Effects of Conductivity Asymmetry Between the Northern and Southern Latitudes on Toroidal and Poloidal Mode

When the ionosphere has significant differences between the northern and southern conductance, it is well known that quarter-wave modes may occur as standing shear Alfvén waves. Owing to the finite conductivity at the ionospheric boundary, quarter-wave modes tend to have strong damping and energy dissipation, which indicates that we should pay attention to time-dependent feature of such transient modes. We study how quarter-waves are excited in both of toroidal and poloidal modes by adopting a 3-D dipole wave model. Unlike the previous studies that mainly showed peak frequency of the wave, we investigate the effects of 1) dipolar geometry, 2) ratios of bandwidth and peak frequency, 3) differential feature of toroidal and poloidal modes, respectively. It is presented how quarter-wave modes depend on various conditions above by examining time histories of electric and magnetic fields in the MHD dipole model where a broad range of ionospheric conductivities is considered.