Now showing items 1-20 of 34

• #### A Mechanism For Current Sheet Thinning In The Growth Phase Of Magnetospheric Substorms

The thinning of the near-Earth current sheet during the growth phase of magnetospheric substorms is a fundamental problem of space physics. It is a characteristic of the slow, steady evolution of the magnetosphere during the growth phase, during which the bulk kinetic energy of the solar wind is transformed into and stored as magnetic field energy in the magnetotail lobes. The thin near-Earth current sheet at the end of the growth phase provides the conditions for the onset of the expansion phase, and is fundamentally important to understand the physical mechanism for the onset of the rapid evolution during which the stored energy is released. I propose that current sheet thinning occurs because of the evacuation of a 'magnetic flux reservoir' in the near-Earth magnetotail by convection to replace magnetic flux that is eroded on the dayside by magnetic reconnection. My hypothesis is able to predict basic properties of current sheet thinning, such as the location, temporal evolution, and dynamics of this process. I examined this new mechanism both conceptually and quantitatively. My conceptual considerations enabled the prediction of the location and duration of current sheet thinning. This location is largely independent of the detailed state of the magnetosphere. I examined this mechanism quantitatively through the use of a three-dimensional ideal MHD simulation. I was able to predict the duration of the growth phase by considering the time needed to deplete our proposed 'magnetic flux reservoir.' The simulation demonstrates the global increase of the current density in this reservoir, despite the removal of magnetic flux---which one would otherwise expect to lead to a decrease of current---as well as even greater local amplifications of the current density. The simulation results are even more significant because the model does not include other effects of the real magnetosphere that contribute to a further increase of the tail current. The increase in current density and thinning are found to be consistent with the amount of flux removed from the system. In addition, I have found a new explanation for the very thin bifurcated current sheets that have been reported in recent publications.
• #### A study of one-dimensional nonlinear hydromagnetic waves and collisionless shocks

A variety of nonlinear hydromagnetic waves have been observed in the collisionless solar wind plasma. A comprehensive theoretical study of nonlinear hydromagnetic waves, including rotational discontinuities and collisionless shocks, is carried out in this thesis by means of both analytical solutions and numerical simulations. Nonlinear hydromagnetic waves are governed by the interplay of the dispersion process, the collisionless dissipation process and the nonlinear steepening process. The purpose of this thesis is to understand the nonlinear behavior of hydromagnetic waves in terms of these fundamental processes. It is shown that the rotational discontinuity structures observed in the solar wind and at the magnetopause are nonlinear Alfven wave solutions of the collisionless two-fluid plasma equations. In these nonlinear wave solutions, nonlinear steepening is self-consistently balanced by dispersion. Collisionless viscous dissipation is the dominant dissipation in high Mach number shocks, which converts the flow energy into thermal energy. Hybrid simulations show that the collisionless viscous dissipation can result from the reflection and pitch-angle scattering of incoming ions flowing through the magnetic structures in the shock transition region. Collisionless dissipations in hydromagnetic shocks is governed by the magnetic structures in the shock transition region. The dissipation in turn can modify the wave structures and balance the nonlinear steepening. However, such delicate balance of the dispersion, dissipation, and nonlinear steepening has been observed to break down momentarily in high Mach number quasi-parallel shocks. This leads to the so-called cyclic shock front reformation seen in the hybrid simulations. The shock front reformation can be explained in terms of momentary off-balance between the dispersion-dissipation on the one hand and the nonlinear steepening on the other hand. The off-balance occurs after a significant fraction of incoming ions are reflected. Each off-balance lasts a few ion gyro periods, which governs the shock front reformation time scale.
• #### A study of the magnetosphere-ionosphere coupling processes

Magnetosphere-ionosphere (M-I) coupling processes are studied by using numerical modeling. An M-I coupling model of substorms on the ionospheric recombination time scale (tens of seconds) is developed. The model is two-dimensional (2-D) and time-dependent from which several signatures of substorms can be obtained and understood. The model is then extended to northward interplanetary magnetic field (IMF) conditions to study the effects of the M-I coupling on the high-latitude convection. Based on the model results, a mechanism for the origin of distorted two-cell ionospheric convection is proposed. The ionospheric and ground signatures of multiple field-aligned current sheets originating from dayside flux transfer events have been modeled. The interaction between Alfven waves and field aligned potential drops is studied by using a local model.
• #### A theory of field-aligned current generation from the plasma sheet and the poleward expansion of aurora substorms

This dissertation reports a study of the generation of field-aligned currents in the plasma sheet in terms of magnetosphere-ionosphere coupling. For the study, the plasma sheet and the ionosphere are treated as two-dimensional layers by height integration. In the magnetosphere between them, the Alfven wave transition time through this region is assumed to be zero. The ionospheric momentum is allowed to be transferred to the plasma sheet. Both linear analyses and numerical simulation are performed to study the field-aligned current generation. In the linear analysis, evolution from initial perturbations is studied. Zero order configurations are steady state without field-aligned currents. The field-aligned currents are treated as a perturbed quantity and linearly related with the other perturbed quantities. One result for the linear waves is that the magnetohydrodynamics (MHD) fast mode and Alfven mode are coupled through the ionospheric Hall current. The Hall current causes the dawn-dusk asymmetry: a westward-travelling wave is amplified on the region 1 current system, while an eastward-travelling wave is amplified elsewhere. The expansion phase of the magnetospheric substorm after the onset is numerically simulated on the near-earth plasma sheet. The inner edge of the plasma sheet is taken as the outflow boundary. As the initial condition, an enhanced earthward magnetospheric convection is assumed to cause a finite pressure increase at the inner edge of the plasma sheet. The numerical results are as follows. An MHD fast-mode wave is generated. It propagates tailward accompanied by the field-aligned currents. The wave propagation and the field-aligned currents account for the poleward expansion of the aurora and the region 1 field-aligned current during the expansion phase of the substorm. The region 1 field-aligned currents are linked with the dusk to dawn current on this wave, which is driven by the dynamo mechanism of the wave. The ionospheric Hall current causes asymmetry of the wave, and hence, of the field-aligned current distribution. This asymmetry accounts for the stronger field-aligned current in the premidnight sector.
• #### An Interdisciplinary Computational Study Of Magnetosphere-Ionsphere Coupling And Its Visual And Thermal Impact In The Auroral Region

A three-dimensional, three-fluid simulation (ions, electrons, and neutrals) was explicitly parallelized, facilitating the study of small-scale magnetospheric-ionospheric (M-I) coupling processes. The model has ionization and recombination, self-consistently (semi-empirically) determined collision frequencies, and a height resolved ionosphere. Inclusion of ion inertial terms in the momentum equation enables the propagation of Alfven waves. Investigation at small scales required large system domains, and thus fast parallel computers. The model was explicitly parallelized---enabling investigations of M-I coupling processes on very small temporal and spatial scales. The generation, reflection, and propagation of Alfven waves is of importance to the understanding of M-1 coupling processes---it is, in fact, the primary means of communication of physical processes in the coupled system. Alfvenic reflections were modeled for two different boundary conditions, and it was shown that the deformation of the current layer was Alfvenic in character. Visualizations of the data obtained appear to be consistent with the visual characteristics of actual discrete aurora in nature. The model reproduces qualitatively, and semi-quantitatively, in a self-consistent manner, some the behaviors of the formation and time-evolution of discrete arcs. These include the narrowness of arcs; electric fields extending parallel outward from the arcs; and fast (plasma) flows in the region of discrete arcs. Large-scale models---due to inevitable limitations of computational resources---need to make large-scale averages of computed properties. In regions of active small-scale structure, significant under-representation of the Joule heating occurs. It has been shown that the under-representation of the Joule heating in the region of active aurora can be as large as a factor of 8. This work includes a computer-based study of a quantitative approximation of this underrepresentation of the Joule heating by global, large-scale models and experimental observations.
• #### An investigation of cusp latitude magnetosphere-ionosphere physics: A time series analysis approach

The shocked solar wind plasma of the magnetosheath has direct access to the Earth's high-latitude ionosphere and upper atmosphere only through the magnetospheric cusps. The interaction of solar and terrestrial plasmas and fields in these regions has made them an obvious choice for the study of coupling processes in the geospace environment. Some of the information regarding these processes is manifest in the transmission and generation of wave energy, a portion of which can be detected by ground-based magnetometers. In the present day, records of the magnetic field are stored in a digital format; therefore, some form of signal processing is required to extract meaningful physical information from them. This thesis is aimed at the physical characterization of the cusp region through the careful application of digital time series analysis techniques to ground-based magnetometer records. It is demonstrated that judicious application of signal processing techniques can yield new, physically meaningful results from ground-based magnetometer records, and aid in the understanding of disparate reports from groups using different analysis techniques on like data. Characterization of the cusp region is couched in terms of three specific, open problems of the physics of magnetic perturbations in the cusp: (1) the coherence of localized pulsations, (2) the spatiotemporal nature of the cusp magnetic spectrum, and (3) the ground-based magnetic determination of the separatrix. The first problem is addressed by assuming that localized pulsations are coherent only over some finite spatial extent. A statistical measure of interstation coherence is developed to estimate an upper bound of ${\cal O}$(200 km) for the coherence length of this class of pulsations. The second problem is addressed by examining the ultra low frequency polarization spectrum. An information theoretic measure is established as a quantitative means of discriminating the spatial passage of the cusp by ground-based magnetic means. This procedure replaces previous determinations which were made "by-eye." Finally, separatrix identification is addressed by applying the statistical interstation coherence measure to pulsations presumably representative of a magnetic field line resonance. The analysis indicates that a determination is not possible to a resolution better than ${\cal O}$3(300 km).
• #### Analysis Of Methods For Solar Wind Propagation From Lagrangian Point L1 To Earth

The Lagrangian point L1 is situated about 1.5 million kilometers sunwards from Earth and provides a unique orbiting point for satellites, placing them constantly upstream in the solar wind, allowing for prediction of solar wind conditions impacting Earth's magnetosphere. Short-term forecasting of geomagnetic activity requires extrapolation of solar wind data from L1 to Earth (typical propagation time around 1 hour), as does any research in interactions between the solar wind and the magnetosphere during intervals when no Earth-orbiting satellites are in the solar wind. To accurately predict propagation delays it is necessary to take the geometry of incoming solar wind structures into account. Estimating the orientation of solar wind structures currently has to be done using single satellite measurements, which will likely remain the case for another decade or more, making it important to optimize single satellite techniques for solar wind propagation. In this study a comprehensive analysis of 8 different single satellite propagation methods was performed, each involving several variable parameters. 4 of these used electric field calculations and had not previously been tested for solar wind propagation. Large amounts of data were propagated from a satellite near L1 to target satellites near Earth for comparison to measured data, using specific test scores to evaluate relative performance between methods and parameter values. Electric field methods worked well for continuous data but did not predict arrival time of discontinuities (abrupt transitions) as accurately as methods based on magnetic field data, one of which delivered the best results on all accounts. This method had also been found to give best results in a previous study, but optimal parameter values were significantly different with the larger data set used here. Propagating 6,926 discontinuities it was found that on average they arrive about 30 seconds later than predicted (about 1% of the propagation time). Barring a systematic error in velocity data or delay calculations the offset suggests an asymmetry in the geometry of solar wind structures. While this idea is physically plausible it was not unambiguously supported by the data.
• #### Analysis of optical observations and three-dimensional hybrid code simulation of the CRRES plasma injection experiments in space

The Combined Release and Radiation Effects Satellite (CRRES) was a NASA funded campaign designed to study a variety of plasma processes in the Earth's space environment. An analysis of optical data from three CRRES plasma injection experiments, in conjunction with results from a three-dimensional hybrid code simulation, have provided new insights into small-scale coupling processes in the ionosphere. The results have direct application to auroral processes, comets, and other similar geophysical/astrophysical systems.
• #### Applications Of A Time-Dependent Polar Ionosphere Model For Radio Modification Experiments

A time-dependent self-consistent ionosphere model (SLIM) has been developed to study the response of the polar ionosphere to radio modification experiments, similar to those conducted at the High-Frequency Active Auroral Research Program (HAARP) facility in Gakona, Alaska. SCIM solves the ion continuity and momentum equations, coupled with average electron and ion gas energy equations; it is validated by reproducing the diurnal variation of the daytime ionosphere critical frequency, as measured with an ionosonde. Powerful high-frequency (HF) electromagnetic waves can drive naturally occurring electrostatic plasma waves, enhancing the ionospheric reflectivity to ultra-high frequency (UHF) radar near the HF-interaction region as well as heating the electron gas. Measurements made during active experiments are compared with model calculations to clarify fundamental altitude-dependent physical processes governing the vertical composition and temperature of the polar ionosphere. The modular UHF ionosphere radar (MUIR), co-located with HAARP, measured HF-enhanced ion-line (HFIL) reflection height and observed that it ascended above its original altitude after the ionosphere had been HF-heated for several minutes. The HFIL ascent is found to follow from HF-induced depletion of plasma surrounding the F-region peak density layer, due to temperature-enhanced transport of atomic oxygen ions along the geomagnetic field line. The lower F-region and topside ionosphere also respond to HF heating. Model results show that electron temperature increases will lead to suppression of molecular ion recombination rates in the lower F region and enhancements of ambipolar diffusion in the topside ionosphere, resulting in a net enhancement of slant total electron content (TEC); these results have been confirmed by experiment. Additional evidence for the model-predicted topside ionosphere density enhancements via ambipolar diffusion is provided by in-situ measurements of ion density and vertical velocity over HAARP made by a Defense Meteorological Satellite Program (DMSP) satellite.
• #### Building Blocks Of Self -Organized Criticality

Why are we having difficulty developing economical nuclear fusion? How can a squirrel cause a statewide power blackout? How do correlations arise in a random complex system? How are these questions related? This thesis addresses these questions through a study of self-organized criticality (SOC). Among the systems that have been proposed as SOC are confined fusion plasmas, the Earth's magnetosphere and earthquake faults. SOC describes how large-scale complex behavior can emerge from small-scale simple interactions. The essence of SOC is that many dynamical systems, regardless of underlying physics, share a common nonlinear mechanism: local gradients grow until exceeding some critical gradient and then relax in events called avalanches. Avalanches range in size from very small to system-wide. Interactions of many avalanches over long times result in robust statistical and dynamical signatures that are surprisingly similar in many different physical systems. Two of the more well-known signatures are power law scaling of probability distribution functions (PDFs) and power spectra. Of particular interest in the literature for approximately a century are 1/f spectra. I studied the SOC running sandpile model and applied the results to confined and space plasmas. My tools were power spectra, PDFs and rescaled range ( R/S) analysis. I found that SOC systems with random external forcing store memory of previous states in their local gradients and can have dynamical correlations over very long time scales regardless of how weak the external forcing is. At time scales much longer than previously thought, the values of the slope of the power spectra, beta and the Hurst exponent, H, are different from the values found for white noise. As forcing changes, beta changes in the range 0.4 $<f> &lap;</f>$ beta &le; 1 but the Hurst exponent remains relatively constant, H &ap; 0.8. The same physics that produces a 1/f spectrum at strong forcing produces a f -0.4 spectrum at weaker forcing. Small amounts of diffusive spreading added to the two dimensional SOC sandpile greatly decreases the frequency and maximum size of large transport events. More diffusion increases the frequency of large events to values much greater than for systems without diffusion.
• #### 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.
• #### Dependence of the ionospheric convection pattern on the conductivity and the southward IMF

Electric field measurements from the DE-2 satellite were used to determine the location of the convection reversal boundary and the potential around this boundary under a combination of interplanetary magnetic field (IMF) and auroral electrojet conditions. The electric potential is obtained by the integration of the electric fields. The convection reversal boundary is defined in this study as where the potential has its absolute maximum and minimum values. The data were sorted into 18 categories according to two levels of the negative IMF $B\sb{z},$ three ranges of IMF $B\sb{y},$ and two substorm phases. The data were fit with both continuous and discontinuous boundaries to get a functional representation of boundary potentials and locations. A simple model is constructed by solving the Laplace's equation in order to illustrate the obtained boundary potentials and locations. The results show that the enhanced electric field in the midnight sector is associated with an intense westward electrojet current. It can also be seen that the convection reversal boundary is found to be discontinuous near midnight. The discontinuous convection reversal boundary on the dayside is related to the merging near dayside cusp region. The discontinuous convection reversal boundary on the nightside is related to the conductivity enhancement. The intrusion of the dawn cell into the dusk cell is due to nonuniformity of the Hall conductivity in the ionosphere. Another model is constructed by solving the current continuity equation with field-aligned current and nonuniform conductivity added. It can be found that a secondary convection reversal, which is detached from the dusk-cell convection reversal, appears in the evening-midnight sector within the polar cap when the IMF $B\sb{y}$ is positive and the conductivity is nonuniform. This convection reversal is attributed to be created by the B $\times$ V dynamo. Also, the inclusion of the region 2 field-aligned current leads to an enhancement of the electric field in the region between the region 1 and region 2 currents.
• #### Design And Implementation Of A Meteor Tracking Retrofit System For The Hf Radar At Kodiak Island, Alaska

The HF radar at Kodiak Island, Alaska, is part of the SuperDARN network of radars, and was originally designed to detect echoes from ionospheric field-aligned density irregularities. A new digital receiver has been implemented on the radar to allow provide the capabilities required for increased range resolution detection of meteor echoes. A meteor detection algorithm has also been implemented to detect meteor echoes with a range resolution of 4.5 km. The algorithm measures the velocity, range, and altitude of the echoes. This data can be used to derive information about the meteor region winds. The design and implementation of the receiver, the design and implementation of the meteor detection algorithm, and some meteor region wind data derived from the new system are presented. <p>
• #### 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.
• #### 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.
• #### 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.
• #### Influence Of The Kelvin -Helmholtz Instability On The Plasma Transport At The Magnetospheric Boundary

The Kelvin-Helmholtz (KH) instability has long been suggested as a mechanism for viscous interaction at the magnetospheric boundary but it was not expected to produce significant mass transport. Satellite observations show that the density, temperature, particle pressure and total pressure of the plasma sheet are strongly correlated with those of the solar wind on a time scale of ~2 hours. I present a systematic 2-D study of reconnection in KH flow vortices using MHD and Hall-MHD approximations depending on magnetosheath and magnetospheric plasma and field properties. The presented results show that the Kelvin-Helmholtz instability can be a major plasma transport mechanism during times of strongly northward IMF providing a source of plasma into the low latitude boundary layer and plasma sheet on a time scale of ~2 hours. I have also analyzed Equator-S and Cluster satellite data at the dawnside magnetospheric flank and compared these results with MHD simulations in order to distinguish signatures caused by Kelvin-Helmholtz instability. In addition I have discussed typical ionospheric signatures caused by KH instability.
• #### 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.
• #### 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.