to a simple model, significant $\rm HNO\sb3$ production was expected. No increase in $\rm HNO\sb3$ column abundance was measured. From this set of data, it was not possible to determine whether significant amounts of $\rm N\sb2O\sb5$ were converted to $\rm HNO\sb3$ by this heterogeneous reaction. Better estimates of the $\rm N\sb2O\sb5$ and aerosol profile, and a more continuous set of $\rm HNO\sb3$ measurements, are needed to determine if $\rm HNO\sb3$ was actually produced.
• #### Nonlinear analysis of the ground-based magnetometer network

When the first magnetometer was created by Frederick Gauss in 1833, scientists gained a powerful tool for studying the structure, dynamics, and strength of the Earth's magnetic field: the magnetosphere. Since Gauss' time, the world's scientific community has established ground-based magnetometer stations around the globe in an effort to study local and global perturbations and patterns of the Earth's magnetic field. The main focus of this network has been monitoring the magnetic flux and impact from the Sun's constant outflow of radiation and particles known as the solar wind, as well as its more violent eruptive events. There has been little work, by comparison, into the signals and correlations within the network itself. Since the Earth's field can roughly be mapped to a dipole and disturbances often have a large scale structure, one can surmise there should be some correlation between stations based on their relative positions to one another. What that correlation is or represents is not clear. To investigate this possible correlation and its nature, a set of nonlinear analytic methods were conducted on magnetic data collected from stations scattered across North America over an 18 year period. The analysis was focused on searching for spatial and temporal correlations of nonperiodic signals in the magnetometer network. The findings from that analysis suggest there exist nonlocal correlations between stations that are dependent on position, which could be useful in the development of a space weather risk assessment.
• #### Observations and generation mechanisms of slow-mode waves in the magnetosheath

The interaction of solar wind with the geomagnetic field leads to the formation of the bow shock, magnetosheath, and magnetopause. Magnetohydrodynamic (MHD) slow-mode structures with a plasma density enhancement and magnetic field depression have been observed to appear frequently in the inner magnetosheath. In addition, the slow-mode structures usually consist of slow-mode waves with a smaller length scale. These slow-mode structures and waves are studied in this thesis through satellite observations and numerical simulations. We find, through satellite observations, that some of the slow-mode structures are associated with Alfven waves in the solar wind. On the other hand, simulations show that slow-mode waves are generated through the interactions between the bow shock and interplanetary shocks, magnetosonic waves, rotational discontinuities, or Alfven waves. The generated slow-mode waves stay in the inner magnetosheath for a long time (about 15 minutes) before the wave energy is convected away tailward. Of particular importance are the interactions between the bow shock and interplanetary rotational discontinuities or Alfven waves. These interactions generate a region with an enhanced plasma density and depressed magnetic field, which is very similar to the slow-mode structures observed in the inner magnetosheath. Based on observations and simulations, it is suggested that the interactions of various types of solar wind fluctuations with the bow shock may lead to the frequent appearance of slow-mode structures and waves in the inner magnetosheath. The generated slow-mode structures have strong pressure variations, and may impinge on the magnetopause as strong pressure pulses.
• #### Observations Of Metal Concentrations In E-Region Sporadic Thin Layers Using Incoherent-Scatter Radar

This thesis has used incoherent-scatter radar data from the facility at Sondrestrom, Greenland to determine the ion mass values inside thin sporadic-E layers in the lower ionosphere. Metallic positively-charged ions of meteoric origin are deposited in the earth's upper atmosphere over a height range of about 85-120 km. Electric fields and neutral-gas (eg N2, O, O2) winds at high latitudes may produce convergent ion dynamics that results in the re-distribution of the background altitude distribution of the ions to form thin (1-3 km) high-density layers that are detectable with radar. A large database of experimental radar observations has been processed to determine ion mass values inside these thin ion layers. The range resolution of the radar was 600 meters that permitted mass determinations at several altitude steps within the layers. Near the lower edge of the layers the ion mass values were in the range 20-25 amu while at the top portion of the layers the mass values were generally in the range 30-40 amu. The numerical values are consistent with in-situ mass spectrometer data obtained by other researchers that suggest these layers are mainly composed of a mixture or Mg +, Si+, and Fe + ions. The small tendency for heavier ions to reside at the top portion of the layers is consistent with theory. The results have also found new evidence for the existence of complex-shaped multiple layers; the examples studied suggest similar ion mass values in different layers that in some cases are separated in altitude by several km.
• #### Ozone depletion and biologically relevant ultraviolet radiation

An atmospheric radiative transfer model is used to calculate surface spectral ultraviolet irradiance under cloud-free conditions, and compared with measurements made at Lauder, New Zealand (45$\sp\circ{S}$, 170$\sp\circ{E})$ before and after the eruption of Mt. Pinatubo, and including a snow-covered surface. The ratios of diffuse to direct irradiance depend critically on solar elevation, surface albedo, and aerosol extinction. Ozone changes have pronounced effects on the global UVB irradiance, but have only a minor effect on these ratios. The comparison suggests that the ultraviolet radiation exposure can be computed with confidence for clear sky conditions, if the appropriate atmospheric pressure and temperature profiles, ozonesonde data, surface albedo, and aerosol optical properties are available. The total ozone abundances are derived by using ground-based UV irradiance measurements and compared with TOMS in Antarctica and the Arctic from 1990 to 1994. The comparisons show that they are generally in good agreement. Possible reasons for the discrepancies between the two methods are discussed. The equivalent cloud optical depths are also inferred from these data. Ozone depletion can also increase the penetration of ultraviolet radiation into the aquatic system. A coupled atmosphere-ocean radiative transfer model is used to investigate the effect of ozone depletion on UV penetration through the atmosphere and into the underlying water column. Comparisons between model computations and in situ measurements of irradiances made in Antarctic water show good agreement in the UV spectral range between 300 and 350 nm. The ratio of UVB (280-320 nm) to total (280-700 nm) irradiance also compared well. For a given ozone reduction the largest relative increase of UVB radiation arriving at the surface and penetrating to various depths in the ocean occurs at large solar zenith angles. At high latitudes the most pronounced increase in UVB exposure due to an ozone depletion occurs in the early spring, when ozone depletion is expected to be the most severe. The sensitivities of irradiance reflectance and diffuse attenuation coefficients to solar zenith angle, sky conditions, and chlorophyll concentration are discussed by using a coupled atmosphere-ocean radiative transfer model. The irradiance reflectance is sensitive to solar zenith angle, cloud cover, and chlorophyll concentration; the diffuse attenuation coefficient is sensitive to solar zenith angle and chlorophyll concentration, but less sensitive to sky conditions.
• #### Particle simulations of magnetic field reconnection and applications to flux transfer events

Basic plasma processes associated with driven collisionless magnetic reconnection at the Earth's dayside magnetopause are studied on the basis of particle simulations. A two-and-one-half-dimensional (2$1\over2$-D) electromagnetic particle simulation model with a driven inflow boundary and an open outflow boundary is developed for the present study. The driven inflow boundary is featured with a driving electric field for the vector potential, while the open outflow boundary is characterized by a vacuum force free condition for the electrostatic potential. The major findings are as follows. (1) The simulations exhibit both quasi-steady single X-line reconnection (SXR) and intermittent multiple X line reconnection (MXR). The MXR process is characterized by repeated formation and convection of magnetic islands (flux tubes or plasmoids). (2) Particle acceleration in the MXR process occurs mainly in O line regions as particles are trapped within magnetic islands, not in X line regions. The MXR process results in a power law particle energy spectrum of $f(E)\sim E\sp{-4}$. (3) Field-aligned particle heat fluxes and intense plasma waves associated with the collisionless magnetic reconnection process are also observed. (4) When applied to the dayside magnetopause, simulation results show that the MXR process tends to generate a simultaneous magnetic field perturbation on both sides of the dayside magnetopause, resembling the observed features of two-regime flux transfer events (FTEs). (5) An intrusion of magnetosheath plasma bulge into the magnetosphere due to the formation of magnetic islands may lead to the layered structures observed in magnetospheric FTEs. (6) In the current sheet, the enhanced tearing mode instability caused by the driving force applied at the driven inflow boundary creates an energy source at a specific wavenumber range with $k\sb{z}L\sim$ 0.3 in the modal spectrum of the magnetic field $B\sb{x}$ component. An inverse cascade of the modal spectrum of $B\sb{x}$ leads to the formation of the large-scale ordered magnetic island structures observed in the simulations. (7) In addition, the results of a theoretical study show that the tearing mode instability, and hence the magnetic reconnection at the dayside magnetopause, do not exhibit strong dependence on the magnetosheath $\beta$ values.
• #### Plasma dynamics of the Earth magnetopause-boundary layer and its coupling to the polar ionosphere

In this thesis, the plasma dynamics of the Earth magnetopause-boundary layer and its coupling to the polar ionosphere are studied by using computer simulations. First, the plasma dynamics and structure of the magnetopause-boundary layer are studied by a two-dimensional incompressible magnetohydrodynamic simulation code. It is found that the Kelvin-Helmholtz instability with driven boundary conditions at the magnetopause can lead to the formation of plasma vortices observed in the magnetopause-boundary layer. In the later stage of development, a density plateau is formed in the central part of the boundary layer. Second, the coupling of plasma vortices formed in the boundary layer to the polar ionosphere is studied based on a magnetosphere-ionosphere coupling model. The finite ionospheric conductivity provides a dragging force to the plasma flow and leads to the decay of plasma vortices in the boundary layer. In the model, the ionospheric conductivity is allowed to be enhanced due to accelerated electrons precipitating in upward field-aligned current regions. The competing effect of the formation and decay of vortices leads to the formation of strong vortices only in limited regions. Several enhanced conductivity regions are formed along the post-noon auroral oval, which may account for the observed auroral bright spots. In addition, the evolution of localized plasma vortices, as well as magnetic flux ropes, along magnetic field lines is studied. The evolution leads to the generation of large-amplitude Alfven waves, which carry field-aligned currents and provide the link for the coupling of plasma vortices and magnetic flux ropes in the magnetosphere to the polar ionosphere.

• #### Poker flat incoherent scatter radar investigations of the nighttime E-region

Plasma within the ionosphere affects technology, such as long distance communications and satellite navigation, by scattering and altering the propagation of radio waves sent through the ionosphere. Understanding the structure and dynamics of the ionosphere that may interfere with modern technology is therefore an important aspect of Space Weather research. In this thesis, the average characteristics and dynamics of the nighttime E-region (90-150 km in altitude) are investigated during auroral disturbances and near extreme solar minimum. The near-continuous data on electron density obtained with the Poker Flat Incoherent Scatter Radar (PFISR) near Fairbanks, Alaska are utilized. A number of correlation analyses between E-region electron content and AE index are performed in order to examine the influence of geomagnetic conditions on the E-region in relation to time of the day as well as seasonal and solar cycle effects. It is shown that E-region electron content and AE index exhibit significant positive correlation, particularly near local magnetic midnight, with greater correlation generally occurring in spring and autumn. The midnight feature is interpreted as an indication that the electrojet system near midnight is mostly controlled by electric conductance. The presented statistical results on the current-conductance relationship utilizing a new dataset strengthen conclusions derived from previous studies. The extent of E-region contribution to the total electron content (TEC) is also estimated and investigated for various conditions for the first time using the full altitude profile of PFISR. The estimates ranged between 5%-60% and more active periods generally displayed a more significant contribution from the E-region to TEC. Additionally, using the AE index as an indicator of auroral disturbance onset, the evolution of auroral density enhancements is explored using the superposed epoch analysis technique. The behavior of E-region electron content, peak density, height of peak density, and layer thickness is considered and three discernible phases in response of the thick E-layer to auroral disturbances are found. The observations are consistent with a scenario in which an initially soft and broad auroral electron energy spectrum quickly hardens and narrows during the main response and then slowly softens and becomes more broad during recovery.
• #### Proton transport and auroral optical emissions

The hydrogen lines are the characteristic emissions of proton aurora and have been used to study the impact of protons upon the atmosphere. Observations of hydrogen emission on the long wavelength side of the unshifted lines were not explained by previous theories. To explain the observed optical emissions, a numerical code is developed to solve the one dimensional, steady state, linearly coupled transport equations of H$\sp+$/H in a dipole magnetic field. For the first time, the mirror force is included in the transport equations to produce backscattered particles which are responsible for emission at wavelengths longward of the unshifted lines. Both downward and upward particle intensities of H$\sp+$/H are calculated. The mirror reflectivities of energy and particles are defined, and their dependences on proton input spectra and pitch angle distributions are studied. The results show that the mirror reflectivity increases both with characteristic energy and with pitch angle of the input proton flux, but is more sensitive to angular distributions than to energy spectra. Energy deposition rate, ionization rate, H$\sb\alpha$, H$\sb\beta$ and Nitrogen First Negative bands emission rates and profiles are calculated. Calculated fluxes of H$\sp+$/H and emission properties of Hydrogen Balmer lines are compared with a rocket measurement. The efficiency for production of the Balmer lines and the Nitrogen First Negative bands is obtained in terms of the energy input rate and the H$\sp+$ particle flux. A Doppler shift of about 3.0 A toward the blue for magnetic-zenith profiles of H$\sb\alpha$ is obtained, compared with observational results of $6.0 \pm 2.0$ A. The calculated emissions on the red side of the unshifted hydrogen atomic emission lines when convolved with the instrumental function accounts for the observed emissions on the long wavelength side of the unshifted hydrogen Balmer lines.
• #### Radial and azimuthal dynamics of the io plasma torus

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.

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

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.