Pi 2 source region in the magnetosphere deduced from CPMN data

Abstract

In order to investigate the source region of Pi 2 and its propagation mechanism in the magnetosphere, we have developed a three-dimensional (3D) propagation model of MHD waves in the magnetosphere and compared it with observational results presented by Uozumi et al. [Propagation characteristics of Pi 2 magnetic pulsations observed at ground high latitudes. J. Geophys. Res. 109, A08203, doi:10.1029/2003JA009898]. We have assumed the propagation mechanism of Pi 2 as follows: fast mode waves are firstly generated at the Pi 2 source region, propagate from there in a 3D manner, and then excite shear Alfvén mode waves via mode conversion. Thus excited Alfvén mode waves then propagate to the Earth, and are observed as Pi 2 pulsations at ground stations. Based on the above propagation, we have assumed the propagation path of Pi 2 starting from a Pi 2 source region located on the magnetic equatorial plane to a ground station, and calculated the Alfvén transit time or time of flight ( TOF) of MHD waves propagating in the magnetosphere. The model for the 3D spatial distribution of the Alfvén speed and the fast mode speed (assumed identical, under cold-plasma condition) is constructed based on realistic magnetic field and plasma density models of the magnetosphere. Time differences in model TOFs among our ground stations have been compared with the observed differences in the Pi 2 maximum-power times among the ground stations presented by Uozumi et al. [Propagation characteristics of Pi 2 magnetic pulsations observed at ground high latitudes. J. Geophys. Res. 109, A08203, doi:10.1029/2003JA009898]. As a result, it is suggested that the most probable source location of Pi 2 is 9 R E and 22.5 MLT on the equatorial plane in the magnetotail. This is the first study that deduced the Pi 2 source location in a quantitative manner. The consistency between the observational and the numerical estimation result supports our assumption shown in the above as regards the energy transfer mechanisms of Pi 2s globally observed in the high-latitude region. It is suggested that the generation of fast mode waves in the near-Earth region at the onsets of substorms is the essential process for the global Pi 2 occurrence.