Gem: The Generation Of Falling-Tone Chorus And Scattering Of Particles By Chirped Waves

Plasma waves called 'chorus' are thought to be one of the most important waves for creating the high-energy electrons in the radiation belts from lower energy populations and are also believed to be involved in their loss through scattering into the atmosphere. Chorus waves are radio waves in the acoustic frequency range. If converted to acoustic waves they sound like whistling or chirping, hence the name. Fine structures in the waveform called rising and falling tones are the source of these chirping sounds. The rising tones have received a lot of attention, with several mechanisms suggested for their formation. Not so for the falling tones. This project will investigate further a new mechanism for the generation of falling tones and examine the interaction of radiation belts electron with chorus - both rising and falling tones. Research into these waves benefits not only the field of space physics but also plasma physics and fusion. In addition the project will provide training for a postdoctoral student and research support for an early-career scientist (the proposer) at the New Jersey Institute of Technology. There are significant differences in the mechanisms that have been proposed to generate chorus (focusing mostly on rising tones) compared to the new mechanism suggested for falling tones. Most current theories conclude that plasma density variations and/or inhomogeneity of the magnetic field are necessary to generate chorus. In fact, for rising tone chorus, simulations show that spectral characteristics of the chorus are directly related to the geomagnetic field inhomogeneity. However, the falling tone waves have much more oblique wave normal angles and, as a result, more resonances with plasma electrons are possible. Competition between resonances that cause growth and damping of the waves can make them marginally stable, resulting in nonlinear effects that produce chirping. This new picture of falling tone chorus generation along with scattering of electrons by chorus will be investigated through the combined use of analytic model developments and computational particle in cell (PIC) and Tristan-mp code runs. The roles of various plasma parameters will be determined from runs of a 1D Vlasov code, which solves for the time evolution of the electron distribution.