Local Time Dependence of Preliminary Impulse Propagation and the Implications on Density Distribution in the Magnetosphere

The preliminary impulse of sudden commencements is a unique phenomenon in the interaction between the solar wind and the magnetosphere in which the impact of an enhanced solar wind pressure produces a strong pulse propagating throughout the magnetosphere. This propagation provides an opportunity for ground observers both to study the fundamental process of wave propagation in the magnetosphere and to understand the state of the magnetosphere. Recently, observations have provided tangible evidence that preliminary impulses propagate to the ground mainly by means of MHD waves. Clear discontinuity of the arrival time of preliminary impulses is found at the plasmapause latitude, where the plasma density varies significantly. The propagation velocities and patterns are also likely to vary across the noon-dusk sector because the plasma density is also a function of local time. Our study is aimed to catalog a number of preliminary reverse impulse (PRI) events for varying local times in hopes to confirming the process by which the wave signal propagates. We have examined several PRI events in ground records at a variety of local times. The PRI arrival time for local times close to noon is in good agreement with the estimation of arrival time from MHD wave propagation and in contradiction to the prediction by the Earth-ionosphere waveguide model. The polarization pattern is also found to vary consistently with local time, a direct result of the station's location with respect to the Hall current vortex. We further apply travel-time magnetoseismology to find that density distribution implied by these observations. A larger variation in arrival time for local times closer to dusk implies that the density inversion can be complicated by the existence of the plasmatail or drainage plumes.