THEMIS multipoint observations of Pi2 pulsations inside and outside the plasmasphere

Abstract

[1] Much evidence indicates that Pi2 pulsations in the inner magnetosphere can be explained as radially trapped fast-mode waves in the plasmasphere. Most of these studies, however, are based on observations from a single spacecraft in the plasmasphere and low-latitude ground stations within several hours of local time. Here we present simultaneous observations of two Pi2 pulsations on 26 January 2009 by three THEMIS probes (P3, P4, and P5) and the Ascension Island (ASC) ground station (L ∼ 1.002, MLT ∼ 23). By using plasma density and spacecraft potential measurements, we determined that during this time P4 and P5 were located inside the plasmasphere and P3 was located outside it. It is found that fast-mode components (compressional component Bz, azimuthal component Ey) observed by P4 and P5 have a high correlation with the H component at ASC while those at P3 do not. The Bz oscillations at P4 are nearly in phase with ground Pi2 while those at P5 near the plasmapause are almost out of phase with the H component on the ground during the two Pi2 events. The Ey -H cross phases at P4 and P5 are nearly constant at −90° for both Pi2 events. These multipoint observations strongly suggest that fast-mode waves trapped in the plasmasphere are the source of the low-latitude Pi2 pulsations.