摘要：The propagation of compressional Pi 2 waves in the inner magnetosphere is investigated by analyzing the onset delay times between the ground and the geosynchronous altitude.We use the compressional component (northward) of magnetic data from low-latitude stations and the geosynchronous satellite ETS-VIII (GMLat.= -10.8°, GMLon.= 217.5°).The onset delays are determined by a cross-correlation analysis, and we analyzed the events with high waveform correlations (correlation coefficient greater than 0.75).Some of these high-correlation events have the properties of propagating waves; Pi 2 waveforms at the ground stations and the satellite were synchronized with each other when the data were shifted by onset delays.The results of the statistical analysis show that 87% of the Pi 2 onsets at a ground station (Kuju, GMLat.= 26.13°, GMLon.= 202.96°) were delayed from the Pi 2 onsets at ETS-VIII, and the average of the delay times was 29 sec.This clearly shows Pi 2 onsets (initial perturbations of Pi 2) propagated from the geosynchronous altitude to the low-latitude ground.The delay times tended to be larger around the midnight sector than around the dawn and dusk sectors.These results are consistent with two-dimensional propagation of fast waves estimated by the model of Uozumi et al.(J Geophys Res 114:A11207, 2009).The delay times are nearly identical to the travel time of fast waves from geosynchronous altitude to the low-latitude ground, and the local time variation of the delay shows the azimuthal propagation along the geosynchronous orbit.We conclude that the initial compressional perturbations of Pi 2 waves propagate radially and longitudinally as a fast wave in the inner magnetosphere.
