### Recent Submissions

• #### A wiggle around a giant: exploring the hot electrons within the Io torus

Jupiter exhibits a fundamental rotational periodicity known as System IV that has no widely-accepted explanation, yet is easily observed in ultraviolet emission from the plasma torus generated by the innermost Galilean moon Io. This periodicity around Jupiter maps to a persistent, radially independent subcorotation within the Io plasma torus. In this thesis, we explore the origin and consequences of this periodicity. Using an equatorial chemistry and diffusive transport model, we demonstrate that a prescribed hot electron population produces a coherent wave of heightened energy flow that induces a consistent subcorotation. This additional hot electron population is consistent with the energization produced at high latitude by parallel electric fields induced by Alfvén waves propagating to the planet. The radial independence of this period means that while the generating mechanisms are likely in close proximity to the Io flux tube, the periodicity has dramatic consequences for plasma out at Europa, the next moon outwards and a subject of intense scientific curiosity.
• #### Properties and formation mechanisms of foreshock transients

Interactions between bow shock-reflected (foreshock) ions with solar wind particles can lead to the formation of foreshock transients, frequently observed upstream of the bow shock. Foreshock transients, such as foreshock bubbles, hot flow anomalies, and spontaneous hot flow anomalies, display heated, tenuous cores with large flow deflections bounded by compressional boundaries or shocks. They aid in particle acceleration at the bow shock and their significant dynamic pressure depletions can disturb the magnetosphere-ionosphere system. Thus, studying foreshock transients will expand our understanding of shocks and of the solar wind-planetary magnetosphere coupling throughout the universe. The dissertation presents an observational statistical study and numerical simulations of foreshock transients to investigate their solar wind conditions, properties, and formation mechanisms. The statistical study shows that occurrences of foreshock transients are higher for lower magnetic field strengths and higher solar wind speeds and Alfvén Mach numbers. A geometrical approach with a model bow shock reveals that they typically span up to 3 Earth radii along and extend up to 6 Earth radii from the bow shock. The study also finds that foreshock transients with local density enhancements (substructures), in otherwise density-depleted cores, are often larger in size than those without substructures, and provide variances to the dynamic pressure profiles that could further deform the bow shock surface and lead to more complicated geoeffects. In addition, foreshock transients with and without substructures occur in the same solar wind conditions, thereby implying that substructures are an inherent property of foreshock transients during their evolution. In numerical simulations where injected foreshock ions perform partial gyrations around a discontinuity to generate Hall currents that change the magnetic field topology, foreshock bubbles form from thin discontinuities, and hot flow anomalies form from thick discontinuities. The simulations prove that the foreshock ion current configuration, controlled by the magnetic field change the foreshock ions experience within their gyromotions, determines the magnetic field profile of the structure. Furthermore, the parameter scan results show that the initial foreshock ion distribution types determine how easily foreshock ions can cross the discontinuity and thus how strong the structure forms; parameters that increase their densities or speeds perpendicular to the magnetic field across the discontinuity leads to stronger Hall currents and more significant magnetic field variations. We also find that the initial foreshock ion densities, thermal speeds, and beam speeds all positively and linearly correlate with the expansion speeds and the density compression ratios of the formed structures. This, thereby, provides a possibility to quantify the role of these parameters in the formation and expansion model of foreshock transients and to forecast their aforementioned particle acceleration and geoeffects.
• #### Exospheric neutral density study using XMM-Newton soft x-ray observations and MHD-based magnetosheath model

Studying interactions between the solar wind and the Earth is important, and one future satellite mission, called Solar wind Magnetosphere Ionosphere Link Explorer (SMILE), will be launched in late 2024 or early 2025 to study these interactions. SMILE will image the magnetosheath using the charge exchange process and extract the location and motion of the bow shock and magnetopause. The subsolar magnetopause is typically around 10 Earth radii, and neutral density in this outer exosphere is poorly understood. For that reason, we study the neutral density around 10 Earth radii. We estimated the exospheric density at 10 Earth radii using XMM-Newton astrophysics observations, one taken during solar minimum and five taken during solar maximum. For the solar minimum case study, the lower limit of the exospheric density was estimated to be 36.8 ± 11.7 cm⁻³ at 10 Earth radii subsolar point. For solar maximum case studies, we estimated neutral density to be in the range of 42.5 - 65.1 cm⁻³ at 10 Earth radii subsolar point. This suggests weak dependence of neutral density on solar activity but more statistical analysis is needed. The neutral density behavior of the outer exosphere will help us understand the Earth's atmospheric loss due to the dynamic space environment and thus, infer the entire evolutionary history of the Earth's atmosphere as well as of other planetary atmospheres. Along with neutral density, plasma number density, velocity, and temperature in the magnetosheath are key parameters for predicting soft X-ray images. We developed a user-friendly model of magnetosheath parameters to help derivation of these parameters in future wide field-of-view soft X-ray missions. The model parameterizes number density, velocity, magnetic field, and temperature, by using OpenGGCM MHD simulation results as seed data. We made a suite of magnetosheath models, by compiling pre-existing magnetosheath models (analytic, gas-dynamic) with our MHD-based model. This parameterized model is expected to enable researchers to reconstruct expected soft X-ray images and also use these images for analysis of observed images from future satellite missions including SMILE.
• #### Phase effects on turbulent transport in the magnetic confinement of plasmas for nuclear fusion

With climate change effects on the rise, the global energy infrastructure requires revision. We first provide a brief review of common energy resources as well as their safety and climate effects. We then compare and contrast nuclear fission and fusion based energy schemes. Difficulties based on the requirements of the fusion triple product, as well as the fast neutrons from the deuterium and tritium reaction are also discussed. The lack of sufficient experimental controls in enhanced confinement modes like the I-mode and the H-mode, lead to difficulties satisfying the restrictions imposed by the Greenwald density limit. These combined with several operational needs like ash and impurity removal, enhanced density control, the ability to access other confinement modes at reduced energy thresholds, motivates the search for a barrier capable of variable energy and density confinement. Self consistent models suggest that unique phase relationships exist between different turbulent instabilities and plasma profiles like temperature and density, that determine the turbulent transport of the quantity. Two common instabilities, driven by the electron and ion temperature gradient, and their unique phase relations are used to arrive at a net phase relation for temperature and for density. Then, using electron and ion radio frequency heating, the difference in phase of the turbulent transport may be locally changed, altering transport dynamics. Methods to increase core temperature while simultaneously increasing density transport, thereby avoiding the Greenwald limit, are discussed. The proposed transport controls are based upon characteristics of the localized radio frequency heating including amplitude, location, and duration. These parameters determine the power deposited in the plasma, and therefore the local ratios of the electron and ion temperature driven instabilities. Aspects of each parameter's effect on radial transport are summarized, with the strongest phase barrier allowing for a ∼ 15% increase of core ion temperature and ∼ 30% decrease of core density.
• #### The distribution of nitric oxide at 150 km

"The objectives of this thesis are to determine the morphology of nitric oxide at the altitude of 150 km and to determine what drives the observed variability. Those objectives are accomplished by characterizing satellite observations of nitric oxide at that altitude and comparing them with those at 106 km, the altitude of peak density. The global distribution of nitric oxide and its response to geomagnetic activity vary between the two altitudes. At 150 km, nitric oxide is most abundant at high latitudes in the sunlit summer hemisphere, in contrast to nitric oxide at 106 km, which is most abundant at high latitudes in the winter hemisphere. The high-latitude component of nitric oxide at both altitudes is associated with geomagnetic activity, although the primary production mechanisms differ between the two altitudes. At 106 km, high-latitude nitric oxide density enhancements are driven by particle precipitation. At 150 km, nitric oxide at high latitudes is enhanced by increased temperatures arising from Joule heating. Enhancements at 150 km occur more rapidly than those at 106 km. At both altitudes, the response of nitric oxide to geomagnetic activity exhibits a seasonal variation that is attributed to seasonal variations in the production mechanisms"--Leaf iii
• #### Determination of the diffusion coefficient for trimethylaluminum in the thermosphere at altitudes 120 to 180 km

"The object of this work is to determine the diffusion coefficient (D) of trimethylaluminum (TMA) in the lower thermosphere as a function of altitude (h). This is done by measuring the dispersion of chemiluminescent TMA that is released in discrete quantities, or puffs, from sounding rockets at altitudes 120 to 180 km. Diffusing TMA, which glows in contact with atmospheric oxygen, is observed with stereoscopic ground-based imaging. Brightness profiles across a puff are found to be Gaussian in shape, with width parameter [sigma](t, h) that increases with age (t) of the puff leading to D = [sigma]² (t, h)/2t, independent of time, which is in good agreement with some past results. For example D = (2.5 ± 0.2) x 10³m²s⁻¹ at an altitude of 128 km for the state of the thermosphere at that time. A constant A links three altitude-dependent terms, the diffusion coefficient, temperature and density, at a particular location of the atmosphere, via D(h) = ATS (h)/n(h). It is determined from this study to be A=(4.42±0.05)x10¹⁸(m·s)⁻¹ for s = 0.75. Using these values for A and s, and temperatures and the densities determined from the MSIS-90 thermospheric model, diffusion coefficients for TMA can be determined at other locations and under different geomagnetic conditions"--Leaf iii
• #### Plasma transport and magnetic flux circulation in Saturn's magnetosphere

The magnetospheres of outer planets are very different than the terrestrial magnetosphere. The magnetosphere of Saturn is rapidly rotating, and has its own plasma source. Enceladus located around 4Rs is the main source of plasma. The strong magnetic field of Saturn's magnetosphere picks up the plasma which experiences a strong centrifugal force in the non-inertial reference frame. The plasma produced in the inner magnetosphere has to be transported radially outward and lost to the solar wind. The transport of plasma in Saturn's magnetosphere is not fully understood. It is believed that transport is centrifugally-driven, occurring via flux tube interchange motions in the inner magnetosphere and via plasmoid expulsion in the magnetotail due to reconnection. It has been found that these mechanisms are not sufficient to explain the transport. We tried to determine different possible transport mechanisms that could exist in the outer planetary magnetosphere. Ma et al. (2019a) showed the low-specific entropy plasma with a narrow distribution in Saturn's inner magnetosphere and suggests a significant nonadiabatic cooling process during the inward motion while high specific entropy suggests the nonadiabatic heating during the outward transport. We have estimated the outward plasma transport rate about 55 kg s⁻¹. The calculation of magnetic flux transport and analysis of magnetic field data indicates that plasma transport in the Saturn magnetosphere could be dominated by small scale magnetic reconnection.
• #### Modeling supraglacial lake drainage and its effects on the seasonal evolution of the subglacial drainage system in a tributary glacier setting

This work aims to gain a better understanding of the relationship between glacier motion and water distributed through subglacial drainage systems. A numerical scheme (GlaDS) is used to model both inefficient and efficient drainage systems to see which dominates after the draining of a supraglacial lake on a synthetic glacier that is made up of an outline that features a main branch and a tributary. The geometry is based on the surgetype Black Rapids Glacier (Ahtna Athabascan name: Da lu'itsaa'den) in Alaska, where a lake develops in the higher ablation zone, and drains rapidly early in the melt season. It has also been observed that this lake drainage causes a twofold or threefold speed-up of the main branch, with some acceleration of the lower-lying Loket tributary. This speed-up can be considered a surrogate for a surge, which also initiates in the main branch, while, during times of quiescence, the ice flow on the tributary is dominant. We investigate the effects of varying timing and volume inputs of lake drainage with a focus on its effects beneath the tributary. We find that the response of the glacier depends on the seasonal timing, the amount of water from the draining lake, and its location on or near the margins of the glacier. Results show that an inefficient drainage system is the cause of the glacier speed-up, both when the lake drains rapidly or when there is an extended time in drainage, at any time of the season. The speed signals vary throughout the glacier depending on the location of the lake relative to that of an evolved efficient drainage system.
• #### Control of internal transport barriers in magnetically confined tokamak fusion plasmas

In the Tokamak plasma, for fusion to be possible, we have to maintain a very high temperature and density at the core at the same time keeping them low at the edge to protect the machine. Nature does not favor gradients. Gradients are source of free energy that causes instability. But we require a large gradient to get energy from plasma fusion. We therefore, apply a huge magnetic field on the order of few Tesla (1 T-10 T) that confines the plasma in the core, maintaining gradients. Due to gradients in density of charged particles (ions and electrons), there is an electric field in the plasma. Heat and particle transport takes place from core to edge mainly through anomalous transport while the E x B velocity sheer acts to reduce the transport of heat and particles. The regime at which the E x B velocity shear exceeds the maximum linear instability growth rate, as a result, the transport of particles and heat gets locally reduced is termed as the formation of a transport barrier. This regime can be identified by calculating the transport coefficients in the local region. Sometimes it can be observed in the edge where it is called an edge barrier while if it is near the core it is an internal transport barrier. There is a positive feedback loop between gradients and transport barrier formation. External heating and current drives play an important role to control such barriers. Auxiliary heating like neutral beam injection (NBI) and radio frequency (RF) heating can be used at a proper location (near the core of the plasma) to trigger or (far outside from the core) to destroy those barriers. Barrier control mechanism in the burning plasmas in international thermonuclear test reactor (ITER) parameter scenarios employing fusion power along with auxiliary heating source and pellets are studied. Continuous bombardment with pellets in the interval of a fraction of a second near the core of the burning plasma results in a stronger barrier. Frozen pellets along with auxiliary heating are found to be helpful to control the barriers in the tokamak plasmas. Active control mechanism for transport barriers using pellets and auxiliary heating in one of tokamaks in United States (DIII-D) parameter scenarios are presented in which intrinsic hysteresis is used as a novel control tool. During this process, a small background NBI power near the core assists in maintaining the profile. Finally, a self-sustained control mechanism in the presence of core heating is also explored in Japanese tokamak (JT-60SA) parameter scenarios. Centrally peaked narrow NBI power is mainly absorbed by ions with a smaller fraction by the electrons. Heat exchange between the electron and ion channels and heat conduction in the electron channel are found to be the main processes that govern this self control effect. A strong barrier which is formed in the ion channel is found to play the main role during the profile steepening while the burst after the peaked core density is found to have key role in the profile relaxation.
• #### Regional modeling of Greenland's outlet glaciers with the parallel ice sheet model

The most recent report from the Intergovernmental Panel on Climate Change cites ice sheet dynamics as the greatest source of uncertainty for predicting current and future rates of sea level rise. This has prompted the development and use of ice sheet models that are capable of simulating the flow and evolution of ice sheets and their corresponding sea level contribution. In the Arctic, the Greenland ice sheet appears to be responding to a warming climate more quickly than expected. In order to determine sea level contribution from Greenland, it is necessary to capture the regional dynamics of the fast flowing outlet glaciers that drain the ice sheet. This work has developed a novel regional model capable of simulating an outlet glacier, and its associated drainage basin, as a mode of using the Parallel Ice Sheet Model. Specifically, it focuses on modeling the Jakobshavn Isbrae as a demonstration. The Jakobshavn Isbrae is one of the world's fastest flowing outlet glaciers, and accounts for nearly 5% of ice loss from the Greenland Ice Sheet. Additionally, the Jakobshavn Isbrae has been widely studied for several decades, and a wealth of remotely sensed and in situ data is available in this region. These data are used as model input and for model validation. We have completed a parameter study in this work to examine the behavior of the regional model. The purpose of this study was not to tune the model to match observations, but rather to look at the influence of parameter choices on the ice dynamics. Model results indicate that we have identified the subset of the model parameter space that is appropriate for modeling this outlet glacier. Additionally, we are able to produce some of this more interesting features that have been observed at Jakobshavn, such as the development and disintegration of a floating ice tongue and the distribution of observed surface velocities. We validate these model results by comparison with recent spatially rich measurements of ice surface speeds, as well as ice geometry.
• #### Transient spatiotemporal chaos in a Morris-Lecar neuronal ring network collapses to either the rest state or a traveling pulse

Transient spatiotemporal dynamics exists in an electrically coupled Morris-Lecar neuronal ring network, a theoretical model of an axo-axonic gap junction network. The lifetime of spatiotemporal chaos was found to grow exponentially with network size. Transient dynamics regularly collapses from a chaotic state to either the resting potential or a traveling pulse, indicating the existence of a chaotic saddle. For special conditions, a chaotic attractor can arise in the Morris-Lecar network to which transient chaos can collapse. The short-term outcome of a Morris-Lecar ring network is determined as a function of perturbation configuration. Perturbing small clusters of nearby neurons in the network consistently induced chaos on a resting network. Perturbation on a chaotic network can induce collapse in the network, but transient chaos becomes more resistant to collapse by perturbation when greater external current is applied.
• #### Two-dimensional Bernstein-Greene-Kruskal modes in a magnetized plasma with kinetic effects from electrons and ions

Electrostatic structures are observed in various of space environments including the auroral acceleration region, the solar wind region and the magnetosphere. The Bernstein-Greene-Kruskal (BGK) mode, one of the non-linear solutions to the Vlasov-Poisson system, is a potential explanation to these phenomena. Specifically, two dimensional (2D) BGK modes can be constructed through solving the Vlasov-Poisson-Ampère system with the assumption of a uniform ion background. This thesis discusses the existence and features of the 2D BGK modes with kinetic effects from both electrons and ions. Specifically, we construct electron or ion BGK modes with finite temperature ratio between ions and electrons. More general cases, the electron-ion 2D BGK mode with the participation of both non-Boltzmann electron and ion distributions are constructed and analyzed as well.
• #### Solar magnetic fields: source, evolution, and interaction with planetary magnetospheres

Magnetized plasmas with twisted and filamented magnetic fields are pervasive throughout the heliosphere. In the solar magnetic field, photospheric convection on scale sizes from granules to differential rotation is responsible for driven magnetic reconnection. These reconnection sites are closely related to the magnetic topology, which is highly complex as the magnetic field is structured by a network of many thousands of magnetic flux concentrations. The coronal plasma overlying this "magnetic carpet" is the source of the solar wind flow, which has been found to be turbulent as close to the sun as our observations can currently resolve. At 1 AU, observations have also revealed a highly structured solar wind which we posit in this thesis originates in the corona rather than forming in-transit. Further, the solar wind-magnetosphere interaction depends on variability in the solar wind. When the boundary between solar wind plasma and magnetospheric plasma is unstable to the growth of Kelvin-Helmholtz waves, driven magnetic reconnection can occur on the magnetopause boundary. Such reconnection allows magnetic field to thread the boundary and transport can take place. We quantify the solar wind interaction for a corotation dominated system in terms of the mass and momentum transport driven by Kelvin-Helmholtz instabilities. Model-data comparisons are performed in this thesis using both the magnetohydrodynamic and hybrid-kinetic approaches for fluid simulations.
• #### Risk analysis of Cordova's microgrid from a complex systems viewpoint

Cordova is a town of approximately 2,000 people located on the southern coast of Alaska. A power grid for a town this size, with a large seasonal fishing economy, is considered a moderate to large sized microgrid in terms of power produced. Understanding the vulnerabilities and risks of failures in such a grid is important for planning and operations and investigating these characteristics in the context of complex system dynamics is novel. The analysis of Cordova's microgrid is a case study relevant to a large class of microgrid communities that could benefit from this work. Our analysis of this grid began by looking at the distribution of all outages from 2003 - 2017 by size, followed by splitting up outages based on certain characteristics and again looking at outage size distribution based on different characteristics. Following this we correlated the outages with different weather patterns and then with the hourly load demand on the system. After doing these analyses we developed a risk metric to give a single numerical value to the risk of an outage occurring during certain time periods and under certain conditions. We looked at risk in the summer versus the winter due to the summer having a much larger load demand, and we also looked at the risk before and after all cables in the grid were buried underground. This gives us an idea of when/under what circumstances the most outages are likely to occur and allows us to run our model of the system, make changes, and determine if those changes were beneficial to the system or not.
• #### The effect of rate, frequency, and form of migration on host parasite population dynamics

What is the effect of migration on host-parasite population dynamics? Animals live in a landscape where they move between patches. They are also locked in host-parasite conflicts. Host-parasite interactions are modeled with consumer resource functions. I constructed models using two different consumer resource functions (the Lotka Volterra system and the Saturating Type II system). The first model was a conservative system. The second was dissipative and more biologically realistic. I examined the effect of rate of migration, time between migration events, and form of migration. I found that the time between migration events had the largest effect on the synchronization in host-parasites population dynamics between the patches. Decreased time between migration events increased the fraction of simulation to completely synchronize and decreased the time it took to do so. In the first model, I observed simulations with a low rate of migration took a long time to synchronization and with a high rate of migration took a short time to synchronize. There was a phase transition between these two amounts of time it took to synchronize. In the second model, simulations done at low rates of migration did not synchronize while with increased migration rates the fraction of simulations to synchronize increased. I found in some simulations of parasite only migration that the patches synchronized faster. My results imply that parasite only migration to islands could have a greater impact on the extinction risk on islands further from the mainland than other forms of migration.
• #### Studying auroral microphysics using multiple optically tracked rocket sub-payloads

There is insufficient knowledge of scale length parameters associated with ionospheric plasma structures. Using a novel technique combining rocket-based instrument data with ground-based optical and instrumental data measurements, ISINGLASS attempts to determine the spatial scale lengths over which parameter differences in auroral arcs present in the upper ionosphere. Determination of such scale lengths has the propensity to strengthen preexisting models of magnetosphere-ionosphere interactions. While analysis is not complete and the extent of such scale lengths is still unknown, after completion of the experiment phase of the mission, differences in measurements have been found that cannot be accounted for through experimental error. This shows the existence of a critical scale length within the distances measured, and the techniques used present a reliable method with which to launch a future campaign.
• #### 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.
• #### Characteristics of dayside auroral displays in relation to magnetospheric processes

The use of dayside aurorae as a ground based monitor of magnetopause activity is explored in this thesis. The origin of diffuse (OI) 630.0 nm emissions in the midday auroral oval is considered first. Analysis of low altitude satellite records of precipitating charged particles within the cusp show an unstructured electron component that will produce a 0.5-1 kR 630.0 nm emission throughout the cusp. Distribution of the electrons is controlled by the requirement of charge neutrality in the cusp, predicting a diffuse 630.0 nm background even if the magnetosheath plasma is introduced into the magnetosphere in discrete merging events. Cusp electron fluxes also contain a structured component characterized by enhancements in the electron energy and energy flux over background values in narrow regions a few 10's of kilometers in width. These structured features are identified as the source of the transient midday arcs. An auroral model is developed to study the morphology of (OI) 630.0 nm auroral emissions produced by the transient arcs. The model demonstrates that a diffuse 630.0 nm background emission is produced by transient arcs due to the long lifetime of the O$(\sp1D)$ state. Two sources of diffuse 630.0 nm background emissions exist in the cusp which may originate in discrete merging events. The conclusion is that persistent 630.0 nm emissions cannot be interpreted as prima facie evidence for continuous particle transport from the magnetosheath across the magnetopause boundary and into the polar cusp. The second subject that is considered is the analysis of temporal and spatial variations of the diffuse 557.7 nm pulsating aurora in relation to the 630.0 nm dominated transient aurora. Temporal variations at the poleward boundary of the diffuse 557.7 nm aurora correlate with the formation of the 630.0 nm transient aurorae suggesting that the two events are related. The character of the auroral variations is consistent with the behavior of particle populations reported during satellite observations of flux transfer events near the dayside magnetopause. An interpretation of the events in terms of impulsive magnetic reconnection yields a new observation that relates the poleward moving transient auroral arcs in the midday sector to the flux transfer events.
• #### Two- and three-dimensional study of the Kelvin-Helmholtz instability, magnetic reconnection and their mutual interaction at the magnetospheric boundary

Magnetic reconnection and the Kelvin-Helmholtz (KH) instability regulate the transport of magnetic flux, plasma, momentum and energy from the solar wind into the magnetosphere. In this thesis, I use two-dimensional and three-dimensional MHD simulations to investigate the KH instability, magnetic reconnection, and their relationship. Two basic flow and magnetic field configurations are distinguished at the Earth's magnetopause: (1) configurations where the difference in plasma velocity between the two sides of the boundary $\Delta$v (velocity shear) is parallel to the difference of the magnetic field $\Delta$b (magnetic shear), and (2) configurations where the velocity shear is perpendicular to the magnetic shear. For configuration (1), either magnetic reconnection is modified by the shear flow, or the KH instability is modified by the magnetic shear and resistivity. The evolution of the basic configuration (2) requires three dimensions. In this case, both processes can operate simultaneously in different planes. If the KH instability grows faster initially, it can wrap up the current layer and thereby initiate a very fast and turbulent reconnection process. The resulting magnetic turbulence can provide the first explanation of often very turbulent structures of the magnetopause current layer. For the first time, it is quantitatively confirmed that the KH instability operates at the magnetospheric boundary at low latitudes.
• #### 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.