Thesis (Ph. D.)--University of Iowa, 2008. / Thesis supervisor: William Daughton. Includes bibliographical references (leaves 202-207).

Thesis (Ph.D.)--Montana State University--Bozeman, 2007. / Typescript. Chairperson, Graduate Committee: Richard Canfield. Includes bibliographical references (leaves 83-88).

In highly conductive plasmas described by the ideal magnetohydrodynamics (MHD), magnetic field lines are frozen-in to the plasma. The contrary process takes place when the localized non-ideal and diffusive effects allow the field lines to break and reform, and therefore, called "magnetic reconnection" process. Magnetic reconnection is well recognized as an important plasma process capable of converting enormous amounts of stored magnetic energy to both thermal energy and bulk acceleration of the plasma. Single-fluid MHD model of this process can not explain the rate of magnetic reconnection observed in the space and laboratory plasmas, but the two-fluid model has raised the promises of explaining the magnetic reconnection satisfactorily. This thesis by employing the two-fluid MHD model of the magnetic reconnection studies theoretically this process.

Solutions of the magnetohydrodynamic (MHD) equations are very important for modelling laboratory, space and astrophysical plasmas, for example the solar and stellar coronae, as well as for modelling many of the dynamic processes that occur in these different plasma environments such as the fundamental process of magnetic reconnection. Our previous understanding of the behavior of plasmas and their associated dynamic processes has been developed through two-dimensional (2D) models. However, a more realistic model should be three-dimensional (3D), but finding 3D solutions of the MHD equations is, in general, a formidable task. Only very few analytical solutions are known and even calculating solutions with numerical methods is usually far from easy. In this thesis, 3D solutions which model magnetic reconnection and rigidly rotating magnetized coronae are presented. For magnetic reconnection, a 3D stationary MHD model is used. However, the complexity of the problem meant that so far no generic analytic solutions for reconnection in 3D exist and most work consists of numerical simulations. This has so far hampered progress in our understanding of magnetic reconnection. The model used here allows for analytic solutions at least up to a certain order of approximation and therefore gives some better insight in the significant differences between 2D and 3D reconnection. Three-dimensional numerical solutions are also obtained for this model. Rigidly rotating magnetized coronae, on the other hand, are modeled using a set of magnetohydrostatic (MHS) equations. A general theoretical framework for calculating 3D MHS solutions outside massive rigidly rotating central bodies is presented. Under certain assumptions, the MHS equations are reduced to a single linear partial differential equation referred to as the fundamental equation of the theory. As a first step, an illustrative case of a massive rigidly rotating magnetized cylinder is considered, which somehow allows for analytic solutions in a certain domain of validity. In general, the fundamental equation of the theory can only be solved numerically and hence numerical example solutions are presented. The theory is then extended to include a more realistic case of massive rigidly rotating spherical bodies. The resulting fundamental equation of the theory in this case is too complicated to allow for analytic solutions and hence only numerical solutions are obtained using similar numerical methods to the ones used in the cylindrical case.

During the late 19th and into the 20th Century, the Tisza River’s vast floodplain-wetland systems were largely disconnected by levees, facilitating "reclamation" for agriculture and resulting in an estimated loss of over 90% of historical wetlands. While levees have been successful in preventing catastrophic flooding for a century, Lower Tisza flood stages continue to rise partially due to aggradation and increased roughness on the confined floodplain. The decrease in the Tisza's current floodway carrying capacity has reduced the flood-protection level of the Tisza's aging levee system. Recently in Hungary, "Room for the River" policies have gained more prominence. For the first of three papers for this dissertation, I assessed eight potential floodplain-reconnection scenarios that would provide more room for the river between Csongrád, Hungary and the Hungary-Serbia border. A novel framework using hydrodynamic and geospatial modeling was used to perform planning-level evaluations of the tradeoffs between floodplain-reconnection scenarios and enhancement of the existing levee system. The scenarios evaluated include levee removal and levee setbacks to strategically reconnect historical wetlands while reducing flood levels. Scenario costs and human population impacts were also assessed. Impacts of reconnecting the Lower Tisza floodplain were compared to heightening levees, the prevailing strategy over the previous century. From a purely construction-cost perspective, heightening Lower Tisza levees is potentially the most cost-effective and politically expedient solution (i.e., impacts the least number of people). However, levee heightening does not solve the long-term problem of reduced flood conveyance, nor does it result in wetland reconnection or enhancement of other floodplain ecosystem services. The suite of reconnection options evaluated provides engineers, planners, and decision makers a framework from which they can further evaluate potential flood-risk reduction options. At least three of the eight reconnection scenarios (setting the western levee back, 1500-meter, and 2000-meter setbacks) along the Lower Tisza demonstrated that floodplain-wetland reconnection is possible while achieving the objectives of minimizing impacts on human populations and reducing flood heights. The Illinois River has a similar history to the Tisza. Levees were constructed, and wetlands were drained during the late 19th and early 20th Centuries. By the mid-1920’s, Illinois River levee systems became increasingly difficult for private landowners and the state to maintain as commodity prices fell and flood levels increased. However, the 1928 U.S. Flood Control act shifted a substantial portion of the burden of flood mitigation from local landowners to the federal government, preventing the dissolution of levee districts. While these levee systems have facilitated floodplain agricultural production and development for the last century, disconnecting the river from its floodplain has led to concerns about the negative impacts of levees on the physical and biological systems of the Illinois River Valley. Recent studies have emphasized approaches that would result in setting back or removing levees in order to naturalize portions of large river-floodplain systems, including the Illinois. The costs and benefits of such projects have shown potential restoration benefits may outweigh potential costs, but these studies have not demonstrated the specific levee districts which have the highest reconnection potential from an economic standpoint. The second paper for this dissertation used geospatial methods to fill this gap by assessing the National Commodity Crops Productivity Index (NCCPI) soil values and agricultural production and profit values for corn and soybeans in 32 individual levee districts along a 235-km segment of the Lower Illinois River. In general, soil productivity index values were lower for Illinois River levee districts compared to the county averages in which the districts are located. Over the five-year study period from 2010-2014, the total agricultural profits in the levee districts ranged from $18-61 million. Several levee districts have relatively low per hectare agricultural values when compared to wetland benefit studies, indicating these protected floodplain areas may be suitable for reconnection. For the third and final dissertation paper I used a novel hydrodynamic, geospatial, economic, and habitat suitability framework to assess the tradeoffs of strategically reconnecting the 125-km La Grange Segment (LGS) of the Lower Illinois River to its floodplain in order to decrease flood risk, improve floodplain habitats, and limit the costs of reconnection. Costs included building-associated losses, lost agricultural profits, and total levee removal and construction costs. Modeled scenarios demonstrated that while flood heights and environmental benefits are maximized through the most aggressive levee setbacks and removals, these scenarios also have the highest economic costs. However, the tradeoff of implementing lower-cost scenarios is that there would be less flood-height reduction and less floodplain habitat available. Several levee districts had high potential for reconnection based on limiting potential damages as well as providing suitable floodplain habitat. To implement large-scale strategic floodplain reconnection along the LGS, opportunity costs ranged from $1.1-$4.3 billion. As such, payments for ecosystem services will likely be necessary to compensate landowners for building losses and decreased long-term agricultural production that result in an overall flood-reduction benefit, increased floodplain wetlands, and most-soil plant habitat.

Plasma, a gas of charged particles exhibiting collective behaviour, can be found everywhere in our vast Universe. The characteristics of plasma in very distant parts of the Universe can be similar to characteristics in our solar system and near-Earth space. We can therefore gain an understanding of what happens in astrophysical plasmas by studying processes occurring in near Earth space, an environment much easier to reach. Large volumes in space are filled with plasma and when different plasmas interact distinct boundaries are often created. Many important physical processes, for example particle acceleration, occur at these boundaries. Thus, it is very important to study and understand such boundaries. In Paper I we study magnetic nulls, regions of vanishing magnetic fields, that form inside boundaries separating plasmas with different magnetic field orientations. For the first time, a statistical study of magnetic nulls in the Earth’s nightside magnetosphere has been done by using simultaneous measurements from all four Cluster spacecraft. We find that magnetic nulls occur both in the magnetopause and the magnetotail. In addition, we introduce a method to determine the reliability of the type identification of the observed nulls. In the manuscript of Paper II we study a different boundary, the shocked solar wind plasma in the magnetosheath, using the new Magnetospheric Multiscale mission. We show that a region of strong current in the form of a current sheet is forming inside the turbulent magnetosheath behind a quasi-parallel shock. The strong current sheet can be related to the jets with extreme dynamic pressure, several times that of the undisturbed solar wind dynamic pressure. The current sheet is also associated with electron acceleration parallel to the background magnetic field. In addition, the current sheet satisfies the Walén relation suggesting that plasmas on both sides of the current region are magnetically connected. We speculate on the formation mechanisms of the current sheet and the physical processes inside and around the current sheet.

The electron physics in the collisionless magnetic reconnection is studied using data from the Polar spacecraft. Among the types of discontinuities in space plasmas, the Electron Diffusion Region (EDR) at the center of the reconnection has the theoretically unique properties that its thickness is of order of the electron gyroradius, and in such a region electrons are demagnetized with a non-gyrotropic pressure tensor. These unusual properties of EDRs reflect the expected violation of guiding center theory for electrons and are exploited in this thesis. We use four dimensionless, diagnostic, single spacecraft observables derived from theoretical properties of EDRs to locate them. They are electron agyrotropy, out-of-interconnection-plane electron Mach number, and dimensionless thresholds for electric field strengths parallel and perpendicular to the magnetic field. These observables are constructible using the electron density, bulk velocity, pressure tensor, and the electromagnetic field data. With a 3-year survey using particle data from a slower version of Hydra's moment producing system, M3, the vast preponderance of these dimensionless parameters are below unity, which is consistent with the theoretical expectations for most space plasmas being strongly magnetized. The unusual outliers with the demagnetization parameters over unity (<1%) in the distribution are geophysically distributed near the magnetopause within 8-9 Re shells and collected as potential reconnection sites, although a number of other possibilities are also considered in this thesis such as data processing anomalies, systematic effects of data acquisition and aliasing. It is shown that plasma particle data with the highest time resolution possible are needed to improve the time aliasing issues, and to sense the rapidly changing and short scale current structure like the EDR. We use a recently developed algorithm G3/T1, which reduces the aliasing time of the 3D analysis of products of the Polar Hydra Hot Plasma Analyzer by a factor of 12. With this new technique, we have found that among these outliers some demagnetization signatures are ameliorated by higher time cadences and the ones caused by time aliasing effects are ruled out. The moment recoveries of G3/T1 at a 2.3 cadence are in excellent agreement with input distribution models over a considerably wide range of density, Mach number, electron anisotropy, and agyrotropy, provided that a suitable accurate inventory can be made in advance for the bulk velocity of these distributions. The 2 candidate reconnection events analyzed in this thesis by G3/T1 processing techniques demonstrate: (1) strong out-of-interconnection-plane electron flows along the separatrices also observed in 2D PIC simulations; (2) significant electron agyrotropy enhancements framing high thermal Mach number flow, proving excellent consistency with the agyrotropy islands predicted by PIC simulations of asymmetric reconnection geometry; and (3) measured thermal electron gyroscale current channels in patterns that are supported by PIC simulation models as resolved examples of EDRs with direct measures of the electron demagnetization.

This thesis deals with the global aspects of plasmaconvection in the magnetosphere as measured by the low-altitudepolar orbiting Astrid-2 and FAST satellites. The major focus ison the electric field measurements, but they are alsocomplemented by magnetic field, ion and electron particle data,which is fundamental for the understanding of theelectrodynamics of the high-latitude auroral ovals and polarcap, which are the regions analysed here. The essential subjectof this thesis is the so-called magnetic reconnection processthat drives plasma convection in the Earth's magnetosphere. Itis shown that the ionospheric convection, being intimatelycoupled to the magnetospheric convection, responds in about15-25 min depending on geomagnetic activity after the arrivalof the solar wind at the magnetopause. It also responds on alonger time scale, around 55-75 min, which is interpreted asthe unloading of solar wind energy previously stored in thelarge-scale current system of the magnetotail. These resultshave been found previously using ionospheric parameters such asthe auroral electrojet AL index. What is new is that these sameresults are reproduced by using a discrete set of cross-polarpotential measurements. Using an extensive set of electric andmagnetic field data combined with particle precipitation datafrom the FAST satellite, it is shown that the reconnectionprocess can also be applied to explain features of sunwardplasma convection in the polar cap with a likely antiparallelmerging site in the lobe magnetopause region. The lobereconnection is found to depend strongly on IMF Byand to coexist with dayside subsolar merging.Finally, a comparison is performed between the Weimer electricfield model and Astrid-2 electric field data. Empiricalelectric field models are important in understanding thecomplete convection pattern at any one time, something, whichcannot be provided by measurements from single satellites. <b>Keywords:</b>Satellite measurements, electric fields,magnetosphere, magneticreconnection, plasma convection, lobecell convection, empirical electric field models.

<p>This thesis deals with the global aspects of plasmaconvection in the magnetosphere as measured by the low-altitudepolar orbiting Astrid-2 and FAST satellites. The major focus ison the electric field measurements, but they are alsocomplemented by magnetic field, ion and electron particle data,which is fundamental for the understanding of theelectrodynamics of the high-latitude auroral ovals and polarcap, which are the regions analysed here. The essential subjectof this thesis is the so-called magnetic reconnection processthat drives plasma convection in the Earth's magnetosphere. Itis shown that the ionospheric convection, being intimatelycoupled to the magnetospheric convection, responds in about15-25 min depending on geomagnetic activity after the arrivalof the solar wind at the magnetopause. It also responds on alonger time scale, around 55-75 min, which is interpreted asthe unloading of solar wind energy previously stored in thelarge-scale current system of the magnetotail. These resultshave been found previously using ionospheric parameters such asthe auroral electrojet AL index. What is new is that these sameresults are reproduced by using a discrete set of cross-polarpotential measurements. Using an extensive set of electric andmagnetic field data combined with particle precipitation datafrom the FAST satellite, it is shown that the reconnectionprocess can also be applied to explain features of sunwardplasma convection in the polar cap with a likely antiparallelmerging site in the lobe magnetopause region. The lobereconnection is found to depend strongly on IMF B_yand to coexist with dayside subsolar merging.Finally, a comparison is performed between the Weimer electricfield model and Astrid-2 electric field data. Empiricalelectric field models are important in understanding thecomplete convection pattern at any one time, something, whichcannot be provided by measurements from single satellites.</p><p><b>Keywords:</b>Satellite measurements, electric fields,magnetosphere, magneticreconnection, plasma convection, lobecell convection, empirical electric field models.</p>

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