09:00 〜 10:30
[PEM17-P10] Simulation study of ULF wave propagation in the inner magnetosphere using a spectral method
Ultra Low Frequency (ULF) waves are low-frequency oscillations observed in the Earth’s inner magnetosphere with periods ranging from a few seconds to a few hundred seconds. One type of ULF waves, called Pc4-5, has periods ranging from tens to hundreds of seconds (Jacobs et al, 1964) and can be described by the magnetohydrodynamic (MHD) approximation. The excitation and propagation processes of ULF waves have been extensively studied. Field line resonance (FLR) is a process in which fast mode and Alfvén waves, which are excited and propagate outside the magnetosphere, resonate with each other. This process accounts for the magnetic field line oscillations that are associated with Pc4-5 phenomena (Dungey, 1954; Chen & Hasegawa, 1974; Southwood, 1974).
We have performed a one-dimensional simulation of FLR along the magnetic field lines using the ideal MHD model and the spectral method with fixed field line end conditions. This boundary condition means the field displacement is small in the ionosphere (Southwood and Kivelson, 1981). We use a cylindrical coordinate system and do not consider the curvature of the magnetic field lines. Our method differs from Mann et al., 1995 which used a spectral method as we consider thermal pressure and background magnetic field gradient.
The simulation results showed two different frequencies of oscillations. The high-frequency oscillation has a period that depends on beta and longitude that depends on wavelength, while the low-frequency oscillation does not have these dependencies. From these results, we inferred that the high-frequency wave was a fast wave and the low-frequency wave was an Alfven wave. Moreover, we confirmed that the longitudinal structure causes the interaction between toroidal and poloidal modes. The effect of field lines being fixed at the ionosphere boundary also added more complexity. The fixed field line end conditions effect showed that oscillations with different wavelengths in the direction of the magnetic field lines interact with each other. This means that the fundamental, second-harmonic, and third-harmonic modes affect each other. We suggest that this effect may be observable when the plasma pressure is relatively high, with plasma beta around 1. The advantage of using the spectral method in the above results is that it clearly demonstrates that the interaction factor is the fixed field line end condition caused by the ionosphere.
We have performed a one-dimensional simulation of FLR along the magnetic field lines using the ideal MHD model and the spectral method with fixed field line end conditions. This boundary condition means the field displacement is small in the ionosphere (Southwood and Kivelson, 1981). We use a cylindrical coordinate system and do not consider the curvature of the magnetic field lines. Our method differs from Mann et al., 1995 which used a spectral method as we consider thermal pressure and background magnetic field gradient.
The simulation results showed two different frequencies of oscillations. The high-frequency oscillation has a period that depends on beta and longitude that depends on wavelength, while the low-frequency oscillation does not have these dependencies. From these results, we inferred that the high-frequency wave was a fast wave and the low-frequency wave was an Alfven wave. Moreover, we confirmed that the longitudinal structure causes the interaction between toroidal and poloidal modes. The effect of field lines being fixed at the ionosphere boundary also added more complexity. The fixed field line end conditions effect showed that oscillations with different wavelengths in the direction of the magnetic field lines interact with each other. This means that the fundamental, second-harmonic, and third-harmonic modes affect each other. We suggest that this effect may be observable when the plasma pressure is relatively high, with plasma beta around 1. The advantage of using the spectral method in the above results is that it clearly demonstrates that the interaction factor is the fixed field line end condition caused by the ionosphere.