The main ionospheric trough (MIT) is a key density feature in the mid-latitude ionosphere and characterizing its structure is important for understanding GPS radio signal scintillation and HF wave propagation. While a number of previous studies have statistically investigated the properties of the trough, they have only examined its latitudinal cross sections, and have not considered the instantaneous two-dimensional structure of the trough. In this work, we developed an automatic optimization-based method for identifying the trough in Total Electron Content (TEC) maps and quantified its agreement with the algorithm developed in (Aa et al., 2020). Using the newly developed method, we created a labeled dataset and statistically examined the two-dimensional structure of the trough. Specifically, we investigated how Kp affects the trough’s occurrence probability at different local times. At low Kp, the trough tends to form in the postmidnight sector, and with increasing Kp, the trough occurrence probability increases and shifts premidnight. We explore the possibility that this is due to increased occurrence of troughs formed by subauroral polarization streams (SAPS). Additionally, using SuperDARN convection maps and solar wind data, we characterized the MIT's dependence on the interplanetary magnetic field (IMF) clock angle.

Particles trapped in the magnetosphere are naturally accelerated by the exchange of electromagnetic and kinetic energy, resulting in relativistic plasma populations. Through a number of processes, these particles can be scattered into the atmosphere and lost to interactions. Such precipitating particles can affect radio communications, ozone chemistry, and thermal structures. For these reasons, it is important to characterize loss mechanisms and quantify precipitation rates. This thesis examines one particular loss mechanism known as current sheet scattering (CSS). If interactions are negligible, charged particles in a magnetic field have approximately conserved quantities that characterize their motion provided the background field changes sufficiently slowly over space and time. The first of these ‘adiabatic invariants,’ the magnetic moment, is related to the particle’s mirror point along its bounce trajectory—the location at which the particle reverses direction in its journey from weaker to stronger B. In the equatorial region of the near-Earth magnetotail, where the radius of field line curvature of the magnetic field can become comparable to the gyroradius of ≈ 100 keV electrons, the homogeneity conditions needed for conservation of the magnetic moment of this population are broken. Upon passing through this location, known as the current sheet, these particles experience a chaotic change in their magnetic moment, and thus an alteration of their mirror point. This is the phenomenon of CSS. If the resulting mirror point lies within the atmosphere, the particle will most likely be lost through interactions. CSS is often investigated for highly relativistic electrons. However, recent observations suggest that this mechanism may account for a significant proportion of precipitating electrons between 100 and 300 keV during the substorm growth phase, a common space weather event wherein magnetic field lines in the near-Earth magnetotail become highly stretched. In this thesis, we test the efficacy of CSS as a loss mechanism for < 300 keV electrons by developing a relativistic charged particle tracer capable of solving complex trajectories in realistic magnetospheric magnetic field models. We then find distributional characteristics through Monte Carlo methods, comparing simulated ratios of loss- to total-flux with observations of the same quantities for a single substorm event. These observations are obtained by comparison of in situ measurements made by THEMIS (Time History of Events and Macroscale Interactions during Substorms) with ionospheric energy flux remotely sensed by PFISR (Poker Flat Incoherent Scatter Radar). Given an input distribution from THEMIS satellite measurements, we find agreement between observed and simulated loss- to total-flux ratios within an order of magnitude, with closer agreement for electrons between 100 and 300 keV. This implies CSS can explain a significant proportion of observed precipitation for the event studied and demonstrates its role as a prominent radiation belt loss mechanism. In particular, these findings suggest that the measured loss flux of < 300 keV electrons during such events can be immediately related to the geometry of the near-Earth magnetotail. This is further supported by a parametric study of initially field aligned distributions spawned at various nightside locations, showing a low-energy peak in the loss- to total-flux ratio at the boundary between the outermost radiation belt and the magnetotail. Measurements of particle orientation taken from THEMIS are low resolution, and agreement between simulated and observed loss- to total-flux ratios can be increased by assuming a more field aligned distribution for electrons below 100 keV. This suggests the presence of other physical processes besides CSS that may preferentially structure the pitch angle distributions of low energy electrons to be field aligned. Additional analysis is needed to identify these possible mechanisms. In summary, findings from this work support the role of CSS as an important contributor to < 300 keV electron loss during the substorm growth phase. Though there is an underestimation of loss for < 100 keV electrons, it is known that the empirical magnetic field models employed overestimate the radius of curvature in the current sheet. Furthermore, the dawn-dusk electric field has been neglected, though it has the possibility to produce field aligned electrons through current sheet acceleration. The inclusion of these effects in future studies may further improve agreement between simulation and observations.

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第19508号 / 理博第4168号 / 新制||理||1599(附属図書館) / 32544 / 京都大学大学院理学研究科地球惑星科学専攻 / (主査)教授 田口 聡, 教授 家森 俊彦, 教授 余田 成男 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

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. / 2999-01-01

The ionosphere is an electrically charged atmospheric region which is coupled to the sun, the magnetosphere, and the neutral atmosphere. The ionospheric state can significantly impact technological systems, especially those which utilize radio frequency energy. By studying ionospheric disturbances, it is possible to gain a deeper understanding of not only the ionosphere itself, but also the natural and technological systems it is coupled to. This dissertation research utilizes high frequency (HF) radio remote sensing techniques to study three distinct types of ionospheric disturbances. First, ground magnetometers and a new mid latitude SuperDARN HF radar at Blackstone, Virginia are used to observe magnetospheric Pi2 ultra low frequency (ULF) pulsations in the vicinity of the plasmapause. Prior to these pulsations, two Earthward moving fast plasma flows were detected by spacecraft in the magnetotail. Signatures of inner magnetospheric compression observed by the Blackstone radar provide conclusive evidence that the plasma flow bursts directly generated the ground Pi2 signature via a compressional wave. This mechanism had previously been hypothesized, but never confirmed. Next, ten SuperDARN radars in the North American Sector are used to investigate the sources and characteristics of atmospheric gravity waves (AGW) associated medium scale traveling ionospheric disturbances (MSTIDs) at both midlatitudes and high latitudes. Consistent with prior studies, the climatological MSTID population in both latitudinal regions was found to peak in the fall and winter and have a dominant equatorward propagation direction. Prior studies suggested these MSTIDs were caused by mechanisms associated with auroral and space weather activity; however, it is shown here that the AE and Sym-H indices are poorly correlated with MSTID observations. A new, multi-week timescale of MSTID activity is reported. This leads to the finding that MSTID occurrence is highly correlated with an index representative of polar vortex activity, possibly controlled by a filtering mechanism that is a function of stratospheric neutral wind direction. Finally, a case study of a radio blackout of transionospheric HF communications caused by an X2.9 class solar flare is presented. This study demonstrates the potential of a novel technique employing signals of opportunity and automated receiving networks voluntarily created by an international community of amateur radio operators. / Ph. D.

As humanity continues its expansion into space, the understanding of the near-Earth space environment has never been more critical. As the ionosphere and thermosphere form the boundary between Earth's atmosphere and outer space, characterization of these regions is critical to understanding geospace. The Ionospheric Connection Explorer (ICON), launched in 2019, sought to establish the effects of forcing on the ionosphere and thermosphere from below and above, in part by using observations of ultraviolet airglow, which have long been used as a tool for making remote sensing observations of the upper atmosphere. The Extreme Ultraviolet Spectrometer (EUV) instrument was included on ICON to measure atmospheric airglow between 54 and 88 nm in order to estimate the density and structure of the ionosphere. In this work, we analyze the EUV observations throughout the ICON mission, characterizing the signal observed at various wavelengths during normal operations and during nadir and lunar calibrations. We use the ICON EUV data to develop the first algorithm for retrieval of neutral densities from EUV airglow. / Doctor of Philosophy / As humanity continues its expansion into space, the understanding of the near-Earth space environment has never been more critical. The neutral (thermosphere) and charged (ionosphere) particles in the upper atmosphere, around the altitude where satellite orbit, play a key role as the boundary between Earth and space. The Ionospheric Connection Explorer (ICON), launched in 2019, sought to establish how the ionosphere and thermosphere change over time. It measured the density of particles using light emitted from the atmosphere by chemical reactions (airglow). Extreme Ultraviolet (EUV) light is highly energetic, almost as much as X-rays, and the EUV airglow emitted by the atmosphere at certain can be used to detect O^+. In this work, we examine the measurements from the ICON EUV detector at various wavelengths to determine what other particles can be seen. Notably, we find that the measurements contain information about neutral atomic oxygen and molecular nitrogen. We develop a technique for using the EUV airglow brightness to measure the amount of O and N_2, the first of its kind.

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The mechanisms involved in the transfer of mass and energy from the solar wind to any planetary magnetosphere is considered an important topic in space physics. With the use of the Mercury spacecraft MESSENGER's data, it has been possible to study these processes in an environment different, yet similar, to Earth's. These data have resulted in new knowledge advancing not only the extraterrestrial space plasma research, but also the general space physics field.   This thesis aims to investigate mechanisms for the transfer of mass and energy into Mercury’s magnetosphere, and magnetospheric regions affected by, and processes directly driven by, these. The work includes the Kelvin-Helmholtz instability (KHI) at the magnetopause, which is one of the main drivers for mass and energy transfer on Earth, the low-latitude boundary layer (LLBL), which is in direct connection to the magnetopause and proposed to be affected by the KHI, magnetospheric ultra-low frequency (ULF) waves driven by the KHI, and isolated magnetic field structures in the magnetosheath as possible analogues to the Earth magnetosheath plasmoids and jets.   Kelvin-Helmholtz waves (KHW) and the LLBL are identified and characterized. The KHWs are observed almost exclusively on the duskside magnetopause, something that has not been observed on Earth. In contrast, the LLBL shows an opposite asymmetry. Results suggest that the KHI and LLBL are connected, possibly by the LLBL creating the asymmetry observed for the KHWs.   Isolated changes of the total magnetic field strength in the magnetosheath are identified. The similar properties of the solar wind and magnetosheath negative magnetic field structures suggest that they are analogues to diamagnetic plasmoids found on Earth. No clear analogues to paramagnetic plasmoids are found.     Distinct magnetospheric ULF wave signatures are detected frequently in close connection to KHWs. Results from the polarization analysis on the dayside ULF waves indicate that the majority of these are most probably driven by the KHI. In general, likely KHI driven ULF waves are observed frequently in the Hermean magnetosphere.  Although similar in many aspects, Mercury and Earth show fundamental differences in processes and structures, making Mercury a highly interesting planet to study to increase our knowledge of Earth-like planets. / <p>QC 20170519</p>

Although lightning-generated whistlers have been studied for nearly a century, there are still questions to be answered. It is clear that, at least in a certain frequency range, these waves significantly contribute to the overall wave intensity in the inner magnetosphere. They also influence distribution functions of energetic particles in the Van Allen radiation belts. Thanks to the on-board implemented neural network for automated whistler detection, we can relate electromagnetic wave and energetic particle flux measurements performed by the low-altitude DEMETER spacecraft with the number and dispersion of whistlers detected during a certain time interval. We distinguish the cases with high and low whistler occurrence and we use this information to determine the overall effect of lightning-generated whistlers. Powered by TCPDF (www.tcpdf.org)