Thesis (Ph.D.)--Boston University / PLEASE NOTE: Boston University Libraries did not receive an Authorization To Manage form for this thesis or dissertation. It is therefore not openly accessible, though it may be available by request. If you are the author or principal advisor of this work and would like to request open access for it, please contact us at open-help@bu.edu. Thank you. / Due to the direct magnetic connection of the high-latitude ionosphere to the outer magnetosphere, a great deal of knowledge of the physics and properties of magnetospheric regions and the fundamental plasma processes operating within them can be learned from studying low-altitude particle measurements. In this thesis the temporal and spatial aspects of the low-altitude auroral particle precipitation are investigated using a unique set of particle flux observations from two Defense Meteorological Satellite Program (DMSP) spacecraft in the same orbit but with varying time separation. Three different topics are investigated in this study: auroral stability, the accuracy of the Newell-Meng criteria for region identification, and the relative importance of various magnetopause reconnection models. In the first part the prevalent timescales and spatial dimensions of low-altitude auroral formations are examined using both electron and ion data. It is found that spatial scales larger than 50-100 km are stable for up to 1.5 minutes, while smaller size features vary more rapidly. In the second topic we explore quantitative and qualitative aspects of the Newell-Meng criteria. The flexibility and limitations of the numerical values used are examined with case and statistical studies; all but one are found to be sufficiently robust. Additionally, an expansion of the criteria to include a distinction between open and closed magnetic field line geometries is considered. The last part concentrates on the evaluation of currently proposed models of magnetopause reconnection, based on a case study of ion and electron low-altitude particle reconnection signatures. We conclude that a unique combination of the multiple x-line and bursty single x-line reconnection models is required for a full interpretation of the data. This scenario also provides a comprehensive mechanism for the formation of the low-latitude boundary layer on both open and closed field lines. Finally, the common conclusion of all three studies is that two-point measurements add considerably to our understanding of the low-altitude auroral environment and thereby, the remote processes governing its dynamics. / 2031-01-01

The Juno Waves instrument detected new broadband plasma wave emissions on the first three successful passes over the low altitude polar regions of Jupiter on Days 240 and 346 of 2016 and Day 033 of 2017. This study investigated the characteristics of these emissions and found similarities to whistler-mode auroral hiss observed at Earth, including the funnel-shaped frequency-time features. The electron cyclotron frequency was much higher than both the emission frequencies for all three days and the local plasma frequency, which was assumed to be 20 – 40 kHz. The electric to magnetic field (E/cB) ratio was around three near the start of each event and then decreased to one for the remaining duration of each pass. Spin modulation phase shifts were found on two of the three days (Day 240 and Day 033), indicating wave propagation up to the assumed plasma frequency. A correlation of the electric field spectral densities with the flux of up-going 20 to 800 keV electron beams on all three days were found, with correlation coefficients of 0.59, 0.72, and 0.34 for Day 240, Day 346, and Day 033 respectively. We conclude that the emissions are propagating in the whistler-mode and are driven by energetic up-going electron beams along the polar cap magnetic field lines.

Earth's plasmasphere persists as an extension of the ionosphere into space. The toroidal region of plasma is shaped by electric and magnetic forces in the terrestrial magnetosphere. As a dense population of cold plasma, the plasmasphere interacts with particles in the hot ring current and energetic radiation belts. Evolution of plasmaspheric density under the driving influence of the solar wind crosses many physical scales. Convective erosion during geomagnetic storms occurs on the order of hours, reducing the size of the plasmasphere by forming an abrupt plasmapause density gradient that varies in radial and diurnal location. The history of geomagntic activity determines the presence of morphological structures as small as notches and as large as plumes. Plasma of atmospheric origin is carried sunward by convection through drainage plumes towards the magnetopause where it can diminish the effectiveness of magnetic reconnection. Long-lived plumes are sustained by a higher rate of refilling than typically observed during plasmasphere recovery from geomagnetic disturbances. The response of the plasmasphere, then, is an integral part of the feedback cycle between the magnetosphere and ionosphere in the exchange of energy and particles. This thesis aims to address three questions concerning the nature of the plasmasphere through the development of empirical and physics-based models under recent observations provided by the Van Allen Probes (RBSP-A & -B). First, what is the distribution of density content in the plasmasphere? For a two year period with full MLT coverage by RBSP, the upper-hybrid resonance frequency in plasma wave spectra is used to identify sudden changes consistent with the plasmapause feature and to calculate the magnetic equatorial electron density. Plasmapause encounter radial locations for both spacecraft are correlated with a geomagnetic activity index showing significant scatter around a linear fit. On average, the predicted plasmapause location does account for the separation between the saturated plasmasphere and the depleted plasmatrough. A density threshold corresponding to the plasmapause boundary is used to sort RBSP measurements into these two classified plasma regions. Model profiles are developed for each region and compared to the results from previous missions. The importance of solar wind properties in regulating the severity of plasmasphere erosion is demonstrated. Second, how does the plasmapause form and vary with geomagnetic activity? The two-dimensional plasmasphere density model, RAM-CPL, is employed to simulate two geomagnetic storms observed by the RBSP spacecraft. Inner-magnetospheric convection is parameterized by the Kp-index and solar wind properties. The performance of RAM-CPL is evaluated by the correspondence between virtual and actual plasmapause encounters. Overall, RAM-CPL achieved good agreement with RBSP observations of the plasmapause to within 0.5 L and measurements of electron density to within one order of magnetude inside the plasmasphere. An empirical model of ring current-ionosphere feedback was included to account for asymmetric erosion, but did not contribute significantly in the MLT sectors of interest when compared to electric field measurements. The difference in background activity level during quiet conditions between the two convection parameterizations was found to lead to 1 L difference in plasmapause location for each simulation trial. Solar wind driven simulations produce sharper and deeper erosion of the plasmapause at the onset of a geomagnetic storm, but also allow for larger recovery of the plasmasphere when compared to Kp-index driven simulations. Third, what is the role of the ionosphere in sustaining the plasmasphere? Four geomagnetic events are observed by RBSP in opposing MLT sectors to exhibit undisturbed plasmasphere refilling following significant erosion of the plasmapause. RAM-CPL simulations of the strongest storm parameterized by solar wind properties shows the full evolution of plasmasphere density from the narrowing of a sunward plume at the onset of erosion, that begins to corotate into a duskside bulge as activity diminishes, to the outward recovery of the plasmapause over several days. A piecewise empirical model of plasmasphere refilling is composed from profiles of equatorial electron density and the observed correlation between the Kp-index and plasmapause location. The RAM-CPL timescale of refilling mediates the increase in density from plasmatrough to plasmasphere levels matching RBSP measurements during the quiet period after the storm. Density observations of the other geomagnetic events are consistent with reports of a two-stage refilling process.

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>

Whistlers have, for many years, been used as probes of the ionosphere and magnetosphere. Whistlers received on the ground have been shown (Smith [1961], Helliwell [1965]) to have propagated, in almost all cases, through ducts of enhanced ionisation aligned along the magnetic field direction. Analysis of these whistlers, using for example the Ho and Bernard [1973] method, allows determination of the L-value of the field line along which the signal has propagated, the equatorial electron density and the time of the initiating lightning strike. Satellite received whistlers, known as fractional-hop whistlers, are not restricted to propagating through ducts and, in this case, ducted whistlers are probably rarer than unducted whistlers. Analysis of these whistlers is consequently much more difficult as the propagation path is often not known. This study is an attempt to understand some of the characteristics of whistlers received on the 18182 satellite at low latitudes during October 1976. Haselgrove's [1954] ray tracing equations, together with realistic density and magnetic field models, have been used to determine the ray paths and travel times. The whistler dispersions, calculated from the travel times, are compared with the results obtained from analysis of the 18182 data. Values given by the density models used were also compared with density values obtained from other models and values recorded by ionosondes during the same period and at locations close to the latitude and longitude of the 18182 satellite. Another part of this study considers the cyclotron resonance interaction between ducted whistler mode waves and energetic electrons. During this interaction, electrons can diffuse into the loss cone and will then precipitate into the upper atmosphere causing secondary ionisation. This ionisation patch modifies the earthionosphere wave guide and can be observed as phase and/or amplitude perturbations on VLF transmitter signals, known as Trimpi events (Helliwell et al [1973], Dowden and Adams [1988], 1nan and Carpenter [1987]) . Trimpi events and associated whistlers were observed at Marion Island (46°53" 5, 37°52" E, L = 2.63) during May 1996. Analysis of the associated whistler groups confirms that the Trimpi events can be explained by the above mentioned cyclotron resonance interaction and subsequent electron precipitation. During this process the whistlers were propagating towards Marion Island while the electrons were propagating away. The electrons must therefore have mirrored in the northern hemisphere before precipitating near Marion Island causing the observed Trimpi. The calculated time delays are shown to confirm this process. During the unusual 2-hour period of observation, the Trimpi associated whistler groups were, in all cases, followed by a second, fainter whistler group which has been called a whistler 'ghost' . The dispersion of whistlers within this second whistler group are shown to be the same as those within the initial whistler group indicating that these whistlers must have propagated through common ducts at different times and hence must have been caused by different atmospheric discharges. It is thought that during the wave-particle interaction, which caused the observed Trimpi, some of the energetic electrons may have precipitated into the northern hemipshere triggering this second discharge. The timing between the two whistler groups is such that, if the above triggering is correct, the interaction must have taken place about 10° from the equatorial plane . / Thesis (Ph.D.)-University of Natal, 1997.

A magnetohydrodynamic (MHD) cavity-resonance model is developed to study the ultra low frequency (ULF) response in the magnetosphere to an external compressional impulse. It is assumed that the magnitude of the impulse is small enough such that non-linear terms remain negligible. The MHD differential equations are derived in a cold, non-uniform plasma imbedded in a cylindrical ambient field geometry and are solved using numerical finite difference integration methods. The crucial feature of the model is that it allows for the investigation of the response within the magnetospheric cavity to an impulse that has both temporal and spatial form. There is strong observational evidence that low-latitude Pi 2 pulsations have, or are associated with, a global propagation mechanism. Evidence alluding to the global nature of low-latitude Pi 2 is the characteristically low azimuthal (or axial) wavenumbers, (Irnl ;S 1 ). Further evidence of the global nature of Pi 2 is the lack of arrival time difference between globally separate events, as well as the similarity in the spectral content of globally separate events. As an application, the cavity-resonance model is applied to investigate the Pi 2 pulsation event. The cavity-resonance waves are excited by an impulsive perturbation at the magnetopause which is centred about the midnight meridian. The excitation signal is chosen representing the causal Pi 2 mechanism thought to be associated with the sudden, short circuiting of the cross-tail current to the auroral oval. Various aspects of the cavity-resonance wave modes are investigated and the appropriateness of this type of modelling for -the study of Pi 2 is evaluated. Numerical integration and well as Fourier and Laplace methods are used to investigate the transmission of the impulsive signal through the magnetosphere. Coupling between the isotropic (cavity) and the transverse Alfven (resonance) mode is studied. The effect of the plasmapause is considered. Longitudinal variations of polarization as well as the latitudinal phase variations of the perturbed fields are computed. Computational results are compared with observational features of the Pi 2 event. / Thesis (Ph.D.)-University of Natal, Durban, 1993.

Thesis (Ph. D.)--University of Texas at Austin, 2008. / Vita. Includes bibliographical references.

Ganymède, une lune de Jupiter, est le plus grand et le plus massif des satellites de notre système solaire. Cet objet a été observé depuis la Terre, notamment grâce au télescope Hubble (HST), et in situ par la sonde Galileo. Grâce à ces observations, une atmosphère très ténue, ou exosphère,principalement composée d'hydrogène, d'oxygène et d'oxygène moléculaire, a été détectée au voisinage de Ganymède. Ganymède est l'unique lune du système solaire possédant son propre champ magnétique intrinsèque, qui, en interagissant avec le plasma magnétosphérique jovien, génère unemini-magnétosphère. Cette magnétosphère est imbriquée dans celle de Jupiter. C'est le seul cas connu d'interaction entre deux magnétosphères. Galileo est l'une des seules sondes spatiales ayant investigué l'environnement complexe de Ganymède. La prochaine mission spatiale qui étudiera ce satellite estune mission européenne de l'ESA : JUICE (JUpiter ICy moon Exploration). Dans le cadre de cette mission, et dans un but de mieux connaître ce satellite, mon travail de thèse a consisté à modéliser l'environnement global neutre et ionisé de Ganymède.La première partie de mon travail de thèse a été consacrée à l'étude de l'exosphère de Ganymède à l'aide d'un modèle 3D Monte-Carlo. J'ai parallélisé ce modèle afin d'améliorer ses performances et d'enrichir la physique décrite par le modèle. Les résultats sont comparés à ceux d'autres modèles, ainsi que les observations effectuées par le HST et Galileo. L'environnement ionisé, en particulier la magnétosphère de Ganymède, a été ensuite étudié à l'aide d'un modèle hybride parallèle 3D, notamment en se plaçant dans les conditions d'observations de Ganymède par Galileo. Les résultats sont globalement cohérents avec les observations, et concordent avec ceux d'autres modèles, maismontrent néanmoins une nécessité d'améliorer significativement la résolution spatiale du modèle. De ce fait, une partie significative de mon travail de thèse a été dédiée au développement et à l'implémentation d'une approche multi-grilles au sein du modèle hybride, pour améliorer la résolution spatiale d'un facteur 2 dans le voisinage proche du satellite. Enfin, les résultats obtenus avec ce modèle optimisé sont confrontés aux observations de Galileo. / Jupiter’s moon Ganymede is the biggest and most massive satellite of our solar system. Thisobject has been observed from the Earth, with the Hubble Space Telescope (HST), and through in situ measurements by Galileo spacecraft. Thanks to these observations, a very tenuous atmosphere, or exosphere, has been detected at Ganymede. It is mainly composed of atomic hydrogen, atomic oxygen, and molecular oxygen. Ganymede is the only moon of the solar system to have its own intrinsic magnetic field, which generates a minimagnetosphere interacting with the magnetospheric jovian plasma. This magnetosphere is embedded in the jovian magnetosphere. It is the only known case of interaction between two magnetospheres. Galileo is the only mission that has investigated the complex ionized environment of Ganymede. The next space mission dedicated to investigate the Jovian magnetosphere and its galilean satellite is an European mission from ESA : JUICE (Jupiter ICy moons Explorer). In the frame of this mission, and to prepare future observations at Ganymede, my thesis work has consisted in modeling the global neutral and ionized environment of Ganymede. The first part of my thesis work has been dedicated to the study of Ganymede’s exosphere with a 3D Monte-Carlo model. I have parallelized this model to improve its performance and to enrich the physics described by the model. Results have been compared to those of other models, and to HST and Galileo observations. The ionized environment, in particular the magnetosphere of Ganymede, has then been studied with a 3D parallel hybrid model,considering the observation conditions of Galileo. Results are globally consistent with the observations and with other models, but show the necessity to significantly improve the spatial resolution. Therefore, a significant part of my work has been dedicated to the development of a multi-grid approach in the hybrid model, to divide by 2 the spatial resolution at the vicinity of Ganymede. Finally, results obtained with the optimized model are compared to Galileo observations.

Energetic electrons undergo significant flux variations in the Earth’s outer radiation belt, where magnetospheric waves play an important role in changing the energetic electron dynamics. In particular, electromagnetic ion cyclotron (EMIC) waves are suggested to drive efficient pitch angle scattering of relativistic electrons, which results in relativistic electron precipitation into the upper atmosphere. Such precipitation provides an important source of energy input into the upper atmosphere, where precipitating electrons can affect atmospheric chemistry and ionization. However, the quantitative role of EMIC waves in energetic electron precipitation in various regions of the magnetosphere is not fully understood. This dissertation aims to answer outstanding open questions on the characteristics and quantification of EMIC-driven precipitation, such as the spatial extent and the energy range of electron precipitation. The relationship between EMIC waves and electron precipitation is evaluated by analyzing magnetic conjunction events when EMIC waves are detected in the magnetosphere by near-equatorial satellites (Van Allen Probes, GOES) and precipitating electrons are measured by Low-Earth-Orbiting satellites (POES, FIREBIRD). Quasi-linear theory is used to quantify the role of various observed magnetospheric waves (e.g., EMIC waves, plasmaspheric hiss, magnetosonic waves) in the electron precipitation. Several in-depth case analyses show that EMIC waves are the main driver of the observed relativistic electron precipitation, while other waves play a minor role. The precipitation events were clearly identified within L shell of ~7.5, favorably near the dusk and night sectors. The analysis shows that each precipitation event was localized on average spatial scales of ~0.3 L, suggesting that the resonance conditions are satisfied in a very localized region of the magnetosphere. The electron precipitation was observed at the expected relativistic (> ~MeV) energies; however, the minimum energy of efficient electron precipitation was newly found to extend down to at least ~200–300 keV. The quantitative analysis using multi-point measurements combined with theoretical calculations in this dissertation provides a more comprehensive understanding of EMIC-driven precipitation, which is a critical electron loss process in the magnetosphere. Moreover, the results are helpful to improve currently existing models of radiation belt, ring current and atmosphere dynamics, as well as theories of wave-particle interactions.