Nonlinear amplification and generation of very low frequency waves in the near-earth space environment [electronic resource]
Keywords: Electrical Engineering.

Whistler-mode waves are intense electromagnetic radio waves that are known to be key drivers of energetic particle dynamics in the Earth's radiation belts. Despite many decades of active research, fundamental questions still remain concerning the interaction of whistler-mode waves with energetic electrons, especially in regard to nonlinear phenomena including wave amplification. The Siple Transmitter Experiment, which operated in Antarctica from 1973 to 1988, provided researchers a rare opportunity to perform controlled experiments on nonlinear wave-particle interactions. Very low frequency waves in the few kilohertz range radiated by the transmitter at Siple Station propagate into the magnetosphere where they interact with energetic electrons. The interactions modify the waves, amplifying them and generating new frequency components. The modified waves are then observed with radio receivers on the ground at the magnetic conjugate point in the northern hemisphere. The Siple Experiment produced many new and fascinating observations of wave-particle interactions; however, during the experiment, available data analysis tools allowed for only a fraction of the data collected to be analyzed. Moreover, theoretical understanding of the physical processes driving the interactions was limited at the time. We present statistical analysis of data collected in 1986 from the Siple Experiment to quantify nonlinear growth rates and total nonlinear growth of very low frequency waves injected into the magnetosphere, which are useful for bounding theoretical discussion and numerical simulation of wave-particle interactions. We consider a specific amplification phenomenon, termed preferential magnetospheric amplification, by quantifying the preferential effect of injected rising versus falling swept-frequency waves and by examining the theoretical mechanism involved using a Vlasov-Maxwell numerical simulation. The modeling efforts, validated by the data, reveal that the total amplification depends on the linear growth rate and the onset and duration of nonlinear growth, which for typical hot plasma parameters favors injected rising swept frequency waves. In addition to the amplification process, wave-particle interactions can also generate new very-low frequency waves called triggered emissions. These triggered emissions have not been yet well characterized and pose significant challenges for theoretical interpretation and numerical modeling. We present a comprehensive description of triggered emission behavior observed in 1986 by quantifying time and frequency changes and demonstrating the significant dependence of the emission behavior on the frequency-time format of the injected signal. The triggered emission characteristics can be used to evaluate the validity of current numerical models and provide a framework for describing wave behavior in the magnetosphere. Finally, we demonstrate the application of modern machine learning techniques for predicting observations of transmissions from the Siple experiment as a suggested approach to further study of the large sets of data in the field of space physics.