Thumbnail Image

Publication or External Link





Research on ionospheric modifications led to numerous studies of practical importance. This thesis highlights work carried out on this subject. Following an overview of plasma physics and ionospheric modification in Chapter 1, three main topics are discussed.

Chapter 2 examines the effects of long-term HF heating of the ionosphere. It was found that the plasma expands and transports along Earth's magnetic field line, resulting in the formation of plasma ``tubes'' referred to as artificial ionospheric ducts. While a computational model of HF-heated plasma transport was previously presented, experimental observations of artificial ducts and comparison against the model were missing. A study was conducted by performing several ionospheric heating experiments at the HAARP facility during different ionospheric conditions and times of day, and recording their effects with instruments on-board overflying satellites. The work culminated in the first large collection of satellite observations of HF-induced ionospheric ducts and plasma transport. Modeling comparisons against a representative subset of observations established the basic physics picture of ducts and their physical characteristics. Moreover, we present the first observations of HF wave focusing by ionospheric density depletions.

Chapter 3 deals with the study of Ionospheric Current Drive (ICD), a method for generating Ultra Low Frequency (ULF) and Extreme Low Frequency (ELF) waves using modulated heating of the F region ionosphere. A wave generation/propagation model of ICD had been presented along with successful proof of concept experiments. However test comparisons of observations and theory had not been performed. To this end, we carried out a set of parameter-sweep simulation studies that reveal the conditions for maximum generation efficiency. With these considerations, we show that the frequency dependence of the generated wave amplitudes predicted by the model is in qualitative agreement with the proof of concept experimental results. Future work will be necessary to fully understand the frequency response of ICD generated waves.

Chapter 4 deals with the generation of Very Low Frequency (VLF) electromagnetic (``whistler'') waves. Artificial whistler waves were known to be generated by modulated heating of the polar electrojet. We present the very first clear experimental observations of whistler waves generated by continuous heating of the upper ionosphere. The proposed generation mechanism relies on parametrically excited Lower-Hybrid (LH) waves and their non-linear conversion to whistler waves. After generalizing an existing LH-whistler conversion model, we present simulation results that are in good agreement with the observed whistler wave spectrum. A combination of the qualitative discussions and simulation results explains major peculiarities observed in the spectrum.

With regard to the first topic, the author took part in experimental planning, processed satellite data, performed simulation runs of an adopted plasma transport computational model, and compared simulation results with satellite observations. In the second topic the author adopted the ICD generation computational model, performed parameter-sweep simulation runs, and compared simulation results with experimental observations. In the last topic, the author took part in experimental planning, processed satellite data, wrote and benchmarked a computational code for an adopted mathematical model of LH-whistler conversion, performed simulation runs after generalizing the computational model, and compared simulation results against satellite observations.