ELF/VLF Whistler-Mode Waves in Magnetic Ducts

Abstract

This dissertation presents a comprehensive investigation into the propagation and behavior of extremely-low frequency (ELF) and very-low frequency (VLF) whistler-mode waves within small-scale irregularities of the Earth\u27s magnetospheric magnetic field, known as magnetic ducts. These irregularities, observed by NASA\u27s Magnetospheric Multiscale (MMS) mission, are categorized as high-magnetic ducts (HBDs), where the magnetic field inside the duct is stronger than the surrounding field, and low-magnetic ducts (LBDs), where the field is weaker. Analytical criteria for wave ducting within these structures are derived and validated through two-dimensional, time-dependent electron-magnetohydrodynamic (EMHD) simulations, demonstrating the ability of these ducts to guide waves effectively. A detailed analysis of LBDs reveals their susceptibility to electromagnetic energy leakage. However, a non-leaking propagation condition is identified, whereby the duct width aligns with an integer multiple of the perpendicular wavelengths of the waves inside it. This condition provides a practical method for identifying non-leaking eigenmodes, with the number of such modes being proportional to the duct\u27s width and the ambient magnetic field magnitude. The propagation of these non-leaking waves observed by MMS is successfully modeled using EMHD simulations. The study also explores the phenomenon of mode switching between two whistler-mode waves with the same frequency and parallel wavelength but different perpendicular wavenumbers in regions with transverse inhomogeneities in plasma density and magnetic field. Analytical criteria for this mode switching are established and confirmed via simulations, revealing its dependence on critical values of plasma density and magnetic field within finite-sized transition layers. Additionally, new findings on magnetic shelf structures are presented, where observations from MMS are compared with similar waves guided by density shelves observed by the Van Allen Probes. Simulations demonstrate that magnetic shelves, akin to density shelves, effectively guide whistler-mode waves along the ambient magnetic field with minimal attenuation. The parameters of the guided waves are shown to depend on the characteristics of the shelves, with distinct differences and similarities between magnetic and density shelf ducting explored. Finally, a statistical analysis of 687 ELF/VLF wave events within B-ducts is conducted, examining spatial distributions and underlying mechanisms across different magnetospheric regions. Differences between dawn-side flank and night-side magnetotail events are identified, highlighting the influence of plasma density, magnetic field extrema, and wave parameters on B-duct formation and wave propagation. These findings enhance the understanding of ELF/VLF wave dynamics in Earth\u27s space environment, providing a framework for future studies of wave-particle interactions and ducted wave phenomena