Typescript. / Thesis (Ph. D.)--University of Hawaii, 1969. / Bibliography: leaves [217]-224. / xiii, 224 l illus

Thesis (Ph. D.)--George Mason University, 2007. / Title from PDF t.p. (viewed Jan. 21, 2008). Thesis director: Michael E. Summers, Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Computational Sciences and Informatics. Vita: p. 204. Includes bibliographical references (p.193-203). Also available in print.

Thesis (Ph.D)---University of Adelaide, 1954.

Thesis (M.A.)--Boston University / A study of ionospheric scintillations by the reception of signals from the Russian satellite 1962 Cosmos I was undertaken. The data were obtained at the Sagamore Hill Radio Observatory, Hamilton, Massachusetts, of the Air Force Cambridge Research Laboratories. The satellite transmitted at a frequency of 20 Mc/s and it was found possible to receive the first two harmonics at 40 Mc/s and 60 Me/s. A high percentage of scintillation was observed on all nighttime transits of the satellite. No marked variation of scintillation index with elevation angle was observed. The scintillation index varied with wavelength as lambda^(0.78) for magnetically quiet days and as lambda^(0.36) for magnetically disturbed days. A pronounced latitude dependence of scintillations was observed. The onset of scintillations occurred at a sub-ionospheric latitude (referred to a height of 300 km) of 35 degrees North for magnetically disturbed days. A zone of almost uniform scintillation exists beyond a latitude of 41 degrees North. On certain occasions the satellite passed through clouds of irregularities, the size of which were about 550 km. A model of the non-uniform distribution of irregularities in the ionosphere is proposed as a result of this study.

After a brief historical introduction, the apparatus used in the investigation of the ionosphere in Grahamstown, is described with special reference to the recording equipment, which consists essentially of a camera focussed on the screen of a C.R.O. with z-axis deflection. The equations of the magneto-ionic theory are quoted. The basic formulae of the theories of layer formation in an isothermal (Chapman's),and linear temperature gradient (Gledhill and Szendrei) atmosphere are also quoted.

Thesis advisor: Michael J. Naughton / Thesis advisor: Keith M. Groves / The low-latitude ionosphere is characterized by large-scale instabilities in the post-sunset hours due to the distinct geometry of the earth’s magnetic field lines at the equator. The magnetic field lines are horizontal at the equator contributing to the high vertical drift velocity of the plasma bubbles growing from the bottomside of the ionospheric F-region. The phenomenon, commonly known as equatorial spread F, is an important problem in aeronomy as it can cause radio wave scintillation effects representing the most critical impacts of space weather on man-made technologies, such as satellite communications and global navigation satellite systems (GNSS). Here, we present results from an artificial ionospheric modification experiment as well as from naturally occurring instabilities in the equatorial ionosphere. An artificial plasma cloud was created in the bottomside of the ionospheric F-layer during the Metal Oxide Space Cloud (MOSC) experiment in May 2013 to study the interactions of artificial ionization with the background plasma under the hypothesis that the artificial plasma might suppress the occurrence of natural instabilities. While the suppression hypothesis remains open to debate, the propagation results confirm that the injection of artificial ionization in the lower F–region causes dramatic changes to the ambient HF propagation environment. We also calculate various parameters needed to evaluate the growth rate of Rayleigh- Taylor instability created in the F-region bottomside of the ionosphere from the thirteen days of High-Frequency (HF) radar data during the MOSC campaign. These parameters have been used to calculate the growth rate to predict the diurnal variability of the spread F occurrence. The growth rate has also been calculated from model ionospheric profiles optimized by ray-tracing techniques to match actual delays as observed in the oblique HF links. The calculated growth rate provides a close prediction of spread F development as seen in its correlation with the ground scintillation observations. With regard to natural processes, data from the Air Force Research Laboratory (AFRL) / the National Aeronautics and Space Administration (NASA) Communications/Navigations Outage Forecasting System (C/NOFS) satellite mission has been analyzed to investigate the characteristics of equatorial ionospheric irregularities from in situ observations. We present a comprehensive investigation on the variation of apex-altitude distribution of equatorial ionospheric irregularities with solar activity supported by modeling, simulation and comparisons with ground- and space-based in situ density observations. We also analyze Physics Based Model (PBMOD) ionospheric model results to determine if a physics-based model can reproduce the observed dependence of bubble height on solar activity. / Thesis (PhD) — Boston College, 2019. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

Abstract This thesis will present an investigation into the variability of the maximum height of the ionospheric F2 layer, hmF2, with hour, season and latitude over the South African region. The dependence of hmF2 on solar and magnetic activity is also investigated. Data from three South African stations, namely Madimbo (22.4 S, 26.5 E), Grahamstown (33.3 S, 26.5 E) and Louisvale (28.5 S, 21.2 E) were used in this study. Initial results indicate that hmF2 shows a larger variability around midnight than during daytime for all the seasons. Monthly median values for hmF2 were used in all cases to illustrate the variability, and the International Reference Ionosphere (IRI) model has been used to investigate hmF2 predictability over South Africa. This research represents the initial steps towards a predictive model for the hmF2 parameter, with the long term aim of developing a new global hmF2 predictive model for the IRI. It is believed that this work will contribute signi cantly towards this aim through the understanding of the hmF2 parameter over a region that has not previously been investigated.

Abstract The high-latitude polar ionosphere is characterized by two regions, the polar cap and the auroral oval. In the polar cap, the geomagnetic field lines are open and connect to the solar wind, whereas the field lines in the auroral oval are closed and map to the plasma sheet and the plasma sheet boundary layer in the magnetosphere. The two substantially different magnetic and plasma domains are separated by a separatrix, the polar cap boundary (PCB), which is an ionospheric projection of the open-closed field line boundary (OCB) in the magnetosphere. In this thesis, a new method to determine the location of the PCB in the nightside ionosphere based on electron temperature measurements by EISCAT incoherent scatter radars is introduced. Comparisons with other PCB proxies like poleward boundary of the auroral emissions, poleward edge of the auroral electrojets and poleward boundary of energetic particle precipitation show general agreement. By applying the method to several events together with other supporting ground-based and space-borne observations, dynamic processes and phenomena in the vicinity of the PCB and inside the auroral oval are studied. The main results include the following. During substorm expansion, the PCB moves poleward in a burstlike manner with individual bursts separated by 2–10 min, indicating impulsive reconnection in the magnetotail. In one event, a possible signature of the high-altitude counterpart of the Earthward flowing field-aligned current of the Hall current system at the magnetotail reconnection site is observed. Investigation of the relation between the auroral activity and the local reconnection rate estimated from the EISCAT measurements reveals direct association between individual auroral poleward boundary intensifications (PBIs) and intensifications in the ionospheric reconnection electric field within the same MLT sector. The result confirms earlier suggestions of positive correlation between PBIs and enhanced flux closure in the magnetotail. In another event, quiet-time bursty bulk flows (BBFs) and their ionospheric signatures are studied. The BBFs are found to be consistent with the so called "bubble" model with Earthward fast flows inside the region of depleted plasma density (bubble). The tailward return flows show an interesting asymmetry in plasma density. Whereas the duskside return flows show signatures of a depleted wake, consistent with a recent suggestion, no similar feature is seen for the dawnside return flows, but rather increase in density. The BBFs are associated with auroral streamers in the conjugate ionosphere, consistently with previous findings. The related ionospheric plasma flow patterns are interpreted as ionospheric counterpart of the BBF flows, excluding the dawnside return flows which could not be identified in the ionosphere. The BBFs and streamers are found to appear during an enhanced reconnection electric field in the magnetotail.

published_or_final_version / Physics / Doctoral / Doctor of Philosophy

published_or_final_version / Physics / Master / Master of Science