Formation of a magnetic duct by the compressional component of ULF oscillation: a ray tracing study of whistler-mode wave propagation

We investigate the propagation of whistler-mode waves in ultralow frequency (ULF) wave-derived magnetic ducts using ray tracing simulations in a two-dimensional dipole coordinate system on the meridian plane. The refractive index modulation caused by the ULF wave affects whistler-mode wave propagation. In this study, a magnetic duct structure at an L value (magnetic shell) of 6 is considered by a waveform of the fundamental mode with compressional and poloidal components. The total magnetic field strength increases depending on the ULF wave phase, forming duct structures that correspond to a decrease in the refractive index and vice versa. We model the refractive index distribution and compute the ray paths of whistler-mode waves under the settings of each ULF wave phase. We assume whistler-mode waves with a frequency of 20 or 70% of the electron gyrofrequency at the equator. The initial wave normal angle is 0 degrees at the equator. The results show duct propagation, while the frequency at which the ducting of whistler-mode waves switches with the duct structure varies with the ULF wave phase. The present study depicts the duct propagation caused by the ULF wave and suggests that ULF waves should cause spatial and temporal modulation of the whistler-mode wave via the duct propagation process. These features could help in the study of inner magnetospheric processes and aurora physics.