Now showing items 23-42 of 91

• #### Effects of gravity waves on the polar oxygen and hydroxyl airglow

The effect of gravity waves on the OH (87 km) and O$\sb2$ (95 km) airglow emissions was examined using spectroscopic airglow data. The data was obtained from Longyearbyen, Svalbard (78$\sp\circ$N) and Fairbanks, Alaska (64$\sp\circ$N) using Ebert-Fastie spectrometers and a system of Meridian Scanning Photometers. The spectrometers scanned in wavelength from 8200A to 8750A which included the airglow emissions from the OH(6-2) Meinel band and the O$\sb2$(0-1) atmospheric band. The analysis was done by fitting a synthetic spectrum to the data and thereby the rotational temperature was calculated as well as the band intensity of each of the emissions. The rotational temperatures were assumed to represent the temperature of the emission region. Gravity waves were assumed to modify the density and temperature of the atmosphere in the region of the airglow emissions. These modifications were measured as fluctuations in the band intensity and rotational temperatures of the two emissions. In order to compare the data with theoretical models, it was necessary to calculate two parameters. The parameter $\eta$ is defined as the ratio of the amplitudes of the fluctuations in intensity and temperature. The other parameter is the phase angle between the fluctuations in intensity and temperature. These parameters were found to vary with wave period. The variations in $\eta$ and phase agreed fairly well, for long period waves, with the most recent models. None of the models agree with the observed values of $\eta$ and phase for short period waves. The second part of this thesis examines the vertical and horizontal wavelengths, the phase speeds, and the propagation directions of several specific gravity wave examples. During a 60 hour period of data taken from Svalbard, three well defined gravity waves were observed. The propagation directions implied a moving source south of the observing station.
• #### Electron transport and optical emissions in the aurora

A one-dimensional, steady state auroral model is developed based on a linear electron transport calculation. A set of cross sections for electron neutral collisions describing elastic scattering, energy loss, and photon emission is compiled and used in conjunction with a discrete ordinate transport code. Calculated electron intensities are compared with in situ rocket measurements. Auroral optical emissions that result from direct electron impact on neutrals are calculated for synthetic and observed electron spectra. A systematic dependence of the brightness of auroral features on energy flux, characteristic energy, and atmospheric composition is found and parameterized. A method for interpreting the brightness and the ratio of brightnesses of certain auroral emissions in terms of the energy flux, characteristic energy, and relative oxygen density is described. Application of this method to auroral images acquired by nadir viewing instruments aboard a satellite is discussed and the distribution of energy flux, characteristic energy, and ionospheric conductances over the auroral oval is determined. Emissions that are suitable for analysing auroral spectra in terms of the atomic oxygen abundance in the auroral zone are identified.
• #### Electrostatic ion cyclotron waves in barium injection experiments in space

Electrostatic ion cyclotron waves are investigated in a charge-generated barium-shaped plasma directed parallel to the earth’s magnetic field. The barium plasma is generated as a result of a barium shape charge release in the upper F₂ region of the ionosphere undergoing photoionization, Using a differential velocity distribution given by Stenbaek-Nielsen et al., [1984], this situation has been modeled based on the condition of collisionless plasma. The instabilities were studied for cases with and without an ambient oxygen ion background. It was concluded that fast ionization in excess of photoionization due to the excitation of electrons by electrostatic ion cyclotron waves was not feasible for the ejection directed along the earth’s magnetic field nor would there be any contribution to Alfven’s critical velocity mechanism if the injection was directed perpendicular to the magnetic field.
• #### Equilibrium structure and dynamics of near-earth plasma sheet during magnetospheric substorms

A magnetofrictional method and MHD simulation are used to study MHD equilibria and dynamic evolution of the Earth's magnetosphere during a substorm growth phase. I suggest that the new "entropy anti-diffusion instability" associated with plasma transport across field lines leads to an enhanced entropy gradient and accelerates the formation of a thin current sheet during the final substorm growth phase. Based upon the MHD simulations with a pressure diffusion term, I confirm that entropy anti-diffusion instability can lead to a very thin current sheet with $B\sb{z} < 0.5nT$ and thickness $<$1000km in the near-earth magnetotail ($x \sim -8$ to $-20R\sb{e}$) during the growth phase of substorm. The formation of the thin current sheet can explain the observed explosive growth phase of substorms. In the study of magnetotail equilibrium configurations, it is found that the profile of the magnetic field strength B$\sb{z}$ component in the equatorial plane is mainly determined by the entropy $S(A)\ (S = pV\sp\gamma)$ on magnetic flux tubes. I obtain self-consistent equilibria of the Earth's magnetosphere with very strong lobe fields and a monotonically increasing $B\sb{z}$ component towards the Earth. It is also confirmed that an enhancement of the lobe flux favors the formation of a current sheet during the early substorm growth phase. However, my results do not support the notion that a critical amount of the lobe flux is required for a collapse of the tail current sheet.
• #### Excitation of the ionized nitrogen molecule in the aurora

An understanding of the excitation mechanism of the ionospheric molecules during auroral activity is of vital importance for the overall ionospheric understanding including its interaction with the magnetosphere. In this thesis we study two emissions originating from the excited nitrogen molecule ion. The first negative (0,1) emission at 4278 Å originating from the B state, and Meinel (2,0) emission at 7852 Å originating from the A state during moderate to strong aurora have been observed with an imaging spectrograph at Poker Flat, Alaska. The B state has a short lifetime compared to the inverse collision frequency at auroral altitudes, while the A state can be deactivated during collisions at altitudes near 95 km. The B state can be populated by an up-welling of N₂ into sunlit regions. Both processes are expected to depend on auroral activity. If none of the processes are present we expect a constant ratio between the two emissions. Data for three nights have been studied and a constant ratio was found at all times. Thus neither deactivation of the A state or up-welling of the ion seem to appear during the observations presented here. The values of the ratio for the three nights are 2.53 plus-minus 0.38, 3.05 plus minus 0.22, and 3.40 plus minus 1.10, respectively.

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

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.

• #### Hot flow anomalies at earth's bow shock and their magnetospheric-ionospheric signatures

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

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

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

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

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

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.
• #### Longwave Radiative Transfer In The Atmosphere: Model Development And Applications

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.