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Growth and Evolution of Electromagnetic Ion Cyclotron Waves in the Presence of Multiple Ion Species in the Earth's Magnetosphere
Full text linkElectromagnetic ion cyclotron (EMIC) waves are generated by a fundamental plasma instability and interact with multiple particle populations in the Earth's magnetosphere. This dissertation describes the application of spacecraft data and linear theory of electromagnetic waves to investigating the evolution of EMIC wave properties in the presence of multiple magnetospheric ion species. In particular, the role of the low-energy heavy ion species on the wave properties is explored. A case study describes spacecraft measurements of EMIC wave activity, the multiple ion species (hot protons, cold protons, and cold He+) present during the wave activity, and the methods for performing thorough characterization and analysis of the wave observations. I show that the observed wave characteristics are not typical of such waves as established from linear cold plasma theory. By using the full range of the observations and applying them to modeling of linear wave growth, I then show that wave properties evolve in the presence of sufficient free energy (low density hot protons), high density cold protons, and warm He+ (~10 eV). Parametric study of linear wave growth using the observed multiple ion properties as a reference point implies that EMIC waves evolve due to these warm plasma effects of the heavier ion species, and may also evolve due to non-local generation and propagation. This motivates a global multi-spacecraft magnetospheric study of the dominant cold/warm ions (H+, He+, and O+) to establish their typical properties (composition, densities, and temperatures) at different magnetospheric locations and to determine where such cold/warm ions can lead to similar evolution of warm plasma EMIC waves assuming the hot proton free energy is available. I then apply these results to successfully explain typical growth rates and properties of EMIC waves observed in each MLT sector. The results show how our findings on cold/warm ion properties can be used in future studies of EMIC wave generation and properties, including the effect of the waves on scattering of relativistic radiation belt electrons, cold ion heating, and hot ion precipitation to the ionosphere resulting in the proton aurora
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The Effectiveness of EMIC Wave-Driven Relativistic Electron Pitch Angle Scattering in Outer Radiation Belt Depletion
Full text linkThe dynamic variability of Earth's outer radiation belt is due to the competition among various particle transport, acceleration, and loss processes. The following dissertation investigates electron resonance with Electromagnetic Ion Cyclotron (EMIC) waves as a potentially dominant mechanism driving relativistic electron loss from the radiation belts. EMIC waves have been previously studied as contributors to relativistic electron flux depletion. However, assumed limitations on the pitch angle and energy ranges within which scattering takes place leave uncertainties regarding the capability of the mechanism to explain sudden loss of core electron populations of the outer radiation belt. By introducing new methods to analyze EMIC wave-driven scattering signatures and relativistic electron precipitation events through a multi-point observation approach, this dissertation reveals the effectiveness of EMIC waves to drive losses of outer radiation belt electrons with a new resolution. The research that composes this dissertation focuses on three key areas of the EMIC wave-relativistic electron relationship. A chapter comparing a single EMIC wave event with a pitch angle scattering signature shows that these waves can cause scattering of electrons at energies and pitch angles predicted by the wave-particle resonance condition. This initial study establishes the motivation and methodological groundwork for a statistical study which provides evidence for the common occurrence of these scattering signatures and shows that the energies and pitch angles affected by EMIC waves are often within the core radiation belt population. A subsequent study then links scattering signatures to observations of relativistic electron precipitation events, revealing a significant coincidence rate between EMIC waves and precipitation events. These three investigations together provide the first quantifiable tracing of relativistic electron precipitation events back to the driving EMIC wave, through verified scattering signatures. The results support EMIC wave-relativistic electron resonant interaction theory and provide strong quantitative evidence that EMIC waves can effectively drive losses of core radiation belt electrons.The new knowledge gained here benefits the space physics community by informing space weather modelers and forecasters of the conditions that increase the efficiency of EMIC wave-driven radiation belt losses, and by introducing new and effective ways of identifying and analyzing EMIC wave-driven scattering to be used in future investigations
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Magnetospheric Particle Injections and their Relation to Impulsive, Localized Electric Fields
Full text linkEnergetic particle injections in the near-Earth plasma sheet are critical for supplying particles and energy to the radiation belts and ring current. Their origin, however, has been elusive due to the lack of equatorial, multi-point observations. After the launch of NASA's Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission in 2007, intense electric fields and elevated energetic particle fluxes have been observed to accompany localized (1-4 RE wide) bursty bulk flows and to propagate from the mid-tail regions (at geocentric radial distances R > 25RE) towards Earth, up to and at times inside of geosynchronous orbit (GEO, R=6.6RE). Motivated by these observations, I model simultaneous multi-point observations of electron injections using guiding center approximation in prescribed but realistic electric and magnetic fields to better understand the nature of their acceleration. Additionally, I perform a statistical analysis of the electron and ion injections to better understand their properties observationally. I find a good correlation between injections and azimuthally localized fast flows, dipolarization fronts and impulsive, dawn-dusk electric field increases. This correlation is present regardless of distance, from inside GEO out to 30 RE. The findings are inconsistent with the classical concept of injections forming from an azimuthally wide injection boundary moving earthward from ~9-12 RE to GEO under an enhanced, large-scale, duskward electric field. Modeling of electron injections assuming a localized, impulsive, potential electric field transported from mid-tail to near-Earth at bursty flow speeds of ~400 km/s successfully reproduces the observations at multiple spacecraft. Addition of a small, inductive electric field component, related to the dipolarizing magnetic field consistent with observations, further improves the agreement between modeled and observed electron spectra. The impulsive, localized, and vortical nature of the earthward-propagating electromagnetic pulse is what makes this model particularly effective in reproducing both the injection and the dispersed decrease in energy flux often observed simultaneously with the injection but at lower energies (~10-30 keV). The results suggest that particle acceleration and transport towards the inner magnetosphere can be thought of as a superposition of individual bursts of varying intensity and cadence depending on global geomagnetic activity levels
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Electron driven instabilities around dipolarizing flux bundles in Earth’s magnetotail
Full text linkDipolarization fronts (DFs) are transient phenomena in the magnetotail with various types of waves observed in their vicinity. The potential effect of these fluctuations on particle distributions and energy conversion near DFs is poorly understood. In this study, we aim to determine the relations between waves and dipolarization fronts. Using observational data from Time History of Events and Macroscale Interactions during Substorms (THEMIS), we established whistler wave event database and electron cyclotron harmonic (ECH) wave event database near dipolarization fronts from year 2007-2017. We find that electron temperature anisotropies are well-constrained by the marginal stability thresholds of whistler instability and electron firehose instability. During earthward transport of electrons, a significant portion of the suprathermal electron energy flux is evacuated in the form of whistler wave Poynting flux. Later we investigate the generation mechanism of ECH waves near dipolarization fronts. We find that moderately oblique (with wave normal angle at around 70⁰) ECH waves are frequently observed behind DFs and they are driven unstable by low energy electron beams. By performing a parametric survey of beam-driven ECH waves, we demonstrate that these waves are unstable under a wide range of plasma conditions. This work emphasizes the importance of wave-particle interaction in the magnetotail
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Energetic Electron Losses Driven by Whistler-Mode Waves in the Inner Magnetosphere: ELFIN observations and theoretical models
Full text linkResonant interactions between energetic radiation belt electrons and equatorially-generated whistler-mode waves are widely studied because they yield either electron acceleration or precipitation -- where electrons are scattered and lost into the Earth's atmosphere -- both of which are fundamental to space weather forecasting, which is an increasingly relevant challenge as society scales up its reliance on space technologies. This dissertation investigates the mechanisms that govern the effectiveness of electron losses from Earth's radiation belts driven by whistler-mode waves using novel electron precipitation measurements from the ELFIN CubeSats. A culmination of innovative engineering efforts and a refactored satellite operations program has allowed ELFIN to obtain over 12,500 high-quality, low-altitude electron measurements of the radiation belts. These measurements are uniquely capable of resolving the bounce loss cone, allowing us to probe the physics that drive electron precipitation in great detail. We first present a test particle simulation that directly compares ELFIN-measured electron precipitation with equatorial electron and wave measurements by the THEMIS and MMS spacecraft during magnetic conjunctions, confirming the importance of mid-high latitude wave-power. Next, we demonstrate that test particle simulations combined with an empirical wave amplitude model adequately approximate statistical ELFIN observations at the dawn, day, and dusk MLT sectors, but they significantly underestimate relativistic (>500$keV) electron losses on the nightside. To resolve this discrepancy, we additionally use quasi-linear diffusion simulation methods to find that considering wave obliquity, wave frequency, and plasma density together are required to recover the energetic portion (>100 keV) of precipitating electron spectra without overestimating the loss contributions from the quasi-linear regime (~100 keV). We conclude by presenting the ranges of wave and plasma characteristics necessary for the incorporation of accurately modeled electron loss rates into modern radiation belt models. This unlocks the potential to remotely sense equatorial wave properties using electron precipitation measurements, but also calls for future \textit{in situ} satellite experiments to more deeply understand the interconnected role of energetic electron losses in atmospheric, ionospheric, and magnetospheric dynamics
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Excitation of Electron Cyclotron Harmonic Waves in Earth's Magnetotail
Full text linkThis dissertation investigates the generation mechanism, spatial distribution and characteristics of electrostatic electron cyclotron harmonic (ECH) waves under different plasma sheet conditions, and quantifies the role of these waves in producing the diffuse aurora. THEMIS observations from five magnetotail seasons, along with ray-tracing, and electron diffusion codes have been utilized towards that goal. By modeling the wave growth and quasi-linear pitch-angle diffusion of electrons with realistic parameters for the magnetic field, loss-cone distribution and wave intensity (obtained from observations as a function of magnetotail location), we estimate the loss-cone fill ratio and the contribution of auroral energy flux from wave-induced electron precipitation. We conclude that ECH waves are the dominant driver of electron precipitation in the middle to outer magnetotail
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On the Generation and Expulsion of Plasmoids in Earth's Magnetotail
Full text linkPlasmoids have been observed over a broad distance along Earth's magnetotail, from X= -30 RE to -200 RE (X points positively sunward along the sun-Earth line). As described in the near-Earth-neutral-line (NENL) substorm model, reconnection at the NENL causes a plasmoid to be formed and ejected tailward. Because distant-tail (X<-100 RE) plasmoids are correlated one-to-one with large, isolated substorms, they are reliable remote signatures of substorms. Such a correlation, however, does not exist between mid-tail (-100 RE < X <-30 RE) plasmoids and substorms. Also, as indicated in recent studies, magnetic reconnection may be quite localized rather than extending across the entire magnetotail in the dawn-dusk direction. Hence, plasmoid formation and evolution are not well explained by the two-dimensional NENL substorm model. In this dissertation, I will reconcile these seemingly inconsistent observations and describe the formation and expulsion of plasmoids from a three-dimensional perspective using multi-point observations of mid-tail plasmoids and other reconnection-generated structures (dipolarization fronts and anti-dipolarization fronts). To understand the formation and expulsion of plasmoids in three dimensions, I investigate the following unresolved questions: What are the three-dimensional configurations of plasmoids in the near-Earth region, the mid-tail, and the distant-tail? Is local lobe reconnection required for plasmoid ejection? How does a plasmoid that originates near Earth evolve while propagating tailward? My results reveal that a mid-tail plasmoid is typically localized and its azimuthal extent increases with increasing substorm intensity. Local lobe reconnection is not always necessary for plasmoid ejection, and thus a plasmoid can grow due to continuous reconnections on closed field lines. Reconnection produced not only plasmoids, but also anti-dipolarization front (ADF), which shares similar observed properties with plasmoids but represents an interface between the reconnected hot outflow and the ambient plasma sheet plasma. In this dissertation, I present a case study suggesting that an ADF could evolve into a plasmoid
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Characteristics of Energetic Charged Particle Isotropy Boundaries in Earth's Magnetosphere
Full text linkIn this dissertation, I investigate the observational characteristics of 50 keV to ∼5 MeV electron and 50 keV to ∼2 MeV proton isotropy boundaries in Earth’s magnetosphere. Viewed from Low Earth Orbit, the isotropy boundary (IB) is the magnetic latitude poleward of which persistently isotropic pitch-angle distributions (Jprec/Jperp ∼ 1) are first detected, representing a fundamental transition from an adiabatic “inner magnetosphere” to a non-adiabatic “outer magnetosphere.” The IB is a near-instantaneous tracer of the equatorial magnetospheric field configuration, and provides a means to remote-sense its evolution under any geomagnetic conditions.
Here, I use particle data from the ELFIN mission to characterize the IB distribution in local time, energy, geomagnetic activity, and ≥50 keV precipitation from isotropic particles. I find these IBs primarily exhibit negative energy-latitude dispersion patterns consistent with equatorialmagnetic field-line curvature (FLC) scattering, with a 10%-30% chance of any particular energy channel exhibiting mesoscale-embedded positive dispersion structures, associated with wave-particle interactions and localized Bz gradients. The lowest latitude and most energetic IBs were in the pre-midnight sector, consistent with the location of maximal cross-tail current-sheet thinning. I identify that electron and proton IBs form the lower-latitude boundary of an FLC-dominated transition
region, separating the outer radiation belt/ring current from the inner edge of the plasma sheet (“PS2ORB” and “PS2RC”), resulting in perpetual loss of electrons and protons exceeding typical plasma sheet energies. I show this ≥50 keV precipitation is often sufficiently intense and distributed to produce ionization enhancements over a range of altitudes at auroral/sub-auroral latitudes.
Lastly, I use the information-theoretic technique of Mutual Information (MI) to characterize the drivers of IB characteristics in the solar wind and magnetosphere. From these observables, I construct an empirical predictive model of IB properties, which had previously never been reported for electrons, and extends the previously reported <200 keV proton IB models up to MeV energies. These results demonstrate a deep connection between the IB latitude, particle precipitation, andthe evolution of the magnetosphere
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The Role of Dipolarizing Flux Bundles in Magnetotail Dynamics
Full text linkTransient (~40s time scale) flux tubes carrying strong northward magnetic field and earthward flow are frequently observed in the magnetotail. Here I use the term "dipolarizing flux bundles" (DFBs) to describe these flux tubes. Thought to be generated by tail reconnection at XGSM=~-20 RE, DFBs travel earthward towards the inner edge of the plasma sheet, bringing significant changes to the near-earth magnetotail. After twenty years of extensive study on DFBs, numerous questions regarding their role in magnetotail dynamics remain. The THEMIS mission, which enables multi-point observations at different tail locations, provides the opportunity to answer many of these questions. With a statistical study based on THEMIS data, I explore the dipolarizing flux bundle's role in several aspects of magnetotail dynamics---its importance in magnetotail flux transport, the mechanisms controlling its motion, the modifications its motion makes to the ambient plasma, its current system, and the modifications it makes to the global magnetotail current system. First, I establish that DFBs are the major, high-efficiency magnetic flux carriers in near-earth magnetotail convection. Therefore, the DFB flux transport properties I found (e.g., DFBs transport more magnetic flux during substorm time) may shape all near-earth magnetotail convection. Then I investigate how a DFB's earthward motion modifies the plasma inside it as well as the ambient plasma. Dipolarizing flux bundle motion results in a total pressure buildup inside the ~1000km-thin DF layer; the buildup exerts a tailward force from there to retard DFB motion. This motion also builds up thermal pressure, the distribution of which requires the existence of field-aligned currents (FACs) near the DFB. To confirm the existence of these FACs, I infer the DFB-associated current system from magnetic field variations. The magnetic field variations are consistent with region-2-sense (towards/away from earth to the DFB's dusk/dawn side) FACs immediately earthward of the DFB and region-1-sense (opposite to region-2-sense) FACs inside the DF layer. Such a FAC configuration is similar to that of a substorm current wedge (SCW). In addition, the amount of current carried by several DFBs is sufficient to form a typical ~1 MA SCW. Therefore, I suggest that dipolarizing flux bundles are "wedgelets"---building blocks of a substorm current wedge
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Mechanisms of Plasma Energization during Magnetotail Reconnection in Earth’s Magnetosphere
Full text linkMagnetic reconnection, a process characterized by the mixing of distinct magnetic flux tubes within a magnetized plasma, results in the exchange of energy between the electromagnetic fields of these flux tubes and the confined plasma. This phenomenon is observed across various astrophysical entities, playing a pivotal role in the energization and global transport of plasma. On Earth, magnetic reconnection is integral to the dynamics of the magnetotail, facilitating the depressurization of the tail during earthward convection, markedly elevating the temperature of magnetotail plasma, and generating field-aligned particle beams that pro- mote wave generation and growth. Observations across the magnetotail and under varying levels of geomagnetic activity have demonstrated magnetic reconnection’s importance and prevalence. It significantly influences global magnetospheric plasma energy transport during both quiescent and active periods through episodic magnetic reconnection events that pro- pel heated plasma ejecta in both earthward and tailward directions. Given its central role in global plasma transport and energization within the magnetotail this Ph.D. thesis seeks to elucidate key aspects of magnetotail reconnection to enhance our understanding of the magnetotail’s evolution.Investigating the dynamics of magnetotail reconnection presents a multifaceted challenge. This Ph.D. thesis leverages ongoing inner-magnetosphere missions and advanced magnetic field models to dissect three critical aspects of this issue. 1) Assess whether global plasma convection during geomagnetic storms is influenced by intermittent or quasi-continuous mag- netotail reconnection. While prior studies have primarily concentrated on relatively quiet conditions, revealing two distinct plasma transport modes in the magnetotail—substorms characterized by irregular reconnection at approximately 25-30 Earth radii (RE) and steady magnetospheric convection featuring more consistent reconnection beyond 40 RE —a detailed investigation during intense storm conditions is notably absent. This analysis aims to cate- gorize magnetotail reconnection during storms as either intermittent or continuous, offering significant insights into energy transport mechanisms under such conditions. 2) Examine the frequency and impact of near-Earth magnetotail reconnection during storms, a phenomenon less understood compared to non-storm conditions traditionally located beyond 25 RE. With the recent solar cycle prompting more frequent storms, emerging evidence supports the oc- currence of very-near-Earth reconnection (VNERX). This study explores VNERX events and their contribution to storm dynamics. 3) Study the heating processes that take place during magnetotail reconnection. Simulations and observations show that most of the par- ticle energy gained through reconnection is in the form of thermal energy. In the near-Earth tail, gradients in plasma pressure can transfer the kinetic energy from reconnection ejecta into plasma heat far away from the x-line. However, in the mid-tail, where reconnection occurs most frequently, these gradients are diminished. We study whether particle heating is still significant in magnetotail reconnection at these distances, and what factors control that heating. The heating process of mid-tail reconnection has important implications towards understanding the thermal profile of the plasma sheet, a major source of plasma for the inner magnetosphere and the storm-associated ring current
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