Thesis (M.Sc.Eng.) 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. / The study of wave-wave interactions in the ionosphere is important for designing communication systems, satellite systems, and spacecraft. Ionospheric research also informs laser and magnetic fusion plasma physics. This thesis focuses on two nonlinear wave-wave interactions in the ionosphere. The first interaction is a nonlinear mode conversion. Very Low Frequency (VLF) waves transmitted from the ground travel through the ionosphere as injected whistler waves. The whistler waves interact with naturally-occurring density fluctuations in the ionosphere and are mode converted into lower hybrid waves. The lower hybrid waves accelerate electrons along the geomagnetic field and the resulting beam mode Langmuir waves are detectable by radar. This type of mode conversion may combine additively with a four wave interaction with the same VLF wave as its source. Data collected at the Arecibo Observatory in Puerto Rico during the occurrence of spread F and sporadic E was analyzed. Plasma line enhancements may indicate the nonlinear mode conversion both separately from and in conjunction with the four wave interaction. The second nonlinear wave-wave interaction is the parametric decay instability (PDI) excited by High Frequency (HF) heater waves at the High Frequency Active Auroral Research Program facility in Gakona, Alaska. Resonant PDI cascades downwards, resulting in up-shifted ion line enhancements as detected by radar. This process has been detected in the presence of down-shifted ion line enhancements which may be caused by beating between PDI-produced Langmuir waves, or by naturally occurring ionospheric currents. / 2031-01-01

Internal waves are prominent fluid phenomena in both the atmosphere and ocean. Because internal waves have the ability to transfer a large amount of energy, they contribute to the global distribution of energy. This causes internal waves to influence global climate patterns and critical ocean mixing. Therefore, studying internal waves provides additional insight in how to model geophysical phenomena that directly impact our lives. There is a myriad of fluid phenomena with which internal waves can interact, including other internal waves. Equipment and processes are developed to perform laboratory experiments analyzing the interaction of two colliding nonresonant internal waves. Nonresonant interactions have not been a major focus in previous research. The goal of this study is to visualize the flow field, compare qualitative results to Tabaei et al., and determine the energy partition to the second-harmonic for eight unique interaction configurations. When two non-resonant internal waves collide, harmonics are formed at the sum and difference of multiples of the colliding waves' frequencies. In order to create the wave-wave interaction, two identical wave generators were designed and manufactured. The interaction flow field is visualized using synthetic schlieren and the energy entering and leaving the interaction region is analyzed. It is found that the energy partitioned to the harmonics is much more dependent on the general direction the colliding waves approach each other than on the angle. Depending on the configurations, between 0.5 and 7 percent of the energy within the colliding waves is partitioned to the second-harmonics. Interactions in which the colliding waves have opposite signed vertical wavenumber partition much more energy to the harmonics. Most of the energy entering the interaction is dissipated by viscous forces or leaves the interaction within the colliding waves. For all eight configurations studied, 5 to 8 percent of the energy entering the interaction has an unknown fate.

Electromagnetic waves can be used to transmit information over long distances and are therefore often employed for communication purposes. The electromagnetic waves are reflected off material objects on their paths and interact with the medium through which they propagate. For instance, the plasma in the ionosphere can refract and even reflect radio waves propagating through it. By increasing the power of radio waves injected into the ionosphere, the waves start to modify the plasma, resulting in the generation of a wide range of nonlinear processes, including turbulence, in particular near the reflection region. By systematically varying the injected radio waves in terms of frequency, power, polarisation, duty cycle, inclination, etc. the ionosphere can be used as an outdoor laboratory for investigating fundamental properties of the near-Earth space environment as well as of plasma turbulence. In such ionospheric modification experiments, it has been discovered that the irradiation of the ionosphere by powerful radio waves leads to the formation of plasma density structures and to the emission of secondary electromagnetic radiation, a phenomenon known as stimulated electromagnetic emission. These processes are highly repeatable and have enabled systematic investigations of the nonlinear properties of the ionospheric plasma. In this thesis we investigate features of the plasma density structures and the secondary electromagnetic radiation. In a theoretical study we analyse a certain aspect of the formation of the plasma structures. The transient dynamics of the secondary radiation is investigated experimentally in a series of papers, focussing on the initial stage as well as on the decay. In one of the papers we use the transient dynamics of the secondary radiation to reveal the intimate relation between certain features of the radiation and structures of certain scales. Further, we present measurements of unprecedentedly strong secondary radiation, attributed to stimulated Brillouin scattering, and report measurements of the secondary radiation using a novel technique imposed on the transmitted radio waves.

Nonlinear wave-wave interactions and wave breaking contribute to nearshore wave energy dissipation. These factors can be analyzed by the principles of wave self-similarity. The equilibrium range can be shown in wind-driven wave spectra that exist in the form ( ) and However, the appropriate methods used to determine this loss of energy are controversial. This study examines an approach that reinvestigates the self-similarity principles. Wave spectra with lower peak periods are dominated by nonlinear wave-wave interactions which produce a scaling in shallow water. This thesis investigates the relative role of spectral similarity in different conditions in the nearshore region of the U.S. Army Corps of Engineers Field Research Facility in Duck, North Carolina. The results show young sea waves (wave spectra in which the propagation speed of waves at the spectral peak is much smaller than the wind speed) are dominated by nonlinear wave-wave interactions in the nearshore while older waves (wave spectra in which the propagation speed of waves at the spectral peak is equal to or greater than the wind speed) are dominated by wave breaking in deep water. Furthermore, nearshore wave models need to incorporate the self-similarity concept in deep and shallow water to better understand and quantify important aspects of wave physics in shallow water.

L'Océan Austral est une zone clef pour la circulation océanique tant à cause de l'intensité du courant circumpolaire antarctique qu'en tant que région de formation des masses d'eaux abyssales de l'océan global. Pour modéliser l'océan et prévoir les changements climatiques futurs, il est important de comprendre les processus de mélange diapycnal qui lient ces eaux abyssales aux couches supérieures. Dans l'Océan Austral, des courants profonds et intenses s'écoulent sur une topographie accidentée, ce qui génère des ondes internes de relief très énergétiques. Actuellement, la dissipation de l'énergie induite par ces ondes de relief est la candidate principale pour expliquer les forts taux de mélange observés à ces latitudes. L'objet du présent travail de thèse est de comprendre comment les ondes internes de relief sont dissipées et affectent la circulation et le mélange diapycnal dans l'océan abyssal. Nous examinons l'impact de ces ondes sur le mélange profond au moyen d'une combinaison d'expertise de terrain, de simulations non hydrostatiques bi-dimensionnelles et de calculs théoriques. Sur la gamme de paramètres étudiés, nous montrons, en présence des ondes de relief, une intensification du taux de dissipation d'énergie cinétique turbulente sur une profondeur de 1000 m au-dessus de la topographie, atteignant typiquement ~20 mW/m2. Nous montrons également comment les ondes participent à des interactions triadiques impliquant des oscillations inertielles qui sont amplifiées par intéractions résonantes contrôlées par les ondes de relief. Finalement, nous préparons de futures études tri-dimensionnelles en concevant un cadre numérique et en décrivant des outils théoriques adaptés à ce problème. Nos résultats préliminaires en trois dimensions montrent qui le confinement méridien de la topographie réduit significativement l'émissions d'ondes internes de relief. / The Southern Ocean plays a key role in global ocean circulation by connecting the major ocean basins with the intense Antarctic Circumpolar Current and as a formation region for abyssal water masses of the global ocean. Understanding the diapycnal mixing processes that link these abyssal waters to the overlying layers is essential both for ocean modelling and for predicting future climate change. In the Southern Ocean, deep reaching currents impinge on rough topography and create highly energetic internal lee waves. The dissipation of the energy of these internal lee waves is the main candidate for explaining the high mixing rates between waters of different densities observed at these latitudes. The purpose of this study is to understand the fate of the internal lee wave energy and how it affects the circulation and diapycnal mixing in the abyssal ocean. We first study the impact of internal lee waves on deep mixing with the combination of field expertise, two-dimensional non hydrostatic numerical simulations and theoretical developments. Over the range of parameters studied, an enhanced bottom turbulent kinetic energy dissipation is observed in the bottom 1000 m, typically reaching $sim$ 20 mW.m$^{-2}$. We further show that internal lee waves undergo non-dissipative wave-wave interactions that can be rationalized as resonant triad interactions between the bottom emitted internal lee waves, inertial oscillations and linear combinations of these two waves. We then build a three-dimensional model configuration and specific diagnostic methods that pave the way for future investigations in three dimensions. Preliminary results with the three-dimensional numerical configuration show that the meridional confinement of the topography notably reduces the emission of internal lee waves.