Now showing items 36-55 of 91

• #### Kelvin-Helmholtz Instability And Magnetic Reconnection At The Earth's Magnetospheric Boundary

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

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

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.

• #### Magnetic Reconnection As A Chondrule Heating Mechanism

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

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

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

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

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

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

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.