Multipoint observation of fast mode waves trapped in the dayside plasmasphere

Abstract

Multipoint observations of a dayside Pc4 pulsation event provide evidence of fast mode waves trapped in the plasmasphere (plasmaspheric cavity mode or virtual resonance). Time History of Events and Macroscale Interactions during Substorms (THEMIS)‐A, the primary source of data for the present study, was moving outward near noon and detected poloidal oscillations, characterized by the azimuthal electric field component Ey and the radial and compressional magnetic field components Bx and Bz. The structure of the plasmasphere was constructed from the mass density radial profile estimated from the frequency of toroidal standing Alfvén waves observed at this spacecraft. The outer edge of the plasmapause (the maximum of the equatorial Alfvén velocity VAeq) was located at L ∼ 7, and the minimum of VAeq was located at L ∼ 4, forming a potential well structure required for mode trapping. Relative to the ground magnetic pulsations observed in the H component at a low‐latitude station (L = 1.5), the Ey component exhibited a broad amplitude maximum around L ∼ 3.5 and maintained a nearly constant phase from L = 2 to L = 5. In contrast, the Bz component exhibited an amplitude minimum and switched its phase by 180° at L = 3.8. This radial mode structure is consistent with theoretical models of mode trapping. Also, the Ey and Bz components oscillated ±90° out of phase, as is expected for radially standing waves.