The Evolution of Poloidal Alfvén Waves in the Dipolar Magnetic Field

Poloidal Alfven wave with high azimuthal wave number (m >> 1) in the Earth's magnetosphere is an important wave phenomenon to understand the wave-particle interaction since it is capable of interacting with the ring current ions via drift-bounce resonances. In this study, we examine temporal characteristics of the poloidal mode using MHD wave models with high grid resolution. Comparisons between 3-D simple box model and dipole model clearly show that the geometry of the magnetic field plays a significant role in determining temporal and spatial wave properties. We impose various standing poloidal Alfven waves with different azimuthal wave numbers and harmonics, and investigate their evolution in time. Our results show that poloidal Alfven waves are highly transient phenomena such that the initial poloidal wave energy is quickly transferred to the toroidal mode energy, which is opposite to the (toroidal) field line resonance. The time scale of the brevity in dipolar geometry turns out to be much shorter than that in the box model. Our results indicate that such time-dependent behavior of the poloidal mode wave is significantly affected by the geometry, therefore it should be taken into account when examining the wave-particle interaction. It also suggests that prolonged poloidal mode oscillations could be observed only if there is continuous wave excitation via the wave-particle interaction.