09:30 〜 09:45
[PEM13-03] Raytracing study of whistler-mode waves in the magnetic duct formed by the compressional component of ULF waves
キーワード：whistler-mode wave、duct、ULF wave、ray tracing simulation
Whistler-mode waves are essential for radiation belt dynamics as they cause the acceleration of electrons from kiloelectron volts to megaelectron volts and the loss of radiation belt electrons through wave-particle interaction. Recent studies have highlighted the significance of duct propagation of whistler-mode waves in the radiation belt loss process, as resonant energy increases to the relativistic energy in the high latitude region where ducting whistler-mode waves can reach. The density duct caused by electron density structure has been studied for decades. On the other hand, it is clear from the dispersion relation that the refractive index is affected not only by the cold electron density but also by the magnetic field intensity.
This study investigates the propagation of whistler-mode waves in ULF wave-derived magnetic ducts using two-dimensional ray-tracing simulations in the dipole coordinate system on the meridian plan. Assuming a magnetic duct structure at L=6, we consider the dipole field and a waveform of ULF wave oscillation of the fundamental mode with compressional and poloidal components. The refractive index variations inside the duct are then calculated from the magnetic field variations caused by the modeled compressional component of the ULF wave. We assume standing ULF waves and use a Gaussian function for the duct's shape in the meridian plane, while the modeled ULF waveform and the scale factor give the rate of change of the magnetic field and the duct width. The refractive index structure is modeled every eighth of a ULF wave period, and we compute the ray path of whistler-mode waves under the settings of every ULF wave period. The background magnetic field increases or decreases with each ULF phase, forming duct structures corresponding to a decrease or an increase of the refractive index depending on the wave phase of the modeled ULF wave.
We assume whistler-mode waves whose frequency is 0.2 Ωce or 0.7 Ωcd, where Ωce is the electron gyrofrequency at the equator of L=6, and 0-degree initial wave normal angle at the equator. The simulation results demonstrate duct propagations, while the frequency at which ducting of whistler-mode waves is observed switches depending on the duct structure caused by the modeled ULF wave. The whistler-mode waves of 0.2 Ωce/0.7 Ωce are ducted by the refractive index increased/decreased duct, and the arrival magnetic latitudes of the whistler-mode waves are near 27° and more than 40°, respectively. We also confirm the propagation of whistler-mode waves inside the ducts in other harmonics.
This study investigates the propagation of whistler-mode waves in ULF wave-derived magnetic ducts using two-dimensional ray-tracing simulations in the dipole coordinate system on the meridian plan. Assuming a magnetic duct structure at L=6, we consider the dipole field and a waveform of ULF wave oscillation of the fundamental mode with compressional and poloidal components. The refractive index variations inside the duct are then calculated from the magnetic field variations caused by the modeled compressional component of the ULF wave. We assume standing ULF waves and use a Gaussian function for the duct's shape in the meridian plane, while the modeled ULF waveform and the scale factor give the rate of change of the magnetic field and the duct width. The refractive index structure is modeled every eighth of a ULF wave period, and we compute the ray path of whistler-mode waves under the settings of every ULF wave period. The background magnetic field increases or decreases with each ULF phase, forming duct structures corresponding to a decrease or an increase of the refractive index depending on the wave phase of the modeled ULF wave.
We assume whistler-mode waves whose frequency is 0.2 Ωce or 0.7 Ωcd, where Ωce is the electron gyrofrequency at the equator of L=6, and 0-degree initial wave normal angle at the equator. The simulation results demonstrate duct propagations, while the frequency at which ducting of whistler-mode waves is observed switches depending on the duct structure caused by the modeled ULF wave. The whistler-mode waves of 0.2 Ωce/0.7 Ωce are ducted by the refractive index increased/decreased duct, and the arrival magnetic latitudes of the whistler-mode waves are near 27° and more than 40°, respectively. We also confirm the propagation of whistler-mode waves inside the ducts in other harmonics.